zstd: BuildDict fails with RLE table (#951)

* zstd: BuildDict fails with RLE table

We cannot build a useable table if RLE. Add a fake entry to generate valid tables.

* Prevent offsets longer than dict to be selected.

zstd/dict.go

@@ -273,6 +273,9 @@ func BuildDict(o BuildDictOptions) ([]byte, error) {
 		enc.Encode(&block, b)
 		addValues(&remain, block.literals)
 		litTotal += len(block.literals)
+		if len(block.sequences) == 0 {
+			continue
+		}
 		seqs += len(block.sequences)
 		block.genCodes()
 		addHist(&ll, block.coders.llEnc.Histogram())
@@ -286,6 +289,9 @@ func BuildDict(o BuildDictOptions) ([]byte, error) {
 			if offset == 0 {
 				continue
 			}
+			if int(offset) >= len(o.History) {
+				continue
+			}
 			if offset > 3 {
 				newOffsets[offset-3]++
 			} else {
@@ -336,6 +342,9 @@ func BuildDict(o BuildDictOptions) ([]byte, error) {
 	if seqs/nUsed < 512 {
 		// Use 512 as minimum.
 		nUsed = seqs / 512
+		if nUsed == 0 {
+			nUsed = 1
+		}
 	}
 	copyHist := func(dst *fseEncoder, src *[256]int) ([]byte, error) {
 		hist := dst.Histogram()
@@ -358,6 +367,28 @@ func BuildDict(o BuildDictOptions) ([]byte, error) {
 			fakeLength += v
 			hist[i] = uint32(v)
 		}
+
+		// Ensure we aren't trying to represent RLE.
+		if maxCount == fakeLength {
+			for i := range hist {
+				if uint8(i) == maxSym {
+					fakeLength++
+					maxSym++
+					hist[i+1] = 1
+					if maxSym > 1 {
+						break
+					}
+				}
+				if hist[0] == 0 {
+					fakeLength++
+					hist[i] = 1
+					if maxSym > 1 {
+						break
+					}
+				}
+			}
+		}
+
 		dst.HistogramFinished(maxSym, maxCount)
 		dst.reUsed = false
 		dst.useRLE = false