Bug 1970973 - part1 : add a method to advance more than 32 bits for BitReader. r=media-playback-reviewers,karlt a=pascalc

Differential Revision: https://phabricator.services.mozilla.com/D253259

dom/media/BitReader.cpp

@@ -6,6 +6,8 @@
 
 #include "BitReader.h"
 
+#include "mozilla/Unused.h"
+
 namespace mozilla {
 
 BitReader::BitReader(const mozilla::MediaByteBuffer* aBuffer)
@@ -194,4 +196,16 @@ uint32_t BitReader::GetBitLength(const mozilla::MediaByteBuffer* aNAL) {
   return size;
 }
 
+size_t BitReader::AdvanceBits(size_t aNum) {
+  const size_t advanceBits = std::min(aNum, BitsLeft());
+  size_t temp = advanceBits;
+  while (temp > 0) {
+    uint32_t readBits = temp > 32 ? 32 : temp;
+    // TODO : return error if reading less bits than expectation in bug 1972401.
+    Unused << ReadBits(readBits);
+    temp -= readBits;
+  }
+  return advanceBits;
+}
+
 }  // namespace mozilla

dom/media/BitReader.h

@@ -31,6 +31,12 @@ class BitReader {
   // Limited to unsigned 64 bits.
   CheckedUint64 ReadULEB128();
 
+  // Advance bits and return the actual number of bits forwarded. Unlike
+  // ReadBits, which can only read up to 32 bits, this function does not limit
+  // how many bits it can advance. If fewer bits are available than requested,
+  // it will only advance the available bits.
+  size_t AdvanceBits(size_t aNum);
+
   // Return the number of bits parsed so far;
   size_t BitCount() const;
   // Return the number of bits left.

dom/media/platforms/agnostic/bytestreams/H265.cpp

@@ -158,13 +158,8 @@ Result<HVCCConfig, nsresult> HVCCConfig::Parse(
       const uint8_t* currentPtr =
           aExtraData->Elements() + reader.BitCount() / 8;
       H265NALU nalu(currentPtr, nalUnitLength);
-      // ReadBits can only read at most 32 bits at a time.
-      uint32_t nalSize = nalUnitLength * 8;
-      while (nalSize > 0) {
-        uint32_t readBits = nalSize > 32 ? 32 : nalSize;
-        reader.ReadBits(readBits);
-        nalSize -= readBits;
-      }
+      uint32_t nalBitsLength = nalUnitLength * 8;
+      Unused << reader.AdvanceBits(nalBitsLength);
       // Per ISO_IEC-14496-15-2022, 8.3.2.1.3 Semantics, NALU should only be
       // SPS/PPS/VPS or SEI, ignore all the other types of NALU.
       if (nalu.IsSPS() || nalu.IsPPS() || nalu.IsVPS() || nalu.IsSEI()) {