// -*- mode: c++; c-basic-offset: 4; flycheck-clang-language-standard: "c++14"; -*-

/*
 * Copyright (c) 2012-2017 Chad Austin
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy,
 * modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#pragma once

#include <assert.h>
#include <stdarg.h>
#include <stddef.h>
#include <string.h>
#include <math.h>
#include <limits.h>
#include <algorithm>
#include <cstdio>
#include <limits>

#ifndef SAJSON_NO_STD_STRING
#include <string> // for convenient access to error messages and string values.
#endif

#if defined(__GNUC__) || defined(__clang__)
#define SAJSON_LIKELY(x) __builtin_expect(!!(x), 1)
#define SAJSON_UNLIKELY(x) __builtin_expect(!!(x), 0)
#define SAJSON_ALWAYS_INLINE __attribute__((always_inline))
#define SAJSON_UNREACHABLE() __builtin_unreachable()
#else
#define SAJSON_LIKELY(x) x
#define SAJSON_UNLIKELY(x) x
#define SAJSON_ALWAYS_INLINE __forceinline
#define SAJSON_UNREACHABLE() assert(!"unreachable")
#endif

namespace sajson {
    namespace internal {
        // This template utilizes the One Definition Rule to create global arrays in a header.
        // This trick courtesy of Rich Geldreich's Purple JSON parser.
        template<typename unused=void>
        struct globals_struct {
            static const unsigned char parse_flags[256];
        };
        typedef globals_struct<> globals;

        // bit 0 (1) - set if: plain ASCII string character
        // bit 1 (2) - set if: whitespace
        // bit 4 (0x10) - set if: 0-9 e E .
        template<typename unused>
        const uint8_t globals_struct<unused>::parse_flags[256] = {
         // 0    1    2    3    4    5    6    7      8    9    A    B    C    D    E    F
            0,   0,   0,   0,   0,   0,   0,   0,     0,   2,   2,   0,   0,   2,   0,   0, // 0
            0,   0,   0,   0,   0,   0,   0,   0,     0,   0,   0,   0,   0,   0,   0,   0, // 1
            3,   1,   0,   1,   1,   1,   1,   1,     1,   1,   1,   1,   1,   1,   0x11,1, // 2
            0x11,0x11,0x11,0x11,0x11,0x11,0x11,0x11,  0x11,0x11,1,   1,   1,   1,   1,   1, // 3
            1,   1,   1,   1,   1,   0x11,1,   1,     1,   1,   1,   1,   1,   1,   1,   1, // 4
            1,   1,   1,   1,   1,   1,   1,   1,     1,   1,   1,   1,   0,   1,   1,   1, // 5
            1,   1,   1,   1,   1,   0x11,1,   1,     1,   1,   1,   1,   1,   1,   1,   1, // 6
            1,   1,   1,   1,   1,   1,   1,   1,     1,   1,   1,   1,   1,   1,   1,   1, // 7

         // 128-255
            0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,
            0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,
            0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,
            0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0
        };

        inline bool is_plain_string_character(char c) {
            //return c >= 0x20 && c <= 0x7f && c != 0x22 && c != 0x5c;
            return (globals::parse_flags[static_cast<unsigned char>(c)] & 1) != 0;
        }

        inline bool is_whitespace(char c) {
            //return c == '\r' || c == '\n' || c == '\t' || c == ' ';
            return (globals::parse_flags[static_cast<unsigned char>(c)] & 2) != 0;
        }
    }

    enum type: uint8_t {
        TYPE_INTEGER = 0,
        TYPE_DOUBLE = 1,
        TYPE_NULL = 2,
        TYPE_FALSE = 3,
        TYPE_TRUE = 4,
        TYPE_STRING = 5,
        TYPE_ARRAY = 6,
        TYPE_OBJECT = 7,
    };

    static const size_t TYPE_BITS = 3;
    static const size_t TYPE_MASK = (1 << TYPE_BITS) - 1;
    static const size_t VALUE_MASK = size_t(-1) >> TYPE_BITS;

    static const size_t ROOT_MARKER = VALUE_MASK;

    inline type get_element_type(size_t s) {
        return static_cast<type>(s & TYPE_MASK);
    }

    inline size_t get_element_value(size_t s) {
        return s >> TYPE_BITS;
    }

    inline size_t make_element(type t, size_t value) {
        //assert((value & ~VALUE_MASK) == 0);
        //value &= VALUE_MASK;
        return static_cast<size_t>(t) | (value << TYPE_BITS);
    }

    class string {
    public:
        string(const char* text, size_t length)
            : text(text)
            , _length(length)
        {}

        const char* data() const {
            return text;
        }

        size_t length() const {
            return _length;
        }

#ifndef SAJSON_NO_STD_STRING
        std::string as_string() const {
            return std::string(text, text + _length);
        }
#endif

    private:
        const char* const text;
        const size_t _length;

        string(); /*=delete*/
    };

    class literal : public string {
    public:
        explicit literal(const char* text)
            : string(text, strlen(text))
        {}
    };

    struct object_key_record {
        size_t key_start;
        size_t key_end;
        size_t value;
    };

    struct object_key_comparator {
        object_key_comparator(const char* object_data)
            : data(object_data)
        {}

        bool operator()(const object_key_record& lhs, const string& rhs) const {
            const size_t lhs_length = lhs.key_end - lhs.key_start;
            const size_t rhs_length = rhs.length();
            if (lhs_length < rhs_length) {
                return true;
            } else if (lhs_length > rhs_length) {
                return false;
            }
            return memcmp(data + lhs.key_start, rhs.data(), lhs_length) < 0;
        }

        bool operator()(const string& lhs, const object_key_record& rhs) const {
            return !(*this)(rhs, lhs);
        }

        bool operator()(const object_key_record& lhs, const
                object_key_record& rhs)
        {
            const size_t lhs_length = lhs.key_end - lhs.key_start;
            const size_t rhs_length = rhs.key_end - rhs.key_start;
            if (lhs_length < rhs_length) {
                return true;
            } else if (lhs_length > rhs_length) {
                return false;
            }
            return memcmp(data + lhs.key_start, data + rhs.key_start,
                    lhs_length) < 0;
        }

        const char* data;
    };

    class refcount {
    public:
        refcount()
            : pn(new size_t(1))
        {}

        refcount(const refcount& rc)
            : pn(rc.pn)
        {
            ++*pn;
        }

        ~refcount() {
            if (--*pn == 0) {
                delete pn;
            }
        }

        size_t count() const {
            return *pn;
        }

    private:
        size_t* pn;

        refcount& operator=(const refcount&) = delete;
    };

    class mutable_string_view {
    public:
        mutable_string_view()
            : length_(0)
            , data(0)
        {}

        mutable_string_view(size_t length, char* data)
            : length_(length)
            , data(data)
        {}

        size_t length() const {
            return length_;
        }

        char* get_data() const {
            return data;
        }

    private:
        size_t length_;
        char* data;
    };

    union integer_storage {
        enum {
            word_length = 1
        };

        static void store(size_t* location, int value) {
            integer_storage is;
            is.i = value;
            *location = is.u;
        }

        int i;
        size_t u;
    };
    static_assert(sizeof(integer_storage) == sizeof(size_t), "integer_storage must have same size as one structure slot");

    union double_storage {
        enum {
            word_length = sizeof(double) / sizeof(size_t)
        };

