Update section behavior

sections.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"regexp"
+	"strings"
 
 	"github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/text"
@@ -10,6 +11,11 @@ import (
 
 var sectionHeadingRe = regexp.MustCompile(`(?m)^#{1,6} `)
 
+// headingTextRe matches an ATX heading at the start of a section. The
+// heading text is everything after the `#`s and the required space, on the
+// first line.
+var headingTextRe = regexp.MustCompile(`^(#{1,6})\s+([^\n]*)`)
+
 // splitSections splits raw markdown into sections.
 // Section 0 is any content before the first heading.
 // Each subsequent section begins at a heading line and runs to the next.
@@ -64,3 +70,31 @@ func joinSections(sections [][]byte) []byte {
 	}
 	return buf.Bytes()
 }
+
+// sectionHeading returns the heading level (1..6) and trimmed text of a
+// section produced by splitSections. Returns level=0 for the pre-heading
+// section (index 0).
+func sectionHeading(section []byte) (level int, text string) {
+	m := headingTextRe.FindSubmatch(section)
+	if m == nil {
+		return 0, ""
+	}
+	return len(m[1]), strings.TrimSpace(string(m[2]))
+}
+
+// sectionSpanEnd returns the exclusive end index of the section span starting
+// at start: the first following section whose heading level is <= start's.
+// Sections nested deeper than start (its subsections) belong to the span, so
+// editing a `##` covers every `###`+ under it up to the next `##` or `#`.
+func secionSpanEnd(sections [][]byte, start int) int {
+	level, _ := sectionHeading(sections[start])
+	if level == 0 {
+		return start + 1
+	}
+	for i := start + 1; i <= len(sections); i++ {
+		if l, _ := sectionHeading(sections[i]); l > 0 && l <= level {
+			return i
+		}
+	}
+	return len(sections)
+}