From 56cf7c69a948ee04100b8363206b51d680bc4664 Mon Sep 17 00:00:00 2001
From: Mirek Kratochvil <exa.exa@gmail.com>
Date: Thu, 17 Jul 2025 22:21:23 +0200
Subject: [PATCH] have diffs (for good manners)

---
 Main.hs | 29 ++++++++++++++++++++++++++---
 Opts.hs | 25 +++++++++++++++++++++++--
 2 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/Main.hs b/Main.hs
index b8e2ce4..5ea9b57 100644
--- a/Main.hs
+++ b/Main.hs
@@ -182,12 +182,19 @@ expand n = go
     go [] = []
     go (x@(Conflict m1 o1 y1):xs) =
       case go xs of
-        (Conflict m2 o2 y2:xs') ->
+        (Conflict m2 o2 y2:xs') | n > 0 ->
           Conflict (m1 ++ m2) (o1 ++ o2) (y1 ++ y2) : xs'
         (Ok a:Conflict m2 o2 y2:xs')
-          | length a <= n ->
+          | length a < n ->
             Conflict (m1 ++ a ++ m2) (o1 ++ a ++ o2) (y1 ++ a ++ y2) : xs'
         xs' -> x : xs'
+    go (x@(Replace o1 n1):xs) = case go xs of
+      (Replace o2 n2:xs') | n > 0 ->
+          Replace (o1++o2) (n1++n2): xs'
+      (Ok a:Replace o2 n2:xs')
+        | length a < n ->
+          Replace (o1++a++o2) (n1++a++n2): xs'
+      xs' -> x : xs'
     go (x:xs) = x : go xs
 
 resolve :: Config -> Merged -> Merged
@@ -217,6 +224,8 @@ merge cfg@Config {..} ms ys =
     . regroup
     $ align (chunks ms) (chunks ys)
 
+diff Config{..} = expand cfgContext . chunks
+
 {-
  - front-end
  -}
@@ -239,7 +248,11 @@ format Config {..} h = go False
             , cfgLabelEnd
             ]
       go True xs
-    go _ _ = error "bad format (replace)"
+    go _ (Replace o n:xs) = do
+      hPutStr h
+        $ mconcat
+            [cfgLabelStart, Toks.glue o, cfgLabelDiff, Toks.glue n, cfgLabelEnd]
+      go True xs
 
 runCmd :: Command -> Config -> IO ()
 runCmd CmdDiff3 {..} cfg =
@@ -278,6 +291,16 @@ runCmd CmdGitMerge {..} cfg = do
   if or conflicts
     then exitWith (ExitFailure 1)
     else exitSuccess
+runCmd CmdDiff {..} cfg = do
+  withSystemTempDirectory "werge-diff" $ \workdir -> do
+    let [fOld, fNew, fDiff] = map (workdir </>) ["old", "new", "diff"]
+    for_ [(diffOld, fOld), (diffNew, fNew)] $ \(path, tmp) ->
+      bracketFile path ReadMode $ \h -> hSplitToFile cfg h tmp
+    rundiff fOld fNew fDiff
+    conflicted <- pdiff fDiff >>= format cfg stdout . diff cfg
+    if conflicted
+      then exitWith (ExitFailure 1)
+      else exitSuccess
 
 main :: IO ()
 main = catch go bad
diff --git a/Opts.hs b/Opts.hs
index dc6987f..dcb3330 100644
--- a/Opts.hs
+++ b/Opts.hs
@@ -110,6 +110,7 @@ data Config = Config
   , cfgConflicts :: ConflictMask
   , cfgLabelStart :: String
   , cfgLabelMyOld :: String
+  , cfgLabelDiff :: String
   , cfgLabelOldYour :: String
   , cfgLabelEnd :: String
   } deriving (Show)
@@ -154,10 +155,10 @@ config = do
       $ long "expand-context"
           <> short 'C'
           <> metavar "N"
-          <> value 1
+          <> value 2
           <> showDefault
           <> help
-               "Consider changes that are at most N tokens apart to be a single change. Zero may cause bad resolutions of near conflicting edits"
+               "Consider changes that are at less than N tokens apart to be a single change; 0 turns off conflict expansion, 1 may cause bad resolutions of near conflicting edits"
   cfgResolution <-
     option (eitherReader resolutionMode)
       $ long "resolve"
@@ -182,6 +183,11 @@ config = do
       $ long "label-mo"
           <> metavar "\"|||||\""
           <> help "Separator of local edits and original"
+  labelDiff <-
+    optional . strOption
+      $ long "label-diff"
+          <> metavar "\"|||||\""
+          <> help "Separator for old and new version"
   labelOldYour <-
     optional . strOption
       $ long "label-oy"
@@ -198,6 +204,8 @@ config = do
           bool "<<<<<" "\ESC[1;37m<\ESC[0;31m" color `fromMaybe` labelStart
       , cfgLabelMyOld =
           bool "|||||" "\ESC[1;37m|\ESC[1;30m" color `fromMaybe` labelMyOld
+      , cfgLabelDiff =
+          bool "|||||" "\ESC[1;37m|\ESC[0;32m" color `fromMaybe` labelDiff
       , cfgLabelOldYour =
           bool "=====" "\ESC[1;37m=\ESC[0;32m" color `fromMaybe` labelOldYour
       , cfgLabelEnd =
@@ -215,6 +223,10 @@ data Command
       { gmFiles :: Maybe [FilePath]
       , gmDoAdd :: Bool
       }
+  | CmdDiff
+    { diffOld :: FilePath
+    , diffNew :: FilePath
+    }
   deriving (Show)
 
 cmdDiff3 :: Parser Command
@@ -252,6 +264,12 @@ cmdGitMerge = do
       ]
   pure CmdGitMerge {..}
 
+cmdDiff :: Parser Command
+cmdDiff = do
+  diffOld <- strArgument $ metavar "OLDFILE" <> help "Original file version"
+  diffNew <- strArgument $ metavar "NEWFILE" <> help "File version with changes"
+  pure CmdDiff {..}
+
 -- TODO have some option to output the (partially merged) my/old/your files so
 -- that folks can continue with external program or so (such as meld)
 cmd :: Parser Command
@@ -264,6 +282,9 @@ cmd =
         , command "git"
             $ info cmdGitMerge
             $ progDesc "Automerge unmerged files in git conflict"
+        , command "diff"
+            $ info cmdDiff
+            $ progDesc "Highlight differences between two files"
         ]
 
 parseOpts :: IO (Config, Command)