#if defined(_M_IX86) || defined(__i386__) || defined(_X86_)
        static double load(const size_t* location) {
            return *reinterpret_cast<const double*>(location);
        }
        static void store(size_t* location, double value) {
            *reinterpret_cast<double*>(location) = value;
        }
#else
        static double load(const size_t* location) {
            double_storage s;
            for (unsigned i = 0; i < double_storage::word_length; ++i) {
                s.u[i] = location[i];
            }
            return s.d;
        }

        static void store(size_t* location, double value) {
            double_storage ns;
            ns.d = value;

            for (int i = 0; i < ns.word_length; ++i) {
                location[i] = ns.u[i];
            }
        }

        double d;
        size_t u[word_length];
#endif
    };
    // TODO: reinstate with c++03 implementation
    //static_assert(sizeof(double_storage) == sizeof(double), "double_storage should have same size as double");

    class value {
    public:
        explicit value(type value_type, const size_t* payload, const char* text)
            : value_type(value_type)
            , payload(payload)
            , text(text)
        {}

        type get_type() const {
            return value_type;
        }

        // valid iff get_type() is TYPE_ARRAY or TYPE_OBJECT
        size_t get_length() const {
            assert_type_2(TYPE_ARRAY, TYPE_OBJECT);
            return payload[0];
        }

        // valid iff get_type() is TYPE_ARRAY
        value get_array_element(size_t index) const {
            assert_type(TYPE_ARRAY);
            size_t element = payload[1 + index];
            return value(get_element_type(element), payload + get_element_value(element), text);
        }

        // valid iff get_type() is TYPE_OBJECT
        string get_object_key(size_t index) const {
            assert_type(TYPE_OBJECT);
            const size_t* s = payload + 1 + index * 3;
            return string(text + s[0], s[1] - s[0]);
        }

        // valid iff get_type() is TYPE_OBJECT
        value get_object_value(size_t index) const {
            assert_type(TYPE_OBJECT);
            size_t element = payload[3 + index * 3];
            return value(get_element_type(element), payload + get_element_value(element), text);
        }

        // valid iff get_type() is TYPE_OBJECT
        value get_value_of_key(const string& key) const {
            assert_type(TYPE_OBJECT);
            size_t i = find_object_key(key);
            assert_in_bounds(i);
            return get_object_value(i);
        }

        // valid iff get_type() is TYPE_OBJECT
        // return get_length() if there is no such key
        size_t find_object_key(const string& key) const {
            assert_type(TYPE_OBJECT);
            const object_key_record* start = reinterpret_cast<const object_key_record*>(payload + 1);
            const object_key_record* end = start + get_length();
            const object_key_record* i = std::lower_bound(start, end, key, object_key_comparator(text));
            return (i != end
                    && (i->key_end - i->key_start) == key.length()
                    && memcmp(key.data(), text + i->key_start, key.length()) == 0)? i - start : get_length();
        }

        // valid iff get_type() is TYPE_INTEGER
        int get_integer_value() const {
            assert_type(TYPE_INTEGER);
            integer_storage s;
            s.u = payload[0];
            return s.i;
        }

        // valid iff get_type() is TYPE_DOUBLE
        double get_double_value() const {
            assert_type(TYPE_DOUBLE);
            return double_storage::load(payload);
        }

        // valid iff get_type() is TYPE_INTEGER or TYPE_DOUBLE
        double get_number_value() const {
            assert_type_2(TYPE_INTEGER, TYPE_DOUBLE);
            if (get_type() == TYPE_INTEGER) {
                return get_integer_value();
            } else {
                return get_double_value();
            }
        }

        // valid iff get_type() is TYPE_INTEGER or TYPE_DOUBLE
        // returns true if out is modified written.
        // returns false if the value is a non-integral double
        // or out of range of a 53-bit integer.
        bool get_int53_value(int64_t* out) const {
            // Make sure the output variable is always defined to avoid any
            // possible situation like
            // https://gist.github.com/chadaustin/2c249cb850619ddec05b23ca42cf7a18
            *out = 0;

            assert_type_2(TYPE_INTEGER, TYPE_DOUBLE);
            if (get_type() == TYPE_INTEGER) {
                *out = get_integer_value();
                return true;
            } else if (get_type() == TYPE_DOUBLE) {
                double v = get_double_value();
                if (v < -(1LL << 53) || v > (1LL << 53)) {
                    return false;
                }
                int64_t as_int = static_cast<int64_t>(v);
                if (as_int != v) {
                    return false;
                }
                *out = as_int;
                return true;
            } else {
                return false;
            }
        }

        // valid iff get_type() is TYPE_STRING
        size_t get_string_length() const {
            assert_type(TYPE_STRING);
            return payload[1] - payload[0];
        }

        // valid iff get_type() is TYPE_STRING
        // WARNING: calling this function and using the return value as a
        // C-style string (that is, without also using get_string_length())
        // will cause the string to appear truncated if the string has
        // embedded NULs.
        const char* as_cstring() const {
            assert_type(TYPE_STRING);
            return text + payload[0];
        }

#ifndef SAJSON_NO_STD_STRING
        // valid iff get_type() is TYPE_STRING
        std::string as_string() const {
            assert_type(TYPE_STRING);
            return std::string(text + payload[0], text + payload[1]);
        }
#endif

        const size_t* _internal_get_payload() const {
            return payload;
        }


    private:
        void assert_type(type expected) const {
            assert(expected == get_type());
        }

        void assert_type_2(type e1, type e2) const {
            assert(e1 == get_type() || e2 == get_type());
        }

        void assert_in_bounds(size_t i) const {
            assert(i < get_length());
        }

        const type value_type;
        const size_t* const payload;
        const char* const text;
    };

    enum error {
        ERROR_SUCCESS,
        ERROR_OUT_OF_MEMORY,
        ERROR_UNEXPECTED_END,
        ERROR_MISSING_ROOT_ELEMENT,
        ERROR_BAD_ROOT,
        ERROR_EXPECTED_COMMA,
        ERROR_MISSING_OBJECT_KEY,
        ERROR_EXPECTED_COLON,
        ERROR_EXPECTED_END_OF_INPUT,
        ERROR_UNEXPECTED_COMMA,
        ERROR_EXPECTED_VALUE,
        ERROR_EXPECTED_NULL,
        ERROR_EXPECTED_FALSE,
        ERROR_EXPECTED_TRUE,
        ERROR_MSSING_EXPONENT,
        ERROR_ILLEGAL_CODEPOINT,
        ERROR_INVALID_UNICODE_ESCAPE,
        ERROR_UNEXPECTED_END_OF_UTF16,
        ERROR_EXPECTED_U,
        ERROR_INVALID_UTF16_TRAIL_SURROGATE,
        ERROR_UNKNOWN_ESCAPE,
        ERROR_INVALID_UTF8,
    };

