%  Copyright (C) 2002-2003 David Roundy
%  This program is free software; you can redistribute it and/or modify
%  it under the terms of the GNU General Public License as published by
%  the Free Software Foundation; either version 2, or (at your option)
%  any later version.
%  This program is distributed in the hope that it will be useful,
%  but WITHOUT ANY WARRANTY; without even the implied warranty of
%  GNU General Public License for more details.
%  You should have received a copy of the GNU General Public License
%  along with this program; see the file COPYING.  If not, write to
%  the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
%  Boston, MA 02110-1301, USA.

{-# OPTIONS_GHC -cpp -fglasgow-exts -fno-warn-orphans #-}
-- , TypeOperators #-}

#include "gadts.h"

module Darcs.Patch.Commute ( fromPrims,
                             merge, elegant_merge,
                             merger, unravel,
                             public_unravel, mangle_unravelled,
                             CommuteFunction, Perhaps(..),
                             -- for other commutes:

import Control.Monad ( MonadPlus, mplus, msum, mzero )

import Darcs.Patch.FileName ( FileName, fn2fp, fp2fn )
import Darcs.Patch.Info ( invert_name, idpatchinfo )
import Darcs.Patch.Patchy ( Commute(..), Invert(..) )
import Darcs.Patch.Core ( Patch(..), Named(..),
                          join_patchesFL )
import Darcs.Patch.Prim ( Prim(..), FromPrims(..),
                          Conflict(..), Effect(..),
                          is_filepatch, sort_coalesceFL,
                          FilePatchType(..), DirPatchType(..),
                          is_hunk, modernizePrim )
import qualified Data.ByteString.Char8 as BC (pack, last)
import qualified Data.ByteString       as B (null, ByteString)
import Data.Maybe ( isJust )
import Data.List ( intersperse, sort )
import Darcs.Patch.Permutations ( head_permutationsRL, head_permutationsFL )
import Printer ( text, vcat, ($$) )
import Darcs.Patch.Patchy ( invertRL )
import Darcs.Patch.Show ( showPatch_ )
import Data.List ( nubBy )
import Darcs.Sealed ( unsafeUnseal )
import Darcs.Utils ( nubsort )
#include "impossible.h"
import Darcs.Sealed ( Sealed(..), mapSeal )
import Darcs.Ordered ( mapFL, mapFL_FL, unsafeCoerceP,
                             FL(..), RL(..),
                             (:/\:)(..), (:<)(..), (:\/:)(..), (:>)(..),
                             lengthFL, mapRL,
                             reverseFL, reverseRL, concatFL,
                             MyEq, unsafeCompare

--import Darcs.ColorPrinter ( traceDoc )
--import Printer ( greenText )
\section{Commuting patches}

\subsection{Composite patches}

Composite patches are made up of a series of patches intended to be applied
sequentially.  They are represented by a list of patches, with the first
patch in the list being applied first.


The first way (of only two) to change the context of a patch is by
commutation, which is the process of changing the order of two sequential
The commutation of patches $P_1$ and $P_2$ is represented by
\[ P_2 P_1 \commutes {P_1}' {P_2}'. \]
Here $P_1'$ is intended to describe the same change as $P_1$, with the
only difference being that $P_1'$ is applied after $P_2'$ rather than
before $P_2$.
The above definition is obviously rather vague, the reason being that what
is the ``same change'' has not been defined, and we simply assume (and
hope) that the code's view of what is the ``same change'' will match those
of its human users.  The `$\commutes$' operator should be read as something
like the $==$ operator in C, indicating that the right hand side performs
identical changes to the left hand side, but the two patches are in
reversed order.  When read in this manner, it is clear that commutation
must be a reversible process, and indeed this means that commutation
\emph{can} fail, and must fail in certain cases.  For example, the creation
and deletion of the same file cannot be commuted.  When two patches fail to
commutex, it is said that the second patch depends on the first, meaning
that it must have the first patch in its context (remembering that the
context of a patch is a set of patches, which is how we represent a tree).
\footnote{The fact that commutation can fail makes a huge difference in the
whole patch formalism.  It may be possible to create a formalism in which
commutation always succeeds, with the result of what would otherwise be a
commutation that fails being something like a virtual particle (which can
violate conservation of energy), and it may be that such a formalism would
allow strict mathematical proofs (whereas those used in the current
formalism are mostly only hand waving ``physicist'' proofs).  However, I'm
not sure how you'd deal with a request to delete a file that has not yet
been created, for example.  Obviously you'd need to create some kind of
antifile, which would annihilate with the file when that file finally got
created, but I'm not entirely sure how I'd go about doing this.
$\ddot\frown$ So I'm sticking with my hand waving formalism.}

