zstd: Minor decoder improvements

zstd/bitreader.go

@@ -50,16 +50,23 @@ func (b *bitReader) getBits(n uint8) int {
 	if n == 0 /*|| b.bitsRead >= 64 */ {
 		return 0
 	}
-	return b.getBitsFast(n)
+	return int(b.get32BitsFast(n))
 }
 
-// getBitsFast requires that at least one bit is requested every time.
+// get32BitsFast requires that at least one bit is requested every time.
 // There are no checks if the buffer is filled.
-func (b *bitReader) getBitsFast(n uint8) int {
+func (b *bitReader) get32BitsFast(n uint8) uint32 {
 	const regMask = 64 - 1
 	v := uint32((b.value << (b.bitsRead & regMask)) >> ((regMask + 1 - n) & regMask))
 	b.bitsRead += n
-	return int(v)
+	return v
+}
+
+func (b *bitReader) get16BitsFast(n uint8) uint16 {
+	const regMask = 64 - 1
+	v := uint16((b.value << (b.bitsRead & regMask)) >> ((regMask + 1 - n) & regMask))
+	b.bitsRead += n
+	return v
 }
 
 // fillFast() will make sure at least 32 bits are available.

zstd/fse_decoder.go

@@ -379,7 +379,7 @@ func (s decSymbol) final() (int, uint8) {
 // This can only be used if no symbols are 0 bits.
 // At least tablelog bits must be available in the bit reader.
 func (s *fseState) nextFast(br *bitReader) (uint32, uint8) {
-	lowBits := uint16(br.getBitsFast(s.state.nbBits()))
+	lowBits := br.get16BitsFast(s.state.nbBits())
 	s.state = s.dt[s.state.newState()+lowBits]
 	return s.state.baseline(), s.state.addBits()
 }

zstd/seqdec.go

@@ -278,7 +278,7 @@ func (s *sequenceDecs) decode(seqs int, br *bitReader, hist []byte) error {
 			mlState = mlTable[mlState.newState()&maxTableMask]
 			ofState = ofTable[ofState.newState()&maxTableMask]
 		} else {
-			bits := br.getBitsFast(nBits)
+			bits := br.get32BitsFast(nBits)
 			lowBits := uint16(bits >> ((ofState.nbBits() + mlState.nbBits()) & 31))
 			llState = llTable[(llState.newState()+lowBits)&maxTableMask]
 
@@ -326,7 +326,7 @@ func (s *sequenceDecs) updateAlt(br *bitReader) {
 		s.offsets.state.state = s.offsets.state.dt[c.newState()]
 		return
 	}
-	bits := br.getBitsFast(nBits)
+	bits := br.get32BitsFast(nBits)
 	lowBits := uint16(bits >> ((c.nbBits() + b.nbBits()) & 31))
 	s.litLengths.state.state = s.litLengths.state.dt[a.newState()+lowBits]