encoding/base64: correct DecodedLen overestimate for unpadded encodings

While we're at it, add tests for EncodedLen and DecodedLen.

Fixes #14803.

Change-Id: I200c72cf11c51669b8d9f70c6e57ece359f7ae61
Reviewed-on: https://go-review.googlesource.com/20649Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/base64/base64.go

@@ -459,7 +459,7 @@ func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
 func (enc *Encoding) DecodedLen(n int) int {
 	if enc.padChar == NoPadding {
 		// Unpadded data may end with partial block of 2-3 characters.
-		return (n*6 + 7) / 8
+		return n * 6 / 8
 	}
 	// Padded base64 should always be a multiple of 4 characters in length.
 	return n / 4 * 3

src/encoding/base64/base64_test.go

@@ -234,6 +234,51 @@ func TestDecodeCorrupt(t *testing.T) {
 	}
 }
 
+func TestEncodedLen(t *testing.T) {
+	for _, tt := range []struct {
+		enc  *Encoding
+		n    int
+		want int
+	}{
+		{RawStdEncoding, 0, 0},
+		{RawStdEncoding, 1, 2},
+		{RawStdEncoding, 2, 3},
+		{RawStdEncoding, 3, 4},
+		{RawStdEncoding, 7, 10},
+		{StdEncoding, 0, 0},
+		{StdEncoding, 1, 4},
+		{StdEncoding, 2, 4},
+		{StdEncoding, 3, 4},
+		{StdEncoding, 4, 8},
+		{StdEncoding, 7, 12},
+	} {
+		if got := tt.enc.EncodedLen(tt.n); got != tt.want {
+			t.Errorf("EncodedLen(%d): got %d, want %d", tt.n, got, tt.want)
+		}
+	}
+}
+
+func TestDecodedLen(t *testing.T) {
+	for _, tt := range []struct {
+		enc  *Encoding
+		n    int
+		want int
+	}{
+		{RawStdEncoding, 0, 0},
+		{RawStdEncoding, 2, 1},
+		{RawStdEncoding, 3, 2},
+		{RawStdEncoding, 4, 3},
+		{RawStdEncoding, 10, 7},
+		{StdEncoding, 0, 0},
+		{StdEncoding, 4, 3},
+		{StdEncoding, 8, 6},
+	} {
+		if got := tt.enc.DecodedLen(tt.n); got != tt.want {
+			t.Errorf("DecodedLen(%d): got %d, want %d", tt.n, got, tt.want)
+		}
+	}
+}
+
 func TestBig(t *testing.T) {
 	n := 3*1000 + 1
 	raw := make([]byte, n)