go map
// 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.// 元素个数，len(map)count int // # live cells == size of map. Must be first (used by len() builtin)// 标记是否有线程正在写flags uint8// buckets 的对数 log_2B uint8 // log_2 of # of buckets (can hold up to loadFactor * 2^B items)// overflow的bucket大致数量noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details// 哈希的种子，它能为哈希函数的结果引入随机性，这个值在创建哈希表时确定，并在调用哈希函数时作为参数传入hash0 uint32 // hash seed// buckets 数组，大小为 2^Bbuckets unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.// 旧map，不为nil时表示正在扩容oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing// 扩容进度，小于此地址的 buckets 是已经迁移完成的nevacuate uintptr // progress counter for evacuation (buckets less than this have been evacuated)extra *mapextra // optional fields
}
buckets 是一个指针，最终它指向的是一个结构体：
// 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 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.
}
运行时:
type bmap struct {topbits [8]uint8keys [8]keytypevalues [8]valuetypepad uintptroverflow uintptr
}
bmap 就是我们常说的“桶”，桶里面会最多装 8 个 key，这些 key 之所以会落入同一个桶，是因为它们经过哈希计算后，哈希结果是“一类”的。在桶内，又会根据 key 计算出来的 hash 值的高 8 位来决定 key 到底落入桶内的哪个位置（一个桶内最多有8个位置）。
