在使用map的過程中，有兩個問題是經常會遇到的：讀寫衝突和遍歷無序性。爲什麼會這樣呢，底層是怎麼實現的呢？帶着這兩個問題，我簡單的瞭解了一下map的增刪改查及遍歷的實現。

結構

hmap

type hmap struct {
    // Note: the format of the hmap is also encoded in cmd/compile/internal/gc/reflect.go.
    // Make sure this stays in sync with the compiler's definition.
    count     int // 有效數據的長度# live cells == size of map.  Must be first (used by len() builtin)
    flags     uint8 // 用於記錄hashmap的狀態
    B         uint8  // 2^B = buckets的數量log_2 of # of buckets (can hold up to loadFactor * 2^B items)
    noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
    hash0     uint32 // 隨機的hash種子

    buckets    unsafe.Pointer // buckets數組array of 2^B Buckets. may be nil if count==0.
    oldbuckets unsafe.Pointer // 老的buctedts數據，map增長的時候會用到
    nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated)

    extra *mapextra // 額外的bmap數組optional fields
}

mapextra

 type mapextra struct {
    // If both key and value do not contain pointers and are inline, then we mark bucket
    // type as containing no pointers. This avoids scanning such maps.
    // However, bmap.overflow is a pointer. In order to keep overflow buckets
    // alive, we store pointers to all overflow buckets in hmap.extra.overflow and hmap.extra.oldoverflow.
    // overflow and oldoverflow are only used if key and value do not contain pointers.
    // overflow contains overflow buckets for hmap.buckets.
    // oldoverflow contains overflow buckets for hmap.oldbuckets.
    // The indirection allows to store a pointer to the slice in hiter.
    overflow    *[]*bmap
    oldoverflow *[]*bmap

    // nextOverflow holds a pointer to a free overflow bucket.
    nextOverflow *bmap
}

bmap

type bmap struct {
    // tophash generally contains the top byte of the hash value
    // for each key in this bucket. If tophash[0] < minTopHash,
    // tophash[0] is a bucket evacuation state instead.
    tophash [bucketCnt]uint8
    // Followed by bucketCnt keys and then bucketCnt values.
    // NOTE: packing all the keys together and then all the values together makes the
    // code a bit more complicated than alternating key/value/key/value/... but it allows
    // us to eliminate padding which would be needed for, e.g., map[int64]int8.
    // Followed by an overflow pointer.
}

stringStruct

type stringStruct struct {
    str unsafe.Pointer
    len int
}

hiter

map遍歷時用到的結構，startBucket+offset設定了開始遍歷的地址，保證map遍歷的無序性

type hiter struct {
  // key的指針
    key         unsafe.Pointer // Must be in first position.  Write nil to indicate iteration end (see cmd/internal/gc/range.go).
  // 當前value的指針
    value       unsafe.Pointer // Must be in second position (see cmd/internal/gc/range.go).
    t           *maptype
  // 指向map的指針
    h           *hmap
  // 指向buckets的指針
    buckets     unsafe.Pointer // bucket ptr at hash_iter initialization time
  // 指向當前遍歷的bucket的指針
    bptr        *bmap          // current bucket
  // 指向map.extra.overflow
    overflow    *[]*bmap       // keeps overflow buckets of hmap.buckets alive
  // 指向map.extra.oldoverflow
    oldoverflow *[]*bmap       // keeps overflow buckets of hmap.oldbuckets alive
  // 開始遍歷的bucket的索引
    startBucket uintptr        // bucket iteration started at
  // 開始遍歷bucket上的偏移量
    offset      uint8          // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
    wrapped     bool           // already wrapped around from end of bucket array to beginning
    B           uint8
    i           uint8
    bucket      uintptr
    checkBucket uintptr
}

這裏的keys和values、*overflow三個變量在結構體中並沒有體現，但是在源碼過程中，一直有爲他們預留位置，所以這裏的示意圖中就展示出來了，keys和values其實8個長度的數組

demo

我們簡單寫個demo，通過go tool 來分析一下底層所對應的函數

func main() {
    m := make(map[interface{}]interface{}, 16)
    m["111"] = 1
    m["222"] = 2
    m["444"] = 4
    _ = m["444"]
    _, _ = m["444"]
    delete(m, "444")

