const (
?? ?// Maximum number of key/elem pairs a bucket can hold.
?? ?bucketCntBits = 3
?? ?bucketCnt ? ? = 1 << bucketCntBits

?? ?// Maximum average load of a bucket that triggers growth is 6.5.
?? ?// Represent as loadFactorNum/loadFactorDen, to allow integer math.
?? ?loadFactorNum = 13
?? ?loadFactorDen = 2
)

// 在元素?cái)?shù)量大于8且元素?cái)?shù)量大于負(fù)載因子(6.5)*桶總數(shù)，就要進(jìn)行擴(kuò)容
func overLoadFactor(count int, B uint8) bool {
?? ?return count > bucketCnt && uintptr(count) > loadFactorNum*(bucketShift(B)/loadFactorDen)
}

// overflow buckets 太多
func tooManyOverflowBuckets(noverflow uint16, B uint8) bool {
?? ?if B > 15 {
?? ??? ?B = 15
?? ?}
?? ?return noverflow >= uint16(1)<<(B&15)
}

func hashGrow(t *maptype, h *hmap) {
?? ?// 判斷是bucket多還是overflow多，然后根據(jù)這兩種情況去申請(qǐng)新buckets空間
?? ?bigger := uint8(1)
?? ?if !overLoadFactor(h.count+1, h.B) {
?? ??? ?bigger = 0
?? ??? ?h.flags |= sameSizeGrow
?? ?}
?? ?oldbuckets := h.buckets
?? ?newbuckets, nextOverflow := makeBucketArray(t, h.B+bigger, nil)

?? ?flags := h.flags &^ (iterator | oldIterator)
?? ?if h.flags&iterator != 0 {
?? ??? ?flags |= oldIterator
?? ?}
?? ?// commit the grow (atomic wrt gc)
? // 更新最新的bucket總數(shù)、將原桶標(biāo)記為舊桶（后面判斷是否在搬遷就是通過這個(gè)進(jìn)行判斷的）
?? ?h.B += bigger
?? ?h.flags = flags
?? ?h.oldbuckets = oldbuckets
?? ?h.buckets = newbuckets
? // 初始化搬遷進(jìn)度為0
?? ?h.nevacuate = 0
? // 初始化新桶overflow數(shù)量為0
?? ?h.noverflow = 0

? // 將原來extra的overflow掛載到old overflow，代表這已經(jīng)是舊的了
?? ?if h.extra != nil && h.extra.overflow != nil {
?? ??? ?// Promote current overflow buckets to the old generation.
?? ??? ?if h.extra.oldoverflow != nil {
?? ??? ??? ?throw("oldoverflow is not nil")
?? ??? ?}
?? ??? ?h.extra.oldoverflow = h.extra.overflow
?? ??? ?h.extra.overflow = nil
?? ?}
? // extra指向下一塊空閑的overflow空間
?? ?if nextOverflow != nil {
?? ??? ?if h.extra == nil {
?? ??? ??? ?h.extra = new(mapextra)
?? ??? ?}
?? ??? ?h.extra.nextOverflow = nextOverflow
?? ?}
}

func growWork(t *maptype, h *hmap, bucket uintptr) {
?? ?// make sure we evacuate the oldbucket corresponding
?? ?// to the bucket we're about to use
?? ?evacuate(t, h, bucket&h.oldbucketmask())

?? ?// evacuate one more oldbucket to make progress on growing
?? ?if h.growing() {
?? ??? ?evacuate(t, h, h.nevacuate)
?? ?}
}

?? ??? ?// xy contains the x and y (low and high) evacuation destinations.
?? ??? ?var xy [2]evacDst
?? ??? ?x := &xy[0]
?? ??? ?x.b = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
?? ??? ?x.k = add(unsafe.Pointer(x.b), dataOffset)
?? ??? ?x.e = add(x.k, bucketCnt*uintptr(t.keysize))

?? ??? ?if !h.sameSizeGrow() {
?? ??? ??? ?// Only calculate y pointers if we're growing bigger.
?? ??? ??? ?// Otherwise GC can see bad pointers.
?? ??? ??? ?y := &xy[1]
? ? ? // newBit在擴(kuò)容的情況下等于1<<(B-1)
?? ??? ??? ?y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
?? ??? ??? ?y.k = add(unsafe.Pointer(y.b), dataOffset)
?? ??? ??? ?y.e = add(y.k, bucketCnt*uintptr(t.keysize))
?? ??? ?}

