Kotlin Functions (Extension Function, Infix Function, Inline Function, Interface Function, Scope Function, Tail Recursion)

Extension Function.kt

fun main() {
    println("asdf".isDigits)
    println("asdf1234".isDigits)
    println("123456".isDigits)

    val content: String? = null
    println(content.orEmpty())

    String.print("hello")
}

// extension value
val String.isDigits: Boolean get() = this.matches(Regex("^[0-9]*$"))

// extension function
fun <T> T?.orDefault(default: T): T {
    return this ?: default
}

// extension companion object
fun String.Companion.print(value: String) {
    println("$$ $value")
}

Infix Function.kt

fun main() {
//    val decimal1 = Decimal(5, 7)
//    val decimal2 = Decimal(6, 7)
//    println(decimal1 + decimal2)

    println("hello " * 5)
}

operator fun String.times(i: Int): String {
    return buildString {
        repeat(i) {
            append(this@times)
        }
    }
}


class Decimal(private val fraction: Int, private val denomination: Int) {

    infix operator fun plus(other: Decimal): Decimal {
        return if (this.denomination == other.denomination) {
            Decimal(this.fraction + other.fraction, this.denomination)
        } else {
            val fraction = (other.denomination * this.fraction) + (this.denomination * other.fraction)
            val denomination = this.denomination * other.denomination
            Decimal(fraction, denomination)
        }
    }

    override fun toString(): String {
        return "$fraction/$denomination => ${fraction.toDouble() / denomination}"
    }
}

Inline Function.kt

fun main() {
    doSomething {
        println("doSomething")
    }

    /* println("before")
     println("doSomething")
     println("after")*/
}

inline fun doSomething(block: () -> Unit) {
    println("before")
    block()
    println("after")
}

Interface Function.kt

fun interface SetOnClickListener {
    fun click()
}

fun main() {
//    val clicked = object : SetOnClickListener {
//        override fun click() {
//            println("click")
//        }
//    }
    val clicked = SetOnClickListener { println("click") }
    clicked.click()
}

Scope Function.kt

fun main() {
    // apply
    val car = Car().apply {
        color = "black"
    }

    // also
    val car2 = Car().also { car ->
        car.color
    }
    var a = 5
    var b = 6
    a = b.also { b = a }

    // let
    val text: String? = ""
    val content = text?.let { "i am not null" }.orDefault("i am null")
//    println(content)

    // with
    with(car) {
        println(color)
    }

    // run
    car.run {
        drive()
    }
}

class Car {
    var color = "red"
    var name = ""

    fun drive() = println("drive")
}

Tail Recursion.kt

import java.math.BigInteger

tailrec fun fibonacci(n: Int, a: BigInteger = BigInteger.ZERO, b: BigInteger = BigInteger.ONE): BigInteger {
    return if (n == 0) {
        a
    } else {
        fibonacci(n - 1, a + b, a)
    }
}

fun main() {
    println(fibonacci(10000))
}