Function EstimateBlockSize

s2/encode.go:64–100

EstimateBlockSize will perform a very fast compression without outputting the result and return the compressed output size. The function returns -1 if no improvement could be achieved. Using actual compression will most often produce better compression than the estimate.

(src []byte)

// The function returns -1 if no improvement could be achieved.
// Using actual compression will most often produce better compression than the estimate.
func EstimateBlockSize(src []byte) (d int) {
	if len(src) <= inputMargin || int64(len(src)) > 0xffffffff {
		return -1
	}
	if len(src) <= 1024 {
		const sz, pool = 2048, 0
		tmp, ok := estblockPool[pool].Get().(*[sz]byte)
		if !ok {
			tmp = &[sz]byte{}
		}
		race.WriteSlice(tmp[:])
		defer estblockPool[pool].Put(tmp)

		d = calcBlockSizeSmall(src, tmp)
	} else {
		const sz, pool = 32768, 1
		tmp, ok := estblockPool[pool].Get().(*[sz]byte)
		if !ok {
			tmp = &[sz]byte{}
		}
		race.WriteSlice(tmp[:])
		defer estblockPool[pool].Put(tmp)

		d = calcBlockSize(src, tmp)
	}

	if d == 0 {
		return -1
	}
	// Size of the varint encoded block size.
	d += (bits.Len64(uint64(len(src))) + 7) / 7

	if d >= len(src) {
		return -1
	}
	return d
}

// EncodeBetter returns the encoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire encoded block.