for ; b != nil; b = b.overflow(t) {
? // 找到key開始位置k，和value開始位置e
?? ?k := add(unsafe.Pointer(b), dataOffset)
?? ?e := add(k, bucketCnt*uintptr(t.keysize))
? // 遍歷bucket中元素進(jìn)行搬遷
?? ?for i := 0; i < bucketCnt; i, k, e = i+1, add(k, uintptr(t.keysize)), add(e, uintptr(t.elemsize)) {
? ? // 獲取tophash，若發(fā)現(xiàn)是空，說明已經(jīng)搬遷過。并標(biāo)記為空且已搬遷
?? ??? ?top := b.tophash[i]
?? ??? ?if isEmpty(top) {
?? ??? ??? ?b.tophash[i] = evacuatedEmpty
?? ??? ??? ?continue
?? ??? ?}
?? ??? ?if top < minTopHash {
?? ??? ??? ?throw("bad map state")
?? ??? ?}
?? ??? ?k2 := k
? ? // 若key為引用類型就解引用
?? ??? ?if t.indirectkey() {
?? ??? ??? ?k2 = *((*unsafe.Pointer)(k2))
?? ??? ?}
? ? // X指的就是原序號(hào)桶
? ? // Y指的就是擴(kuò)容情況下，新的最高位為1的時(shí)候應(yīng)該去的桶
?? ??? ?var useY uint8
?? ??? ?if !h.sameSizeGrow() {
?? ??? ??? ?// Compute hash to make our evacuation decision (whether we need
?? ??? ??? ?// to send this key/elem to bucket x or bucket y).
?? ??? ??? ?hash := t.hasher(k2, uintptr(h.hash0))
? ? ? // 若正在迭代，且key為NaNs。是不是使用Y就取決最低位是不是1了
?? ??? ??? ?if h.flags&iterator != 0 && !t.reflexivekey() && !t.key.equal(k2, k2) {
?? ??? ??? ??? ?useY = top & 1
?? ??? ??? ??? ?top = tophash(hash)
?? ??? ??? ?} else {
? ? ? ? // 如果最高位為1，那么就應(yīng)該選Y桶
?? ??? ??? ??? ?if hash&newbit != 0 {
?? ??? ??? ??? ??? ?useY = 1
?? ??? ??? ??? ?}
?? ??? ??? ?}
?? ??? ?}

?? ??? ?if evacuatedX+1 != evacuatedY || evacuatedX^1 != evacuatedY {
?? ??? ??? ?throw("bad evacuatedN")
?? ??? ?}

? ? // 標(biāo)記X還是Y搬遷，并依此獲取到搬遷目標(biāo)桶
?? ??? ?b.tophash[i] = evacuatedX + useY?
?? ??? ?dst := &xy[useY] ? ? ? ? ? ? ? ??

? ? // 若目標(biāo)桶已經(jīng)超出桶容量，就分配新overflow
?? ??? ?if dst.i == bucketCnt {
?? ??? ??? ?dst.b = h.newoverflow(t, dst.b)
?? ??? ??? ?dst.i = 0
?? ??? ??? ?dst.k = add(unsafe.Pointer(dst.b), dataOffset)
?? ??? ??? ?dst.e = add(dst.k, bucketCnt*uintptr(t.keysize))
?? ??? ?}
? ? // 更新元素目標(biāo)桶對(duì)應(yīng)的tophash
? ? // 采用與而非取模，應(yīng)該是出于性能目的
?? ??? ?dst.b.tophash[dst.i&(bucketCnt-1)] = top
? ? // 復(fù)制key到目標(biāo)桶
?? ??? ?if t.indirectkey() {
?? ??? ??? ?*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
?? ??? ?} else {
?? ??? ??? ?typedmemmove(t.key, dst.k, k) // copy elem
?? ??? ?}
? ? // 復(fù)制value到目標(biāo)桶
?? ??? ?if t.indirectelem() {
?? ??? ??? ?*(*unsafe.Pointer)(dst.e) = *(*unsafe.Pointer)(e)
?? ??? ?} else {
?? ??? ??? ?typedmemmove(t.elem, dst.e, e)
?? ??? ?}
? ??
? ? // 更新目標(biāo)桶元素?cái)?shù)量、key、value位置
?? ??? ?dst.i++
?? ??? ?// These updates might push these pointers past the end of the key or elem arrays. ??
? ? // That's ok, as we have the overflow pointer at the end of the bucket to protect against pointing past the end of the bucket.
?? ??? ?dst.k = add(dst.k, uintptr(t.keysize))
?? ??? ?dst.e = add(dst.e, uintptr(t.elemsize))
?? ?}
}

        // Unlink the overflow buckets & clear key/elem to help GC.
        if h.flags&oldIterator == 0 && t.bucket.ptrdata != 0 {
      // 計(jì)算當(dāng)前原bucket所在的開始位置b
            b := add(h.oldbuckets, oldbucket*uintptr(t.bucketsize))
            // Preserve b.tophash because the evacuation
            // state is maintained there.
      // 從開始位置加上key-value的偏移量，那么ptr所在的位置就是實(shí)際key-value的開始位置
            ptr := add(b, dataOffset)
      // n是總bucket大小減去key-value的偏移量，就key-value占用空間的大小
            n := uintptr(t.bucketsize) - dataOffset
      // 清理從ptr開始的n個(gè)字節(jié)
            memclrHasPointers(ptr, n)
        }

func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
  // 更新進(jìn)度
    h.nevacuate++
    // Experiments suggest that 1024 is overkill by at least an order of magnitude.
    // Put it in there as a safeguard anyway, to ensure O(1) behavior.
  // 向后看，更新已經(jīng)完成的進(jìn)度
    stop := h.nevacuate + 1024
    if stop > newbit {
        stop = newbit
    }
    for h.nevacuate != stop && bucketEvacuated(t, h, h.nevacuate) {
        h.nevacuate++
    }
  // 若是完成搬遷，就釋放掉old buckets、重置搬遷狀態(tài)、釋放原bucket掛載到extra的overflow指針
    if h.nevacuate == newbit { // newbit == # of oldbuckets
        // Growing is all done. Free old main bucket array.
        h.oldbuckets = nil
        // Can discard old overflow buckets as well.
        // If they are still referenced by an iterator,
        // then the iterator holds a pointers to the slice.
        if h.extra != nil {
            h.extra.oldoverflow = nil
        }
        h.flags &^= sameSizeGrow
    }
}