%I should add that one using the inversion relationship of sequential
%patches, one can avoid having to provide redundant definitions of

% There is another interesting property which is that a commutex's results
% can't be affected by commuting another thingamabopper.

data Perhaps a = Unknown | Failed | Succeeded a

instance  Monad Perhaps where
    (Succeeded x) >>= k =  k x
    Failed   >>= _      =  Failed
    Unknown  >>= _      =  Unknown
    Failed   >> _       =  Failed
    (Succeeded _) >> k  =  k
    Unknown  >> k       =  k
    return              =  Succeeded
    fail _              =  Unknown

instance  MonadPlus Perhaps where
    mzero                 = Unknown
    Unknown `mplus` ys    = ys
    Failed  `mplus` _     = Failed
    (Succeeded x) `mplus` _ = Succeeded x

toMaybe :: Perhaps a -> Maybe a
toMaybe (Succeeded x) = Just x
toMaybe _ = Nothing

toPerhaps :: Maybe a -> Perhaps a
toPerhaps (Just x) = Succeeded x
toPerhaps Nothing = Failed

clever_commute :: CommuteFunction -> CommuteFunction
clever_commute c (p1:<p2) =
    case c (p1 :< p2) of
    Succeeded x -> Succeeded x
    Failed -> Failed
    Unknown -> case c (invert p2 :< invert p1) of
               Succeeded (p1' :< p2') -> Succeeded (invert p2' :< invert p1')
               Failed -> Failed
               Unknown -> Unknown

speedy_commute :: CommuteFunction
speedy_commute (p1 :< p2) -- Deal with common case quickly!
    | p1_modifies /= Nothing && p2_modifies /= Nothing &&
      p1_modifies /= p2_modifies = Succeeded (unsafeCoerceP p2 :< unsafeCoerceP p1)
    | otherwise = Unknown
    where p1_modifies = is_filepatch_merger p1
          p2_modifies = is_filepatch_merger p2

instance Commute p => Commute (Named p) where
    commute (NamedP n1 d1 p1 :> NamedP n2 d2 p2) =
        if n2 `elem` d1 || n1 `elem` d2
        then Nothing
        else do (p2' :> p1') <- commute (p1 :> p2)
                return (NamedP n2 d2 p2' :> NamedP n1 d1 p1')
    merge (NamedP n1 d1 p1 :\/: NamedP n2 d2 p2)
        = case merge (p1 :\/: p2) of
          (p2' :/\: p1') -> NamedP n2 d2 p2' :/\: NamedP n1 d1 p1'
    list_touched_files (NamedP _ _ p) = list_touched_files p

instance Conflict p => Conflict (Named p) where
    list_conflicted_files (NamedP _ _ p) = list_conflicted_files p
    resolve_conflicts (NamedP _ _ p) = resolve_conflicts p

everything_else_commute :: MaybeCommute -> CommuteFunction
everything_else_commute c x = eec x
    eec :: CommuteFunction
    eec (PP px :< PP py) = toPerhaps $ do y' :< x' <- commutex (px :< py)
                                          return (PP y' :< PP x')
    eec (ComP NilFL :< p1) = Succeeded (unsafeCoerceP p1 :< (ComP NilFL))
    eec (p2 :< ComP NilFL) = Succeeded (ComP NilFL :< unsafeCoerceP p2)
    eec (ComP (p:>:ps) :< p1) = toPerhaps $ do
                              (p1' :< p') <- c (p :< p1)
                              (p1'' :< ComP ps') <- c (ComP ps :< p1')
                              return (p1'' :< ComP (p':>:ps'))
    eec (patch2 :< ComP patches) =
        toPerhaps $ do (patches' :< patch2') <- ccr (patch2 :< reverseFL patches)
                       return (ComP (reverseRL patches') :< patch2')
        where ccr :: FORALL(x y) (Patch :< RL Patch) C(x y) -> Maybe ((RL Patch :< Patch) C(x y))
              ccr (p2 :< NilRL) = seq p2 $ return (NilRL :< p2)
              ccr (p2 :< p:<:ps) = do (p' :< p2') <- c (p2 :< p)
                                      (ps' :< p2'') <- ccr (p2' :< ps)
                                      return (p':<:ps' :< p2'')
    eec _xx =
        msum [
              clever_commute commute_recursive_merger       _xx
             ,clever_commute other_commute_recursive_merger _xx