    class document {
    public:
        explicit document(const mutable_string_view& input, type root_type, const size_t* root)
            : input(input)
            , root_type(root_type)
            , root(root)
            , error_line(0)
            , error_column(0)
            , error_code(ERROR_SUCCESS)
            , error_arg(0)
        {
            formatted_error_message[0] = 0;
        }

        explicit document(const mutable_string_view& input, size_t error_line, size_t error_column, const error error_code, int error_arg)
            : input(input)
            , root_type(TYPE_NULL)
            , root(0)
            , error_line(error_line)
            , error_column(error_column)
            , error_code(error_code)
            , error_arg(error_arg)
        {
            formatted_error_message[ERROR_BUFFER_LENGTH - 1] = 0;
            int written = has_significant_error_arg()
                ? snprintf(formatted_error_message, ERROR_BUFFER_LENGTH - 1, "%s: %d", _internal_get_error_text(), error_arg)
                : snprintf(formatted_error_message, ERROR_BUFFER_LENGTH - 1, "%s", _internal_get_error_text());
            (void)written;
            assert(written >= 0 && written < ERROR_BUFFER_LENGTH);
        }

        document(const document&) = delete;
        void operator=(const document&) = delete;

        document(document&& rhs)
            : input(rhs.input)
            , root_type(rhs.root_type)
            , root(rhs.root)
            , error_line(rhs.error_line)
            , error_column(rhs.error_column)
            , error_code(rhs.error_code)
            , error_arg(rhs.error_arg)
        {
            // Yikes... but strcpy is okay here because formatted_error is
            // guaranteed to be null-terminated.
            strcpy(formatted_error_message, rhs.formatted_error_message);
            // should rhs's fields be zeroed too?
        }

        bool is_valid() const {
            return root_type == TYPE_ARRAY || root_type == TYPE_OBJECT;
        }

        value get_root() const {
            return value(root_type, root, input.get_data());
        }

        size_t get_error_line() const {
            return error_line;
        }

        size_t get_error_column() const {
            return error_column;
        }

#ifndef SAJSON_NO_STD_STRING
        std::string get_error_message_as_string() const {
            return formatted_error_message;
        }
#endif

        const char* get_error_message_as_cstring() const {
            return formatted_error_message;
        }

        /// WARNING: Internal function which is subject to change
        error _internal_get_error_code() const {
            return error_code;
        }

        /// WARNING: Internal function which is subject to change
        int _internal_get_error_argument() const {
            return error_arg;
        }

        /// WARNING: Internal function which is subject to change
        const char* _internal_get_error_text() const {
            switch (error_code) {
                case ERROR_SUCCESS: return "no error";
                case ERROR_OUT_OF_MEMORY: return  "out of memory";
                case ERROR_UNEXPECTED_END: return  "unexpected end of input";
                case ERROR_MISSING_ROOT_ELEMENT: return  "missing root element";
                case ERROR_BAD_ROOT: return  "document root must be object or array";
                case ERROR_EXPECTED_COMMA: return  "expected ,";
                case ERROR_MISSING_OBJECT_KEY: return  "missing object key";
                case ERROR_EXPECTED_COLON: return  "expected :";
                case ERROR_EXPECTED_END_OF_INPUT: return  "expected end of input";
                case ERROR_UNEXPECTED_COMMA: return  "unexpected comma";
                case ERROR_EXPECTED_VALUE: return  "expected value";
                case ERROR_EXPECTED_NULL: return  "expected 'null'";
                case ERROR_EXPECTED_FALSE: return  "expected 'false'";
                case ERROR_EXPECTED_TRUE: return  "expected 'true'";
                case ERROR_MSSING_EXPONENT: return  "missing exponent";
                case ERROR_ILLEGAL_CODEPOINT: return  "illegal unprintable codepoint in string";
                case ERROR_INVALID_UNICODE_ESCAPE: return  "invalid character in unicode escape";
                case ERROR_UNEXPECTED_END_OF_UTF16: return  "unexpected end of input during UTF-16 surrogate pair";
                case ERROR_EXPECTED_U: return  "expected \\u";
                case ERROR_INVALID_UTF16_TRAIL_SURROGATE: return  "invalid UTF-16 trail surrogate";
                case ERROR_UNKNOWN_ESCAPE: return  "unknown escape";
                case ERROR_INVALID_UTF8: return  "invalid UTF-8";
            }

            SAJSON_UNREACHABLE();
        }

        /// WARNING: Internal function exposed only for high-performance language bindings.
        type _internal_get_root_type() const {
            return root_type;
        }

        /// WARNING: Internal function exposed only for high-performance language bindings.
        const size_t* _internal_get_root() const {
            return root;
        }

        const mutable_string_view& _internal_get_input() const {
            return input;
        }

    private:
        bool has_significant_error_arg() const {
            return error_code == ERROR_ILLEGAL_CODEPOINT;
        }

        mutable_string_view input;
        const type root_type;
        const size_t* const root;
        const size_t error_line;
        const size_t error_column;
        const error error_code;
        const int error_arg;

        enum { ERROR_BUFFER_LENGTH = 128 };
        char formatted_error_message[ERROR_BUFFER_LENGTH];
    };

    class single_allocation {
    public:
        class stack_head {
        public:
            stack_head(stack_head&& other)
                : stack_bottom(other.stack_bottom)
                , stack_top(other.stack_top)
            {}

            bool has_allocation_error() {
                return false;
            }

            // check has_allocation_error() immediately after calling
            void push(size_t element) {
                *stack_top++ = element;
            }

            // check has_allocation_error() immediately after calling
            size_t* reserve(size_t amount) {
                size_t* rv = stack_top;
                stack_top += amount;
                return rv;
            }

            // The compiler does not see the stack_head (stored in a local)
            // and the allocator (stored as a field) have the same stack_bottom
            // values, so it does a bit of redundant work.
            // So there's a microoptimization available here: introduce a type
            // "stack_mark" and make it polymorphic on the allocator.  For
            // single_allocation, it merely needs to be a single pointer.

