function startswith(a, b) {
    for(let i=0; i<b.length; i++) {
        // in JS, ['X'] == 'X'
        if((a[i] != b[i]) | (typeof a[i] != typeof b[i])) {
            return false;
        }
    }
    return true;
}


function generate(start, rules, iterations) {
    if(iterations == 0) {
        return start;
    }

    for(let rule of rules) {
        let pos = 0;
        while(pos < start.length) {
            let w = start.slice(pos, pos + rule[0].length);
            if(startswith(w, rule[0])) {
                start.splice(pos, rule[0].length, rule[1])
                pos += rule[0].length;
            } else {
                pos += 1;
            }
        }
    }
    
    // TODO find a better way
    let ret = [];
    for(let x of start) {
        if(typeof x === 'object') {
            // TODO why?
            //ret.concat(x);
            ret = ret.concat(x);
        } else {
            ret.push(x);
        }
    }
    return generate(ret, rules, iterations - 1);
}


function draw_initstate(state) {
    if(!('dir' in state)) {
        state.dir = 0;//Math.PI/2;
    }
}


function draw_forward() {
    return function(p, state) {
        draw_initstate(state);
        if(p.length == 0) {
            p.add(new paper.Point(0, 0));
        }
        var lastp = p.segments[p.segments.length - 1].point;
        p.add(lastp.add(
            new paper.Point(Math.sin(state.dir), Math.cos(state.dir))
        ));
    }
}


function draw_turn(angle) {
    return function(p, state) {
        draw_initstate(state);
        state.dir += angle;
    }
}


function draw_angle_turn(fac) {
    return function(p, state) {
        draw_initstate(state);
        state.dir += state.angle * fac;
    }
}


function draw_angle_init(angle) {
    return function(p, state) {
        state.angle = angle;
    }
}


function draw_angle_add(delta) {
    return function(p, state) {
        state.angle += delta;
    }
}


function draw(word, actions) {
    var p = new paper.Path();
    var state = {};
    for(let w of word) {
        if(w in actions) {
            actions[w](p, state);
        }
    }
    return p;
}