Note that it must be true that

commutex (A^-1 A, P) = Just (P, A'^-1 A')


if commutex (A, B) == Just (B', A')
then commutex (B^-1, A^-1) == Just (A'^-1, B'^-1)

merger_commute :: (Patch :< Patch) -> Perhaps (Patch :< Patch)
merger_commute (Merger _ _ p1 p2 :< pA)
    | unsafeCompare pA p1 = Succeeded (merger "0.0" p2 p1 :< p2)
    | unsafeCompare pA (invert (merger "0.0" p2 p1)) = Failed
merger_commute (Merger _ _
                (Merger _ _ c b)
                (Merger _ _ c' a) :<
                Merger _ _ b' c'')
    | unsafeCompare b' b && unsafeCompare c c' && unsafeCompare c c'' =
        Succeeded (merger "0.0" (merger "0.0" b a) (merger "0.0" b c) :<
                   merger "0.0" b a)
merger_commute _ = Unknown

instance Commute Patch where
    merge (y :\/: z) =
        case actual_merge (y:\/:z) of
        y' -> case commutex (y' :< z) of
                         Nothing -> bugDoc $ text "merge_patches bug"
                                    $$ showPatch_ y
                                   $$ showPatch_ z
                                   $$ showPatch_ y'
                         Just (z' :< _) -> z' :/\: y'
        case elegant_merge (y:\/:z) of
        Just (z' :/\: y') -> z' :/\: y'
        Nothing -> undefined
    commutex x = toMaybe $ msum [speedy_commute x,
                                 clever_commute merger_commute x,
                                 everything_else_commute commutex x
    -- Recurse on everything, these are potentially spoofed patches
    list_touched_files (ComP ps) = nubsort $ concat $ mapFL list_touched_files ps
    list_touched_files (Merger _ _ p1 p2) = nubsort $ list_touched_files p1
                                            ++ list_touched_files p2
    list_touched_files c@(Regrem _ _ _ _) = list_touched_files $ invert c
    list_touched_files (PP p) = list_touched_files p

commute_no_merger :: MaybeCommute
commute_no_merger x =
    toMaybe $ msum [speedy_commute x,
                    everything_else_commute commute_no_merger x]
    bug "commute_no_merger undefined when compiled with GADTs" x

is_filepatch_merger :: Patch C(x y) -> Maybe FileName
is_filepatch_merger (PP p) = is_filepatch p
is_filepatch_merger (Merger _ _ p1 p2) = do
     f1 <- is_filepatch_merger p1
     f2 <- is_filepatch_merger p2
     if f1 == f2 then return f1 else Nothing
is_filepatch_merger (Regrem und unw p1 p2)
    = is_filepatch_merger (Merger und unw p1 p2)
is_filepatch_merger (ComP _) = Nothing

commute_recursive_merger :: (Patch :< Patch) -> Perhaps (Patch :< Patch)
commute_recursive_merger (p@(Merger _ _ p1 p2) :< pA) = toPerhaps $
  do (pA' :< _) <- commutex (undo :< pA)
     commutex (invert undo :< pA')
     (pAmid :< _) <- commutex (invert p1 :< pA)
     (pAx :< p1') <- commutex (p1 :< pAmid)
     assert (pAx `unsafeCompare` pA)
     (_ :<p2') <- commutex (p2 :< pAmid)
     (_ :< p2o) <- commutex (p2' :< invert pAmid)
     assert (p2o `unsafeCompare` p2)
     let p' = if unsafeCompare p1' p1 && unsafeCompare p2' p2
              then p
              else merger "0.0" p1' p2'
         undo' = merger_undo p'
     (_ :< pAo) <- commutex (pA' :< undo')
     assert (pAo `unsafeCompare` pA)
     return (pA' :< p')
    where undo = merger_undo p
commute_recursive_merger _ = Unknown

other_commute_recursive_merger :: (Patch :< Patch) -> Perhaps (Patch :< Patch)
other_commute_recursive_merger (pA':< p_old@(Merger _ _ p1' p2')) =
  toPerhaps $
  do (_ :< pA) <- commutex (pA' :< merger_undo p_old)
     (p1 :< pAmid) <- commutex (pA :< p1')
     (pAmido :< _) <- commutex (invert p1 :< pA)
     assert (pAmido `unsafeCompare` pAmid)
     (_ :< p2) <- commutex (p2' :< invert pAmid)
     (_ :< p2o') <- commutex (p2 :< pAmid)
     assert (p2o' `unsafeCompare` p2')
     let p = if p1 `unsafeCompare` p1' && p2 `unsafeCompare` p2'
             then p_old
             else merger "0.0" p1 p2
         undo = merger_undo p
     assert (not $ pA `unsafeCompare` p1) -- special case here...
     (pAo' :< _) <- commutex (undo :<pA)
     assert (pAo' `unsafeCompare` pA')
     return (p :< pA)
other_commute_recursive_merger _ = Unknown

assert :: Bool -> Maybe ()
assert False = Nothing
assert True = Just ()

type CommuteFunction = FORALL(x y) (Patch :< Patch) C(x y) -> Perhaps ((Patch :< Patch) C(x y))
type MaybeCommute = FORALL(x y) (Patch :< Patch) C(x y) -> Maybe ((Patch :< Patch) C(x y))

