From: Anderson A. Giacomolli <agiacomolli@gmail.com>
Date: Sat, 7 Nov 2026 15:23:09 +0000 (-0300)
Subject: Split long replies between Markdown blocks
X-Git-Url: http://git.giacomolli.uk/onechat/commitdiff_plain/97729fc59a2fbd73e3f8a424cae4c825f5ba885b?ds=sidebyside

Split long replies between Markdown blocks

splitRich cuts the Markdown of a reply into parts of at most 32768
bytes, the limit of the Markdown of one rich message. It cuts after a
blank line outside a code block, so a table, a code block, and a
paragraph stay whole, and each part renders as the reply would. When no
such cut fits, it cuts after a line break outside a code block, and then
inside a code block, which it closes at the end of the part and opens
again with the same fence line in the next part. A single line over the
limit is cut at the limit, never inside a character.

The Bot API allows 32768 characters of rendered text and does not say if
a character is a byte. The bytes of the Markdown are at least the
characters of the text, so a part is never over the limit. The bot does
not use splitRich yet.

Issue: fec23f9
---

diff --git a/markdown.go b/markdown.go
index 48e9ace..1a2f4fe 100644
--- a/markdown.go
+++ b/markdown.go
@@ -169,8 +169,7 @@ func closingFence(lines []string, index int, fence string) int {
 	}
 
 	for end := index + 1; end < len(lines); end++ {
-		trimmed := strings.TrimSpace(lines[end])
-		if strings.HasPrefix(trimmed, fence) && strings.Trim(trimmed, "`") == "" {
+		if closesFence(lines[end], fence) {
 			return end
 		}
 	}
@@ -178,6 +177,12 @@ func closingFence(lines []string, index int, fence string) int {
 	return -1
 }
 
+// closesFence tells if line closes the code block of fence.
+func closesFence(line, fence string) bool {
+	trimmed := strings.TrimSpace(line)
+	return strings.HasPrefix(trimmed, fence) && strings.Trim(trimmed, "`") == ""
+}
+
 // headingText returns the text of an ATX heading line without its marks, or ""
 // when line is not a heading or has no text.
 func headingText(line string) string {
diff --git a/telegram.go b/telegram.go
index 9a24a4e..9dbe86e 100644
--- a/telegram.go
+++ b/telegram.go
@@ -14,6 +14,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode/utf8"
 )
 
 const (
@@ -33,6 +34,12 @@ const (
 	// messageUnits is the most UTF-16 code units of one Telegram message.
 	messageUnits = 4096
 
+	// richMessageBytes is the most bytes of the Markdown of one rich
+	// message. The Bot API allows 32768 characters of rendered text, and
+	// the Markdown of a text has at least as many bytes as the text has
+	// characters.
+	richMessageBytes = 32768
+
 	// typingInterval is how often the bot shows the typing action while a
 	// turn runs. Telegram shows the action for about five seconds.
 	typingInterval = 4 * time.Second
@@ -548,3 +555,83 @@ func prefixEnd(text string, limit int) int {
 
