#!/usr/bin/env node

const MIN_DROP_LENGTH = 12;
const MAX_DROP_LENGTH = 25;

const CHAR_MIN_RANGE = 0x21;
const CHAR_MAX_RANGE = 0x7E;

const DROP_MIN_SPEED = 80;
const DROP_MAX_SPEED = 20;
const DROP_SPAWN_INTERVAL = 40;

const colors = [46, 40, 34, 28, 22, 16].map(index => `\x1b[38;5;${index}m`);
const terminal = {
    clear: () => write('\033[2J'),
    hide: () => write('\x1b[?25l'),
    moveTo: (x, y) => write(`\x1b[${y + 1};${x + 1}H`)
}

function write(data) {
    process.stdout.write(data);
}

function random(min, max) {
    return min + Math.floor((max - min) * Math.random());
}

class Drop {
    constructor() {
        this.length = random(Math.min(MIN_DROP_LENGTH, process.stdout.rows), Math.min(MAX_DROP_LENGTH, process.stdout.rows));
        this.speed = random(DROP_MAX_SPEED, DROP_MIN_SPEED);
        this.chars = new Array(process.stdout.rows).fill(0).map(() => String.fromCharCode(random(CHAR_MIN_RANGE, CHAR_MAX_RANGE)));
        this.head = new Head();
    }

    update() {
        this.draw();
        this.head.slide();

        if(this.head.y - this.length === process.stdout.rows) {
            this.clean();
        }
    }

    draw() {
        for(let y = Math.max(0, this.head.y - process.stdout.rows); y < Math.min(this.length, this.length + (this.head.y - this.length)); y++) {
            terminal.moveTo(this.head.x, this.head.y - y - 1);
            write(colors[Math.floor(y * colors.length / this.length)]);
            write(this.chars[this.head.y - y - 1]);
        }
        if(this.head.y - this.length >= 0) {
            terminal.moveTo(this.head.x, this.head.y - this.length);
            write(' ');
        }
    }

    clean() {
        removeDrop(this);
    }
}

class Head {
    constructor() {
        this.x = random(0, process.stdout.columns);
        this.y = 0;
    }

    slide() {
        this.y += 1;
    }
}

terminal.clear();
terminal.hide();

const drops = [];
const speedIntervals = [];

function addDrop() {
    const drop = new Drop();

    if(!drops[drop.speed]) drops[drop.speed] = [];
    drops[drop.speed].push(drop);

    if(!speedIntervals[drop.speed]) {
        speedIntervals[drop.speed] = setInterval(() => {
            drops[drop.speed].forEach(drop => drop.update());
        }, drop.speed);
    }
}

function removeDrop(drop) {
    drops[drop.speed].splice(drops[drop.speed].indexOf(drop), 1);
    if(!drops[drop.speed].length) {
        clearInterval(speedIntervals[drop.speed]);
        speedIntervals[drop.speed] = null;
    }
}

setInterval(addDrop, DROP_SPAWN_INTERVAL);