{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TemplateHaskell #-}

-- Maintainer  :  dominik.schrempf@gmail.com
-- Stability   :  unstable
-- Portability :  portable
--
-- Creation date: Tue Jun 16 10:18:54 2020.
--
-- Save and load chains. It is easy to save and restore the current state and
-- likelihood (or the trace), but it is not feasible to store all the proposals
-- and so on, so they have to be provided again when continuing a run.

-- |
-- Module      :  Mcmc.Chain.Save
-- Description :  Save and load a Markov chain
-- Copyright   :  (c) Dominik Schrempf, 2021
-- License     :  GPL-3.0-or-later
module Mcmc.Chain.Save
  ( SavedChain (..),
    toSavedChain,
    fromSavedChain,
  )
where

import Control.Monad
import Data.Aeson
import Data.Aeson.TH
import Data.List hiding (cycle)
import Data.Maybe
import qualified Data.Stack.Circular as C
import qualified Data.Vector as VB
import qualified Data.Vector.Unboxed as VU
import Data.Word
import Mcmc.Acceptance
import Mcmc.Chain.Chain
import Mcmc.Chain.Link
import Mcmc.Chain.Trace
import Mcmc.Cycle
import Mcmc.Internal.Random
import Mcmc.Likelihood
import Mcmc.Monitor
import Mcmc.Prior
import Mcmc.Proposal
import Prelude hiding (cycle)

-- | Storable values of a Markov chain.
--
-- See 'toSavedChain'.
data SavedChain a = SavedChain
  { savedId :: Maybe Int,
    savedLink :: Link a,
    savedIteration :: Int,
    savedTrace :: C.Stack VB.Vector (Link a),
    savedAcceptance :: Acceptance Int,
    savedSeed :: VU.Vector Word32,
    savedTuningParameters :: [Maybe (TuningParameter, AuxiliaryTuningParameters)]
  }
  deriving (Eq, Read, Show)

$(deriveJSON defaultOptions ''SavedChain)

-- | Save a chain.
toSavedChain ::
  Chain a ->
  IO (SavedChain a)
toSavedChain (Chain ci it i tr ac g _ _ _ cc _) = do
  g' <- saveGen g
  tr' <- freezeT tr
  return $ SavedChain ci it i tr' ac' g' ts
  where
    ps = ccProposals cc
    ac' = transformKeysA ps [0 ..] ac
    ts =
      [ (\t -> (tTuningParameter t, tAuxiliaryTuningParameters t)) <$> mt
        | mt <- map prTuner ps
      ]

-- | Load a saved chain.
--
-- Recompute and check the prior and likelihood for the last state because the
-- functions may have changed. Of course, we cannot test for the same function,
-- but having the same prior and likelihood at the last state is already a good
-- indicator.
fromSavedChain ::
  PriorFunction a ->
  LikelihoodFunction a ->
  Cycle a ->
  Monitor a ->
  SavedChain a ->
  IO (Chain a)
fromSavedChain pr lh cc mn (SavedChain ci it i tr ac' g' ts)
  | pr (state it) /= prior it =
    let msg =
          unlines
            [ "fromSave: Provided prior function does not match the saved prior.",
              "fromSave: Current prior:" <> show (prior it) <> ".",
              "fromSave: Given prior:" <> show (pr $ state it) <> "."
            ]
     in error msg
  | lh (state it) /= likelihood it =
    error "fromSave: Provided likelihood function does not match the saved likelihood."
  | otherwise = do
    g <- loadGen g'
    tr' <- thawT tr
    return $ Chain ci it i tr' ac g i pr lh cc' mn
  where
    ac = transformKeysA [0 ..] (ccProposals cc) ac'
    tunePs mt p = case mt of
      Nothing -> p
      Just (x, xs) -> either (error . (<> err)) id $ tuneWithTuningParameters x xs p
    err = error "\nfromSavedChain: Proposal with stored tuning parameters is not tunable."
    ps = ccProposals cc
    cc' = cc {ccProposals = zipWith tunePs ts ps}