The second way one can change the context of a patch is by a {\bf merge}
operation.  A merge is an operation that takes two parallel patches and
gives a pair of sequential patches.  The merge operation is represented by
the arrow ``\( \merge \)''.
The result of a merge of two patches, $P_1$ and $P_2$ is one of two patches,
$P_1'$ and $P_2'$, which satisfy the relationship:
\[  P_2 \parallel P_1 \merge {P_2}' P_1 \commutex {P_1}' P_2. \]
Note that the sequential patches resulting from a merge are \emph{required}
to commutex.  This is an important consideration, as without it most of the
manipulations we would like to perform would not be possible.  The other
important fact is that a merge \emph{cannot fail}.  Naively, those two
requirements seem contradictory.  In reality, what it means is that the
result of a merge may be a patch which is much more complex than any we
have yet considered\footnote{Alas, I don't know how to prove that the two
constraints even \emph{can} be satisfied.  The best I have been able to do
is to believe that they can be satisfied, and to be unable to find an case
in which my implementation fails to satisfy them.  These two requirements
are the foundation of the entire theory of patches (have you been counting
how many foundations it has?).}.

\subsection{How merges are actually performed}

The constraint that any two compatible patches (patches which can
successfully be applied to the same tree) can be merged is actually quite
difficult to apply.  The above merge constraints also imply that the result
of a series of merges must be independent of the order of the merges.  So
I'm putting a whole section here for the interested to see what algorithms
I use to actually perform the merges (as this is pretty close to being the
most difficult part of the code).

The first case is that in which the two merges don't actually conflict, but
don't trivially merge either (e.g.\ hunk patches on the same file, where the
line number has to be shifted as they are merged).  This kind of merge can
actually be very elegantly dealt with using only commutation and inversion.

There is a handy little theorem which is immensely useful when trying to
merge two patches.
$ P_2' P_1 \commutex P_1' P_2 $ if and only if $ P_1'^{ -1}
P_2' \commutex P_2 P_1^{ -1} $, provided both commutations succeed.  If
either commutex fails, this theorem does not apply.
This can easily be proven by multiplying both sides of the first
commutation by $P_1'^{ -1}$ on the left, and by $P_1^{ -1}$ on the right.
Besides being used in merging, this theorem is also useful in the recursive
commutations of mergers.  From Theorem~\ref{merge_thm}, we see that the
merge of $P_1$ and $P_2'$ is simply the commutation of $P_2$ with $P_1^{
-1}$ (making sure to do the commutation the right way).  Of course, if this
commutation fails, the patches conflict.  Moreover, one must check that the
merged result actually commutes with $P_1$, as the theorem applies only
when \emph{both} commutations are successful.


elegant_merge :: (Patch :\/: Patch) C(x y)
              -> Maybe ((Patch :/\: Patch) C(x y))
elegant_merge (p1 :\/: p2) =
  case commutex (p1 :< invert p2) of
  Just (ip2':<p1') -> case commutex (p1' :< p2) of
                      Nothing -> Nothing -- should be a redundant check
                      Just (_:<p1o) -> if unsafeCompare p1o p1
                                       then Just (invert ip2' :/\: p1')
                                       else Nothing
  Nothing -> Nothing


Of course, there are patches that actually conflict, meaning a merge where
the two patches truly cannot both be applied (e.g.\ trying to create a file
and a directory with the same name).  We deal with this case by creating a
special kind of patch to support the merge, which we will call a
``merger''.  Basically, a merger is a patch that contains the two patches
that conflicted, and instructs darcs basically to resolve the conflict.  By
construction a merger will satisfy the commutation property (see
Definition~\ref{merge_dfn}) that characterizes all merges.  Moreover the
merger's properties are what makes the order of merges unimportant (which
is a rather critical property for darcs as a whole).

