amp package.

This package contains a CacheURL function that modifies a URL to be
accessed through an AMP cache, and the "AMP armor" data encoding scheme
for encoding data into the AMP subset of HTML.

common/amp/path.go

@@ -0,0 +1,44 @@
+package amp
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"strings"
+)
+
+// EncodePath encodes data in a way that is suitable for the suffix of an AMP
+// cache URL.
+func EncodePath(data []byte) string {
+	var cacheBreaker [9]byte
+	_, err := rand.Read(cacheBreaker[:])
+	if err != nil {
+		panic(err)
+	}
+	b64 := base64.RawURLEncoding.EncodeToString
+	return "0" + b64(cacheBreaker[:]) + "/" + b64(data)
+}
+
+// DecodePath decodes data from a path suffix as encoded by EncodePath. The path
+// must have already been trimmed of any directory prefix (as might be present
+// in, e.g., an HTTP request). That is, the first character of path should be
+// the "0" message format indicator.
+func DecodePath(path string) ([]byte, error) {
+	if len(path) < 1 {
+		return nil, fmt.Errorf("missing format indicator")
+	}
+	version := path[0]
+	rest := path[1:]
+	switch version {
+	case '0':
+		// Ignore everything else up to and including the final slash
+		// (there must be at least one slash).
+		i := strings.LastIndexByte(rest, '/')
+		if i == -1 {
+			return nil, fmt.Errorf("missing data")
+		}
+		return base64.RawURLEncoding.DecodeString(rest[i+1:])
+	default:
+		return nil, fmt.Errorf("unknown format indicator %q", version)
+	}
+}