cmap

support feather

1. support O(1) length method, like the issue decsribed here https://github.com/golang/go/issues/20680

    新增特性，O(1)复杂度的长度方法
   

2. reduce contention between Map operations with muti hash lock (doing), like the issue decsribed here https://github.com/golang/go/issues/21035

    目前sync map 对不同的key操作，对应的都是同一个锁，可以通过多锁的方式，减少竞争。（实现中）
   

In Go 1.9, sync.Map was introduced, but sync.Map dont support length method ,

when you need get the length of your map

获取map长度示例对比

sync.map usage

import (
	"sync"
)

length := 0

myMap.Range(func(_, _ interface{}) bool {
	length++
	return true
})

it will lock your map, and take O(n) times

此操作会触发锁，复杂度O(n)

cmap usage

import (
	"github.com/mojinfu/cmap"
)

length := myCMap.Length()

cmap will not lock your cmap, and take O(1) times

此操作不会触发锁，复杂度O(1)

benchmark

100 times Store(i, i) and Delete(i) in env goos: darwin ; goarch: amd64

| package | ns/op| B/op|allocs/op| | :------:| :------: | :------: | :------: | | sync.Map| 21230 ns/op| 5600 B/op| 499 allocs/op| | cmap.Map |24243 ns/op| 5600 B/op |499 allocs/op|

• it means each Store or Delete action will take another 15ns
• cmap.Map中使用的原子计数器虽然线程安全，是通过底层硬件的支持作为保障的，这使得每次新增Key 删除Key，相对于sync.Map 都将有15ns的额外耗时

| map length | get length in sync.Map time-consuming |store in cmap.Map Extra time consuming | | :------:| :------: | :------: | | map长度 | 使用sync.Map通过range获取长度耗时 |使用CMap存删造成的理论额外耗时 | | 1 | 44.4 ns |15 ns | | 10 | 168 ns |150 ns | | 100 | 1527 ns |1500 ns | | 1000 | 22647 ns |15000 ns | | 10000 | 0.32 ms |0.15 ms | | 100000 | 8.01 ms |1.5 ms |

• it means when programme needs the length of the map(even for one time), use cmap package take the place of sync.map is better.

• 如果程序中需要获取Map长度，务必使用cmap来减少性能损耗。对长度的获取越频繁，使用cmap的必要性就越大。