The job of a merger is basically to undo the two conflicting patches, and
then apply some sort of a ``resolution'' of the two instead.  In the case
of two conflicting hunks, this will look much like what CVS does, where it
inserts both versions into the file.  In general, of course, the two
conflicting patches may both be mergers themselves, in which case the
situation is considerably more complicated.

actual_merge :: (Patch :\/: Patch) -> Patch
actual_merge (ComP the_p1s :\/: ComP the_p2s) =
    join_patchesFL $ mc the_p1s the_p2s
    where mc :: FL Patch -> FL Patch -> FL Patch
          mc NilFL (_:>:_) = NilFL
          mc p1s NilFL = p1s
          mc p1s (p2:>:p2s) = mc (merge_patches_after_patch p1s p2) p2s
actual_merge (ComP p1s :\/: p2) = seq p2 $
                              join_patchesFL $ merge_patches_after_patch p1s p2
actual_merge (p1 :\/: ComP p2s) = seq p1 $ merge_patch_after_patches p1 p2s

actual_merge (p1 :\/: p2) = case elegant_merge (p1:\/:p2) of
                            Just (_ :/\: p1') -> p1'
                            Nothing -> merger "0.0" p2 p1

merge_patch_after_patches :: Patch -> FL Patch -> Patch
merge_patch_after_patches p (p1:>:p1s) =
    merge_patch_after_patches (actual_merge (p:\/:p1)) p1s
merge_patch_after_patches p NilFL = p

merge_patches_after_patch :: FL Patch -> Patch -> FL Patch
merge_patches_after_patch p2s p =
    case commutex (merge_patch_after_patches p p2s :< join_patchesFL p2s) of
    Just (ComP p2s':< _) -> p2s'
    _ -> impossible

Much of the merger code depends on a routine which recreates from a single
merger the entire sequence of patches which led up to that merger (this is,
of course, assuming that this is the complicated general case of a merger
of mergers of mergers).  This ``unwind'' procedure is rather complicated,
but absolutely critical to the merger code, as without it we wouldn't even
be able to undo the effects of the patches involved in the merger, since we
wouldn't know what patches were all involved in it.

Basically, unwind takes a merger such as
M( M(A,B), M(A,M(C,D)))
From which it recreates a merge history:
M( M(A,B), M(A,M(C,D)))
(For the curious, yes I can easily unwind this merger in my head [and on
paper can unwind insanely more complex mergers]---that's what comes of
working for a few months on an algorithm.)  Let's start with a simple
unwinding.  The merger \verb!M(A,B)! simply means that two patches
(\verb!A! and \verb!B!) conflicted, and of the two of them \verb!A! is
first in the history.  The last two patches in the unwinding of any merger
are always just this easy.  So this unwinds to:
What about a merger of mergers? How about \verb!M(A,M(C,D))!.  In this case
we know the two most recent patches are:
But obviously the unwinding isn't complete, since we don't yet see where
\verb!C! and \verb!D! came from.  In this case we take the unwinding of
\verb!M(C,D)! and drop its latest patch (which is \verb!M(C,D)! itself) and
place that at the beginning of our patch train:
As we look at \verb!M( M(A,B), M(A,M(C,D)))!, we consider the unwindings of
each of its subpatches:
A         A
M(A,B)    M(A,M(C,D))
As we did with \verb!M(A,M(C,D))!, we'll drop the first patch on the
right and insert the first patch on the left.  That moves us up to the two
\verb!A!'s.  Since these agree, we can use just one of them (they
``should'' agree).  That leaves us with the \verb!C! which goes first.

The catch is that things don't always turn out this easily.  There is no
guarantee that the two \verb!A!'s would come out at the same time, and if
they didn't, we'd have to rearrange things until they did.  Or if there was
no way to rearrange things so that they would agree, we have to go on to
plan B, which I will explain now.

Consider the case of \verb!M( M(A,B), M(C,D))!.  We can easily unwind the
two subpatches
A         C
M(A,B)    M(C,D)
Now we need to reconcile the \verb!A! and \verb!C!.  How do we do this?
Well, as usual, the solution is to use the most wonderful
Theorem~\ref{merge_thm}.  In this case we have to use it in the reverse of
how we used it when merging, since we know that \verb!A! and \verb!C! could
either one be the \emph{last} patch applied before \verb!M(A,B)! or
\verb!M(C,D)!.  So we can find \verb!C'! using
A^{ -1} C \commutex C' A'^{ -1}
Giving an unwinding of
M( M(A,B), M(C,D) )
There is a bit more complexity to the unwinding process (mostly having to
do with cases where you have deeper nesting), but I think the general
principles that are followed are pretty much included in the above

unwind :: Patch -> RL Patch -- Recreates a patch history in reverse.
unwind (Merger _ unwindings _ _) = unwindings
unwind p = p :<: NilRL;

true_unwind :: Patch -> RL Patch -- Recreates a patch history in reverse.
true_unwind p@(Merger _ _ p1 p2) =
    case (unwind p1, unwind p2) of
    (_:<:p1s,_:<:p2s) -> p :<: p1 :<: reconcile_unwindings p p1s p2s
    _ -> impossible
true_unwind _ = impossible

