我们知道哈希表解决冲突一般要么是开放定址法，要么是再哈希法。Golang采用的是 第一种，但是实现上和教科书式的实现方式不同。这也是很有趣的一个东西，就像之前 看Python的 deque 一样，和教科书不一样，非常高效的实现方式。

通常教科书式的实现方式是，hash值重复的节点组成一个链表，首先我们根据hash值 定位到大概在哪里，然后遍历这个链表。这样有一个缺点，就是不知道这个链表到底有多长。

Golang的实现避免了这种问题，就是采用桶的方式，也就是每个坑后面接固定长度的键值对。

// A header for a Go map.
type hmap struct {
	// Note: the format of the Hmap is encoded in ../../cmd/internal/gc/reflect.go and
	// ../reflect/type.go. Don't change this structure without also changing that code!
	count     int // # live cells == size of map.  Must be first (used by len() builtin)
	flags     uint8
	B         uint8  // 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 // hash seed

	buckets    unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
	oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
	nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated)

	extra *mapextra // optional fields
}

// mapextra holds fields that are not present on all maps.
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.overflow.
	// Overflow is used only if key and value do not contain pointers.
	// overflow[0] contains overflow buckets for hmap.buckets.
	// overflow[1] contains overflow buckets for hmap.oldbuckets.
	// The indirection allows to store a pointer to the slice in hiter.
	overflow [2]*[]*bmap

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

// A bucket for a Go map.
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.
}

不过看到这里我就想吐槽了，Golang虽然标准库设计的很科学，但是很多命名实在是太简单了， 不方便记忆。估计只有写的人自己才能一眼反应过来吧。例如 B uint8 // log_2 of # of buckets (can hold up to loadFactor * 2^B items) 要是我的话，会选择稍微长一点，更容易记住的名字。

buckets 是一个指针，指向的就是 *bmap 这种键值对的数组。tophash[0] 是数组 里的第一个值，相当于链表里的第一个。不过如果冲突到了多于8个怎么办？所以 里面还有 overflow 这样的指针，指向两个 *[]*bmap slice。这样就相当于链表了， 假设冲突已经到了这种层次的话。

下面来看看Golang的map是怎么查找的，我们知道Go的一个坑点就是字典不管找没找到， 都有返回，默认返回value对应类型的 zero object。所以要用类似 a, ok := m["hello"] 的形式，然后判断 ok。麻烦。

// mapaccess1 returns a pointer to h[key].  Never returns nil, instead
// it will return a reference to the zero object for the value type if
// the key is not in the map.
// NOTE: The returned pointer may keep the whole map live, so don't
// hold onto it for very long.
func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
	if raceenabled && h != nil {
		callerpc := getcallerpc(unsafe.Pointer(&t))
		pc := funcPC(mapaccess1)
		racereadpc(unsafe.Pointer(h), callerpc, pc)
		raceReadObjectPC(t.key, key, callerpc, pc)
	}
	if msanenabled && h != nil {
		msanread(key, t.key.size)
	}
    // 如果h里面啥也没有的话，直接返回
	if h == nil || h.count == 0 {
		return unsafe.Pointer(&zeroVal[0])
	}
    // 原来上面结构体里flags是用来做读写状态标识的
	if h.flags&hashWriting != 0 {
		throw("concurrent map read and map write")
	}
    // t是map的类型，Go是编译型语言，所以在编译的时候应该就确定好了
    // 把key的类型确定好，hash算法固定好，直接用就好了
	alg := t.key.alg
	hash := alg.hash(key, uintptr(h.hash0))
	m := uintptr(1)<<h.B - 1
	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.bucketsize)))
	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
		}
		oldb := (*bmap)(add(c, (hash&m)*uintptr(t.bucketsize)))
		if !evacuated(oldb) {
			b = oldb
		}
	}
    // 算出在哪个桶，哈希值取余？
	top := uint8(hash >> (sys.PtrSize*8 - 8))
	if top < minTopHash {
		top += minTopHash
	}
    // 依次遍历，找出对象
	for {
		for i := uintptr(0); i < bucketCnt; i++ {
			if b.tophash[i] != top {
				continue
			}
			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
			if t.indirectkey {
				k = *((*unsafe.Pointer)(k))
			}
			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
			}
		}
		b = b.overflow(t)
		if b == nil {
			return unsafe.Pointer(&zeroVal[0])
		}
	}
}

