Improve performance of buzhash

```
name             old time/op    new time/op    delta
Buzhash2/1K-4       610ns ± 4%     643ns ±16%    ~     (p=0.421 n=8+10)
Buzhash2/1M-4      1.25ms ± 5%    1.16ms ± 4%  -7.31%  (p=0.000 n=10+10)
Buzhash2/16M-4     19.2ms ± 2%    17.5ms ± 2%  -8.73%  (p=0.000 n=9+9)
Buzhash2/100M-4     117ms ± 1%     107ms ± 3%  -8.26%  (p=0.000 n=10+10)

name             old speed      new speed      delta
Buzhash2/1K-4    1.68GB/s ± 4%  1.60GB/s ±14%    ~     (p=0.408 n=8+10)
Buzhash2/1M-4     842MB/s ± 5%   908MB/s ± 3%  +7.86%  (p=0.000 n=10+10)
Buzhash2/16M-4    875MB/s ± 2%   959MB/s ± 2%  +9.57%  (p=0.000 n=9+9)
Buzhash2/100M-4   897MB/s ± 1%   977MB/s ± 3%  +9.02%  (p=0.000 n=10+10)

name             old alloc/op   new alloc/op   delta
Buzhash2/1K-4      1.17kB ± 1%    1.17kB ± 0%  -0.50%  (p=0.006 n=10+10)
Buzhash2/1M-4      1.08MB ± 1%    1.07MB ± 0%    ~     (p=0.739 n=10+10)
Buzhash2/16M-4     17.1MB ± 0%    17.1MB ± 0%    ~     (p=0.579 n=10+10)
Buzhash2/100M-4     106MB ± 0%     106MB ± 0%  -0.01%  (p=0.000 n=9+7)

name             old allocs/op  new allocs/op  delta
Buzhash2/1K-4        3.00 ± 0%      3.00 ± 0%    ~     (all equal)
Buzhash2/1M-4        8.00 ± 0%      8.00 ± 0%    ~     (all equal)
Buzhash2/16M-4       72.0 ± 0%      72.0 ± 0%    ~     (all equal)
Buzhash2/100M-4       406 ± 0%       406 ± 0%    ~     (all equal)
```

License: MIT
Signed-off-by: Jakub Sztandera <[email protected]>


This commit was moved from ipfs/go-ipfs-chunker@79bdab2

Author: Kubuxu

chunker/buzhash.go

@@ -61,17 +61,33 @@ func (b *Buzhash) NextBytes() ([]byte, error) {
 
 	var state uint32 = 0
 
+	if buzMin > len(b.buf) {
+		panic("this is impossible")
+	}
+
 	for ; i < buzMin; i++ {
 		state = bits.RotateLeft32(state, 1)
 		state = state ^ bytehash[b.buf[i]]
 	}
 
-	if b.n+n > len(b.buf) {
-		panic("this is impossible, but gives +9 to performance")
-	}
+	{
+		max := b.n + n - 32 - 1
 
-	for ; state&buzMask != 0 && i < b.n+n; i++ {
-		state = bits.RotateLeft32(state, 1) ^ bytehash[b.buf[i-32]] ^ bytehash[b.buf[i]]
+		buf := b.buf
+		bufshf := b.buf[32:]
+		i = buzMin - 32
+		_ = buf[max]
+		_ = bufshf[max]
+
+		for ; i <= max; i++ {
+			if state&buzMask == 0 {
+				break
+			}
+			state = bits.RotateLeft32(state, 1) ^
+				bytehash[buf[i]] ^
+				bytehash[bufshf[i]]
+		}
+		i += 32
 	}
 
 	res := make([]byte, i)