/**
 * 第 4 章 · 函数与 Lambda — 综合示例
 *
 * 涵盖：
 *   - 单表达式函数
 *   - 默认参数 / 命名参数
 *   - vararg / spread (*)
 *   - infix 中缀函数
 *   - 扩展函数
 *   - 高阶函数 + Lambda
 *   - 闭包
 *   - inline 函数
 */

// ========== 1. 默认参数 + 命名参数 ==========
fun connect(
    host: String = "localhost",
    port: Int = 8080,
    timeout: Int = 5000,
    useTls: Boolean = false
) {
    println("Connecting → $host:$port (timeout=${timeout}ms, tls=$useTls)")
}

// ========== 2. 单表达式函数 ==========
fun max(a: Int, b: Int) = if (a > b) a else b
fun isAdult(age: Int) = age >= 18

// ========== 3. vararg ==========
fun sum(vararg numbers: Int) = numbers.sum()

// ========== 4. infix 中缀函数 ==========
infix fun Int.repeatStr(s: String) = s.repeat(this)

// ========== 5. 扩展函数 ==========
fun String.toCamelCase(): String =
    this.split("_", "-", " ").joinToString("") { word ->
        word.replaceFirstChar { it.uppercase() }
    }

fun List<Int>.average2(): Double = if (isEmpty()) 0.0 else sum().toDouble() / size

// ========== 6. 高阶函数 ==========
fun applyOperation(x: Int, y: Int, op: (Int, Int) -> Int): Int = op(x, y)

inline fun <T> retry(times: Int = 3, block: (attempt: Int) -> T): T {
    var lastError: Throwable? = null
    repeat(times) { attempt ->
        try {
            return block(attempt + 1)
        } catch (e: Exception) {
            lastError = e
            println("  ⚠️ 第 ${attempt + 1} 次失败: ${e.message}")
        }
    }
    throw lastError ?: IllegalStateException("retry exhausted")
}

// ========== 7. 闭包 ==========
fun makeCounter(): () -> Int {
    var count = 0
    return {
        count++
        count
    }
}

// ========== main ==========
fun main() {
    println("=== 1. 默认参数 + 命名参数 ===")
    connect()
    connect("example.com")
    connect(port = 9090)                              // 命名参数跳过 host
    connect(host = "example.com", useTls = true)      // 顺序无所谓

    println("\n=== 2. 单表达式函数 ===")
    println("max(3, 7) = ${max(3, 7)}")
    println("isAdult(20) = ${isAdult(20)}")

    println("\n=== 3. vararg ===")
    println("sum(1,2,3) = ${sum(1, 2, 3)}")
    val nums = intArrayOf(10, 20, 30)
    println("sum(*nums) = ${sum(*nums)}")             // spread 展开

    println("\n=== 4. infix 中缀 ===")
    println(3 repeatStr "Hi ")                        // "Hi Hi Hi "
    println("a" to 1)                                  // Pair("a", 1)，to 也是 infix

    println("\n=== 5. 扩展函数 ===")
    println("hello_kotlin_world".toCamelCase())       // "HelloKotlinWorld"
    println("my-app-name".toCamelCase())              // "MyAppName"
    println(listOf(1, 2, 3, 4, 5).average2())          // 3.0

    println("\n=== 6. 高阶函数 + Lambda ===")
    println(applyOperation(5, 3) { a, b -> a + b })   // 8 (加法 lambda)
    println(applyOperation(5, 3) { a, b -> a * b })   // 15 (乘法 lambda)
    val sub: (Int, Int) -> Int = { a, b -> a - b }
    println(applyOperation(10, 4, sub))               // 6

    println("\n=== 7. retry 高阶函数 ===")
    val result = retry(times = 5) { attempt ->
        if (attempt < 3) throw RuntimeException("假装失败")
        "✅ 成功！第 $attempt 次"
    }
    println(result)

    println("\n=== 8. 闭包：counter 记住 count 变量 ===")
    val counter = makeCounter()
    println(counter())                                // 1
    println(counter())                                // 2
    println(counter())                                // 3

    println("\n=== 9. 集合的高阶函数 ===")
    val list = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    println("偶数: ${list.filter { it % 2 == 0 }}")
    println("平方: ${list.map { it * it }}")
    println("总和: ${list.sum()}")
    println("第一个 > 5: ${list.first { it > 5 }}")
    println("分组奇偶: ${list.groupBy { if (it % 2 == 0) "偶" else "奇" }}")
}