            void reset(size_t new_top) {
                stack_top = stack_bottom + new_top;
            }

            size_t get_size() {
                return stack_top - stack_bottom;
            }

            size_t* get_top() {
                return stack_top;
            }

            size_t* get_pointer_from_offset(size_t offset) {
                return stack_bottom + offset;
            }

        private:
            stack_head() = delete;
            stack_head(const stack_head&) = delete;
            void operator=(const stack_head&) = delete;

            explicit stack_head(size_t* base)
                : stack_bottom(base)
                , stack_top(base)
            {}

            size_t* const stack_bottom;
            size_t* stack_top;

            friend class single_allocation;
        };

        class allocator {
        public:
            allocator() = delete;
            allocator(const allocator&) = delete;
            void operator=(const allocator&) = delete;

            explicit allocator(size_t* buffer, size_t input_size)
                : structure(buffer)
                , structure_end(buffer ? buffer + input_size : 0)
                , write_cursor(structure_end)
            {}

            explicit allocator(std::nullptr_t)
                : structure(0)
                , structure_end(0)
                , write_cursor(0)
            {}

            allocator(allocator&& other)
                : structure(other.structure)
                , structure_end(other.structure_end)
                , write_cursor(other.write_cursor)
            {
                other.structure = 0;
                other.structure_end = 0;
                other.write_cursor = 0;
            }

            ~allocator() = default;

            stack_head get_stack_head() {
                return stack_head(structure);
            }

            size_t get_write_offset() {
                return structure_end - write_cursor;
            }

            size_t* get_write_pointer_of(size_t v) {
                return structure_end - v;
            }

            bool has_allocation_error() {
                return false;
            }

            // check has_allocation_error immediately after calling
            size_t* reserve(size_t size) {
                write_cursor -= size;
                return write_cursor;
            }

            size_t* get_ast_root() {
                return write_cursor;
            }

            void transfer_ownership() {
                structure = 0;
                structure_end = 0;
                write_cursor = 0;
            }

        private:
            size_t* structure;
            size_t* structure_end;
            size_t* write_cursor;
        };

        /// Allocate a single worst-case AST buffer with one word per byte in
        /// the input document.
        single_allocation() = delete;


        /// Write the AST into an existing buffer.  Will fail with an out of
        /// memory error if the buffer is not guaranteed to be big enough for
        /// the document.
        single_allocation(size_t* existing_buffer, size_t size_in_words)
            : existing_buffer(existing_buffer)
            , existing_buffer_size(size_in_words)
        {}

        allocator make_allocator(size_t input_document_size_in_bytes, bool* succeeded) const {
            {
                if (existing_buffer_size < input_document_size_in_bytes) {
                    *succeeded = false;
                    return allocator(nullptr);
                }
                *succeeded = true;
                return allocator(existing_buffer, input_document_size_in_bytes);
            }
        }

    private:
        size_t* existing_buffer;
        size_t existing_buffer_size;
    };

    template<typename Allocator>
    class parser {
    public:
        parser(const mutable_string_view& msv, Allocator&& allocator)
            : input(msv)
            , input_end(input.get_data() + input.length())
            , allocator(std::move(allocator))
            , root_type(TYPE_NULL)
            , error_line(0)
            , error_column(0)
        {}

        document get_document() {
            if (parse()) {
                size_t* ast_root = allocator.get_ast_root();
                allocator.transfer_ownership();
                return document(input, root_type, ast_root);
            } else {
                return document(input, error_line, error_column, error_code, error_arg);
            }
        }

    private:
        struct error_result {
            operator bool() const {
                return false;
            }
            operator char*() const {
                return 0;
            }
        };

        bool at_eof(const char* p) {
            return p == input_end;
        }

        char* skip_whitespace(char* p) {
            // There is an opportunity to make better use of superscalar
            // hardware here* but if someone cares about JSON parsing
            // performance the first thing they do is minify, so prefer
            // to optimize for code size here.
            // * https://github.com/chadaustin/Web-Benchmarks/blob/master/json/third-party/pjson/pjson.h#L1873
            for (;;) {
                if (SAJSON_UNLIKELY(p == input_end)) {
                    return 0;
                } else if (internal::is_whitespace(*p)) {
                    ++p;
                } else {
                    return p;
                }
            }
        }

        error_result oom(char* p) {
            return make_error(p, ERROR_OUT_OF_MEMORY);
        }

        error_result unexpected_end() {
            return make_error(0, ERROR_UNEXPECTED_END);
        }

        error_result unexpected_end(char* p) {
            return make_error(p, ERROR_UNEXPECTED_END);
        }

        error_result make_error(char* p, error code, int arg = 0) {
            if (!p) {
                p = input_end;
            }

            error_line = 1;
            error_column = 1;

            char* c = input.get_data();
            while (c < p) {
                if (*c == '\r') {
                    if (c + 1 < p && c[1] == '\n') {
                        ++error_line;
                        error_column = 1;
                        ++c;
                    } else {
                        ++error_line;
                        error_column = 1;
                    }
                } else if (*c == '\n') {
                    ++error_line;
                    error_column = 1;
                } else {
                    // TODO: count UTF-8 characters
                    ++error_column;
                }
                ++c;
            }

            error_code = code;
            error_arg = arg;
            return error_result();
        }

        bool parse() {
            // p points to the character currently being parsed
            char* p = input.get_data();

            auto stack = allocator.get_stack_head();
            if (SAJSON_UNLIKELY(stack.has_allocation_error())) {
                return oom(p);
            }

            p = skip_whitespace(p);
            if (SAJSON_UNLIKELY(!p)) {
                return make_error(p, ERROR_MISSING_ROOT_ELEMENT);
            }

            // current_base is an offset to the first element of the current structure (object or array)
            size_t current_base = stack.get_size();
            type current_structure_type;
            if (*p == '[') {
                current_structure_type = TYPE_ARRAY;
                stack.push(make_element(current_structure_type, ROOT_MARKER));
                if (SAJSON_UNLIKELY(stack.has_allocation_error())) {
                    return oom(p);
                }
                goto array_close_or_element;
            } else if (*p == '{') {
                current_structure_type = TYPE_OBJECT;
                stack.push(make_element(current_structure_type, ROOT_MARKER));
                if (SAJSON_UNLIKELY(stack.has_allocation_error())) {
                    return oom(p);
                }
                goto object_close_or_element;
            } else {
                return make_error(p, ERROR_BAD_ROOT);
            }

            // BEGIN STATE MACHINE

            size_t pop_element; // used as an argument into the `pop` routine

            if (0) { // purely for structure

            // ASSUMES: byte at p SHOULD be skipped
            array_close_or_element:
                p = skip_whitespace(p + 1);
                if (SAJSON_UNLIKELY(!p)) {
                    return unexpected_end();
                }
                if (*p == ']') {
                    goto pop_array;
                } else {
                    goto next_element;
                }
                SAJSON_UNREACHABLE();

            // ASSUMES: byte at p SHOULD be skipped
            object_close_or_element:
                p = skip_whitespace(p + 1);
                if (SAJSON_UNLIKELY(!p)) {
                    return unexpected_end();
                }
                if (*p == '}') {
                    goto pop_object;
                } else {
                    goto object_key;
                }
                SAJSON_UNREACHABLE();

