From cbcf1476fe0844712412e7fa72a04a7c5e03b64c Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Sat, 16 May 2026 16:25:18 +0300
Subject: [PATCH] highlight: add stateless single-line syntax highlighter for
 diffs

highlight_text is line-stateful (multi-line strings, block comments)
and not appropriate for diff rendering where each line stands alone.
Add highlight_line that handles one line at a time and a
render_diff_line template helper, then wire it into the PR file diff
view. Style the resulting b/u/i tokens through .pr-diff__content.
---
 diff.v                    |   9 ++++
 highlight/highlight.v     | 100 ++++++++++++++++++++++++++++++++++++++
 static/css/gitly.scss     |  24 ++++++++-
 templates/pull_files.html |   2 +-
 4 files changed, 132 insertions(+), 3 deletions(-)

diff --git a/diff.v b/diff.v
index 10a2668..e33f16b 100644
--- a/diff.v
+++ b/diff.v
@@ -2,6 +2,15 @@
 // Use of this source code is governed by a GPL license that can be found in the LICENSE file.
 module main
 
+import veb
+import highlight
+
+// render_diff_line is a template helper that returns the diff line's
+// content with single-line syntax highlighting applied.
+fn render_diff_line(content string, file_path string) veb.RawHtml {
+	return veb.RawHtml(highlight.highlight_line(content, file_path))
+}
+
 struct FileDiff {
 mut:
 	path       string
diff --git a/highlight/highlight.v b/highlight/highlight.v
index 0503711..cd4e7de 100644
--- a/highlight/highlight.v
+++ b/highlight/highlight.v
@@ -133,6 +133,106 @@ pub fn highlight_text(st string, file_path string, commit bool) (string, int, in
 	return res.bytestr(), lines, sloc
 }
 
+// highlight_line returns HTML-escaped, syntax-highlighted markup for a
+// single line of source code. It is stateless across calls (does not
+// track multi-line strings or block comments), so it suits diff rendering
+// where each line is colored independently.
+pub fn highlight_line(content string, file_path string) string {
+	if content.len == 0 {
+		return ''
+	}
+	lang := extension_to_lang(file_path) or { return escape_html(content) }
+	lc := lang.line_comments
+	mut mlc := ''
+	if lang.mline_comments.len >= 2 {
+		mlc = lang.mline_comments[0]
+	}
+	runes := content.bytes()
+	mut res := []u8{cap: runes.len + 16}
+	mut in_string := false
+	mut ss := u8(` `)
+	mut in_line_comment := false
+	for pos := 0; pos < runes.len; pos++ {
+		mut c := runes[pos]
+		if in_line_comment {
+			res << write(c)
+			continue
+		}
+		if in_string {
+			res << write(c)
+			if pos > 0 && runes[pos - 1] == `\\` && ss == `"` {
+				continue
+			}
+			if c == ss {
+				in_string = false
+				res << '</u>'.bytes()
+			}
+			continue
+		}
+		if is_letter(c, lang) {
+			word_start := pos
+			for pos < runes.len && is_letter(runes[pos], lang) {
+				pos++
+			}
+			w := runes[word_start..pos].bytestr()
+			pos--
+			if w in lang.keywords {
+				res << '<b>'.bytes()
+				res << w.bytes()
+				res << '</b>'.bytes()
+			} else {
+				res << w.bytes()
+			}
+			continue
+		}
+		if is_string_token(c, lang) {
+			in_string = true
+			ss = c
+			res << '<u>'.bytes()
+			res << write(c)
+			continue
+		}
+		if mlc != '' && c == mlc[0] && pos + mlc.len <= runes.len
+			&& is_line_comment(runes, pos, mlc) {
+			in_line_comment = true
+			res << '<i>'.bytes()
+			res << write(c)
+			continue
+		}
+		if lc != '' && c == lc[0] && pos + lc.len <= runes.len && is_line_comment(runes, pos, lc) {
+			in_line_comment = true
+			res << '<i>'.bytes()
+			res << write(c)
+			continue
+		}
+		res << write(c)
+	}
+	if in_line_comment {
+		res << '</i>'.bytes()
+	}
+	if in_string {
+		res << '</u>'.bytes()
+	}
+	return res.bytestr()
+}
+
+fn escape_html(s string) string {
+	mut res := []u8{cap: s.len}
+	for i in 0 .. s.len {
+		c := s[i]
+		if c == `<` {
+			res << '&lt;'.bytes()
+		} else if c == `>` {
+			res << '&gt;'.bytes()
+		} else if c == `&` {
+			res << '&amp;'.bytes()
+		} else {
+			res << c
+		}
+	}
+	return res.bytestr()
+}
+
 fn write(c u8) []u8 {
 	mut tmp := []u8{}
 	if c == `<` {
diff --git a/static/css/gitly.scss b/static/css/gitly.scss
index b11ed51..1aedd46 100644
--- a/static/css/gitly.scss
+++ b/static/css/gitly.scss
@@ -1796,11 +1796,13 @@ form {
 	background-color: $white;
 }
 
-.pr-diff__hunk-header td {
+.pr-diff__hunk-header td,
+.pr-diff__hunk-header td code {
 	background-color: #ddf4ff;
 	color: #57606a;
 	padding: 4px 12px;
 	font-size: 12px;
+	font-family: SFMono-Regular, Menlo, Monaco, Consolas, monospace;
 }
 
 .pr-diff__row td {
@@ -1808,6 +1810,8 @@ form {
 	white-space: pre;
 	vertical-align: top;
 	line-height: 1.5;
+	font-family: SFMono-Regular, Menlo, Monaco, Consolas, monospace;
+	font-size: 12px;
 }
 
 .pr-diff__row--context td { background-color: $white; }
@@ -1833,9 +1837,25 @@ form {
 
 	pre {
 		margin: 0;
-		font-family: inherit;
+		font-family: SFMono-Regular, Menlo, Monaco, Consolas, monospace;
+		font-size: 12px;
 		white-space: pre;
 	}
+
+	b {
+		font-weight: 600;
+		color: #cf222e;
+	}
+
+	u {
+		text-decoration: none;
+		color: #0a3069;
+	}
+
+	i {
+		font-style: normal;
+		color: #6e7781;
+	}
 }
 
 .pr-diff__commentrow td {
diff --git a/templates/pull_files.html b/templates/pull_files.html
index cc9d6fa..62a9af5 100644
--- a/templates/pull_files.html
+++ b/templates/pull_files.html
@@ -59,7 +59,7 @@
 											<td class='pr-diff__lineno'>@{dline.old_line_str()}</td>
 											<td class='pr-diff__lineno'>@{dline.new_line_str()}</td>
 											<td class='pr-diff__sign'>@{dline.sign()}</td>
-											<td class='pr-diff__content'><pre>@dline.content</pre></td>
+											<td class='pr-diff__content'><pre>@{render_diff_line(dline.content, fd.path)}</pre></td>
 										</tr>
 										@{render_inline_comments(fd.path, dline, comments_by_key)}
 										@if ctx.logged_in && pr.is_open() && dline.kind != 'context'
-- 
2.39.5