feat(mp4): elst media rate as one signed 16.16 fixed-point number - #552
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ElstEntry splits the media rate over MediaRateInteger and MediaRateFraction, so reading or writing the actual rate required callers to know the split int16/int16 bit layout, including the sign-extension trap when combining a negative fraction half. Add MediaRateFixed32 and SetMediaRateFixed32 to combine and split the halves, handling negative rates correctly.
nchitkara-xai
marked this pull request as ready for review
August 18, 2026 16:25
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@nchitkara-xai This is an improved API. Thanks! |
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Problem
The elst media rate is split across
MediaRateIntegerandMediaRateFraction,two int16 halves of one signed 16.16 number, so reading or writing a rate means
knowing the bit layout, including the trap that sign-extending the fraction
corrupts the integer half when recombining.
Fix
ElstEntry.MediaRateFixed32()andSetMediaRateFixed32()combine and splitthe halves as one signed
int32. The raw fields are unchanged, so decode andencode round-trips are untouched. The getter returns a plain
int32ratherthan
mp4.Fixed32, which is unsigned; the media rate is signed.Tests
Table-driven round-trips in both directions for 1.0, 0.25, 2.5, -1.0, -0.5,
-1.5 and the 0x7fff/-1 extreme; the negative-fraction cases pin the sign
handling.
go test ./...,go vet,gofmt,golangci-lintpass.