func (e *fastBase) UseBlock(enc *blockEnc) {
	enc.reset(e.blk)
	e.blk = enc
}
useBlock will replace the block with the provided one,
but transfer recent offsets from the previous.
package zstd
import (
	"fmt"
	"math/bits"
	"github.com/klauspost/compress/zstd/internal/xxhash"
)
const (
	dictShardBits = 6
)
type fastBase struct {
	cur int32
	maxMatchOff int32
	hist        []byte
	crc         *xxhash.Digest
	tmp         [8]byte
	blk         *blockEnc
	lastDictID  uint32
	lowMem      bool
}
func (e *fastBase) CRC() *xxhash.Digest {
	return e.crc
}
func (e *fastBase) AppendCRC(dst []byte) []byte {
	crc := e.crc.Sum(e.tmp[:0])
	dst = append(dst, crc[7], crc[6], crc[5], crc[4])
	return dst
}
func (e *fastBase) WindowSize(size int) int32 {
	if size > 0 && size < int(e.maxMatchOff) {
		b := int32(1) << uint(bits.Len(uint(size)))
		if b < 1024 {
			b = 1024
		}
		return b
	}
	return e.maxMatchOff
}
func (e *fastBase) Block() *blockEnc {
	return e.blk
}
func (e *fastBase) addBlock(src []byte) int32 {
	if debugAsserts && e.cur > bufferReset {
		panic(fmt.Sprintf("ecur (%d) > buffer reset (%d)", e.cur, bufferReset))
	}
	if len(e.hist)+len(src) > cap(e.hist) {
		if cap(e.hist) == 0 {
			e.ensureHist(len(src))
		} else {
			if cap(e.hist) < int(e.maxMatchOff+maxCompressedBlockSize) {
				panic(fmt.Errorf("unexpected buffer cap %d, want at least %d with window %d", cap(e.hist), e.maxMatchOff+maxCompressedBlockSize, e.maxMatchOff))
			}
			offset := int32(len(e.hist)) - e.maxMatchOff
			copy(e.hist[0:e.maxMatchOff], e.hist[offset:])
			e.cur += offset
			e.hist = e.hist[:e.maxMatchOff]
		}
	}
	s := int32(len(e.hist))
	e.hist = append(e.hist, src...)
	return s
}
func (e *fastBase) ensureHist(n int) {
	if cap(e.hist) >= n {
		return
	}
	l := e.maxMatchOff
	if (e.lowMem && e.maxMatchOff > maxCompressedBlockSize) || e.maxMatchOff <= maxCompressedBlockSize {
		l += maxCompressedBlockSize
	} else {
		l += e.maxMatchOff
	}
	if l < 1<<20 && !e.lowMem {
		l = 1 << 20
	}
	if l < int32(n) {
		l = int32(n)
	}
	e.hist = make([]byte, 0, l)
}