/**
 * 第 3 章 · 基本语法 — 综合示例
 *
 * 涵盖：
 *   - val / var / const val
 *   - 数字类型与显式转换
 *   - 字符串模板（含三引号字符串）
 *   - if 表达式
 *   - when 全场景（值匹配、范围、类型、无参）
 *   - for / while / 范围
 *   - is / as 类型检查
 *
 * 运行：kotlinc BasicSyntax.kt -include-runtime -d bs.jar && java -jar bs.jar
 */

const val APP_NAME = "Kotlin Demo"      // 编译期常量
const val MAX_SCORE = 100

fun main() {
    println("=== 1. 变量 ===")
    val name = "Alice"                  // 类型推断为 String
    var age = 25                        // var 可变
    age += 1
    val score: Double = 88.5            // 显式类型
    println("$name, $age 岁，分数 $score")

    println("\n=== 2. 数字类型 ===")
    val big: Long = 1_000_000_000L
    val pi: Double = 3.14159
    val hex: Int = 0xFF
    println("big=$big, pi=$pi, hex=$hex (十进制 $hex)")

    val i = 100
    val l: Long = i.toLong()            // 必须显式转换
    println("Int $i 转 Long = $l")

    println("\n=== 3. 字符串模板 ===")
    println("Hello, $name!")
    println("$name 长度 = ${name.length}")
    println("一年后 ${age + 1} 岁")

    val sql = """
        SELECT id, name, score
        FROM students
        WHERE score >= $MAX_SCORE * 0.6
    """.trimIndent()
    println("---- 多行字符串 ----")
    println(sql)

    println("\n=== 4. if 表达式 ===")
    val a = 10; val b = 20
    val max = if (a > b) a else b
    println("max($a, $b) = $max")

    println("\n=== 5. when 全场景 ===")
    listOf(0, 5, 50, 999, -10).forEach { x ->
        val cat = when (x) {
            0 -> "零"
            in 1..9 -> "个位数"
            in 10..99 -> "两位数"
            in 100..999 -> "三位数"
            !in 0..999 -> "超出范围"
            else -> "其他"
        }
        println("$x -> $cat")
    }

    // when 当类型派发
    listOf<Any>(1, "hello", 3.14, listOf(1,2,3)).forEach {
        println("${it.javaClass.simpleName}: ${describe(it)}")
    }

    // when 无参数模式
    val score2 = 78
    val grade = when {
        score2 !in 0..100 -> "无效"
        score2 >= 90 -> "A"
        score2 >= 80 -> "B"
        score2 >= 70 -> "C"
        score2 >= 60 -> "D"
        else -> "F"
    }
    println("分数 $score2 -> 等级 $grade")

    println("\n=== 6. for 循环 ===")
    println("升序 1..5:")
    for (i in 1..5) print("$i ")
    println()

    println("半开区间 1 until 5:")
    for (i in 1 until 5) print("$i ")
    println()

    println("倒序 5 downTo 1:")
    for (i in 5 downTo 1) print("$i ")
    println()

    println("步长 1..10 step 2:")
    for (i in 1..10 step 2) print("$i ")
    println()

    println("带索引：")
    val fruits = listOf("apple", "banana", "cherry")
    for ((idx, fruit) in fruits.withIndex()) {
        println("  $idx -> $fruit")
    }

    println("\n=== 7. is / as 类型检查 ===")
    val obj: Any = "hello world"
    if (obj is String) {
        // smart cast: 自动把 obj 当 String 用
        println("是 String，长度 ${obj.length}")
    }

    val any: Any = 123
    val maybeStr: String? = any as? String   // 安全转换
    println("any as? String = $maybeStr")    // null

    println("\n=== 8. try 表达式 ===")
    val parsed: Int = try { "42a".toInt() } catch (e: NumberFormatException) { -1 }
    println("解析失败时返回 = $parsed")
}

fun describe(obj: Any): String = when (obj) {
    is Int -> "整数 $obj，平方是 ${obj * obj}"
    is String -> "字符串 \"$obj\"，长度 ${obj.length}"
    is Double -> "浮点数 $obj，向下取整 ${obj.toInt()}"
    is List<*> -> "列表，含 ${obj.size} 个元素：$obj"
    else -> "未知类型 ${obj::class.simpleName}"
}