// WorkWith is the struct we'll
// be implementing collections for
type WorkWith struct {
	Data    string
	Version int
}

// 基于判斷函數過濾集合，返回符合條件的集合元素
func Filter(ws []WorkWith, f func(w WorkWith) bool) []WorkWith {
	// depending on results, smaller size for result
	// is len == 0
	result := make([]WorkWith, 0)
	for _, w := range ws {
		if f(w) {
			result = append(result, w)
		}
	}
	return result
}
// 基于轉換函數轉換集合元素，返回集合的元素為轉換后的元素
func Map(ws []WorkWith, f func(w WorkWith) WorkWith) []WorkWith {
	// the result should always be the same
	// length
	result := make([]WorkWith, len(ws))
	for pos, w := range ws {
		newW := f(w)
		result[pos] = newW
	}
	return result
}

import "strings"
// LowerCaseData does a ToLower to the
// Data string of a WorkWith
func LowerCaseData(w WorkWith) WorkWith {
	w.Data = strings.ToLower(w.Data)
	return w
}
// IncrementVersion increments a WorkWiths
// Version
func IncrementVersion(w WorkWith) WorkWith {
	w.Version++
	return w
}
// OldVersion returns a closures
// that validates the version is greater than
// the specified amount
func OldVersion(v int) func(w WorkWith) bool {
	return func(w WorkWith) bool {
		return w.Version >= v
	}
}

import (
	"fmt"
	"testing"
)
func TestMap(t *testing.T) {
	ws := []WorkWith{
		{"Example", 1},
		{"Example 2", 2},
	}
	fmt.Printf("Initial list: %#v\n", ws)
	// first lower case the list
	ws = Map(ws, LowerCaseData)
	fmt.Printf("After LowerCaseData Map: %#v\n", ws)
	// next increment all versions
	ws = Map(ws, IncrementVersion)
	fmt.Printf("After IncrementVersion Map: %#v\n", ws)
	// lastly remove all versions older than 3
	ws = Filter(ws, OldVersion(3))
	fmt.Printf("After OldVersion Filter: %#v\n", ws)
}

	ws := []WorkWith{
		{"Example", 1},
		{"Example 2", 2},
	}
	fmt.Printf("Initial list: %#v\n", ws)
	result := Filter(Map(Map(ws, LowerCaseData), IncrementVersion), OldVersion(3))
	fmt.Printf("After OldVersion Filter: %#v\n", result)

func map2[T, U any](data []T, f func(T) U) []U {
    res := make([]U, 0, len(data))
    for _, e := range data {
        res = append(res, f(e))
    }
    return res
}

    // 字符串轉大寫
    words := []string{"war", "cup", "water", "tree", "storm"}
    result := map2(words, func(s string) string {
        return strings.ToUpper(s)
    })
    fmt.Println(result)
    // 生成原集合元素的平方集合
    fmt.Println("-------------------")
    numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
    squares := map2(numbers, func(n int) int {
        return n * n
    })
    fmt.Println(squares)
    // 數值轉為字符串
    fmt.Println("-------------------")
    as_strings := map2(numbers, func(n int) string {
        return strconv.Itoa(n)
    })
    fmt.Printf("%q", as_strings)