From 7f2d008d461d77df15370c7020843f37500a853b Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Sat, 28 Oct 2023 17:16:47 -0700
Subject: [PATCH] Fix incorrectly parsing links with nested `[]`

This markdown input:

```markdown
[[]](https://haskell.org)

[[][]](https://haskell.org)

[[[][]](https://haskell.org)

[[][][]](https://haskell.org)
```

In commonmark-hs, [it used to make this HTML](https://pandoc.org/try/?params=%7B%22text%22%3A%22%5B%5B%5D%5D%28https%3A%2F%2Fhaskell.org%29%5Cn%5Cn%5B%5B%5D%5B%5D%5D%28https%3A%2F%2Fhaskell.org%29%5Cn%5Cn%5B%5B%5B%5D%5B%5D%5D%28https%3A%2F%2Fhaskell.org%29%5Cn%5Cn%5B%5B%5D%5B%5D%5B%5D%5D%28https%3A%2F%2Fhaskell.org%29%22%2C%22to%22%3A%22html5%22%2C%22from%22%3A%22commonmark_x%22%2C%22standalone%22%3Afalse%2C%22embed-resources%22%3Afalse%2C%22table-of-contents%22%3Afalse%2C%22number-sections%22%3Afalse%2C%22citeproc%22%3Afalse%2C%22html-math-method%22%3A%22plain%22%2C%22wrap%22%3A%22auto%22%2C%22highlight-style%22%3Anull%2C%22files%22%3A%7B%7D%2C%22template%22%3Anull%7D):

```html
<p><a href="https://haskell.org">[]</a></p>
<p>[<a href="https://haskell.org">][]</a></p>
<p>[[<a href="https://haskell.org">][]</a></p>
<p>[[]<a href="https://haskell.org">][]</a></p>
```

In commonmark.js, [it makes this instead](https://spec.commonmark.org/dingus/?text=%5B%5B%5D%5D(https%3A%2F%2Fhaskell.org)%0A%0A%5B%5B%5D%5B%5D%5D(https%3A%2F%2Fhaskell.org)%0A%0A%5B%5B%5B%5D%5B%5D%5D(https%3A%2F%2Fhaskell.org)%0A%0A%5B%5B%5D%5B%5D%5B%5D%5D(https%3A%2F%2Fhaskell.org)):

```html
<p><a href="https://haskell.org">[]</a></p>
<p><a href="https://haskell.org">[][]</a></p>
<p>[<a href="https://haskell.org">[][]</a></p>
<p><a href="https://haskell.org">[][][]</a></p>
```

The commonmark.js output seems to be correct according to the specification:

> Brackets are allowed in the [link text](https://spec.commonmark.org/0.30/#link-text)
> only if (a) they are backslash-escaped or (b) they appear as a
> matched pair of brackets, with an open bracket `[`, a sequence of
> zero or more inlines, and a close bracket `]`.
---
 commonmark/src/Commonmark/Inlines.hs | 26 ++++++++++++++++++++++----
 commonmark/test/regression.md        | 16 ++++++++++++++++
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/commonmark/src/Commonmark/Inlines.hs b/commonmark/src/Commonmark/Inlines.hs
index 475a71d..560dc05 100644
--- a/commonmark/src/Commonmark/Inlines.hs
+++ b/commonmark/src/Commonmark/Inlines.hs
@@ -686,6 +686,14 @@ delimsMatch (Chunk open@Delim{} _ opents) (Chunk close@Delim{} _ closets) =
     opents /= closets
 delimsMatch _ _ = False
 
+-- check for balanced `[]` brackets
+bracketChunkToNumber :: Chunk a -> Int
+bracketChunkToNumber (Chunk Delim{ delimType = '[' } _ _) = 1
+bracketChunkToNumber (Chunk Delim{ delimType = ']' } _ _) = -1
+bracketChunkToNumber _ = 0
+bracketMatchedCount :: [Chunk a] -> Int
+bracketMatchedCount chunksinside = sum $ map bracketChunkToNumber chunksinside
+
 processBrackets :: IsInline a
                 => [BracketedSpec a] -> ReferenceMap -> [Chunk a] -> [Chunk a]
 processBrackets bracketedSpecs rm xs =
@@ -783,7 +791,7 @@ processBs bracketedSpecs st =
 
               suffixPos = incSourceColumn closePos 1
 
-          in case parse
+          in case (bracketMatchedCount chunksinside, parse
                  (withRaw
                    (do setPosition suffixPos
                        (spec, constructor) <- choice $
@@ -791,13 +799,13 @@ processBs bracketedSpecs st =
                            specs
                        pos <- getPosition
                        return (spec, constructor, pos)))
-                 "" suffixToks of
-                   Left _ -> -- match but no link/image
+                 "" suffixToks) of
+                   (0, Left _) -> -- match but no link/image
                          processBs bracketedSpecs
                             st{ leftCursor = moveLeft (leftCursor st)
                               , rightCursor = fixSingleQuote $
                                     moveRight (rightCursor st) }
-                   Right ((spec, constructor, newpos), desttoks) ->
+                   (0, Right ((spec, constructor, newpos), desttoks)) ->
                      let left' = case bracketedPrefix spec of
                                       Just _  -> moveLeft left
                                       Nothing -> left
@@ -854,6 +862,16 @@ processBs bracketedSpecs st =
                                             (chunkPos opener)
                                             $ stackBottoms st
                                 }
+                  -- Bracket matched count /= 0
+                  --
+                  -- Links § 6.3 ¶ 2 • 2
+                  -- Brackets are allowed in the link text only if (a) they are
+                  -- backslash-escaped or (b) they appear as a matched pair of
+                  -- brackets, with an open bracket [, a sequence of zero or more
+                  -- inlines, and a close bracket ].
+                   _ ->
+                         processBs bracketedSpecs
+                            st{ leftCursor = moveLeft left }
 
 
        (_, Just (Chunk Delim{ delimType = ']' } _ _))
diff --git a/commonmark/test/regression.md b/commonmark/test/regression.md
index 4b9c3bb..b4183cc 100644
--- a/commonmark/test/regression.md
+++ b/commonmark/test/regression.md
@@ -219,3 +219,19 @@ x
 <pre><code class="language-\">x
 </code></pre>
 ````````````````````````````````
+
+Issue #119
+```````````````````````````````` example
+[[]](https://haskell.org)
+
+[[][]](https://haskell.org)
+
+[[[][]](https://haskell.org)
+
+[[][][]](https://haskell.org)
+.
+<p><a href="https://haskell.org">[]</a></p>
+<p><a href="https://haskell.org">[][]</a></p>
+<p>[<a href="https://haskell.org">[][]</a></p>
+<p><a href="https://haskell.org">[][][]</a></p>
+````````````````````````````````