internal/lz4block: Simplify portable decoder

internal/lz4block/decode_other.go

@@ -1,3 +1,4 @@
+//go:build (!amd64 && !arm && !arm64) || appengine || !gc || noasm
 // +build !amd64,!arm,!arm64 appengine !gc noasm
 
 package lz4block
@@ -10,7 +11,6 @@ func decodeBlock(dst, src, dict []byte) (ret int) {
 	// Restrict capacities so we don't read or write out of bounds.
 	dst = dst[:len(dst):len(dst)]
 	src = src[:len(src):len(src)]
-	dictLen := uint(len(dict))
 
 	const hasError = -2
 	defer func() {
@@ -102,35 +102,17 @@ func decodeBlock(dst, src, dict []byte) (ret int) {
 
 		// Copy the match.
 		if di < offset {
-			// The match is beyond our block, meaning in the dictionary
-			if offset-di > mLen {
-				// The match is entirely contained in the dictionary. Just copy!
-				copy(dst[di:di+mLen], dict[dictLen+di-offset:dictLen+di-offset+mLen])
-				di = di + mLen
-			} else {
-				// The match stretches over the dictionary and our block
-				copySize := offset - di
-				restSize := mLen - copySize
-
-				copy(dst[di:di+copySize], dict[dictLen-copySize:])
-				di = di + copySize
-
-				if di < restSize {
-					// Overlap - we want to copy more than what we have available,
-					// so copy byte per byte.
-					copyFrom := 0
-					endOfMatch := di + restSize
-					for di < endOfMatch {
-						dst[di] = dst[copyFrom]
-						di = di + 1
-						copyFrom = copyFrom + 1
-					}
-				} else {
-					copy(dst[di:di+restSize], dst[0:restSize])
-					di = di + restSize
-				}
+			// The match is beyond our block, meaning the first part
+			// is in the dictionary.
+			fromDict := dict[uint(len(dict))+di-offset:]
+			n := uint(copy(dst[di:di+mLen], fromDict))
+			di += n
+			if mLen -= n; mLen == 0 {
+				continue
 			}
-			continue
+			// We copied n = offset-di bytes from the dictionary,
+			// then set di = di+n = offset, so the following code
+			// copies from dst[di-offset:] = dst[0:].
 		}
 
 		expanded := dst[di-offset:]

internal/lz4block/decode_test.go

@@ -228,6 +228,9 @@ func TestDecodeWithDict(t *testing.T) {
 		// First part in dictionary, rest in dst.
 		{"\x35foo\x09\x00\x401234", "0barbaz", "foobarbazfoo1234"},
 
+		// Copy end of dictionary three times.
+		{"\x08\x04\x00", "---1234", "123412341234"},
+
 		// Same, but >16 bytes before the end,
 		// to test the short match shortcut in the amd64 decoder.
 		{"\x35abc\x09\x00\xf0\x0f0123456789abcdefghijklmnopqrst", "012defghi",