title: Go map

Go map

A map is just a hash table. The data is arranged into an array of buckets. Each bucket contains up to 8 key/elem pairs. The low-order bits of the hash are used to select a bucket. Each bucket contains a few high-order bits of each hash to distinguish the entries within a single bucket.

If more than 8 keys hash to a bucket, we chain on extra buckets.

When the hashtable grows, we allocate a new array of buckets twice as big. Buckets are incrementally copied from the old bucket array to the new bucket array.

Map iterators walk through the array of buckets and return the keys in walk order (bucket #, then overflow chain order, then bucket index). To maintain iteration semantics, we never move keys within their bucket (if we did, keys might be returned 0 or 2 times). When growing the table, iterators remain iterating through the old table and must check the new table if the bucket they are iterating through has been moved ("evacuated") to the new table.


// A header for a Go map.
type hmap struct {
    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

GitHub source code - hmap