Keep options in a fixed order, and say which block a diff hunk is in

Options of a block (server options, a view, a zone) are listed and
written grouped by topic, alphabetical within a group, so the same
option is in the same place everywhere. Top-level statements keep the
order they were added in.

named.conf diffs add the enclosing blocks to each hunk header, like
git's function context: @@ -50,6 +49,8 @@ view "internal" {.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/diffcontext.go

@@ -0,0 +1,136 @@
+package engine
+
+import (
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// hunkHeader matches a unified diff hunk header; the line counts are left
+// out when they are 1.
+var hunkHeader = regexp.MustCompile(`^@@ -(\d+)(?:,\d+)? \+(\d+)(?:,\d+)? @@$`)
+
+// labelHunks adds the blocks a hunk's first change is in to its header,
+// like git does with function names, so a change to named.conf says where
+// it is:
+//
+//	@@ -50,6 +49,8 @@ view "internal" {
+//
+// Removed lines are looked up in the old file, added ones in the new.
+func labelHunks(diff, old, new string) string {
+	if !strings.Contains(diff, "\n@@ ") {
+		return diff
+	}
+	oldCtx, newCtx := blockContext(old), blockContext(new)
+	lines := strings.SplitAfter(diff, "\n")
+	for i, line := range lines {
+		m := hunkHeader.FindStringSubmatch(strings.TrimSuffix(line, "\n"))
+		if m == nil {
+			continue
+		}
+		o, _ := strconv.Atoi(m[1])
+		n, _ := strconv.Atoi(m[2])
+		label := ""
+	hunk:
+		for _, l := range lines[i+1:] {
+			switch {
+			case strings.HasPrefix(l, "-"):
+				label = at(oldCtx, o)
+				break hunk
+			case strings.HasPrefix(l, "+"):
+				label = at(newCtx, n)
+				break hunk
+			case strings.HasPrefix(l, " "):
+				o, n = o+1, n+1
+			case strings.HasPrefix(l, `\`):
+			default:
+				break hunk
+			}
+		}
+		if label != "" {
+			lines[i] = strings.TrimSuffix(line, "\n") + " " + label + "\n"
+		}
+	}
+	return strings.Join(lines, "")
+}
+
+// at returns the context of line n (1-based), or "".
+func at(ctx []string, n int) string {
+	if n < 1 || n > len(ctx) {
+		return ""
+	}
+	return ctx[n-1]
+}
+
+// blockContext returns, for each line of a named.conf file, the blocks
+// open at its start: `view "internal" { zone "example.com" {`, or "" at
+// the top level. It knows named.conf's strings and comments.
+func blockContext(text string) []string {
+	var (
+		out     []string
+		stack   []string
+		stmt    strings.Builder // the current statement, up to a { or ;
+		inStr   bool
+		inBlock bool // in a /* */ comment
+	)
+	label := func() string {
+		if len(stack) == 0 {
+			return ""
+		}
+		return strings.Join(stack, " { ") + " {"
+	}
+	for _, line := range strings.SplitAfter(text, "\n") {
+		if line == "" {
+			continue
+		}
+		out = append(out, label())
+		for i := 0; i < len(line); i++ {
+			c := line[i]
+			switch {
+			case inBlock:
+				if c == '*' && i+1 < len(line) && line[i+1] == '/' {
+					inBlock = false
+					i++
+				}
+			case inStr:
+				stmt.WriteByte(c)
+				if c == '\\' && i+1 < len(line) {
+					stmt.WriteByte(line[i+1])
+					i++
+				} else if c == '"' {
+					inStr = false
+				}
+			case c == '"':
+				inStr = true
+				stmt.WriteByte(c)
+			case c == '#' || c == '/' && i+1 < len(line) && line[i+1] == '/':
+				i = len(line) // a comment to the end of the line
+			case c == '/' && i+1 < len(line) && line[i+1] == '*':
+				inBlock = true
+				i++
+			case c == '{':
+				stack = append(stack, shortLabel(stmt.String()))
+				stmt.Reset()
+			case c == '}':
+				if len(stack) > 0 {
+					stack = stack[:len(stack)-1]
+				}
+				stmt.Reset()
+			case c == ';':
+				stmt.Reset()
+			default:
+				stmt.WriteByte(c)
+			}
+		}
+	}
+	return out
+}
+
+// shortLabel tidies the text before a { for a hunk header.
+func shortLabel(s string) string {
+	s = strings.Join(strings.Fields(s), " ")
+	if len(s) > 60 {
+		s = s[:57] + "..."
+	}
+	return s
+}