            // ASSUMES: byte at p SHOULD NOT be skipped
            structure_close_or_comma:
                p = skip_whitespace(p);
                if (SAJSON_UNLIKELY(!p)) {
                    return unexpected_end();
                }

                if (current_structure_type == TYPE_ARRAY) {
                    if (*p == ']') {
                        goto pop_array;
                    } else {
                        if (SAJSON_UNLIKELY(*p != ',')) {
                            return make_error(p, ERROR_EXPECTED_COMMA);
                        }
                        ++p;
                        goto next_element;
                    }
                } else {
                    assert(current_structure_type == TYPE_OBJECT);
                    if (*p == '}') {
                        goto pop_object;
                    } else {
                        if (SAJSON_UNLIKELY(*p != ',')) {
                            return make_error(p, ERROR_EXPECTED_COMMA);
                        }
                        ++p;
                        goto object_key;
                    }
                }
                SAJSON_UNREACHABLE();

            // ASSUMES: *p == '}'
            pop_object: {
                ++p;
                size_t* base_ptr = stack.get_pointer_from_offset(current_base);
                pop_element = *base_ptr;
                if (SAJSON_UNLIKELY(!install_object(base_ptr + 1, stack.get_top()))) {
                    return oom(p);
                }
                goto pop;
            }

            // ASSUMES: *p == ']'
            pop_array: {
                ++p;
                size_t* base_ptr = stack.get_pointer_from_offset(current_base);
                pop_element = *base_ptr;
                if (SAJSON_UNLIKELY(!install_array(base_ptr + 1, stack.get_top()))) {
                    return oom(p);
                }
                goto pop;
            }

            // ASSUMES: byte at p SHOULD NOT be skipped
            object_key: {
                p = skip_whitespace(p);
                if (SAJSON_UNLIKELY(!p)) {
                    return unexpected_end();
                }
                if (SAJSON_UNLIKELY(*p != '"')) {
                    return make_error(p, ERROR_MISSING_OBJECT_KEY);
                }
                size_t* out = stack.reserve(2);
                if (SAJSON_UNLIKELY(stack.has_allocation_error())) {
                    return oom(p);
                }
                p = parse_string(p, out);
                if (SAJSON_UNLIKELY(!p)) {
                    return false;
                }
                p = skip_whitespace(p);
                if (SAJSON_UNLIKELY(!p || *p != ':')) {
                    return make_error(p, ERROR_EXPECTED_COLON);
                }
                ++p;
                goto next_element;
            }

            // ASSUMES: byte at p SHOULD NOT be skipped
            next_element:
                p = skip_whitespace(p);
                if (SAJSON_UNLIKELY(!p)) {
                    return unexpected_end();
                }

                type value_type_result;
                switch (*p) {
                    case 0:
                        return unexpected_end(p);
                    case 'n':
                        p = parse_null(p);
                        if (!p) {
                            return false;
                        }
                        value_type_result = TYPE_NULL;
                        break;
                    case 'f':
                        p = parse_false(p);
                        if (!p) {
                            return false;
                        }
                        value_type_result = TYPE_FALSE;
                        break;
                    case 't':
                        p = parse_true(p);
                        if (!p) {
                            return false;
                        }
                        value_type_result = TYPE_TRUE;
                        break;
                    case '0':
                    case '1':
                    case '2':
                    case '3':
                    case '4':
                    case '5':
                    case '6':
                    case '7':
                    case '8':
                    case '9':
                    case '-': {
                        auto result = parse_number(p);
                        p = result.first;
                        if (!p) {
                            return false;
                        }
                        value_type_result = result.second;
                        break;
                    }
                    case '"': {
                        size_t* string_tag = allocator.reserve(2);
                        if (allocator.has_allocation_error()) {
                            return oom(p);
                        }
                        p = parse_string(p, string_tag);
                        if (!p) {
                            return false;
                        }
                        value_type_result = TYPE_STRING;
                        break;
                    }

                    case '[': {
                        size_t previous_base = current_base;
                        current_base = stack.get_size();
                        stack.push(make_element(current_structure_type, previous_base));
                        if (stack.has_allocation_error()) {
                            return oom(p);
                        }
                        current_structure_type = TYPE_ARRAY;
                        goto array_close_or_element;
                    }
                    case '{': {
                        size_t previous_base = current_base;
                        current_base = stack.get_size();
                        stack.push(make_element(current_structure_type, previous_base));
                        if (stack.has_allocation_error()) {
                            return oom(p);
                        }
                        current_structure_type = TYPE_OBJECT;
                        goto object_close_or_element;
                    }
                    pop: {
                        size_t parent = get_element_value(pop_element);
                        if (parent == ROOT_MARKER) {
                            root_type = current_structure_type;
                            p = skip_whitespace(p);
                            if (SAJSON_UNLIKELY(p)) {
                                return make_error(p, ERROR_EXPECTED_END_OF_INPUT);
                            }
                            return true;
                        }
                        stack.reset(current_base);
                        current_base = parent;
                        value_type_result = current_structure_type;
                        current_structure_type = get_element_type(pop_element);
                        break;
                    }

                    case ',':
                        return make_error(p, ERROR_UNEXPECTED_COMMA);
                    default:
                        return make_error(p, ERROR_EXPECTED_VALUE);
                }

                stack.push(make_element(value_type_result, allocator.get_write_offset()));
                if (SAJSON_UNLIKELY(stack.has_allocation_error())) {
                    return oom(p);
                }

                goto structure_close_or_comma;
            }

            SAJSON_UNREACHABLE();
        }

        bool has_remaining_characters(char* p, ptrdiff_t remaining) {
            return input_end - p >= remaining;
        }

        char* parse_null(char* p) {
            if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
                make_error(p, ERROR_UNEXPECTED_END);
                return 0;
            }
            char p1 = p[1];
            char p2 = p[2];
            char p3 = p[3];
            if (SAJSON_UNLIKELY(p1 != 'u' || p2 != 'l' || p3 != 'l')) {
                make_error(p, ERROR_EXPECTED_NULL);
                return 0;
            }
            return p + 4;
        }

        char* parse_false(char* p) {
            if (SAJSON_UNLIKELY(!has_remaining_characters(p, 5))) {
                return make_error(p, ERROR_UNEXPECTED_END);
            }
            char p1 = p[1];
            char p2 = p[2];
            char p3 = p[3];
            char p4 = p[4];
            if (SAJSON_UNLIKELY(p1 != 'a' || p2 != 'l' || p3 != 's' || p4 != 'e')) {
                return make_error(p, ERROR_EXPECTED_FALSE);
            }
            return p + 5;
        }

        char* parse_true(char* p) {
            if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
                return make_error(p, ERROR_UNEXPECTED_END);
            }
            char p1 = p[1];
            char p2 = p[2];
            char p3 = p[3];
            if (SAJSON_UNLIKELY(p1 != 'r' || p2 != 'u' || p3 != 'e')) {
                return make_error(p, ERROR_EXPECTED_TRUE);
            }
            return p + 4;
        }

