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// 패키지의 선언은 소스파일의 제일 상단에 위치시켜야 합니다.
// 디렉토리와 패키지를 일치시킬 필요는 없습니다. 소스파일을 임의 디렉토리에 배치해도 무방합니다. 
package my.demo

import java.util.*

// ...

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// 정수 파라미터 2개를 받아 정수값을 리턴하는 함수
fun sum(a: Int, b: Int): Int {
    return a + b
}

fun main(args: Array<String>) {
    print("sum of 3 and 5 is ")
    println(sum(3, 5))
} 
--------------------------------------------------------
sum of 3 and 5 is 8

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// 추론타입을 리턴하는 함수를 표현식과 함께사용
fun sum(a: Int, b: Int) = a + b

fun main(args: Array<String>) {
    println("sum of 19 and 23 is ${sum(19, 23)}")
}
--------------------------------------------------------
sum of 19 and 23 is 42

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// Unit타입(의미있는 값이 리턴하지 않음?? / kotlin.Unit)을 리턴하는 함수
fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}

fun main(args: Array<String>) {
    printSum(-1, 8)
}
--------------------------------------------------------
sum of -1 and 8 is 7

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// Unit타입 선언을 생략
fun printSum(a: Int, b: Int) {
    println("sum of $a and $b is ${a + b}")
}

fun main(args: Array<String>) {
    printSum(-1, 8)
}
--------------------------------------------------------
sum of -1 and 8 is 7

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// 한번만 할당가능한(읽기전용) 지역변수 정의
fun main(args: Array<String>) {
    val a: Int = 1  // immediate assignment
    val b = 2   // `Int` type is inferred
    val c: Int  // Type required when no initializer is provided
    c = 3       // deferred assignment
    println("a = $a, b = $b, c = $c")
}
--------------------------------------------------------	
a = 1, b = 2, c = 3	

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// 여러번 할당가능한 변수 정의
fun main(args: Array<String>) {
    var x = 5 // `Int` type is inferred
    x += 1
    println("x = $x")
}
--------------------------------------------------------
x = 6

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// 한줄 주석
// This is an end-of-line comment

// 블럭 주석(Java와 달리 Kotlin의 블록 주석은 중첩 될 수 있습니다.)
/* This is a block comment
   on multiple lines. */

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fun main(args: Array<String>) {
    var a = 1
    // simple name in template:
    val s1 = "a is $a" 

    a = 2
    // arbitrary expression in template:
    val s2 = "${s1.replace("is", "was")}, but now is $a"
    println(s2)
}
--------------------------------------------------------
a was 1, but now is 2

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//
fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}

fun main(args: Array<String>) {
    println("max of 0 and 42 is ${maxOf(0, 42)}")
}
--------------------------------------------------------
max of 0 and 42 is 42

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// A reference must be explicitly marked as nullable when null value is possible.
// Return null if str does not hold an integer:
fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

    // Using `x * y` yields error because they may hold nulls.
    if (x != null && y != null) {
        // x and y are automatically cast to non-nullable after null check
        println(x * y)
    }
    else {
        println("either '$arg1' or '$arg2' is not a number")
    }    
}


fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("a", "b")
}
--------------------------------------------------------
42
either 'a' or '7' is not a number
either 'a' or 'b' is not a number

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fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)
    
    // ...
    if (x == null) {
        println("Wrong number format in arg1: '${arg1}'")
        return
    }
    if (y == null) {
        println("Wrong number format in arg2: '${arg2}'")
        return
    }

    // x and y are automatically cast to non-nullable after null check
    println(x * y)
}

fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("99", "b")
}
--------------------------------------------------------
42
Wrong number format in arg1: 'a'
Wrong number format in arg2: 'b'

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// The is operator checks if an expression is an instance of a type. If an immutable local variable or property is checked for a specific type, there's no need to cast it explicitly
fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length
    }

    // `obj` is still of type `Any` outside of the type-checked branch
    return null
}


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}
--------------------------------------------------------
'Incomprehensibilities' string length is 21 
'1000' string length is ... err, not a string 
'[java.lang.Object@77459877]' string length is ... err, not a string 

