make hashWith a bit smarter (#238)

It used to return five-byte strings, and now it returns four bytes with
nearly the same number of bits of entropy.

It also avoids the exported vs unexported dance if the name isn't an
identifier, which is now common with import paths.

See the added docs for more details.

hash.go

@@ -151,6 +151,21 @@ var (
 	nameBase64  = base64.NewEncoding(nameCharset)
 )
 
+// These funcs mimic the unicode package API, but byte-based since we know
+// base64 is all ASCII.
+
+func isDigit(b byte) bool { return '0' <= b && b <= '9' }
+func isLower(b byte) bool { return 'a' <= b && b <= 'z' }
+func isUpper(b byte) bool { return 'A' <= b && b <= 'Z' }
+func toLower(b byte) byte { return b + ('a' - 'A') }
+func toUpper(b byte) byte { return b - ('a' - 'A') }
+
+// hashWith returns a hashed version of name, including the provided salt as well as
+// opts.Seed into the hash input.
+//
+// The result is always four bytes long. If the input was a valid identifier,
+// the output remains equally exported or unexported. Note that this process is
+// reproducible, but not reversible.
 func hashWith(salt []byte, name string) string {
 	if len(salt) == 0 {
 		panic("hashWith: empty salt")
@@ -164,12 +179,35 @@ func hashWith(salt []byte, name string) string {
 	d.Write(salt)
 	d.Write(opts.Seed)
 	io.WriteString(d, name)
-	sum := nameBase64.EncodeToString(d.Sum(nil))
-
-	// TODO: Just make the first letter uppercase or lowercase as needed.
-	// This is also not needed for non-names, like import paths.
+	sum := make([]byte, nameBase64.EncodedLen(d.Size()))
+	nameBase64.Encode(sum, d.Sum(nil))
+	sum = sum[:length]
+
+	// Even if we are hashing a package path, we still want the result to be
+	// a valid identifier, since we'll use it as the package name too.
+	if isDigit(sum[0]) {
+		// Turn "3foo" into "Dfoo".
+		// Similar to toLower, since uppercase letters go after digits
+		// in the ASCII table.
+		sum[0] += 'A' - '0'
+	}
+	// Keep the result equally exported or not, if it was an identifier.
+	if !token.IsIdentifier(name) {
+		return string(sum)
+	}
 	if token.IsExported(name) {
-		return "Z" + sum[:length]
+		if sum[0] == '_' {
+			// Turn "_foo" into "Zfoo".
+			sum[0] = 'Z'
+		} else if isLower(sum[0]) {
+			// Turn "afoo" into "Afoo".
+			sum[0] = toUpper(sum[0])
+		}
+	} else {
+		if isUpper(sum[0]) {
+			// Turn "Afoo" into "afoo".
+			sum[0] = toLower(sum[0])
+		}
 	}
-	return "z" + sum[:length]
+	return string(sum)
 }