一个巧妙的迭代器

import (
    "encoding/binary"
)

const batchHeaderLen = 12

const invalidBatchCount = 1<<32 - 1

// Batch is a sequence of Sets and/or Deletes that are applied atomically.
type Batch struct {
    // Data is the wire format of a batch's log entry:
    //   - 8 bytes for a sequence number of the first batch element,
    //     or zeroes if the batch has not yet been applied,
    //   - 4 bytes for the count: the number of elements in the batch,
    //     or "\xff\xff\xff\xff" if the batch is invalid,
    //   - count elements, being:
    //     - one byte for the kind: delete (0) or set (1),
    //     - the varint-string user key,
    //     - the varint-string value (if kind == set).
    // The sequence number and count are stored in little-endian order.
    data []byte
}

// Set adds an action to the batch that sets the key to map to the value.
func (b *Batch) Set(key, value []byte) {
    if len(b.data) == 0 {
        b.init(len(key) + len(value) + 2*binary.MaxVarintLen64 + batchHeaderLen)
    }
    if b.increment() {
        b.data = append(b.data, byte(internalKeyKindSet))
        b.appendStr(key)
        b.appendStr(value)
    }
}

// Delete adds an action to the batch that deletes the entry for key.
func (b *Batch) Delete(key []byte) {
    if len(b.data) == 0 {
        b.init(len(key) + binary.MaxVarintLen64 + batchHeaderLen)
    }
    if b.increment() {
        b.data = append(b.data, byte(internalKeyKindDelete))
        b.appendStr(key)
    }
}

func (b *Batch) init(cap int) {
    n := 256
    for n < cap {
        n *= 2
    }
    b.data = make([]byte, batchHeaderLen, n)
}

// seqNumData returns the 8 byte little-endian sequence number. Zero means that
// the batch has not yet been applied.
func (b *Batch) seqNumData() []byte {
    return b.data[:8]
}

// countData returns the 4 byte little-endian count data. "\xff\xff\xff\xff"
// means that the batch is invalid.
func (b *Batch) countData() []byte {
    return b.data[8:12]
}

func (b *Batch) increment() (ok bool) {
    p := b.countData() // [8:12]
    for i := range p {
        p[i]++
        if p[i] != 0x00 {
            return true
        } //
    }
    // The countData was "\xff\xff\xff\xff". Leave it as it was.
    p[0] = 0xff
    p[1] = 0xff
    p[2] = 0xff
    p[3] = 0xff
    return false
}

func (b *Batch) appendStr(s []byte) {
    var buf [binary.MaxVarintLen64]byte
    n := binary.PutUvarint(buf[:], uint64(len(s)))
    b.data = append(b.data, buf[:n]...)
    b.data = append(b.data, s...)
}

func (b *Batch) setSeqNum(seqNum uint64) {
    binary.LittleEndian.PutUint64(b.seqNumData(), seqNum)
}

func (b *Batch) seqNum() uint64 {
    return binary.LittleEndian.Uint64(b.seqNumData())
}

func (b *Batch) count() uint32 {
    return binary.LittleEndian.Uint32(b.countData())
}

// 返回迭代器
func (b *Batch) iter() batchIter {
    return b.data[batchHeaderLen:]
}

type batchIter []byte

// next returns the next operation in this batch.
// The final return value is false if the batch is corrupt.
func (t *batchIter) next() (kind internalKeyKind, ukey []byte, value []byte, ok bool) {
    p := *t
    if len(p) == 0 {
        return 0, nil, nil, false
    }
    kind, *t = internalKeyKind(p[0]), p[1:]
    if kind > internalKeyKindMax {
        return 0, nil, nil, false
    }
    ukey, ok = t.nextStr()
    if !ok {
        return 0, nil, nil, false
    }
    if kind != internalKeyKindDelete {
        value, ok = t.nextStr()
        if !ok {
            return 0, nil, nil, false
        }
    }
    return kind, ukey, value, true
}

func (t *batchIter) nextStr() (s []byte, ok bool) {
    p := *t
    u, numBytes := binary.Uvarint(p)
    if numBytes <= 0 {
        return nil, false
    }
    p = p[numBytes:] //
    if u > uint64(len(p)) {
        return nil, false
    }
    s, *t = p[:u], p[u:]
    return s, true
}