        static double pow10(int exponent) {
            if (exponent > 308) {
                return std::numeric_limits<double>::infinity();
            } else if (exponent < -323) {
                return 0.0;
            }
            static const double constants[] = {
                1e-323,1e-322,1e-321,1e-320,1e-319,1e-318,1e-317,1e-316,1e-315,1e-314,
                1e-313,1e-312,1e-311,1e-310,1e-309,1e-308,1e-307,1e-306,1e-305,1e-304,
                1e-303,1e-302,1e-301,1e-300,1e-299,1e-298,1e-297,1e-296,1e-295,1e-294,
                1e-293,1e-292,1e-291,1e-290,1e-289,1e-288,1e-287,1e-286,1e-285,1e-284,
                1e-283,1e-282,1e-281,1e-280,1e-279,1e-278,1e-277,1e-276,1e-275,1e-274,
                1e-273,1e-272,1e-271,1e-270,1e-269,1e-268,1e-267,1e-266,1e-265,1e-264,
                1e-263,1e-262,1e-261,1e-260,1e-259,1e-258,1e-257,1e-256,1e-255,1e-254,
                1e-253,1e-252,1e-251,1e-250,1e-249,1e-248,1e-247,1e-246,1e-245,1e-244,
                1e-243,1e-242,1e-241,1e-240,1e-239,1e-238,1e-237,1e-236,1e-235,1e-234,
                1e-233,1e-232,1e-231,1e-230,1e-229,1e-228,1e-227,1e-226,1e-225,1e-224,
                1e-223,1e-222,1e-221,1e-220,1e-219,1e-218,1e-217,1e-216,1e-215,1e-214,
                1e-213,1e-212,1e-211,1e-210,1e-209,1e-208,1e-207,1e-206,1e-205,1e-204,
                1e-203,1e-202,1e-201,1e-200,1e-199,1e-198,1e-197,1e-196,1e-195,1e-194,
                1e-193,1e-192,1e-191,1e-190,1e-189,1e-188,1e-187,1e-186,1e-185,1e-184,
                1e-183,1e-182,1e-181,1e-180,1e-179,1e-178,1e-177,1e-176,1e-175,1e-174,
                1e-173,1e-172,1e-171,1e-170,1e-169,1e-168,1e-167,1e-166,1e-165,1e-164,
                1e-163,1e-162,1e-161,1e-160,1e-159,1e-158,1e-157,1e-156,1e-155,1e-154,
                1e-153,1e-152,1e-151,1e-150,1e-149,1e-148,1e-147,1e-146,1e-145,1e-144,
                1e-143,1e-142,1e-141,1e-140,1e-139,1e-138,1e-137,1e-136,1e-135,1e-134,
                1e-133,1e-132,1e-131,1e-130,1e-129,1e-128,1e-127,1e-126,1e-125,1e-124,
                1e-123,1e-122,1e-121,1e-120,1e-119,1e-118,1e-117,1e-116,1e-115,1e-114,
                1e-113,1e-112,1e-111,1e-110,1e-109,1e-108,1e-107,1e-106,1e-105,1e-104,
                1e-103,1e-102,1e-101,1e-100,1e-99,1e-98,1e-97,1e-96,1e-95,1e-94,1e-93,
                1e-92,1e-91,1e-90,1e-89,1e-88,1e-87,1e-86,1e-85,1e-84,1e-83,1e-82,1e-81,
                1e-80,1e-79,1e-78,1e-77,1e-76,1e-75,1e-74,1e-73,1e-72,1e-71,1e-70,1e-69,
                1e-68,1e-67,1e-66,1e-65,1e-64,1e-63,1e-62,1e-61,1e-60,1e-59,1e-58,1e-57,
                1e-56,1e-55,1e-54,1e-53,1e-52,1e-51,1e-50,1e-49,1e-48,1e-47,1e-46,1e-45,
                1e-44,1e-43,1e-42,1e-41,1e-40,1e-39,1e-38,1e-37,1e-36,1e-35,1e-34,1e-33,
                1e-32,1e-31,1e-30,1e-29,1e-28,1e-27,1e-26,1e-25,1e-24,1e-23,1e-22,1e-21,
                1e-20,1e-19,1e-18,1e-17,1e-16,1e-15,1e-14,1e-13,1e-12,1e-11,1e-10,1e-9,
                1e-8,1e-7,1e-6,1e-5,1e-4,1e-3,1e-2,1e-1,1e0,1e1,1e2,1e3,1e4,1e5,1e6,1e7,
                1e8,1e9,1e10,1e11,1e12,1e13,1e14,1e15,1e16,1e17,1e18,1e19,1e20,1e21,
                1e22,1e23,1e24,1e25,1e26,1e27,1e28,1e29,1e30,1e31,1e32,1e33,1e34,1e35,
                1e36,1e37,1e38,1e39,1e40,1e41,1e42,1e43,1e44,1e45,1e46,1e47,1e48,1e49,
                1e50,1e51,1e52,1e53,1e54,1e55,1e56,1e57,1e58,1e59,1e60,1e61,1e62,1e63,
                1e64,1e65,1e66,1e67,1e68,1e69,1e70,1e71,1e72,1e73,1e74,1e75,1e76,1e77,
                1e78,1e79,1e80,1e81,1e82,1e83,1e84,1e85,1e86,1e87,1e88,1e89,1e90,1e91,
                1e92,1e93,1e94,1e95,1e96,1e97,1e98,1e99,1e100,1e101,1e102,1e103,1e104,
                1e105,1e106,1e107,1e108,1e109,1e110,1e111,1e112,1e113,1e114,1e115,1e116,
                1e117,1e118,1e119,1e120,1e121,1e122,1e123,1e124,1e125,1e126,1e127,1e128,
                1e129,1e130,1e131,1e132,1e133,1e134,1e135,1e136,1e137,1e138,1e139,1e140,
                1e141,1e142,1e143,1e144,1e145,1e146,1e147,1e148,1e149,1e150,1e151,1e152,
                1e153,1e154,1e155,1e156,1e157,1e158,1e159,1e160,1e161,1e162,1e163,1e164,
                1e165,1e166,1e167,1e168,1e169,1e170,1e171,1e172,1e173,1e174,1e175,1e176,
                1e177,1e178,1e179,1e180,1e181,1e182,1e183,1e184,1e185,1e186,1e187,1e188,
                1e189,1e190,1e191,1e192,1e193,1e194,1e195,1e196,1e197,1e198,1e199,1e200,
                1e201,1e202,1e203,1e204,1e205,1e206,1e207,1e208,1e209,1e210,1e211,1e212,
                1e213,1e214,1e215,1e216,1e217,1e218,1e219,1e220,1e221,1e222,1e223,1e224,
                1e225,1e226,1e227,1e228,1e229,1e230,1e231,1e232,1e233,1e234,1e235,1e236,
                1e237,1e238,1e239,1e240,1e241,1e242,1e243,1e244,1e245,1e246,1e247,1e248,
                1e249,1e250,1e251,1e252,1e253,1e254,1e255,1e256,1e257,1e258,1e259,1e260,
                1e261,1e262,1e263,1e264,1e265,1e266,1e267,1e268,1e269,1e270,1e271,1e272,
                1e273,1e274,1e275,1e276,1e277,1e278,1e279,1e280,1e281,1e282,1e283,1e284,
                1e285,1e286,1e287,1e288,1e289,1e290,1e291,1e292,1e293,1e294,1e295,1e296,
                1e297,1e298,1e299,1e300,1e301,1e302,1e303,1e304,1e305,1e306,1e307,1e308
            };
            return constants[exponent + 323];
        }

        std::pair<char*, type> parse_number(char* p) {
            bool negative = false;
            if ('-' == *p) {
                ++p;
                negative = true;

