package main

import "fmt"

func main() {
    // 创建方式一：字面量
    scores := map[string]int{
        "Alice": 95,
        "Bob":   87,
        "Carol": 92,
    }

    // 创建方式二：make
    ages := make(map[string]int)
    ages["Alice"] = 30
    ages["Bob"] = 25

    // 创建方式三：make 指定容量
    data := make(map[string]int, 100)

    fmt.Println(scores)
    fmt.Println(ages)
    fmt.Println(data)

    // 读取
    fmt.Println(scores["Alice"]) // 95

    // 修改
    scores["Alice"] = 98

    // 添加
    scores["Dave"] = 88

    // 删除
    delete(scores, "Bob")

    // 长度
    fmt.Println(len(scores))

    // 遍历（顺序不确定）
    for name, score := range scores {
        fmt.Printf("%s: %d\n", name, score)
    }
}

func main() {
    m := map[string]int{"a": 1, "b": 2}

    // 逗号 ok 模式
    v, ok := m["a"]
    if ok {
        fmt.Println("找到:", v)
    }

    // 不存在的键返回零值
    fmt.Println(m["missing"]) // 0

    // 区分"值为零"和"键不存在"
    v2, ok2 := m["missing"]
    fmt.Println(v2, ok2) // 0, false

    // 常见用法
    if _, exists := m["c"]; !exists {
        fmt.Println("c 不存在")
    }
}

func main() {
    var m map[string]int // nil map

    // 读取 nil map（安全，返回零值）
    fmt.Println(m["key"]) // 0

    // ❌ 写入 nil map 会 panic
    // m["key"] = 1 // panic: assignment to entry in nil map

    // 必须先初始化
    m = make(map[string]int)
    m["key"] = 1 // OK
}

func main() {
    // 用结构体作为键
    type Point struct{ X, Y int }
    distances := map[Point]float64{
        {0, 0}: 0,
        {3, 4}: 5,
    }
    fmt.Println(distances[Point{3, 4}]) // 5

    // 用数组（不是切片）作为键
    pair := map[[2]int]string{
        {1, 2}: "one-two",
        {3, 4}: "three-four",
    }
    fmt.Println(pair[[2]int{1, 2}]) // one-two

    // ❌ 切片不能作为键
    // m := map[[]int]int{} // 编译错误
}

func main() {
    m := map[string]int{
        "a": 1, "b": 2, "c": 3, "d": 4, "e": 5,
    }

    // 遍历键值对
    for k, v := range m {
        fmt.Printf("%s: %d\n", k, v)
    }

    // 只遍历键
    for k := range m {
        fmt.Println(k)
    }

    // 只遍历值
    for _, v := range m {
        fmt.Println(v)
    }

    // 按顺序遍历（需要排序键）
    keys := make([]string, 0, len(m))
    for k := range m {
        keys = append(keys, k)
    }
    sort.Strings(keys)
    for _, k := range keys {
        fmt.Printf("%s: %d\n", k, m[k])
    }
}

package main

import (
    "fmt"
    "strings"
)

func main() {
    // 统计词频
    text := "the quick brown fox jumps over the lazy dog the fox"
    wordCount := make(map[string]int)
    for _, word := range strings.Fields(text) {
        wordCount[word]++
    }
    fmt.Println(wordCount)
    // map[the:3 quick:1 brown:1 fox:2 jumps:1 over:1 lazy:1 dog:1]

    // 分组
    words := []string{"apple", "banana", "avocado", "blueberry", "cherry", "apricot"}
    groups := make(map[byte][]string)
    for _, w := range words {
        key := w[0]
        groups[key] = append(groups[key], w)
    }
    fmt.Println(groups)
    // map[a:[apple avocado] b:[banana blueberry] c:[cherry apricot]]

    // 集合（用 map[T]bool 模拟）
    set := make(map[string]bool)
    set["a"] = true
    set["b"] = true
    set["a"] = true // 不会重复
    fmt.Println(len(set)) // 2
    fmt.Println(set["a"]) // true
    fmt.Println(set["c"]) // false

    // Map 作为缓存
    cache := make(map[string]string)
    getOrCompute := func(key string, compute func() string) string {
        if val, ok := cache[key]; ok {
            return val
        }
        val := compute()
        cache[key] = val
        return val
    }
    result := getOrCompute("db_url", func() string {
        fmt.Println("计算一次...")
        return "postgres://localhost:5432"
    })
    fmt.Println(result)
    result2 := getOrCompute("db_url", func() string {
        fmt.Println("不会执行")
        return ""
    })
    fmt.Println(result2)
}