reconcile_unwindings :: Patch -> RL Patch -> RL Patch -> RL Patch
reconcile_unwindings _ NilRL p2s = p2s
reconcile_unwindings _ p1s NilRL = p1s
reconcile_unwindings p (p1:<:p1s) p2s =
    case [(p1s', p2s')|
          p1s'@(hp1s':<:_) <- head_permutationsRL (p1:<:p1s),
          p2s'@(hp2s':<:_) <- head_permutationsRL p2s,
          hp1s' `unsafeCompare` hp2s'] of
    ((p1':<:p1s', _:<:p2s'):_) ->
        p1' :<: reconcile_unwindings p p1s' p2s'
    [] -> case reverseFL `fmap` put_before p1 (reverseRL p2s) of
          Just p2s' -> p1 :<: reconcile_unwindings p p1s p2s'
          Nothing ->
              case fmap reverseFL $ put_before (headRL p2s) $
                   reverseRL (p1:<:p1s) of
              Just p1s' -> case p2s of
                           hp2s:<:tp2s -> hp2s :<:
                                          reconcile_unwindings p p1s' tp2s
                           NilRL -> impossible
              Nothing ->
                  bugDoc $ text "in function reconcile_unwindings"
                        $$ text "Original patch:"
                        $$ showPatch_ p
    _ -> bug "in reconcile_unwindings"

put_before :: Patch -> FL Patch -> Maybe (FL Patch)
put_before p1 (p2:>:p2s) =
    do p2':<p1' <- commutex (invert p1:<p2)
       commutex (p1:<p2')
       (p2' :>:) `fmap` put_before p1' p2s
put_before _ NilFL = Just NilFL

There are a couple of simple constraints on the routine which determines
how to resolve two conflicting patches (which is called `glump').  These
must be satisfied in order that the result of a series of merges is always
independent of their order.  Firstly, the output of glump cannot change
when the order of the two conflicting patches is switched.  If it did, then
commuting the merger could change the resulting patch, which would be bad.
Secondly, the result of the merge of three (or more) conflicting patches
cannot depend on the order in which the merges are performed.

The conflict resolution code (glump) begins by ``unravelling'' the merger
into a set of sequences of patches.  Each sequence of patches corresponds
to one non-conflicted patch that got merged together with the others.  The
result of the unravelling of a series of merges must obviously be
independent of the order in which those merges are performed.  This
unravelling code (which uses the unwind code mentioned above) uses probably
the second most complicated algorithm.  Fortunately, if we can successfully
unravel the merger, almost any function of the unravelled merger satisfies
the two constraints mentioned above that the conflict resolution code must

instance Conflict Patch where
  commute_no_conflicts (x:>y) = do x' :< y' <- commute_no_merger (y :< x)
                                   return (y':>x')
  resolve_conflicts patch = rcs NilFL $ reverseFL $ flattenFL patch
    where rcs :: FL Patch C(w y) -> RL Patch C(x y) -> [[Sealed (FL Prim C(w))]]
          rcs _ NilRL = []
          rcs passedby (p@(Merger _ _ _ _):<:ps) =
              case commute_no_merger (join_patchesFL passedby:<p) of
              Just (p'@(Merger _ _ p1 p2):<_) ->
                  (map Sealed $ nubBy unsafeCompare $ effect (glump09 p1 p2) : unravel p')
                  : rcs (p :>: passedby) ps
              Nothing -> rcs (p :>: passedby) ps
              _ -> impossible
          rcs passedby (p:<:ps) = seq passedby $
                                  rcs (p :>: passedby) ps
  resolve_conflicts = bug "haven't defined resolve_conflicts with type witnesses."

public_unravel :: Patch C(x y) -> [Sealed (FL Prim C(y))]
public_unravel = bug "Haven't implemented public_unravel with type witnesses."
public_unravel p = map Sealed $ unravel p

unravel :: Patch -> [FL Prim]
unravel p = nubBy unsafeCompare $
            map (sort_coalesceFL . concatFL . mapFL_FL effect) $
            get_supers $ map reverseRL $ new_ur p $ unwind p

get_supers :: [FL Patch] -> [FL Patch]
get_supers (x:xs) =
    case filter (not.(x `is_superpatch_of`)) xs of
    xs' -> if or $ map (`is_superpatch_of` x) xs'
           then get_supers xs'
           else x : get_supers xs'
get_supers [] = []
is_superpatch_of :: FL Patch -> FL Patch -> Bool
x `is_superpatch_of` y | lengthFL y > lengthFL x = False
x `is_superpatch_of` y = x `iso` y
    where iso :: FL Patch -> FL Patch -> Bool
          _ `iso` NilFL = True
          NilFL `iso` _ = False
          a `iso` (b:>:bs) =
              case filter ((`unsafeCompare` b) . headFL) $
                   head_permutationsFL a of
              ((_:>:as):_) -> as `iso` bs
              [] -> False
              _ -> bug "bug in is_superpatch_of"