    for range m {
    }
}

▶ go tool objdump -s "main.main" main | grep CALL
  main.go:4             0x455c74                e8f761fbff              CALL runtime.makemap(SB)                
  main.go:5             0x455ce1                e8da6dfbff              CALL runtime.mapassign(SB)              
  main.go:6             0x455d7b                e8406dfbff              CALL runtime.mapassign(SB)              
  main.go:7             0x455e15                e8a66cfbff              CALL runtime.mapassign(SB)              
  main.go:8             0x455e88                e89363fbff              CALL runtime.mapaccess1(SB)             
  main.go:9             0x455ec4                e84766fbff              CALL runtime.mapaccess2(SB)             
  main.go:10            0x455f00                e85b72fbff              CALL runtime.mapdelete(SB)              
  main.go:12            0x455f28                e804a7ffff              CALL 0x450631                           
  main.go:12            0x455f53                e8b875fbff              CALL runtime.mapiterinit(SB)            
  main.go:12            0x455f75                e88677fbff              CALL runtime.mapiternext(SB)            
  main.go:7             0x455f8f                e81c9cffff              CALL runtime.gcWriteBarrier(SB)         
  main.go:6             0x455f9c                e80f9cffff              CALL runtime.gcWriteBarrier(SB)         
  main.go:5             0x455fa9                e8029cffff              CALL runtime.gcWriteBarrier(SB)         
  main.go:3             0x455fb3                e8f87dffff              CALL runtime.morestack_noctxt(SB)

初始化

makemap

makemap創建一個hmap結構體，並賦予這個變量一些初始的屬性

func makemap(t *maptype, hint int, h *hmap) *hmap {
  // 首先判斷map的大小是否合適
    if hint < 0 || hint > int(maxSliceCap(t.bucket.size)) {
        hint = 0
    }

    // initialize Hmap
  // 初始化hmap結構
    if h == nil {
        h = new(hmap)
    }
  // 生成一個隨機的hash種子
    h.hash0 = fastrand()

    // find size parameter which will hold the requested # of elements
  // 根據hint，也就是map預設的長度，確定B的大小，以使map的裝載係數在正常範圍內，擴容那塊再細講
    B := uint8(0)
    for overLoadFactor(hint, B) {
        B++
    }
    h.B = B

    // allocate initial hash table
    // if B == 0, the buckets field is allocated lazily later (in mapassign)
    // If hint is large zeroing this memory could take a while.
  // 如果B==0，則賦值的時候進行惰性分配，如果B！=0，則分配對應數量的buckets
    if h.B != 0 {
        var nextOverflow *bmap
        h.buckets, nextOverflow = makeBucketArray(t, h.B, nil)
        if nextOverflow != nil {
            h.extra = new(mapextra)
            h.extra.nextOverflow = nextOverflow
        }
    }

    return h
}

makeBucketArray

makeBucketArray初始化了map所需的buckets，最少分配2^b個buckets

func makeBucketArray(t *maptype, b uint8, dirtyalloc unsafe.Pointer) (buckets unsafe.Pointer, nextOverflow *bmap) {
    base := bucketShift(b)
    nbuckets := base
    // 如果b，也就是map比較大的情況，則多分配點數組，給nextOverflow使用
    if b >= 4 {
    // 計算應該多分配的buckets數量
        nbuckets += bucketShift(b - 4)
        sz := t.bucket.size * nbuckets
        up := roundupsize(sz)
        if up != sz {
            nbuckets = up / t.bucket.size
        }
    }
    // 如果不是 dirtyalloc,新分配map空間時，dirtyalloc爲nil
    if dirtyalloc == nil {
    // 申請buckets數組
        buckets = newarray(t.bucket, int(nbuckets))
    } else {
        // dirtyalloc was previously generated by
        // the above newarray(t.bucket, int(nbuckets))
        // but may not be empty.
        buckets = dirtyalloc
        size := t.bucket.size * nbuckets
        if t.bucket.kind&kindNoPointers == 0 {
            memclrHasPointers(buckets, size)
        } else {
            memclrNoHeapPointers(buckets, size)
        }
    }
  // 判斷是否多申請了buckets，多申請的buckets放在nextOverflow裏面以備後用
    if base != nbuckets {
        nextOverflow = (*bmap)(add(buckets, base*uintptr(t.bucketsize)))
        last := (*bmap)(add(buckets, (nbuckets-1)*uintptr(t.bucketsize)))
        last.setoverflow(t, (*bmap)(buckets))
    }
    return buckets, nextOverflow
}

