From ca5217ff94f8a40a703596542a25ba4dbb20b1e3 Mon Sep 17 00:00:00 2001
From: Michael Howell <michael@notriddle.com>
Date: Thu, 26 Oct 2023 09:55:13 -0700
Subject: [PATCH] Track underscore bottom separately mod 3, like asterisk

The reasoning that a failed delimiter means future delimiters will also fail
only applies if the reason they failed was not the multiple-of-three rule.
This was already implemented correctly for asterisks, but not for underscore.

This commit ports [97da298] from commonmark.js to cmark.

[97da298]: https://github.com/commonmark/commonmark.js/commit/97da298feb94e8c404c9c734a8b5965bbcbb398e
---
 src/inlines.c       | 13 ++++++++-----
 test/regression.txt | 28 ++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/src/inlines.c b/src/inlines.c
index b0741383f..1313428d5 100644
--- a/src/inlines.c
+++ b/src/inlines.c
@@ -653,9 +653,11 @@ static void process_emphasis(subject *subj, bufsize_t stack_bottom) {
   delimiter *old_closer;
   bool opener_found;
   int openers_bottom_index = 0;
-  bufsize_t openers_bottom[9] = {stack_bottom, stack_bottom, stack_bottom,
-                                 stack_bottom, stack_bottom, stack_bottom,
-                                 stack_bottom, stack_bottom, stack_bottom};
+  bufsize_t openers_bottom[15] = {stack_bottom, stack_bottom, stack_bottom,
+                                  stack_bottom, stack_bottom, stack_bottom,
+                                  stack_bottom, stack_bottom, stack_bottom,
+                                  stack_bottom, stack_bottom, stack_bottom,
+                                  stack_bottom, stack_bottom, stack_bottom};
 
   // move back to first relevant delim.
   candidate = subj->last_delim;
@@ -675,10 +677,11 @@ static void process_emphasis(subject *subj, bufsize_t stack_bottom) {
         openers_bottom_index = 1;
         break;
       case '_':
-        openers_bottom_index = 2;
+        openers_bottom_index = 2 +
+                (closer->can_open ? 3 : 0) + (closer->length % 3);
         break;
       case '*':
-        openers_bottom_index = 3 +
+        openers_bottom_index = 8 +
                 (closer->can_open ? 3 : 0) + (closer->length % 3);
         break;
       default:
diff --git a/test/regression.txt b/test/regression.txt
index f905677d6..1707956d7 100644
--- a/test/regression.txt
+++ b/test/regression.txt
@@ -212,4 +212,32 @@ x <!A>
 <p>x <!A></p>
 ````````````````````````````````
 
+An underscore that is not part of a delimiter should not prevent another
+pair of underscores from forming part of their own.
+```````````````````````````````` example
+__!_!__
+
+__!x!__
+
+**!*!**
+
+---
+
+_*__*_*
+
+_*xx*_*
+
+_*__-_-
+
+_*xx-_-
+.
+<p><strong>!_!</strong></p>
+<p><strong>!x!</strong></p>
+<p><strong>!*!</strong></p>
+<hr />
+<p><em><em>__</em></em>*</p>
+<p><em><em>xx</em></em>*</p>
+<p><em>*__-</em>-</p>
+<p><em>*xx-</em>-</p>
+````````````````````````````````