Keep generated prose out of Markdown structure

internal/reporttemplate/plain_text.go

@@ -0,0 +1,89 @@
+package reporttemplate
+
+import (
+	"strings"
+	"unicode"
+)
+
+var inlineMarkdownReplacer = strings.NewReplacer(
+	`\`, `\\`,
+	"`", "\\`",
+	"*", "\\*",
+	"_", "\\_",
+	"[", "\\[",
+	"]", "\\]",
+	"(", "\\(",
+	")", "\\)",
+	"<", "\\<",
+	">", "\\>",
+	"!", "\\!",
+	"~", "\\~",
+	"|", "\\|",
+	"&", "\\&",
+)
+
+// plainText preserves prose and paragraph breaks while preventing dynamic
+// values from creating Markdown structure owned by repository templates.
+func plainText(value string) string {
+	value = strings.ReplaceAll(value, "\r\n", "\n")
+	value = strings.ReplaceAll(value, "\r", "\n")
+	lines := strings.Split(value, "\n")
+	for index, line := range lines {
+		lines[index] = escapeMarkdownLine(line)
+	}
+	return strings.Join(lines, "\n")
+}
+
+func escapeMarkdownLine(line string) string {
+	indentEnd := 0
+	spaces := 0
+	hasTab := false
+	for indentEnd < len(line) {
+		switch line[indentEnd] {
+		case ' ':
+			spaces++
+			indentEnd++
+		case '\t':
+			hasTab = true
+			indentEnd++
+		default:
+			goto escaped
+		}
+	}
+
+escaped:
+	indent := line[:indentEnd]
+	if hasTab || spaces >= 4 {
+		indent = ""
+	}
+	return indent + escapeMarkdownBlockPrefix(escapeMarkdownInline(line[indentEnd:]))
+}
+
+func escapeMarkdownInline(value string) string {
+	value = inlineMarkdownReplacer.Replace(value)
+	return strings.Map(func(r rune) rune {
+		if unicode.IsControl(r) {
+			return ' '
+		}
+		return r
+	}, value)
+}
+
+func escapeMarkdownBlockPrefix(value string) string {
+	if value == "" {
+		return value
+	}
+	switch value[0] {
+	case '#', '+', '-', '>', '=':
+		return "\\" + value
+	}
+
+	index := 0
+	for index < len(value) && value[index] >= '0' && value[index] <= '9' {
+		index++
+	}
+	if index > 0 && index < len(value) && value[index] == '.' {
+		return value[:index] + "\\" + value[index:]
+	}
+	return value
+}