Allow editing of individual sections

sections.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"bytes"
+	"regexp"
+)
+
+var sectionHeadingRe = regexp.MustCompile(`(?m)^#{1,6} `)
+
+// 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.
+func splitSections(raw []byte) [][]byte {
+	locs := sectionHeadingRe.FindAllIndex(raw, -1)
+	if len(locs) == 0 {
+		return [][]byte{raw}
+	}
+	sections := make([][]byte, 0, len(locs)+1)
+	prev := 0
+	for _, loc := range locs {
+		sections = append(sections, raw[prev:loc[0]])
+		prev = loc[0]
+	}
+	sections = append(sections, raw[prev:])
+	return sections
+}
+
+// joinSections reassembles sections produced by splitSections.
+func joinSections(sections [][]byte) []byte {
+	return bytes.Join(sections, nil)
+}