headFL :: FL a -> a
headFL (x:>:_) = x
headFL NilFL = bug "bad headFL"

merger :: String -> Patch -> Patch -> Patch
merger "0.0" p1 p2 = Merger undoit unwindings p1 p2
    where fake_p = Merger identity NilRL p1 p2
          unwindings = true_unwind fake_p
          p = Merger identity unwindings p1 p2
          undoit =
              case (is_merger p1, is_merger p2) of
              (True ,True ) -> join_patchesFL $ invertRL $ tailRL $ unwind p
                               where tailRL (_:<:t) = t
                                     tailRL _ = impossible
              (False,False) -> invert p1
              (True ,False) -> join_patchesFL NilFL
              (False,True ) -> join_patchesFL (invert p1 :>: merger_undo p2 :>: NilFL)
merger g _ _ =
    error $ "Cannot handle mergers other than version 0.0\n"++g
    ++ "\nPlease use darcs optimize --modernize with an older darcs."

glump09 :: Patch -> Patch -> Patch
glump09 p1 p2 = fromPrims $ unsafeUnseal $ mangle_unravelled $ map Sealed $ unravel $ merger "0.0" p1 p2


mangle_unravelled :: [Sealed (FL Prim C(x))] -> Sealed (FL Prim C(x))
mangle_unravelled pss = if only_hunks pss
                        then (:>: NilFL) `mapSeal` mangle_unravelled_hunks pss
                        else head pss

only_hunks :: [Sealed (FL Prim C(x))] -> Bool
only_hunks [] = False
only_hunks pss = fn2fp f /= "" && all oh pss
    where f = get_a_filename pss
          oh :: Sealed (FL Prim C(x)) -> Bool
          oh (Sealed (p:>:ps)) = is_hunk p &&
                                 [fn2fp f] == list_touched_files p &&
                                 oh (Sealed ps)
          oh (Sealed NilFL) = True

apply_hunks :: [Maybe B.ByteString] -> FL Prim C(x y) -> [Maybe B.ByteString]
apply_hunks ms (FP _ (Hunk l o n):>:ps) = apply_hunks (rls l ms) ps
    where rls 1 mls = map Just n ++ drop (length o) mls
          rls i (ml:mls) = ml : rls (i-1) mls
          rls _ [] = bug "rls in apply_hunks"
apply_hunks ms NilFL = ms
apply_hunks _ (_:>:_) = impossible

get_old :: [Maybe B.ByteString] -> [Sealed (FL Prim C(x))] -> [Maybe B.ByteString]
get_old mls (ps:pss) = get_old (get_hunks_old mls ps) pss
get_old mls [] = mls

get_a_filename :: [Sealed (FL Prim C(x))] -> FileName
get_a_filename ((Sealed (FP f _:>:_)):_) = f
get_a_filename _ = fp2fn ""

get_hunks_old :: [Maybe B.ByteString] -> Sealed (FL Prim C(x))
              -> [Maybe B.ByteString]
get_hunks_old mls (Sealed ps) =
    apply_hunks (apply_hunks mls ps) (invert ps)

get_hunks_new :: [Maybe B.ByteString] -> Sealed (FL Prim C(x))
              -> [Maybe B.ByteString]
get_hunks_new mls (Sealed ps) = apply_hunks mls ps

get_hunkline :: [[Maybe B.ByteString]] -> Int
get_hunkline = ghl 1
    where ghl :: Int -> [[Maybe B.ByteString]] -> Int
          ghl n pps =
            if any (isJust . head) pps
            then n
            else ghl (n+1) $ map tail pps

make_chunk :: Int -> [Maybe B.ByteString] -> [B.ByteString]
make_chunk n mls = pull_chunk $ drop (n-1) mls
    where pull_chunk (Just l:mls') = l : pull_chunk mls'
          pull_chunk (Nothing:_) = []
          pull_chunk [] = bug "should this be [] in pull_chunk?"

mangle_unravelled_hunks :: [Sealed (FL Prim C(x))] -> Sealed (Prim C(x))
--mangle_unravelled_hunks [[h1],[h2]] = Deal with simple cases handily?
mangle_unravelled_hunks pss =
        if null nchs then bug "mangle_unravelled_hunks"
                     else Sealed (FP filename (Hunk l old new))
    where oldf = get_old (repeat Nothing) pss
          newfs = map (get_hunks_new oldf) pss
          l = get_hunkline $ oldf : newfs
          nchs = sort $ map (make_chunk l) newfs
          filename = get_a_filename pss
          old = make_chunk l oldf
          new = [top] ++ concat (intersperse [middle] nchs) ++ [bottom]
          top    = BC.pack $ "v v v v v v v" ++ eol_c
          middle = BC.pack $ "*************" ++ eol_c
          bottom = BC.pack $ "^ ^ ^ ^ ^ ^ ^" ++ eol_c
          eol_c  = if any (\ps -> not (B.null ps) && BC.last ps == '\r') old
                   then "\r"
                   else ""

