Method writeBlockHuff

flate/huffman_bit_writer.go:987–1174

writeBlockHuff encodes a block of bytes as either Huffman encoded literals or uncompressed bytes if the results only gains very little from compression.

(eof bool, input []byte, sync bool)

// Huffman encoded literals or uncompressed bytes if the
// results only gains very little from compression.
func (w *huffmanBitWriter) writeBlockHuff(eof bool, input []byte, sync bool) {
	if w.err != nil {
		return
	}

	// Clear histogram
	for i := range w.literalFreq[:] {
		w.literalFreq[i] = 0
	}
	if !w.lastHuffMan {
		for i := range w.offsetFreq[:] {
			w.offsetFreq[i] = 0
		}
	}

	const numLiterals = endBlockMarker + 1
	const numOffsets = 1

	// Add everything as literals
	// We have to estimate the header size.
	// Assume header is around 70 bytes:
	// https://stackoverflow.com/a/25454430
	const guessHeaderSizeBits = 70 * 8
	histogram(input, w.literalFreq[:numLiterals])
	ssize, storable := w.storedSize(input)
	if storable && len(input) > 1024 {
		// Quick check for incompressible content.
		abs := float64(0)
		avg := float64(len(input)) / 256
		max := float64(len(input) * 2)
		for _, v := range w.literalFreq[:256] {
			diff := float64(v) - avg
			abs += diff * diff
			if abs > max {
				break
			}
		}
		if abs < max {
			if debugDeflate {
				fmt.Println("stored", abs, "<", max)
			}
			// No chance we can compress this...
			w.writeStoredHeader(len(input), eof)
			w.writeBytes(input)
			return
		}
	}
	w.literalFreq[endBlockMarker] = 1
	w.tmpLitEncoding.generate(w.literalFreq[:numLiterals], 15)
	estBits := w.tmpLitEncoding.canReuseBits(w.literalFreq[:numLiterals])
	if estBits < math.MaxInt32 {
		estBits += w.lastHeader
		if w.lastHeader == 0 {
			estBits += guessHeaderSizeBits
		}
		estBits += estBits >> w.logNewTablePenalty
	}