初始化的過程到此就結束了，比較簡單，就是根據初始化的大小，確定buckets的數量，並分配內存等

查找（mapaccess）

在上面的go tool 分析過程中可以發現

_ = m["444"] 對應 mapaccess1
, = m["444"] 對應 mapaccess2

兩個函數的邏輯大致相同，我們以mapaccess1爲例來分析

mapaccess

func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
  // 如果h還沒有實例化，或者還沒有值，返回零值
    if h == nil || h.count == 0 {
        return unsafe.Pointer(&zeroVal[0])
    }
  // 判斷當前map是否處於 寫 的過程中，讀寫衝突
    if h.flags&hashWriting != 0 {
        throw("concurrent map read and map write")
    }
  // 根據初始化生產的hash隨機種子hash0，計算key的hash值
    alg := t.key.alg
    hash := alg.hash(key, uintptr(h.hash0))
    m := bucketMask(h.B)
  // 根據key的hash值，計算出對應的bucket的位置，計算過程後面圖示
    b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
  // 擴容的過程中，oldbuckets不爲空，所以這時候，這時候需要判斷，目標bucket是否已經遷移完成了，擴容的時候細講
    if c := h.oldbuckets; c != nil {
        if !h.sameSizeGrow() {
            // There used to be half as many buckets; mask down one more power of two.
            m >>= 1
        }
    // 如果目標bucket在擴容中還沒有遷移，則到oldbuckets中找目標bucket
        oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
        if !evacuated(oldb) {
            b = oldb
        }
    }
  // 計算出key的tophash，用於比對
    top := tophash(hash)
    for ; b != nil; b = b.overflow(t) {
        for i := uintptr(0); i < bucketCnt; i++ {
      // 如果tophash不一致，key肯定不同，繼續尋找下一個
            if b.tophash[i] != top {
                continue
            }
      // tophash一直，需要判斷key是否一致
            k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
            if t.indirectkey {
                k = *((*unsafe.Pointer)(k))
            }
      // key也是相同的，則返回對應的value
            if alg.equal(key, k) {
                v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
                if t.indirectvalue {
                    v = *((*unsafe.Pointer)(v))
                }
                return v
            }
        }
    }
    return unsafe.Pointer(&zeroVal[0])
}

overflow

這個函數就是找bmap的overflow的地址，通過結構圖中可以看出，找到bmap結構體的最後一個指針佔用的內存單元就是overflow指向的下一個bmap的地址了

func (b *bmap) overflow(t *maptype) *bmap {
    return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-sys.PtrSize))
}

上面的邏輯比較簡單，但是在這裏有幾個問題需要解決

bucket（bmap結構體）是怎麼確定的
tophash是怎麼確定的
key和value的地址爲什麼是通過偏移來計算的

先放一下buckets和bmap的放大圖

bucket（bmap結構體）是怎麼確定的
```
bucket := hash & bucketMask(h.B)
b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
```
加入B=5，則說明buckets的數量爲2^5 = 32，則取hash的末5位，來計算出目標bucket的索引，圖中計算出索引爲6，所以，在buckets上偏移6個bucket大小的地址，即可找到對應的bucket

tophash是怎麼確定的

func tophash(hash uintptr) uint8 {
    top := uint8(hash >> (sys.PtrSize*8 - 8))
    if top < minTopHash {
        top += minTopHash
    }
    return top
}

每個bucket的tophash數組的長度爲8，所以，這裏直接去hash值的前8位計算出來數值，既是tophash了

key和value的地址爲什麼是通過偏移來計算的
```
k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
```
根據最開始的數據結構分析和上面的bmap圖示，可以看出bmap中所有的key是放在一起的，所有的value是放在一起的，dataoffset是tophash[8]所佔用的大小，所以，key所在的地址也就是 b的地址+dataOffset的偏移+對應的索引i*key的大小，同理value是排列在key的後面的

插入

mapassign

func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
    if h == nil {
        panic(plainError("assignment to entry in nil map"))
    }
    // map併發讀寫的處理，直接拋異常
    if h.flags&hashWriting != 0 {
        throw("concurrent map writes")
    }
  // 根據map的hash種子 hash0，計算key的hash值
    alg := t.key.alg
    hash := alg.hash(key, uintptr(h.hash0))

    // Set hashWriting after calling alg.hash, since alg.hash may panic,
    // in which case we have not actually done a write.
    h.flags |= hashWriting
  // 如果map沒有buckets，就分配（make(map)不指定map長度的時候就會惰性分配buckets）
    if h.buckets == nil {
        h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
    }

again:
  // 根據計算出的hash值，來確定應該插入的bucket在buckets中的索引
    bucket := hash & bucketMask(h.B)
  // 判斷是否在擴容map，growWork是來完成擴容操作的
    if h.growing() {
        growWork(t, h, bucket)
    }
  // 確認bucket的地址
    b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
  // 根據計算出hash二進制前八位的值，作爲tophash使用
    top := tophash(hash)

    var inserti *uint8
    var insertk unsafe.Pointer
    var val unsafe.Pointer
    for {
        for i := uintptr(0); i < bucketCnt; i++ {
      // 循環遍歷tophash數組，如果數組的索引位置爲空，先拿過來使用
            if b.tophash[i] != top {
                if b.tophash[i] == empty && inserti == nil {
                    inserti = &b.tophash[i]
                    insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
                    val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
                }
                continue
            }
      // 找到了tophash數組中找到了當前key的tophash一致的情況
            k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
      // 如果key是指針，獲取指針對應的數據
            if t.indirectkey {
                k = *((*unsafe.Pointer)(k))
            }
      // 判斷這兩個key是否相同，不同繼續尋找
            if !alg.equal(key, k) {
                continue
            }
            // already have a mapping for key. Update it.
            if t.needkeyupdate {
                typedmemmove(t.key, k, key)
            }
      // 根據i找到value應該存放的位置，可以結合結構圖中bmap的數據結構來理解
            val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
            goto done
        }
    // buckets中沒有找到空餘的位置或者相同的key，則到overflow中查找
        ovf := b.overflow(t)
        if ovf == nil {
            break
        }
        b = ovf
    }

