Reverse Polish Notation

rpn.kt

import java.util.Stack
import java.lang.Long.parseLong

fun String.isNumeric(): Boolean {
    return try {
        parseLong(this)
        true
    } catch (e: NumberFormatException) {
        false
    }
}

fun eval(t: String, op1: Long, op2: Long): Long {
    return when (t) {
        "+" -> op1 + op2
        "-" -> op1 - op2
        "*" -> op1 * op2
        "/" -> op1 / op2
        "|" -> op1 or op2
        "&" -> op1 and op2
        "^" -> op1 xor op2
        "<<" -> op1 shl op2.toInt()
        ">>" -> op1 shr op2.toInt()
        else -> throw UnsupportedOperationException(
            "Invalid expression: $op1 $t $op2"
        )
    }
}

fun calc(stack: Stack<Long>, expr: String): Long {
    expr.split("\\s+".toRegex()).forEach {
        if (it.isNumeric()) {
            stack.push(parseLong(it))
        } else {
            if (stack.size < 2) {
                throw UnsupportedOperationException(
                    "Invalid expression: not enough operands"
                )
            }

            val op2 = stack.pop()
            val op1 = stack.pop()

            stack.push(eval(it, op1, op2))
        }
    }

    return stack.pop()
}

fun main() {
    val exprs = arrayOf(
        "7 2 3 * -",
        "1 2 + 4 * 3 +",
        "3 4 + 5 2 - *",
        "7 3 ^ 15 72 - +",
        "12 12 25 | &",
        "1 212 0 << <<",
        "-68 4 212 1 >> + -"
    )

    val stack = Stack<Long>()
    for (expr in exprs) {
        println("$expr = ${calc(stack, expr)}")
    }
}

Output:

7 2 3 * - = 1
1 2 + 4 * 3 + = 15
3 4 + 5 2 - * = 21
7 3 ^ 15 72 - + = -53
12 12 25 | & = 12
1 212 0 << << = 1048576
-68 4 212 1 >> + - = -178