// ❌ 危险：并发读写 map
func unsafe() {
    m := make(map[int]int)
    go func() {
        for i := 0; i < 1000; i++ {
            m[i] = i // 写
        }
    }()
    go func() {
        for i := 0; i < 1000; i++ {
            _ = m[i] // 读
        }
    }()
    // fatal error: concurrent map read and map write
}

package main

import (
    "fmt"
    "sync"
)

type SafeMap struct {
    mu sync.RWMutex
    m  map[string]int
}

func NewSafeMap() *SafeMap {
    return &SafeMap{m: make(map[string]int)}
}

func (s *SafeMap) Get(key string) (int, bool) {
    s.mu.RLock()
    defer s.mu.RUnlock()
    v, ok := s.m[key]
    return v, ok
}

func (s *SafeMap) Set(key string, value int) {
    s.mu.Lock()
    defer s.mu.Unlock()
    s.m[key] = value
}

func (s *SafeMap) Delete(key string) {
    s.mu.Lock()
    defer s.mu.Unlock()
    delete(s.m, key)
}

func (s *SafeMap) Len() int {
    s.mu.RLock()
    defer s.mu.RUnlock()
    return len(s.m)
}

func main() {
    sm := NewSafeMap()
    var wg sync.WaitGroup

    for i := 0; i < 100; i++ {
        wg.Add(2)
        go func(i int) {
            defer wg.Done()
            sm.Set(fmt.Sprintf("key-%d", i), i)
        }(i)
        go func(i int) {
            defer wg.Done()
            sm.Get(fmt.Sprintf("key-%d", i))
        }(i)
    }
    wg.Wait()
    fmt.Println("安全 map 长度:", sm.Len())
}

package main

import (
    "fmt"
    "sync"
)

func main() {
    var m sync.Map

    // 存储
    m.Store("name", "Alice")
    m.Store("age", 30)

    // 读取
    v, ok := m.Load("name")
    if ok {
        fmt.Println("name:", v) // Alice
    }

    // LoadOrStore：如果存在返回已有值，否则存储并返回新值
    actual, loaded := m.LoadOrStore("name", "Bob")
    fmt.Println(actual, loaded) // Alice, true（已有值）

    actual2, loaded2 := m.LoadOrStore("email", "[email protected]")
    fmt.Println(actual2, loaded2) // [email protected], false（新值）

    // 删除
    m.Delete("age")

    // 遍历
    m.Range(func(key, value any) bool {
        fmt.Printf("%v: %v\n", key, value)
        return true // 返回 false 停止遍历
    })

    // LoadAndDelete：原子性读取并删除
    v2, loaded3 := m.LoadAndDelete("name")
    fmt.Println(v2, loaded3) // Alice, true

    // LoadOrStore 的原子性保证
    var wg sync.WaitGroup
    var m2 sync.Map
    for i := 0; i < 100; i++ {
        wg.Add(1)
        go func(i int) {
            defer wg.Done()
            m2.LoadOrStore("counter", 0)
        }(i)
    }
    wg.Wait()
}

// 观察 map 的内存占用
import (
    "fmt"
    "runtime"
)

func main() {
    var m1 runtime.MemStats
    
    runtime.GC()
    runtime.ReadMemStats(&m1)
    
    data := make(map[int]int, 1_000_000)
    for i := 0; i < 1_000_000; i++ {
        data[i] = i
    }
    
    runtime.GC()
    var m2 runtime.MemStats
    runtime.ReadMemStats(&m2)
    
    fmt.Printf("1M 键值对占用: %.2f MB\n", float64(m2.Alloc-m1.Alloc)/1024/1024)
}

import "testing"

// 预分配容量
func BenchmarkMapPrealloc(b *testing.B) {
    for i := 0; i < b.N; i++ {
        m := make(map[int]int, 10000)
        for j := 0; j < 10000; j++ {
            m[j] = j
        }
    }
}

func BenchmarkMapNoPrealloc(b *testing.B) {
    for i := 0; i < b.N; i++ {
        m := make(map[int]int)
        for j := 0; j < 10000; j++ {
            m[j] = j
        }
    }
}