                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                }
            }

            bool try_double = false;

            int i = 0;
            double d = 0.0; // gcc complains that d might be used uninitialized which isn't true. appease the warning anyway.
            if (*p == '0') {
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                }
            } else for (;;) {
                unsigned char c = *p;
                if (c < '0' || c > '9') {
                    break;
                }

                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                }

                unsigned char digit = c - '0';

                if (SAJSON_UNLIKELY(!try_double && i > INT_MAX / 10 - 9)) {
                    // TODO: could split this into two loops
                    try_double = true;
                    d = i;
                }
                if (SAJSON_UNLIKELY(try_double)) {
                    d = 10.0 * d + digit;
                } else {
                    i = 10 * i + digit;
                }
            }

            int exponent = 0;

            if ('.' == *p) {
                if (!try_double) {
                    try_double = true;
                    d = i;
                }
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                }
                for (;;) {
                    char c = *p;
                    if (c < '0' || c > '9') {
                        break;
                    }

                    ++p;
                    if (SAJSON_UNLIKELY(at_eof(p))) {
                        return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                    }
                    d = d * 10 + (c - '0');
                    --exponent;
                }
            }

            char e = *p;
            if ('e' == e || 'E' == e) {
                if (!try_double) {
                    try_double = true;
                    d = i;
                }
                ++p;
                if (SAJSON_UNLIKELY(at_eof(p))) {
                    return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                }

                bool negativeExponent = false;
                if ('-' == *p) {
                    negativeExponent = true;
                    ++p;
                    if (SAJSON_UNLIKELY(at_eof(p))) {
                        return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                    }
                } else if ('+' == *p) {
                    ++p;
                    if (SAJSON_UNLIKELY(at_eof(p))) {
                        return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                    }
                }

                int exp = 0;

                char c = *p;
                if (SAJSON_UNLIKELY(c < '0' || c > '9')) {
                    return std::make_pair(make_error(p, ERROR_MSSING_EXPONENT), TYPE_NULL);
                }
                for (;;) {
                    exp = 10 * exp + (c - '0');

                    ++p;
                    if (SAJSON_UNLIKELY(at_eof(p))) {
                        return std::make_pair(make_error(p, ERROR_UNEXPECTED_END), TYPE_NULL);
                    }

                    c = *p;
                    if (c < '0' || c > '9') {
                        break;
                    }
                }
                exponent += (negativeExponent ? -exp : exp);
            }

            if (exponent) {
                assert(try_double);
                d *= pow10(exponent);
            }

            if (negative) {
                if (try_double) {
                    d = -d;
                } else {
                    i = -i;
                }
            }
            if (try_double) {
                size_t* out = allocator.reserve(double_storage::word_length);
                if (allocator.has_allocation_error()) {
                    return std::make_pair(oom(p), TYPE_NULL);
                }
                double_storage::store(out, d);
                return std::make_pair(p, TYPE_DOUBLE);
            } else {
                size_t* out = allocator.reserve(integer_storage::word_length);
                if (allocator.has_allocation_error()) {
                    return std::make_pair(oom(p), TYPE_NULL);
                }
                integer_storage::store(out, i);
                return std::make_pair(p, TYPE_INTEGER);
            }
        }

        bool install_array(size_t* array_base, size_t* array_end) {
            const size_t length = array_end - array_base;
            size_t* const new_base = allocator.reserve(length + 1);
            if (SAJSON_UNLIKELY(allocator.has_allocation_error())) {
                return false;
            }
            size_t* out = new_base + length + 1;
            size_t* const structure_end = allocator.get_write_pointer_of(0);

            while (array_end > array_base) {
                size_t element = *--array_end;
                type element_type = get_element_type(element);
                size_t element_value = get_element_value(element);
                size_t* element_ptr = structure_end - element_value;
                *--out = make_element(element_type, element_ptr - new_base);
            }
            *--out = length;
            return true;
        }

        bool install_object(size_t* object_base, size_t* object_end) {
            assert((object_end - object_base) % 3 == 0);
            const size_t length_times_3 = object_end - object_base;
            std::sort(
                reinterpret_cast<object_key_record*>(object_base),
                reinterpret_cast<object_key_record*>(object_end),
                object_key_comparator(input.get_data()));

            size_t* const new_base = allocator.reserve(length_times_3 + 1);
            if (SAJSON_UNLIKELY(allocator.has_allocation_error())) {
                return false;
            }
            size_t* out = new_base + length_times_3 + 1;
            size_t* const structure_end = allocator.get_write_pointer_of(0);

            while (object_end > object_base) {
                size_t element = *--object_end;
                type element_type = get_element_type(element);
                size_t element_value = get_element_value(element);
                size_t* element_ptr = structure_end - element_value;

                *--out = make_element(element_type, element_ptr - new_base);
                *--out = *--object_end;
                *--out = *--object_end;
            }
            *--out = length_times_3 / 3;
            return true;
        }

        char* parse_string(char* p, size_t* tag) {
            ++p; // "
            size_t start = p - input.get_data();
            char* input_end_local = input_end;
            while (input_end_local - p >= 4) {
                if (!internal::is_plain_string_character(p[0])) { goto found; }
                if (!internal::is_plain_string_character(p[1])) { p += 1; goto found; }
                if (!internal::is_plain_string_character(p[2])) { p += 2; goto found; }
                if (!internal::is_plain_string_character(p[3])) { p += 3; goto found; }
                p += 4;
            }
            for (;;) {
                if (SAJSON_UNLIKELY(p >= input_end_local)) {
                    return make_error(p, ERROR_UNEXPECTED_END);
                }

                if (!internal::is_plain_string_character(*p)) {
                    break;
                }