    // Did not find mapping for key. Allocate new cell & add entry.

    // If we hit the max load factor or we have too many overflow buckets,
    // and we're not already in the middle of growing, start growing.
  // 判斷是否需要擴容
    if !h.growing() && (overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
        hashGrow(t, h)
        goto again // Growing the table invalidates everything, so try again
    }
    // inerti==nil，表示map的buckets都滿了，則需要新加一個overflow掛載到map和對應的bmap下
    if inserti == nil {
        // all current buckets are full, allocate a new one.
        newb := h.newoverflow(t, b)
        inserti = &newb.tophash[0]
        insertk = add(unsafe.Pointer(newb), dataOffset)
        val = add(insertk, bucketCnt*uintptr(t.keysize))
    }

    // store new key/value at insert position
  // 存儲key value到指定的位置
    if t.indirectkey {
        kmem := newobject(t.key)
        *(*unsafe.Pointer)(insertk) = kmem
        insertk = kmem
    }
    if t.indirectvalue {
        vmem := newobject(t.elem)
        *(*unsafe.Pointer)(val) = vmem
    }
    typedmemmove(t.key, insertk, key)
    *inserti = top
    h.count++

done:
    if h.flags&hashWriting == 0 {
        throw("concurrent map writes")
    }
  // 修改map的flags
    h.flags &^= hashWriting
    if t.indirectvalue {
        val = *((*unsafe.Pointer)(val))
    }
    return val
}

setoverflow

func (h *hmap) newoverflow(t *maptype, b *bmap) *bmap {
    var ovf *bmap
  // 先去找一下預先分配的有沒有剩餘的overflow
    if h.extra != nil && h.extra.nextOverflow != nil {
        // We have preallocated overflow buckets available.
        // See makeBucketArray for more details.
    // 預先分配的有，直接使用預先分配的，然後更新一下 下一個可以用overflow => nextOverflow
        ovf = h.extra.nextOverflow
        if ovf.overflow(t) == nil {
            // We're not at the end of the preallocated overflow buckets. Bump the pointer.
            h.extra.nextOverflow = (*bmap)(add(unsafe.Pointer(ovf), uintptr(t.bucketsize)))
        } else {
            // This is the last preallocated overflow bucket.
            // Reset the overflow pointer on this bucket,
            // which was set to a non-nil sentinel value.
            ovf.setoverflow(t, nil)
            h.extra.nextOverflow = nil
        }
    } else {
        ovf = (*bmap)(newobject(t.bucket))
    }
  // 增加noverflow
    h.incrnoverflow()
    if t.bucket.kind&kindNoPointers != 0 {
        h.createOverflow()
        *h.extra.overflow = append(*h.extra.overflow, ovf)
    }
  // 把當前overflow，掛載到bmap的overflow鏈表後面
    b.setoverflow(t, ovf)
    return ovf
}

overflow指向的就是一個bmap結構，而bmap結構的最後一個地址，存儲的是overflow的地址，通過bmap.overflow可以將bmap的所有overflow串聯起來，hmap.extra.nextOverflow也是一樣的邏輯

擴容

在mapassign函數中可以看到，擴容發生的情況有兩種

overLoadFactor(h.count+1, h.B) || tooManyOverflowBuckets(h.noverflow, h.B)

超過設定的負載值
有太多的overflow

先來看一下這兩個函數

overLoadFactor

func overLoadFactor(count int, B uint8) bool {
  // loadFactorNum = 13; loadFactorDen = 2
    return count > bucketCnt && uintptr(count) > loadFactorNum*(bucketShift(B)/loadFactorDen)
}

uintptr(count) > loadFactorNum*(bucketShift(B)/loadFactorDen) 可以簡化爲 count / (2^B) > 6.5，這個6.5便是代表loadFactor的負載係數

tooManyOverflowBuckets

func tooManyOverflowBuckets(noverflow uint16, B uint8) bool {
    // If the threshold is too low, we do extraneous work.
    // If the threshold is too high, maps that grow and shrink can hold on to lots of unused memory.
    // "too many" means (approximately) as many overflow buckets as regular buckets.
    // See incrnoverflow for more details.
    if B > 15 {
        B = 15
    }
    // The compiler doesn't see here that B < 16; mask B to generate shorter shift code.
    return noverflow >= uint16(1)<<(B&15)
}

