Reduce allocations while hash scanning

scanner/blocks.go

@@ -17,12 +17,15 @@ const StandardBlockSize = 128 * 1024
 var sha256OfNothing = []uint8{0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, 0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55}
 
 // Blocks returns the blockwise hash of the reader.
-func Blocks(r io.Reader, blocksize int) ([]protocol.BlockInfo, error) {
+func Blocks(r io.Reader, blocksize int, sizehint int64) ([]protocol.BlockInfo, error) {
 	var blocks []protocol.BlockInfo
+	if sizehint > 0 {
+		blocks = make([]protocol.BlockInfo, 0, int(sizehint/int64(blocksize)))
+	}
 	var offset int64
+	hf := sha256.New()
 	for {
 		lr := &io.LimitedReader{R: r, N: int64(blocksize)}
-		hf := sha256.New()
 		n, err := io.Copy(hf, lr)
 		if err != nil {
 			return nil, err
@@ -39,6 +42,8 @@ func Blocks(r io.Reader, blocksize int) ([]protocol.BlockInfo, error) {
 		}
 		blocks = append(blocks, b)
 		offset += int64(n)
+
+		hf.Reset()
 	}
 
 	if len(blocks) == 0 {