 	return len(text)
 }
+
+// splitRich cuts the Markdown of a reply into parts of at most limit bytes. It
+// cuts after a blank line outside a code block, so a table, a code block, or a
+// paragraph stays whole. When no such cut fits in a part, it cuts after the
+// last line break outside a code block, then after the last line break in a
+// code block, which it closes at the end of the part and opens again in the
+// next part with the same fence line. A line longer than limit is cut at the
+// limit. It never cuts a character.
+func splitRich(markdown string, limit int) []string {
+	var parts []string
+	for len(markdown) > limit {
+		cut, fence, opening := richCut(markdown, limit)
+		if fence == "" {
+			parts = append(parts, markdown[:cut])
+			markdown = markdown[cut:]
+
+			continue
+		}
+
+		parts = append(parts, markdown[:cut]+fence+"\n")
+		markdown = opening + "\n" + markdown[cut:]
+	}
+
+	return append(parts, markdown)
+}
+
+// richCut returns where splitRich cuts markdown, which is longer than limit.
+// For a cut in a code block, it also returns the fence that closes the block
+// and the line that opens it. Each part before a cut holds text other than
+// space, and a cut in a code block leaves room for the fence, so each cut makes
+// the rest shorter.
+func richCut(markdown string, limit int) (int, string, string) {
+	blank, line, code := 0, 0, 0
+	var fence, opening, inFence, inOpening string
+	for start := 0; start < limit; {
+		end := strings.IndexByte(markdown[start:], '\n')
+		if end < 0 || start+end+1 > limit {
+			break
+		}
+
+		end += start + 1
+		text := strings.TrimRight(markdown[start:end], "\r\n")
+		_, opens := codeFence(text)
+		switch {
+		case fence != "":
+			if closesFence(text, fence) {
+				fence = ""
+			} else if end > len(opening)+1 && end+len(fence)+1 <= limit {
+				code, inFence, inOpening = end, fence, opening
+			}
+		case opens != "":
+			fence, opening = opens, text
+		case strings.TrimSpace(markdown[:end]) == "":
+		case strings.TrimSpace(text) == "":
+			blank = end
+		}
+
+		if fence == "" && strings.TrimSpace(markdown[:end]) != "" {
+			line = end
+		}
+
+		start = end
+	}
+
+	switch {
+	case blank > 0:
+		return blank, "", ""
+	case line > 0:
+		return line, "", ""
+	case code > 0:
+		return code, inFence, inOpening
+	}
+
+	cut := limit
+	for !utf8.RuneStart(markdown[cut]) {
+		cut--
+	}
+
+	return cut, "", ""
+}
diff --git a/telegram_test.go b/telegram_test.go
index 2578b77..30b2fa1 100644
--- a/telegram_test.go
+++ b/telegram_test.go
@@ -409,6 +409,64 @@ func TestSplitEntitiesCutsAnEntityAcrossASplit(t *testing.T) {
 	}, parts)
 }
 
+func TestSplitRich(t *testing.T) {
+	testCases := []struct {
+		name     string
+		markdown string
+		limit    int
+		parts    []string
+	}{
+		{
+			name:     "short reply stays whole",
+			markdown: "| a | b |",
+			limit:    100,
+			parts:    []string{"| a | b |"},
+		},
+		{
+			name:     "cut after a blank line keeps a table whole",
+			markdown: "Intro.\n\n| a | b |\n|---|---|\n| 1 | 2 |\n",
+			limit:    30,
+			parts:    []string{"Intro.\n\n", "| a | b |\n|---|---|\n| 1 | 2 |\n"},
+		},
+		{
+			name:     "blank line in a code block is not a cut",
+			markdown: "Text.\n\n```go\nx\n\ny\n```\n\nEnd.\n",
+			limit:    20,
+			parts:    []string{"Text.\n\n", "```go\nx\n\ny\n```\n\n", "End.\n"},
+		},
+		{
+			name:     "paragraph with no blank line is cut after a line",
+			markdown: "aaa\nbbb\nccc\n",
+			limit:    9,
+			parts:    []string{"aaa\nbbb\n", "ccc\n"},
+		},
+		{
+			name:     "long code block is closed and opened again",
+			markdown: "```sh\none\ntwo\nthree\n```\n",
+			limit:    16,
+			parts:    []string{"```sh\none\n```\n", "```sh\ntwo\n```\n", "```sh\nthree\n```\n"},
+		},
+		{
+			name:     "long line is cut at the limit and not inside a character",
+			markdown: "ab\u00e9cd",
+			limit:    3,
+			parts:    []string{"ab", "\u00e9c", "d"},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			require.Equal(t, tc.parts, splitRich(tc.markdown, tc.limit))
+		})
+	}
+
+	long := strings.Repeat("A paragraph of the reply.\n\n", 2000)
+	parts := splitRich(long, richMessageBytes)
+	require.Len(t, parts, 2)
+	require.Len(t, parts[0], richMessageBytes-richMessageBytes%len("A paragraph of the reply.\n\n"))
+	require.Equal(t, long, strings.Join(parts, ""))
+}
+
 func TestLoadBotConfig(t *testing.T) {
 	testCases := []struct {
 		name      string