interface Disc {
   totalPositions: number,
   initialPosition: number,
}

function* infiniteSequence(){
  var index = 0;
  while(true)
    yield index++;
}

function* filter(iterable:IterableIterator, f:(number)=>boolean){
    for(let item of iterable){
        if(f(item)){
            yield item
        }
    }
}

function toDisc(str:string): Disc {
    const regex:RegExp = /Disc #\d has (\d+) positions; at time=0, it is at position (\d+)./
    const matches = str.match(regex);
    return {totalPositions: parseInt(matches[1], 10), initialPosition: parseInt(matches[2], 10)}
}

function isTimeAllowed(num: number, disc:Disc)
{
    return ((num + disc.initialPosition) % disc.totalPositions) == 0;
}

function getAllowedPositions(iterable:IterableIterator, disc:Disc, discNumber:number ){
    return filter(iterable, num => isTimeAllowed(num + discNumber + 1, disc));
}

declare function require(name:string);
const fs = require("fs");
const lines = fs.readFileSync("day15.txt", "utf8").split("\n");
const discs = lines.filter(s => s !== "").map(toDisc);
const allowedNumbers = discs.reduce(getAllowedPositions, infiniteSequence());
console.log(allowedNumbers.next().value);

function* infiniteSequence(){
  var index = 0;
  while(true)
    yield index++;
}

function* filter(iterable:IterableIterator, f:(number)=>boolean){
    for(let item of iterable){
        if(f(item)){
            yield item
        }
    }
}

const allowedNumbers = discs.reduce(getAllowedPositions, infiniteSequence());
console.log(allowedNumbers.next().value);

function isTimeAllowed(num: number, disc:Disc)
{
    return ((num + disc.initialPosition) % disc.totalPositions) == 0;
}

function getAllowedPositions(iterable:IterableIterator, disc:Disc, discNumber:number ){
    return filter(iterable, num => isTimeAllowed(num + discNumber + 1, disc));
}

Part 2

Another real easy one, as I just had to enter in another line of input to my text file and my code still worked.

Wrap-up

I give myself an A- for this one.  Typescript was fun; I prefer static typing and it was still Javascript that I was familiar with.  I didn’t dive deep enough to really understand what else Typescript can buy me, so that’s why its not a straight up A.  I really liked my solution too, as I got to deal with infinite sequences and reducing filter functions over them.