feat(mp4): elst media rate as one signed 16.16 fixed-point number

CHANGELOG.md

@@ -27,6 +27,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   AudioSpecificConfig fields (object type including the 31-escape, base
   sampling frequency, channel configuration) for every audio object type,
   not just the AAC-LC and HE-AAC types the full decoder supports
+- `ElstEntry.MediaRateFixed32` and `SetMediaRateFixed32` combine and split
+  the media rate halves as one signed 16.16 fixed-point number, so callers
+  no longer need to know the bit layout
 
 ### Changed
 

mp4/elst.go

@@ -23,6 +23,18 @@ type ElstEntry struct {
 	MediaRateFraction int16
 }
 
+// MediaRateFixed32 - the media rate as one signed 16.16 fixed-point number (65536 is the normal rate 1.0).
+// The return type is a plain int32 rather than Fixed32, since Fixed32 is unsigned and the media rate is signed.
+func (e ElstEntry) MediaRateFixed32() int32 {
+	return int32(e.MediaRateInteger)<<16 | int32(uint16(e.MediaRateFraction))
+}
+
+// SetMediaRateFixed32 - set the media rate from a signed 16.16 fixed-point number.
+func (e *ElstEntry) SetMediaRateFixed32(rate int32) {
+	e.MediaRateInteger = int16(rate >> 16)
+	e.MediaRateFraction = int16(uint16(rate))
+}
+
 // DecodeElst - box-specific decode
 func DecodeElst(hdr BoxHeader, startPos uint64, r io.Reader) (Box, error) {
 	data, err := readBoxBody(r, hdr)

mp4/elst_test.go

@@ -29,3 +29,31 @@ func TestElst(t *testing.T) {
 		boxDiffAfterEncodeAndDecode(t, elst)
 	}
 }
+
+func TestElstMediaRateFixed32(t *testing.T) {
+	cases := []struct {
+		integer  int16
+		fraction int16
+		fixed    int32
+	}{
+		{1, 0, 1 << 16},                       // rate 1.0
+		{0, 0x4000, 0x4000},                   // rate 0.25
+		{2, -0x8000, 2<<16 | 0x8000},          // rate 2.5 (fraction bits 0x8000)
+		{-1, 0, -65536},                       // rate -1.0
+		{-1, -0x8000, -0x8000},                // rate -0.5
+		{-2, -0x8000, int32(-2)<<16 | 0x8000}, // negative with fraction bits
+		{0x7fff, -1, 0x7fff<<16 | 0xffff},     // extremes
+	}
+	for _, c := range cases {
+		entry := mp4.ElstEntry{MediaRateInteger: c.integer, MediaRateFraction: c.fraction}
+		if got := entry.MediaRateFixed32(); got != c.fixed {
+			t.Errorf("(%d, %d): got %#x, wanted %#x", c.integer, c.fraction, got, c.fixed)
+		}
+		var back mp4.ElstEntry
+		back.SetMediaRateFixed32(c.fixed)
+		if back.MediaRateInteger != c.integer || back.MediaRateFraction != c.fraction {
+			t.Errorf("%#x: got (%d, %d), wanted (%d, %d)",
+				c.fixed, back.MediaRateInteger, back.MediaRateFraction, c.integer, c.fraction)
+		}
+	}
+}