1、数据结构

// A header for a Go map.
type hmap struct {
	// Note: the format of the hmap is also encoded in cmd/compile/internal/reflectdata/reflect.go.
	// Make sure this stays in sync with the compiler's definition.
	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 elem 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 elem 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
}

// 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 [abi.MapBucketCount]uint8
	// Followed by bucketCnt keys and then bucketCnt elems.
	// NOTE: packing all the keys together and then all the elems together makes the
	// code a bit more complicated than alternating key/elem/key/elem/... but it allows
	// us to eliminate padding which would be needed for, e.g., map[int64]int8.
	// Followed by an overflow pointer.
}

map 的底层结构包含以下几个重要的元素：

• hmap： hmap 是 Go 中 map 类型的实际数据结构，它包含了哈希表的核心信息。例如，桶的指针、大小、负载因子、哈希函数等。

• buckets： buckets 数组存储了 map 中的哈希桶，每个桶内存储着一部分键值对。如果哈希冲突较多，桶会存储多个键值对。

• overflow： 为了处理大量的哈希冲突，Go 中的哈希桶通常还会有一个 overflow 字段，表示某些冲突较严重的元素会被溢出到其他位置。

 2、底层实现

（1）哈希表和桶的结构

        Go 语言的 map 底层实现是一个哈希表，通过链地址法（数组加链表）来解决冲突，它的每个单元是一个桶。它的数组是由一组桶组成，每个桶是一个链式结构，可以存储8个键值对，当发生hash冲突时首先存在桶内，如果桶满了，就在桶后面连接溢出桶来存储键值对。

（2）哈希函数

        Go 使用一个高效的哈希函数将键映射到桶的位置。不同类型的键使用不同的哈希函数，例如字符串和整数会有不同的哈希计算方法。

（3）扩容和负载因子

        负载因子为6.5，代表键值对数量与桶数量比值的上限。如果达到负载因子值或溢出桶的数量超过正常桶的数量（或者超过上限值1<<15），就会发生扩容，新的桶数量是原来的2倍数。（负载因子的计算和桶的倍数不考虑溢出桶的数量）

        Go 的 map 底层哈希表设计采用了渐进式扩容的策略，当扩容时，并不是所有的键值对都会立刻重新哈希。Go 会 分批次地、渐进地将老桶中的元素迁移到新桶中，避免在扩容过程中产生性能瓶颈。