instance Effect Patch where
    effect p@(Merger _ _ _ _) = sort_coalesceFL $ effect $ merger_undo p
    effect p@(Regrem _ _ _ _) = invert $ effect $ invert p
    effect (ComP ps) = concatFL $ mapFL_FL effect ps
    effect (PP p) = effect p
    isHunk p = do PP p' <- return p
                  isHunk p'

modernize_patch :: Patch C(x y) -> Patch C(x y)
modernize_patch p@(Merger _ _ _ _) = fromPrims $ effect p
modernize_patch p@(Regrem _ _ _ _) = fromPrims $ effect p
modernize_patch (ComP ps) = ComP $ filtermv $ mapFL_FL modernize_patch ps
    where filtermv :: FL Patch C(x y) -> FL Patch C(x y)
          filtermv (PP (Move _ b):>:xs) | hasadd xs = filtermv xs
              where hasadd (PP (FP b' AddFile):>:_) | b' == b = True
                    hasadd (PP (DP b' AddDir):>:_) | b' == b = True
                    hasadd (PP (FP b' RmFile):>:_) | b' == b = False
                    hasadd (PP (DP b' RmDir):>:_) | b' == b = False
                    hasadd (_:>:z) = hasadd z
                    hasadd NilFL = False
          filtermv (x:>:xs) = x :>: filtermv xs
          filtermv NilFL = NilFL

modernize_patch (PP p) = fromPrims $ modernizePrim p

instance FromPrims Patch where
    fromPrims (p :>: NilFL) = PP p
    fromPrims ps = join_patchesFL $ mapFL_FL PP ps
    joinPatches = join_patchesFL

new_ur :: Patch -> RL Patch -> [RL Patch]
new_ur p (Merger _ _ p1 p2 :<: ps) =
   case filter ((`unsafeCompare` p1) . headRL) $ head_permutationsRL ps of
   ((_:<:ps'):_) -> new_ur p (p1:<:ps') ++ new_ur p (p2:<:ps')
   _ -> bugDoc $ text "in function new_ur"
              $$ text "Original patch:"
              $$ showPatch_ p
              $$ text "Unwound:"
              $$ vcat (mapRL showPatch_ $ unwind p)

new_ur op ps =
    case filter (is_merger.headRL) $ head_permutationsRL ps of
    [] -> [ps]
    (ps':_) -> new_ur op ps'

headRL :: RL a -> a
headRL (x:<:_) = x
headRL _ = bug "bad headRL"

instance Invert p => Invert (Named p) where
    invert (NamedP n d p)  = NamedP (invert_name n) (map invert_name d) (invert p)
    identity = NamedP idpatchinfo [] identity

instance Invert Patch where
    invert (Merger undo unwindings p1 p2)
        = Regrem undo unwindings p1 p2
    invert (Regrem undo unwindings p1 p2)
        = Merger undo unwindings p1 p2
    invert (PP p) = PP (invert p)
    invert (ComP ps)  = ComP $ invert ps
    identity = ComP NilFL

instance MyEq Patch where
    unsafeCompare = eq_patches
instance MyEq p => MyEq (Named p) where
    unsafeCompare (NamedP n1 d1 p1) (NamedP n2 d2 p2) =
        n1 == n2 && d1 == d2 && unsafeCompare p1 p2

eq_patches :: Patch C(x y) -> Patch C(w z) -> Bool
eq_patches (PP p1) (PP p2) = unsafeCompare p1 p2
eq_patches (ComP ps1) (ComP ps2)
 = eq_FL eq_patches ps1 ps2
eq_patches (ComP NilFL) (PP Identity) = True
eq_patches (PP Identity) (ComP NilFL) = True
eq_patches (Merger _ _ p1a p1b) (Merger _ _ p2a p2b)
 = eq_patches p1a p2a &&
   eq_patches p1b p2b
eq_patches (Regrem _ _ p1a p1b) (Regrem _ _ p2a p2b)
 = eq_patches p1a p2a &&
   eq_patches p1b p2b
eq_patches _ _ = False

eq_FL :: (FORALL(b c d e) a C(b c) -> a C(d e) -> Bool)
      -> FL a C(x y) -> FL a C(w z) -> Bool
eq_FL _ NilFL NilFL = True
eq_FL f (x:>:xs) (y:>:ys) = f x y && eq_FL f xs ys
eq_FL _ _ _ = False