Kotlin functions, lambdas and collections

// A normal function
fun withVat(amount: Double, rate: Double = 0.16): Double {
    return amount * (1 + rate)
}

// One-expression functions can use =
fun square(n: Int) = n * n

// Named arguments make calls readable
fun greet(name: String, title: String = "", shout: Boolean = false): String {
    val text = if (title.isEmpty()) "Habari $name" else "Habari $title $name"
    return if (shout) text.uppercase() else text
}

fun main() {
    println(withVat(1000.0))              // uses the default rate
    println(withVat(1000.0, rate = 0.0))  // named argument
    println(square(12))
    println(greet("Otieno", title = "Dr.", shout = true))
}

fun main() {
    val double = { x: Int -> x * 2 }
    println(double(21))

    // A function that takes another function
    fun applyDiscount(price: Int, rule: (Int) -> Int): Int = rule(price)

    println(applyDiscount(1000) { it - 100 })          // `it` is the single parameter
    println(applyDiscount(1000) { (it * 0.9).toInt() })
}

fun main() {
    val towns = listOf("Nyeri", "Meru", "Embu")          // read-only list
    val cart = mutableListOf("Unga", "Sugar")             // can change
    cart.add("Milk")
    cart.remove("Sugar")
    println("$towns | $cart | first: ${cart.first()} | size: ${cart.size}")

    val unique = setOf("Wanjiku", "Otieno", "Wanjiku")     // no duplicates
    println(unique)

    val prices = mapOf("Unga" to 180, "Sugar" to 150, "Milk" to 60)
    println("Milk costs ${prices["Milk"]}")
    println("Bread costs ${prices["Bread"] ?: "not sold here"}")
    for ((item, price) in prices) println("$item: KSh $price")

    val stock = mutableMapOf("Unga" to 12)
    stock["Rice"] = 20
    stock["Unga"] = stock.getValue("Unga") - 2
    println(stock)
}

data class Product(val name: String, val price: Int, val stock: Int, val category: String)

fun main() {
    val products = listOf(
        Product("Unga 2kg", 180, 12, "Food"),
        Product("Sugar 1kg", 150, 0, "Food"),
        Product("Soap", 120, 30, "Home"),
        Product("Cooking oil 1L", 350, 4, "Food"),
        Product("Matches", 10, 100, "Home"),
    )

    val inStock = products.filter { it.stock > 0 }
    val names = inStock.map { it.name }
    val stockValue = products.sumOf { it.price * it.stock }
    val cheapest = products.minByOrNull { it.price }
    val byCategory = products.groupBy { it.category }
    val sorted = products.sortedByDescending { it.price }
    val anyOut = products.any { it.stock == 0 }

    println("In stock: $names")
    println("Stock value: KSh $stockValue")
    println("Cheapest: ${cheapest?.name}")
    println("Categories: ${byCategory.mapValues { (_, list) -> list.size }}")
    println("Most expensive: ${sorted.first().name}")
    println("Anything sold out? $anyOut")
}

fun Int.toKsh(): String = "KSh " + "%,d".format(this)

fun String.isKenyanPhone(): Boolean {
    val digits = filter { it.isDigit() }
    return Regex("^(0[17]\\d{8}|254[17]\\d{8})$").matches(digits)
}

fun main() {
    println(125000.toKsh())
    println("0712 345 678".isKenyanPhone())
    println("12345".isKenyanPhone())
}