stack-based rpn accumulator

rpn.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var operators = map[string]func(int, int) int{
	"+": func(x, y int) int { return x + y },
	"-": func(x, y int) int { return x - y },
	"*": func(x, y int) int { return x * y },
	"/": func(x, y int) int { return x / y },
	"%": func(x, y int) int { return x % y },
}

var (
	stack            = make([]int, 0, 8)
	errStackEmpty    = errors.New("stack is empty")
	errStackNotClear = errors.New("stack not clear")
)

func getLine() string {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Scan()
	return scanner.Text()
}

func pop() (int, error) {
	if len(stack) < 1 {
		return 0, errStackEmpty
	}
	i := stack[len(stack)-1]
	stack = stack[:len(stack)-1]
	return i, nil
}

func push(i int) {
	stack = append(stack, i)
}

func apply(op func(int, int) int) error {
	right, err := pop()
	left, err := pop()
	if err != nil {
		return err
	}
	push(op(left, right))
	return nil
}

func clear() {
	stack = stack[0:0]
}

func runLine(line string) (int, error) {
	defer clear()
	tokens := strings.Split(line, " ")
	for _, s := range tokens {
		if op, ok := operators[s]; ok {
			if err := apply(op); err != nil {
				return 0, err
			}
			continue
		}
		if i, err := strconv.ParseInt(s, 10, 64); err != nil {
			return 0, err
		} else {
			push(int(i))
		}
	}
	if len(stack) != 1 {
		return 0, errStackNotClear
	}
	return pop()
}

func main() {
	for {
		line := getLine()
		i, err := runLine(line)
		if err != nil {
			fmt.Fprintln(os.Stderr, "error: "+err.Error())
			continue
		}
		fmt.Println(i)
	}
}