rpn.nt.d

module rpn;

macro import std.macro.assert;

import std.algorithm;
import std.stdio;
import std.string;

alias Token = (:add|:sub|:mul|:div|:mod|int);

void main(string[] args) {
  auto rpn = args.map(a => a.parse).array.rpn;
  rpn.case {
    string error: print("failed: $error");
    int i: print("> $i");
  }
}

(int | string) rpn(Token[] tokens) {
  mut int[] stack;
  int pop() {
    int ret = stack[$ - 1];
    stack = stack[0 .. $ - 1];
    return ret;
  }
  for (op in tokens) op.case {
    int i: stack ~= i;
    default:
      if (stack.length < 2) return "not enough tokens for $(op.repr)";
      auto b = pop;
      auto a = pop;
      op.case {
        :add: stack ~= a + b;
        :sub: stack ~= a - b;
        :mul: stack ~= a * b;
        :div: stack ~= a / b;
        :mod: stack ~= a % b;
        default: assert(false);
      }
      // print("$a $(op.repr) $b = $(stack[$ - 1])");
  }
  return pop;
}

Token parse(string arg) {
  if (arg == "+") return :add;
  if (arg == "-") return :sub;
  if (arg == "*") return :mul;
  if (arg == "/") return :div;
  if (arg == "%") return :mod;
  return arg.atoi;
}

string repr(Token token) {
  return token.case(
    :add: "+", :sub: "-", :mul: "*", :div: "/", :mod: "%",
    int i: "$i");
}