通過判斷noverflow的數量來判斷overflow是否太多

我們理解一下這兩種情況擴容的原因

超過設定的負載值
根據key查找的過程中，根據末B位確定bucket，高8位確定tophash，但是查找tophash的過程中，是需要遍歷整個bucket的，所以，最優的情況是每個bucket只存儲一個key，這樣就達到了hash的O(1)的查找效率，但是空間卻大大的浪費了；如果所有的key都存儲到了一個bucket裏面面，就退變成了鏈表，查找效率就變成了O(n)，所以裝載係數就是爲了平衡查找效率和存儲空間的，當裝載係數過大，就需要增加bucket了，來提高查找效率，即增量擴容
有太多的overflow
當bucket的空位全部填滿的時候，裝載係數就達到了8，爲什麼還會有tooManyOverflowBuckets的判斷呢，map不僅有增加還有刪除的操作，當某一個bucket的空位填滿後，開始填充到overflow裏面，這時候再刪除bucket裏面的數據，其實整個過程很有可能並沒有觸發超過負載擴容機制的，（因爲有較多的buckets），但是查找overflow的數據，就首先要遍歷bucket的數據，這個就是無用功了，查找效率就低了，這時候需要不增加bucket數量的擴容，也就是等量擴容

擴容的工作是由hashGrow開始的，但是真正進行遷移工作的是evacuate，由growWork進行d調用；在每一次的maassign和mapdelete的時候，會判斷這個map是否正在進行擴容操作，如果是的，就遷移當前的bucket；所以，map的擴容並不是一蹴而就的，而是一個循序漸進的過程

hashGrow

func hashGrow(t *maptype, h *hmap) {
    // If we've hit the load factor, get bigger.
    // Otherwise, there are too many overflow buckets,
    // so keep the same number of buckets and "grow" laterally.
  // 判斷是等量擴容還是增量擴容
    bigger := uint8(1)
    if !overLoadFactor(h.count+1, h.B) {
        bigger = 0
        h.flags |= sameSizeGrow
    }
  // 爲map根據新的B（h.B+bigger爲新的h.B）重新分配新的buckets和overflow
    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hmap相關的屬性
    h.B += bigger
    h.flags = flags
    h.oldbuckets = oldbuckets
    h.buckets = newbuckets
    h.nevacuate = 0
    h.noverflow = 0
    // 將老的map的extra和nextOverflow更新到新的map結構下面
    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
    }
    if nextOverflow != nil {
        if h.extra == nil {
            h.extra = new(mapextra)
        }
        h.extra.nextOverflow = nextOverflow
    }

    // the actual copying of the hash table data is done incrementally
    // by growWork() and evacuate().
}

hashGrow 這個前菜已經準備完成了，接下來就交給growWork和 evacuate兩個函數來完成的

growWork

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)
    }
}

evacuate

講hmap中的一個bucket搬移到新的buckets中，老的bucket裏key與新的buckets中位置的對應，同樣參考map的查找過程

這裏如何判斷這個bucket是否已經搬移過了呢，主要就是依據evacuated函數來判斷

func evacuated(b *bmap) bool {
    h := b.tophash[0]
    return h > empty && h < minTopHash
}

看了源碼就發現原理很簡單，就是對tophash[0]值的判斷，那麼肯定是在搬移之後設置的這個值，我們通過evacuate函數l哎一探究竟吧

func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
    b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
    newbit := h.noldbuckets()
  // 判斷是否搬移過
    if !evacuated(b) {
        // TODO: reuse overflow buckets instead of using new ones, if there
        // is no iterator using the old buckets.  (If !oldIterator.)

