SpiralMatrix.kt

/*
 * 1 (0,0)
 * 2 (0,1)
 * 3 (0,2)
 * 4 (1,2)
 * 5 (2,2)
 * 6 (2,1)
 * 7 (2,0)
 * 8 (1,0)
 * 9 (1,1)
 */
import java.lang.IllegalArgumentException

fun main() {
    spiralMatrix(4).print()
}

fun spiralMatrix(size: Int): Array<Array<Int>> {
    val defaultValue = -1
    fun fill(
        startY: Int,
        startX: Int,
        startValue: Int,
        endValue: Int,
        array2D: Array<Array<Int>>
    ): Array<Array<Int>> {
        var y = startY
        var x = startX
        var value = startValue
        if (value >= endValue) {
            // Last Value
            array2D[y][x] = value
            return array2D
        }
        while (x + 1 < array2D[y].size && array2D[y][x + 1] == defaultValue) array2D[y][x++] = value++  // Go Right
        while (y + 1 < array2D.size && array2D[y + 1][x] == defaultValue) array2D[y++][x] = value++  // Go Down
        while (x - 1 >= 0 && array2D[y][x - 1] == defaultValue) array2D[y][x--] = value++ // Go Left
        while (y - 1 >= 0 && array2D[y - 1][x] == defaultValue) array2D[y--][x] = value++ // Go Up
        return fill(y, x, value, endValue, array2D)
    }
    return size.takeIf { size > 0 }
        ?.run {
            fill(0, 0, 1, this * this, Array(this) { Array(this) { defaultValue } })
        } ?: throw IllegalArgumentException("Size must be bigger than zero")
}

private fun Array<Array<Int>>.print() {
    forEach { array ->
        array.forEach { value ->
            System.out.print(" ${if (value <= 9) "0$value" else value.toString()} ")
        }
        System.out.println()
    }
}