ShrinkMap

shrinkmap.go

package shrinkmap

import "reflect"

// ShrinkMap shrinks the capacity of the supplied map. It panics if m is not a
// map. ShrinkMap may temporarily require double the amount of memory used by m.
// It is recommended not to hold iterators to the map while it is being shrunk.
// So much fail because of no generics. T_T
func ShrinkMap(m interface{}) interface{} {
	M := reflect.ValueOf(m)
	if M.Kind() != reflect.Map {
		panic("not a map")
	}
	N := reflect.MakeMapWithSize(M.Type(), M.Len())
	Mi := M.MapRange()
	for Mi.Next() {
		K := Mi.Key()
		V := Mi.Value()
		N.SetMapIndex(K, V)
	}
	return N.Interface()
}

shrinkmap_test.go

package shrinkmap

import "fmt"
import "testing"

func TestShrinkMap(_ *testing.T) {
	m := make(map[int]struct{})
	for i := 0; i < 1000; i++ {
		m[i] = struct{}{}
	}
	for i := 0; i < 900; i++ {
		delete(m, i)
	}
	
	// the following line is the same as:
	// {
	// 	_m := make(map[int]struct{}, len(m))
	// 	for k, v := range m {
	//		_m[k] = v
	// 	}
	//	m = _m
	// }
	m = ShrinkMap(m).(map[int]struct{})
	
	fmt.Println(len(m))
}