GoF Interpreter

Interpreter.js

class Context {
    constructor (input) {
        this.input = input;
        this.output = 0;
    }
    startsWith (str) {
        return this.input.substr(0, str.length) === str;
    }
}

class Expression {
    constructor (name, one, four, five, nine, multiplier) {
        this.name = name;
        this.one = one;
        this.four = four;
        this.five = five;
        this.nine = nine;
        this.multiplier = multiplier;
    }
    interpret (context) {
        if (context.input.length == 0) {
            return;
        }
        else if (context.startsWith(this.nine)) {
            context.output += (9 * this.multiplier);
            context.input = context.input.substr(2);
        }
        else if (context.startsWith(this.four)) {
            context.output += (4 * this.multiplier);
            context.input = context.input.substr(2);
        }
        else if (context.startsWith(this.five)) {
            context.output += (5 * this.multiplier);
            context.input = context.input.substr(1);
        }
        while (context.startsWith(this.one)) {
            context.output += (1 * this.multiplier);
            context.input = context.input.substr(1);
        }
    }
}


const roman = "MCMXXVIII"
const context = new Context(roman);
const tree = [];

tree.push(new Expression("thousand", "M", " ", " ", " ", 1000));
tree.push(new Expression("hundred", "C", "CD", "D", "CM", 100));
tree.push(new Expression("ten", "X", "XL", "L", "XC", 10));
tree.push(new Expression("one", "I", "IV", "V", "IX", 1));

for (var i = 0, len = tree.length; i < len; i++) {
    tree[i].interpret(context);
}

console.log(roman + " = " + context.output);
// MCMXXVIII = 1928