chore: migrate mail themes to shared framework and locked upstream inputs

tools/builder/anchors.go

@@ -0,0 +1,42 @@
+package builder
+
+import (
+	"bytes"
+	"fmt"
+	"golang.org/x/net/html"
+	"io"
+)
+
+func BodyAnchors(masked []byte) ([2]int, error) {
+	var anchors [2]int
+	opens, closes, offset := 0, 0, 0
+	z := html.NewTokenizer(bytes.NewReader(masked))
+	for {
+		kind := z.Next()
+		raw := z.Raw()
+		start := offset
+		offset += len(raw)
+		switch kind {
+		case html.ErrorToken:
+			if z.Err() != io.EOF {
+				return anchors, z.Err()
+			}
+			if opens != 1 || closes != 1 || anchors[0] >= anchors[1] {
+				return anchors, fmt.Errorf("expected exactly one static body start and end")
+			}
+			return anchors, nil
+		case html.StartTagToken:
+			tag, _ := z.TagName()
+			if bytes.Equal(tag, []byte("body")) {
+				opens++
+				anchors[0] = offset
+			}
+		case html.EndTagToken:
+			tag, _ := z.TagName()
+			if bytes.Equal(tag, []byte("body")) {
+				closes++
+				anchors[1] = start
+			}
+		}
+	}
+}