        // xy contains the x and y (low and high) evacuation destinations.
    // 吧bucket原先對應的索引賦值給x
        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.v = add(x.k, bucketCnt*uintptr(t.keysize))
        // 如果是增量擴容，擴容後的bucket有變，假如以B=5爲例，B+1= 6，這時候去倒數6位計算bucket的索引，但是倒數第6位只能是0或者1，也就是說索引只能是，x或y（x+newbit）,這裏計算出來y，以備後用
        if !h.sameSizeGrow() {
            // Only calculate y pointers if we're growing bigger.
            // Otherwise GC can see bad pointers.
            y := &xy[1]
            y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
            y.k = add(unsafe.Pointer(y.b), dataOffset)
            y.v = add(y.k, bucketCnt*uintptr(t.keysize))
        }
        // 進行搬移
        for ; b != nil; b = b.overflow(t) {
            k := add(unsafe.Pointer(b), dataOffset)
            v := add(k, bucketCnt*uintptr(t.keysize))
            for i := 0; i < bucketCnt; i, k, v = i+1, add(k, uintptr(t.keysize)), add(v, uintptr(t.valuesize)) {
                top := b.tophash[i]
        // 空的跳過
                if top == empty {
                    b.tophash[i] = evacuatedEmpty
                    continue
                }
                if top < minTopHash {
                    throw("bad map state")
                }
                k2 := k
                if t.indirectkey {
                    k2 = *((*unsafe.Pointer)(k2))
                }
                var useY uint8
                if !h.sameSizeGrow() {
                    // Compute hash to make our evacuation decision (whether we need
                    // to send this key/value to bucket x or bucket y).
          // 判斷hash計算出來，是使用x還是y，等量擴容是使用x
                    hash := t.key.alg.hash(k2, uintptr(h.hash0))
                    if h.flags&iterator != 0 && !t.reflexivekey && !t.key.alg.equal(k2, k2) {
                        // If key != key (NaNs), then the hash could be (and probably
                        // will be) entirely different from the old hash. Moreover,
                        // it isn't reproducible. Reproducibility is required in the
                        // presence of iterators, as our evacuation decision must
                        // match whatever decision the iterator made.
                        // Fortunately, we have the freedom to send these keys either
                        // way. Also, tophash is meaningless for these kinds of keys.
                        // We let the low bit of tophash drive the evacuation decision.
                        // We recompute a new random tophash for the next level so
                        // these keys will get evenly distributed across all buckets
                        // after multiple grows.
                        useY = top & 1
                        top = tophash(hash)
                    } else {
                        if hash&newbit != 0 {
                            useY = 1
                        }
                    }
                }

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

                b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY
                dst := &xy[useY]                 // evacuation destination
                // 如果目標的bucket已經滿了，則新建overflow，掛載到bucket上，並使用這個overflow
                if dst.i == bucketCnt {
                    dst.b = h.newoverflow(t, dst.b)
                    dst.i = 0
                    dst.k = add(unsafe.Pointer(dst.b), dataOffset)
                    dst.v = add(dst.k, bucketCnt*uintptr(t.keysize))
                }
        // 拷貝key value，設置tophash數組的對應索引的值
                dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
                if t.indirectkey {
                    *(*unsafe.Pointer)(dst.k) = k2 // copy pointer
                } else {
                    typedmemmove(t.key, dst.k, k) // copy value
                }
                if t.indirectvalue {
                    *(*unsafe.Pointer)(dst.v) = *(*unsafe.Pointer)(v)
                } else {
                    typedmemmove(t.elem, dst.v, v)
                }
                dst.i++
                // These updates might push these pointers past the end of the
                // key or value 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.v = add(dst.v, uintptr(t.valuesize))
            }
        }
        // Unlink the overflow buckets & clear key/value to help GC.
        if h.flags&oldIterator == 0 && t.bucket.kind&kindNoPointers == 0 {
            b := add(h.oldbuckets, oldbucket*uintptr(t.bucketsize))
            // Preserve b.tophash because the evacuation
            // state is maintained there.
            ptr := add(b, dataOffset)
            n := uintptr(t.bucketsize) - dataOffset
            memclrHasPointers(ptr, n)
        }
    }

    if oldbucket == h.nevacuate {
        advanceEvacuationMark(h, t, newbit)
    }
}

擴容是逐步進行的，一次搬運一個bucket

我們以原先的B=5爲例，現在增量擴容後B=6，但是hash的倒數第6位只能是0或1，也就是說，如果原先計算出來的bucket索引爲6的話，即 00110，那麼新的bucket對應的索引只能是 100110（6+2^5）或 000110（6），x對應的就是6，y對應的就是（6+2^5）；如果是等量擴容，那麼索引肯定就是不變的，這時候就不需要y了

找到對應的新的bucket之後，按順序依次存放就ok了

刪除

mapdelete

刪除的邏輯比較簡單，根據key查找，找到就清空key和value及tophash

func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
    if h == nil || h.count == 0 {
        return
    }
  // 讀寫衝突
    if h.flags&hashWriting != 0 {
        throw("concurrent map writes")
    }
    // 下面一大片的計算hash，查找bucket，查到bucket裏面的key，邏輯一樣，就不重複了
    alg := t.key.alg
    hash := alg.hash(key, uintptr(h.hash0))

