codec/h264/h264dec: added fixedLenBinarization and testing

codec/h264/h264dec/cabac.go

@@ -1,6 +1,8 @@
 package h264dec
 
 import (
+	"math"
+
 	"bitbucket.org/ausocean/av/codec/h264/h264dec/bits"
 	"github.com/pkg/errors"
 )
@@ -830,3 +832,22 @@ func suffix(v, uCoff, k int, signedValFlag bool) []int {
 
 	return s
 }
+
+// Error used by fixedLenBinariztion.
+var errNegativeValue = errors.New("cannot get fixed length binarization of negative number")
+
+// fixedLenBinarization returns the fixed-length (FL) binarization of the syntax
+// element v, given cMax to determine bin length, as specified by section 9.3.2.4
+// of the specifications.
+func fixedLenBinarization(v, cMax int) ([]int, error) {
+	if v < 0 {
+		return nil, errNegativeValue
+	}
+	l := int(math.Ceil(math.Log2(float64(cMax + 1))))
+	r := make([]int, l)
+	for i := l - 1; i >= 0; i-- {
+		r[i] = v % 2
+		v = v / 2
+	}
+	return r, nil
+}

codec/h264/h264dec/cabac_test.go

@@ -215,6 +215,36 @@ func TestUnaryBinarization(t *testing.T) {
 	}
 }
 
+func TestFixedLengthBinarization(t *testing.T) {
+	tests := []struct {
+		v    int
+		cMax int
+		want []int
+		err  error
+	}{
+		{v: 0, cMax: 7, want: []int{0, 0, 0}},
+		{v: 1, cMax: 7, want: []int{0, 0, 1}},
+		{v: 2, cMax: 7, want: []int{0, 1, 0}},
+		{v: 3, cMax: 7, want: []int{0, 1, 1}},
+		{v: 4, cMax: 7, want: []int{1, 0, 0}},
+		{v: 5, cMax: 7, want: []int{1, 0, 1}},
+		{v: 6, cMax: 7, want: []int{1, 1, 0}},
+		{v: 7, cMax: 7, want: []int{1, 1, 1}},
+		{v: -1, cMax: 7, want: nil, err: errNegativeValue},
+	}
+
+	for i, test := range tests {
+		got, err := fixedLenBinarization(test.v, test.cMax)
+		if err != test.err {
+			t.Errorf("did not get expected error for test %d\nGot: %v\nWant: %v\n", i, err, test.err)
+		}
+
+		if !reflect.DeepEqual(test.want, got) {
+			t.Errorf("did not get expected result for test %d\nGot: %v\nWant: %v\n", i, got, test.want)
+		}
+	}
+}
+
 func TestTruncUnaryBinarization(t *testing.T) {
 	tests := []struct {
 		v    int