Optimize chunker

Skip MinSize bytes at the beginning of each chunk. This increases
throughput about 30% percent.

Before:
    $ go test -v -bench . ./chunker
    === RUN TestChunker
    --- PASS: TestChunker (0.60s)
    === RUN TestChunkerReuse
    --- PASS: TestChunkerReuse (2.18s)
    PASS
    BenchmarkChunker          10     111229642 ns/op      94.27 MB/s
    --- BENCH: BenchmarkChunker
        chunker_test.go:168: 6 chunks, average chunk size: 1747626 bytes
        chunker_test.go:168: 6 chunks, average chunk size: 1747626 bytes
    ok      github.com/restic/restic/chunker    4.120s

After:
    $ go test -v -bench . ./chunker
    === RUN TestChunker
    --- PASS: TestChunker (0.48s)
    === RUN TestChunkerReuse
    --- PASS: TestChunkerReuse (1.75s)
    PASS
    BenchmarkChunker          20      81468596 ns/op     128.71 MB/s
    --- BENCH: BenchmarkChunker
        chunker_test.go:168: 6 chunks, average chunk size: 1747626 bytes
        chunker_test.go:168: 6 chunks, average chunk size: 1747626 bytes
    ok      github.com/restic/restic/chunker    4.061s

chunker/chunker.go

@@ -66,6 +66,8 @@ type Chunker struct {
 	count int
 	pos   int
 
+	pre int // wait for this many bytes before start calculating an new chunk
+
 	digest uint64
 }
 
@@ -96,6 +98,8 @@ func (c *Chunker) reset() {
 	c.pos = 0
 	c.count = 0
 	c.slide(1)
+	// do not start a new chunk unless at least MinSize bytes have been read
+	c.pre = MinSize - WindowSize
 }
 
 // Calculate out_table and mod_table for optimization. Must be called only once.
@@ -141,6 +145,7 @@ func (c *Chunker) fill_tables() {
 // calls yield a nil chunk and an io.EOF error.
 func (c *Chunker) Next(dst []byte) (*Chunk, error) {
 	dst = dst[:0]
+
 	for {
 		if c.bpos >= c.bmax {
 			n, err := io.ReadFull(c.rd, c.buf)
@@ -176,6 +181,26 @@ func (c *Chunker) Next(dst []byte) (*Chunk, error) {
 			c.bmax = n
 		}
 
+		// check if bytes have to be dismissed before starting a new chunk
+		if c.pre > 0 {
+			n := c.bmax - c.bpos
+			if c.pre > n {
+				c.pre -= n
+				dst = append(dst, c.buf[c.bpos:c.bmax]...)
+
+				c.count += n
+				c.pos += n
+				c.bpos = c.bmax
+				continue
+			}
+
+			dst = append(dst, c.buf[c.bpos:c.bpos+c.pre]...)
+			c.bpos += c.pre
+			c.count += c.pre
+			c.pos += c.pre
+			c.pre = 0
+		}
+
 		for i, b := range c.buf[c.bpos:c.bmax] {
 			// inline c.slide(b) and append(b) to increase performance
 			out := c.window[c.wpos]
@@ -208,6 +233,7 @@ func (c *Chunker) Next(dst []byte) (*Chunk, error) {
 				c.reset()
 				c.pos = pos
 				c.start = pos
+				c.pre = MinSize - WindowSize
 
 				return chunk, nil
 			}

chunker/chunker_test.go

@@ -43,7 +43,7 @@ var chunks1 = []chunk{
 	chunk{800374, 0x000968473f900000},
 	chunk{2453512, 0x001e197c92600000},
 	chunk{2651975, 0x000ae6c868000000},
-	chunk{237392, 0x00184c5825e18636},
+	chunk{237392, 0x0000000000000001},
 }
 
 // test if nullbytes are correctly split, even if length is a multiple of MinSize.
@@ -79,8 +79,8 @@ func test_with_data(t *testing.T, chnker *chunker.Chunker, chunks []chunk) {
 			}
 
 			if c.Cut != chunk.CutFP {
-				t.Fatalf("Cut fingerprint for chunk %d does not match: expected %016x, got %016x",
-					i, chunk.CutFP, c.Cut)
+				t.Fatalf("Cut fingerprint for chunk %d/%d does not match: expected %016x, got %016x",
+					i, len(chunks)-1, chunk.CutFP, c.Cut)
 			}
 		}
 	}