还有一个 mapaccess2 不知道是干啥的。 > 2019.05.13 更新: 据网友 @rongfeixu 回复，mapaccess1 用于 v := map["bla"] > mapaccess2 用于 v, ok := map["bla"] 这种形式。

mapaccessK 说是给iter用的。 接下来看看赋值操作，赋值操作其实就是先查找，找到了覆盖，没找到新建：

// Like mapaccess, but allocates a slot for the key if it is not present in the map.
func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
	if h == nil {
		panic(plainError("assignment to entry in nil map"))
	}
	if raceenabled {
		callerpc := getcallerpc(unsafe.Pointer(&t))
		pc := funcPC(mapassign)
		racewritepc(unsafe.Pointer(h), callerpc, pc)
		raceReadObjectPC(t.key, key, callerpc, pc)
	}
	if msanenabled {
		msanread(key, t.key.size)
	}
	if h.flags&hashWriting != 0 {
		throw("concurrent map writes")
	}
	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

	if h.buckets == nil {
		h.buckets = newarray(t.bucket, 1)
	}

again:
	bucket := hash & (uintptr(1)<<h.B - 1)
	if h.growing() {
		growWork(t, h, bucket)
	}
	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
	top := uint8(hash >> (sys.PtrSize*8 - 8))
	if top < minTopHash {
		top += minTopHash
	}

	var inserti *uint8
	var insertk unsafe.Pointer
	var val unsafe.Pointer
	for {
		for i := uintptr(0); i < bucketCnt; i++ {
			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
			}
			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
			if t.indirectkey {
				k = *((*unsafe.Pointer)(k))
			}
			if !alg.equal(key, k) {
				continue
			}
            // 如果找到了
			// already have a mapping for key. Update it.
			if t.needkeyupdate {
				typedmemmove(t.key, k, key)
			}
			val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
			goto done
		}
		ovf := b.overflow(t)
		if ovf == nil {
			break
		}
		b = ovf
	}

    // 遍历完了，没找到，那就新建，前面记好了是否有空余处可以插入
	// 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.
	if !h.growing() && (overLoadFactor(int64(h.count), h.B) || tooManyOverflowBuckets(h.noverflow, h.B)) {
		hashGrow(t, h)
		goto again // Growing the table invalidates everything, so try again
	}

	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
	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")
	}
	h.flags &^= hashWriting
	if t.indirectvalue {
		val = *((*unsafe.Pointer)(val))
	}
	return val
}

删除操作操作也是类似，先找，找到了删除，没找到就没动作：

func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
	if raceenabled && h != nil {
		callerpc := getcallerpc(unsafe.Pointer(&t))
		pc := funcPC(mapdelete)
		racewritepc(unsafe.Pointer(h), callerpc, pc)
		raceReadObjectPC(t.key, key, callerpc, pc)
	}
	if msanenabled && h != nil {
		msanread(key, t.key.size)
	}
	if h == nil || h.count == 0 {
		return
	}
	if h.flags&hashWriting != 0 {
		throw("concurrent map writes")
	}

	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 & (uintptr(1)<<h.B - 1)
	if h.growing() {
		growWork(t, h, bucket)
	}
	b := (*bmap)(unsafe.Pointer(uintptr(h.buckets) + bucket*uintptr(t.bucketsize)))
	top := uint8(hash >> (sys.PtrSize*8 - 8))
	if top < minTopHash {
		top += minTopHash
	}
	for {
		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
			}
			if t.indirectkey {
				*(*unsafe.Pointer)(k) = nil
			} else {
				typedmemclr(t.key, k)
			}
			v := unsafe.Pointer(uintptr(unsafe.Pointer(b)) + dataOffset + bucketCnt*uintptr(t.keysize) + i*uintptr(t.valuesize))
			if t.indirectvalue {
				*(*unsafe.Pointer)(v) = nil
			} else {
				typedmemclr(t.elem, v)
			}
            // 标记为空
			b.tophash[i] = empty
			h.count--
			goto done
		}
		b = b.overflow(t)
		if b == nil {
			goto done
		}
	}

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

综上，我们发现，Golang的map是不能并发读写的，只是简单的依靠设置flag， 然后检查并且 throw 来完成的，太不安全了，所以有了 sync.Map 可以愉快的并发读写了。