    // Set hashWriting after calling alg.hash, since alg.hash may panic,
    // in which case we have not actually done a write (delete).
    h.flags |= hashWriting

    bucket := hash & bucketMask(h.B)
    if h.growing() {
        growWork(t, h, bucket)
    }
    b := (*bmap)(add(h.buckets, bucket*uintptr(t.bucketsize)))
    top := tophash(hash)
search:
    for ; b != nil; b = b.overflow(t) {
        for i := uintptr(0); i < bucketCnt; i++ {
            if b.tophash[i] != top {
                continue
            }
            k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
            k2 := k
            if t.indirectkey {
                k2 = *((*unsafe.Pointer)(k2))
            }
            if !alg.equal(key, k2) {
                continue
            }
            // Only clear key if there are pointers in it.
      // 這裏找到了key，如果key是指針，設爲nil，否則清空key對應內存的數據
            if t.indirectkey {
                *(*unsafe.Pointer)(k) = nil
            } else if t.key.kind&kindNoPointers == 0 {
                memclrHasPointers(k, t.key.size)
            }
      // 同理刪除v
            v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
            if t.indirectvalue {
                *(*unsafe.Pointer)(v) = nil
            } else if t.elem.kind&kindNoPointers == 0 {
                memclrHasPointers(v, t.elem.size)
            } else {
                memclrNoHeapPointers(v, t.elem.size)
            }
      // 把tophash設置爲0，並更新count屬性
            b.tophash[i] = empty
            h.count--
            break search
        }
    }

    if h.flags&hashWriting == 0 {
        throw("concurrent map writes")
    }
    h.flags &^= hashWriting
}

遍歷

按一般的思維來考慮，遍歷值需要遍歷buckets數組裏面的每個bucket以及bucket下掛的overflow鏈表即可，但是map存在擴容的情況，這樣就會導致遍歷的難度增大了，我們看一下go是怎麼實現的

根據go tool 的分析，我們可以簡單看一下遍歷時的流程信息

mapiterinit

func mapiterinit(t *maptype, h *hmap, it *hiter) {
    if h == nil || h.count == 0 {
        return
    }

    if unsafe.Sizeof(hiter{})/sys.PtrSize != 12 {
        throw("hash_iter size incorrect") // see cmd/compile/internal/gc/reflect.go
    }
  // 設置iter的屬性
    it.t = t
    it.h = h

    // grab snapshot of bucket state
    it.B = h.B
    it.buckets = h.buckets
    if t.bucket.kind&kindNoPointers != 0 {
        // Allocate the current slice and remember pointers to both current and old.
        // This preserves all relevant overflow buckets alive even if
        // the table grows and/or overflow buckets are added to the table
        // while we are iterating.
        h.createOverflow()
        it.overflow = h.extra.overflow
        it.oldoverflow = h.extra.oldoverflow
    }

    // decide where to start
  // 隨機生成一個種子，並根據這個隨機種子計算出startBucket和offset，保證遍歷的隨機性
    r := uintptr(fastrand())
    if h.B > 31-bucketCntBits {
        r += uintptr(fastrand()) << 31
    }
    it.startBucket = r & bucketMask(h.B)
    it.offset = uint8(r >> h.B & (bucketCnt - 1))

    // iterator state
    it.bucket = it.startBucket

    // Remember we have an iterator.
    // Can run concurrently with another mapiterinit().
    if old := h.flags; old&(iterator|oldIterator) != iterator|oldIterator {
        atomic.Or8(&h.flags, iterator|oldIterator)
    }
    // 開始遍歷
    mapiternext(it)
}

mapiternext

func mapiternext(it *hiter) {
    h := it.h
    if raceenabled {
        callerpc := getcallerpc()
        racereadpc(unsafe.Pointer(h), callerpc, funcPC(mapiternext))
    }
    if h.flags&hashWriting != 0 {
        throw("concurrent map iteration and map write")
    }
    t := it.t
    bucket := it.bucket
    b := it.bptr
    i := it.i
    checkBucket := it.checkBucket
    alg := t.key.alg

