Clarify the semantics of minNonLiteralBlockSize.
diff --git a/encode.go b/encode.go
index 177b953..c57a0f2 100644
--- a/encode.go
+++ b/encode.go
@@ -125,7 +125,7 @@
if len(p) > maxBlockSize {
p, src = p[:maxBlockSize], p[maxBlockSize:]
}
- if len(p) < minBlockSize {
+ if len(p) < minNonLiteralBlockSize {
d += emitLiteral(dst[d:], p)
} else {
d += encodeBlock(dst[d:], p)
@@ -146,12 +146,21 @@
// TODO: implement this fast path.
const inputMargin = 16 - 1
-// minBlockSize is the minimum size of the input to encodeBlock. As above, we
-// want any emitLiteral calls inside encodeBlock's inner loop to use the fast
-// path if possible, which requires being able to overrun by inputMargin bytes.
+// minNonLiteralBlockSize is the minimum size of the input to encodeBlock that
+// could be encoded with a copy tag. This is the minimum with respect to the
+// algorithm used by encodeBlock, not a minimum enforced by the file format.
//
-// TODO: can we make this bound a little tighter, raising it by 1 or 2?
-const minBlockSize = inputMargin
+// The encoded output must start with at least a 1 byte literal, as there are
+// no previous bytes to copy. A minimal (1 byte) copy after that, generated
+// from an emitCopy call in encodeBlock's main loop, would require at least
+// another inputMargin bytes, for the reason above: we want any emitLiteral
+// calls inside encodeBlock's main loop to use the fast path if possible, which
+// requires being able to overrun by inputMargin bytes. Thus,
+// minNonLiteralBlockSize equals 1 + 1 + inputMargin.
+//
+// The C++ code doesn't use this exact threshold, but it could, as discussed at
+// https://groups.google.com/d/topic/snappy-compression/oGbhsdIJSJ8/discussion
+const minNonLiteralBlockSize = 1 + 1 + inputMargin
func hash(u, shift uint32) uint32 {
return (u * 0x1e35a7bd) >> shift
@@ -163,7 +172,7 @@
//
// It also assumes that:
// len(dst) >= MaxEncodedLen(len(src)) &&
-// minBlockSize <= len(src) && len(src) <= maxBlockSize
+// minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
func encodeBlock(dst, src []byte) (d int) {
// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
const maxTableSize = 1 << 14