                ++p;
            }
        found:
            if (SAJSON_LIKELY(*p == '"')) {
                tag[0] = start;
                tag[1] = p - input.get_data();
                *p = '\0';
                return p + 1;
            }

            if (*p >= 0 && *p < 0x20) {
                return make_error(p, ERROR_ILLEGAL_CODEPOINT, static_cast<int>(*p));
            } else {
                // backslash or >0x7f
                return parse_string_slow(p, tag, start);
            }
        }

        char* read_hex(char* p, unsigned& u) {
            unsigned v = 0;
            int i = 4;
            while (i--) {
                unsigned char c = *p++;
                if (c >= '0' && c <= '9') {
                    c -= '0';
                } else if (c >= 'a' && c <= 'f') {
                    c = c - 'a' + 10;
                } else if (c >= 'A' && c <= 'F') {
                    c = c - 'A' + 10;
                } else {
                    return make_error(p, ERROR_INVALID_UNICODE_ESCAPE);
                }
                v = (v << 4) + c;
            }

            u = v;
            return p;
        }

        void write_utf8(unsigned codepoint, char*& end) {
            if (codepoint < 0x80) {
                *end++ = codepoint;
            } else if (codepoint < 0x800) {
                *end++ = 0xC0 | (codepoint >> 6);
                *end++ = 0x80 | (codepoint & 0x3F);
            } else if (codepoint < 0x10000) {
                *end++ = 0xE0 | (codepoint >> 12);
                *end++ = 0x80 | ((codepoint >> 6) & 0x3F);
                *end++ = 0x80 | (codepoint & 0x3F);
            } else {
                assert(codepoint < 0x200000);
                *end++ = 0xF0 | (codepoint >> 18);
                *end++ = 0x80 | ((codepoint >> 12) & 0x3F);
                *end++ = 0x80 | ((codepoint >> 6) & 0x3F);
                *end++ = 0x80 | (codepoint & 0x3F);
            }
        }

        char* parse_string_slow(char* p, size_t* tag, size_t start) {
            char* end = p;
            char* input_end_local = input_end;

            for (;;) {
                if (SAJSON_UNLIKELY(p >= input_end_local)) {
                    return make_error(p, ERROR_UNEXPECTED_END);
                }

                if (SAJSON_UNLIKELY(*p >= 0 && *p < 0x20)) {
                    return make_error(p, ERROR_ILLEGAL_CODEPOINT, static_cast<int>(*p));
                }

                switch (*p) {
                    case '"':
                        tag[0] = start;
                        tag[1] = end - input.get_data();
                        *end = '\0';
                        return p + 1;

                    case '\\':
                        ++p;
                        if (SAJSON_UNLIKELY(p >= input_end_local)) {
                            return make_error(p, ERROR_UNEXPECTED_END);
                        }

                        char replacement;
                        switch (*p) {
                            case '"': replacement = '"'; goto replace;
                            case '\\': replacement = '\\'; goto replace;
                            case '/': replacement = '/'; goto replace;
                            case 'b': replacement = '\b'; goto replace;
                            case 'f': replacement = '\f'; goto replace;
                            case 'n': replacement = '\n'; goto replace;
                            case 'r': replacement = '\r'; goto replace;
                            case 't': replacement = '\t'; goto replace;
                            replace:
                                *end++ = replacement;
                                ++p;
                                break;
                            case 'u': {
                                ++p;
                                if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
                                    return make_error(p, ERROR_UNEXPECTED_END);
                                }
                                unsigned u = 0; // gcc's complaining that this could be used uninitialized. wrong.
                                p = read_hex(p, u);
                                if (!p) {
                                    return 0;
                                }
                                if (u >= 0xD800 && u <= 0xDBFF) {
                                    if (SAJSON_UNLIKELY(!has_remaining_characters(p, 6))) {
                                        return make_error(p, ERROR_UNEXPECTED_END_OF_UTF16);
                                    }
                                    char p0 = p[0];
                                    char p1 = p[1];
                                    if (p0 != '\\' || p1 != 'u') {
                                        return make_error(p, ERROR_EXPECTED_U);
                                    }
                                    p += 2;
                                    unsigned v = 0; // gcc's complaining that this could be used uninitialized. wrong.
                                    p = read_hex(p, v);
                                    if (!p) {
                                        return p;
                                    }

                                    if (v < 0xDC00 || v > 0xDFFF) {
                                        return make_error(p, ERROR_INVALID_UTF16_TRAIL_SURROGATE);
                                    }
                                    u = 0x10000 + (((u - 0xD800) << 10) | (v - 0xDC00));
                                }
                                write_utf8(u, end);
                                break;
                            }
                            default:
                                return make_error(p, ERROR_UNKNOWN_ESCAPE);
                        }
                        break;

                    default:
                        // validate UTF-8
                        unsigned char c0 = p[0];
                        if (c0 < 128) {
                            *end++ = *p++;
                        } else if (c0 < 224) {
                            if (SAJSON_UNLIKELY(!has_remaining_characters(p, 2))) {
                                return unexpected_end(p);
                            }
                            unsigned char c1 = p[1];
                            if (c1 < 128 || c1 >= 192) {
                                return make_error(p + 1, ERROR_INVALID_UTF8);
                            }
                            end[0] = c0;
                            end[1] = c1;
                            end += 2;
                            p += 2;
                        } else if (c0 < 240) {
                            if (SAJSON_UNLIKELY(!has_remaining_characters(p, 3))) {
                                return unexpected_end(p);
                            }
                            unsigned char c1 = p[1];
                            if (c1 < 128 || c1 >= 192) {
                                return make_error(p + 1, ERROR_INVALID_UTF8);
                            }
                            unsigned char c2 = p[2];
                            if (c2 < 128 || c2 >= 192) {
                                return make_error(p + 2, ERROR_INVALID_UTF8);
                            }
                            end[0] = c0;
                            end[1] = c1;
                            end[2] = c2;
                            end += 3;
                            p += 3;
                        } else if (c0 < 248) {
                            if (SAJSON_UNLIKELY(!has_remaining_characters(p, 4))) {
                                return unexpected_end(p);
                            }
                            unsigned char c1 = p[1];
                            if (c1 < 128 || c1 >= 192) {
                                return make_error(p + 1, ERROR_INVALID_UTF8);
                            }
                            unsigned char c2 = p[2];
                            if (c2 < 128 || c2 >= 192) {
                                return make_error(p + 2, ERROR_INVALID_UTF8);
                            }
                            unsigned char c3 = p[3];
                            if (c3 < 128 || c3 >= 192) {
                                return make_error(p + 3, ERROR_INVALID_UTF8);
                            }
                            end[0] = c0;
                            end[1] = c1;
                            end[2] = c2;
                            end[3] = c3;
                            end += 4;
                            p += 4;
                        } else {
                            return make_error(p, ERROR_INVALID_UTF8);
                        }
                        break;
                }
            }
        }

        mutable_string_view input;
        char* const input_end;
        Allocator allocator;

        type root_type;
        size_t error_line;
        size_t error_column;
        error error_code;
        int error_arg; // optional argument for the error
    };

    template<typename AllocationStrategy, typename StringType>
    document parse(const AllocationStrategy& strategy, const StringType& string) {
        mutable_string_view input(string);

        bool success;
        auto allocator = strategy.make_allocator(input.length(), &success);
        if (!success) {
            return document(input, 1, 1, ERROR_OUT_OF_MEMORY, 0);
        }

        return parser<typename AllocationStrategy::allocator>(
            input,
            std::move(allocator)
        ).get_document();
    }
}