next:
  // b==nil說明bucket.overflow鏈表已經遍歷完成了，遍歷下一個bucket
    if b == nil {
    // 遍歷到了開始的bucket，而且startBucket被遍歷過了，則說明整個map遍歷完成了
        if bucket == it.startBucket && it.wrapped {
            // end of iteration
            it.key = nil
            it.value = nil
            return
        }
    // 如果hmap正在擴容，則判斷當前遍歷的bucket是否搬移完了，搬移完了，使用新得bucket，否則使用oldbucket
        if h.growing() && it.B == h.B {
            // Iterator was started in the middle of a grow, and the grow isn't done yet.
            // If the bucket we're looking at hasn't been filled in yet (i.e. the old
            // bucket hasn't been evacuated) then we need to iterate through the old
            // bucket and only return the ones that will be migrated to this bucket.
            oldbucket := bucket & it.h.oldbucketmask()
            b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
            if !evacuated(b) {
                checkBucket = bucket
            } else {
                b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
                checkBucket = noCheck
            }
        } else {
            b = (*bmap)(add(it.buckets, bucket*uintptr(t.bucketsize)))
            checkBucket = noCheck
        }
        bucket++
    // 遍歷到了數組末尾，從數組頭繼續遍歷
        if bucket == bucketShift(it.B) {
            bucket = 0
            it.wrapped = true
        }
        i = 0
    }
  // 遍歷當前bucket或者bucket.overflow裏面的數據
    for ; i < bucketCnt; i++ {
    // 通過offset與i，確定正在遍歷的bucket的tophash的索引
        offi := (i + it.offset) & (bucketCnt - 1)
        if b.tophash[offi] == empty || b.tophash[offi] == evacuatedEmpty {
            continue
        }
    // 根據偏移量i，確定key和value的地址
        k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
        if t.indirectkey {
            k = *((*unsafe.Pointer)(k))
        }
        v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.valuesize))
        if checkBucket != noCheck && !h.sameSizeGrow() {
      // 說明增量擴容中，需要進一步判斷
            // Special case: iterator was started during a grow to a larger size
            // and the grow is not done yet. We're working on a bucket whose
            // oldbucket has not been evacuated yet. Or at least, it wasn't
            // evacuated when we started the bucket. So we're iterating
            // through the oldbucket, skipping any keys that will go
            // to the other new bucket (each oldbucket expands to two
            // buckets during a grow).
            if t.reflexivekey || alg.equal(k, k) {
        // 數據還沒有從oldbucket遷移到新的bucket裏面，判斷這個key重新計算後是否與oldbucket的索引一致，不一致則跳過
                // If the item in the oldbucket is not destined for
                // the current new bucket in the iteration, skip it.
                hash := alg.hash(k, uintptr(h.hash0))
                if hash&bucketMask(it.B) != checkBucket {
                    continue
                }
            } else {
                // Hash isn't repeatable if k != k (NaNs).  We need a
                // repeatable and randomish choice of which direction
                // to send NaNs during evacuation. We'll use the low
                // bit of tophash to decide which way NaNs go.
                // NOTE: this case is why we need two evacuate tophash
                // values, evacuatedX and evacuatedY, that differ in
                // their low bit.
                if checkBucket>>(it.B-1) != uintptr(b.tophash[offi]&1) {
                    continue
                }
            }
        }
        if (b.tophash[offi] != evacuatedX && b.tophash[offi] != evacuatedY) ||
            !(t.reflexivekey || alg.equal(k, k)) {
      // 這裏的數據不是正在擴容中的數據，可以直接使用
            // This is the golden data, we can return it.
            // OR
            // key!=key, so the entry can't be deleted or updated, so we can just return it.
            // That's lucky for us because when key!=key we can't look it up successfully.
            it.key = k
            if t.indirectvalue {
                v = *((*unsafe.Pointer)(v))
            }
            it.value = v
        } else {
            // The hash table has grown since the iterator was started.
            // The golden data for this key is now somewhere else.
            // Check the current hash table for the data.
            // This code handles the case where the key
            // has been deleted, updated, or deleted and reinserted.
            // NOTE: we need to regrab the key as it has potentially been
            // updated to an equal() but not identical key (e.g. +0.0 vs -0.0).
      // 在遍歷開始之後，這個map進行了擴容，數據可能不正確，重新查找獲取一下
            rk, rv := mapaccessK(t, h, k)
            if rk == nil {
                continue // key has been deleted
            }
            it.key = rk
            it.value = rv
        }
        it.bucket = bucket
        if it.bptr != b { // avoid unnecessary write barrier; see issue 14921
            it.bptr = b
        }
        it.i = i + 1
        it.checkBucket = checkBucket
        return
    }
  // 遍歷bucket.overflow鏈表
    b = b.overflow(t)
    i = 0
    goto next
}

整體思路如下：

首先從buckets數組中，隨機確定一個索引，作爲startBucket，然後確定offset偏移量，作爲起始key的地址
遍歷當前bucket及bucket.overflow，判斷當前bucket是否正在擴容中，如果是則跳轉到3，否則跳轉到4
加入原先的buckets爲0，1，那麼擴容後的新的buckets爲0，1，2，3，此時我們遍歷到了buckets[0]，發現這個bucket正在擴容，那麼找到bucket[0]所對應的oldbuckets[0]，遍歷裏面的key，這時候是遍歷所有的嗎？當然不是，而是僅僅遍歷那些key經過hash，可以散列到bucket[0]裏面的部分key；同理，當遍歷到bucket[2]的時候，發現bucket正在擴容，找到oldbuckets[0]，然後遍歷裏面可以散列到bucket[2]的那些key
遍歷當前這個bucket即可
繼續遍歷bucket下面的overflow鏈表
如果遍歷到了startBucket，說明遍歷完了，結束遍歷

參考文章

《深度解密Go語言之 map》

Go map原理剖析

結構