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fun getStringLength(obj: Any): Int? {
    if (obj !is String) return null

    // `obj` is automatically cast to `String` in this branch
    return obj.length
}


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    }
    printLength("Incomprehensibilities")
    printLength(1000)
    printLength(listOf(Any()))
}
--------------------------------------------------------
'Incomprehensibilities' string length is 21 
'1000' string length is ... err, not a string 
'[java.lang.Object@77459877]' string length is ... err, not a string 

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fun getStringLength(obj: Any): Int? {
    // `obj` is automatically cast to `String` on the right-hand side of `&&`
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}


fun main(args: Array<String>) {
    fun printLength(obj: Any) {
        println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
    }
    printLength("Incomprehensibilities")
    printLength("")
    printLength(1000)
}
--------------------------------------------------------
'Incomprehensibilities' string length is 21 
'' string length is ... err, is empty or not a string at all 
'1000' string length is ... err, is empty or not a string at all 

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fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    for (item in items) {
        println(item)
    }
}
--------------------------------------------------------
apple
banana
kiwi

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fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    for (index in items.indices) {
        println("item at $index is ${items[index]}")
    }
}
--------------------------------------------------------
item at 0 is apple
item at 1 is banana
item at 2 is kiwi

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fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    var index = 0
    while (index < items.size) {
        println("item at $index is ${items[index]}")
        index++
    }
}
--------------------------------------------------------
item at 0 is apple
item at 1 is banana
item at 2 is kiwi

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fun describe(obj: Any): String =
when (obj) {
    1          -> "One"
    "Hello"    -> "Greeting"
    is Long    -> "Long"
    !is String -> "Not a string"
    else       -> "Unknown"
}

fun main(args: Array<String>) {
    println(describe(1))
    println(describe("Hello"))
    println(describe(1000L))
    println(describe(2))
    println(describe("other"))
}
--------------------------------------------------------
One
Greeting
Long
Not a string
Unknown

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// in 연산자를 사용하여 숫자가 범위 내에 있는지 확인
fun main(args: Array<String>) {
    val x = 10
    val y = 9
    if (x in 1..y+1) {
        println("fits in range")
    }
}
--------------------------------------------------------
fits in range

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// in 연산자를 사용하여 숫자가 범위 내에 있는지 확인
fun main(args: Array<String>) {
    val list = listOf("a", "b", "c")

    if (-1 !in 0..list.lastIndex) {
        println("-1 is out of range")
    }
    if (list.size !in list.indices) {
        println("list size is out of valid list indices range too")
    }
}
--------------------------------------------------------
-1 is out of range
list size is out of valid list indices range too

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// IntRange 반복
fun main(args: Array<String>) {
    for (x in 1..5) {
        print(x)
    }
}
--------------------------------------------------------
12345

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// or over a progression
fun main(args: Array<String>) {
    for (x in 1..10 step 2) {
        print(x)
    }
    for (x in 9 downTo 0 step 3) {
        print(x)
    }
}
--------------------------------------------------------
135799630

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// 컬렉션 반복
fun main(args: Array<String>) {
    val items = listOf("apple", "banana", "kiwi")
    for (item in items) {
        println(item)
    }
}
--------------------------------------------------------
apple
banana
kiwi

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// in 연산자를 사용하여 컬렉션에 개체가 포함되어 있는지 확인
fun main(args: Array<String>) {
    val items = setOf("apple", "banana", "kiwi")
    when {
        "orange" in items -> println("juicy")
        "apple" in items -> println("apple is fine too")
    }
}
--------------------------------------------------------
apple is fine too

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// 람다 식을 사용하여 콜렉션 필터링 및 매핑
fun main(args: Array<String>) {
    val fruits = listOf("banana", "avocado", "apple", "kiwi")
    fruits
    .filter { it.startsWith("a") }
    .sortedBy { it }
    .map { it.toUpperCase() }
    .forEach { println(it) }
}
--------------------------------------------------------
APPLE
AVOCADO