advent.js

input
  .split('\n')
  .map((e, i, a) => i && a[i] - a[i - 1])
  .filter(n => n > 0)
  .length
// 1527

// create a new array, then scan the input, adding each element to its coresponding index and the next two indexes as well
// is the number of additions any different than looking ahead and behind for each of them? 
// for 1st strategy, for each element, I read 3 and write once.
// for 2nd strategy, for each element, I read once and write three times.
// 1st: number + undefined = NaN, which will be filtered out when compared to 0
input
  .split('\n')
  .map(Number)
  .map((e, i, a) => a[i] + a[i+1] + a[i+2])
  .map((e, i, a) => i && a[i] - a[i - 1])
  .filter(n => n > 0)
  .length
// 1575

advent2.js

input.reduce((result, command) => {
  let [ direction, distance ] = command.match(/([a-z]+)\s(\d+)/).slice(1)
  switch(direction){
    case "forward": result.x += +distance; break
    case "up": result.y -= +distance; break
    case "down": result.y += +distance
  }
  return result
}, {x: 0, y: 0})
// -> Object { x: 1925, y: 879 }
// 1925 * 879 = 1692075

// Part two
input.reduce((result, command) => {
  let [ direction, distance ] = command.match(/([a-z]+)\s(\d+)/).slice(1)
  switch(direction){
    case "up": result.aim -= +distance; break
    case "down": result.aim += +distance; break
    default: /* 'forward' */
      result.x += +distance
      result.y += +distance * result.aim
  }
  return result
}, {x: 0, y: 0, aim: 0})
// -> Object { x: 1925, y: 908844, aim: 879 }
// 1925 * 908844 = 1749524700

day3.js

// for each of the digits, sum the column, then check if the sum is greater than half the length
// this tells you if the number of ones is greater than the number of zeros

input = input
  .split('\n')
  .map(e => e.split('').map(Number))
  
result = input
  .reduce((prev,curr) => prev.map((e, i) => e + curr[i]))
  .map(e => Number(e > (input.length / 2)))
  .join('')
  
 gamma = parseInt(result, 2)
 episilon = gamma ^ parseInt("".padEnd(result.length, 1), 2)

candidates = input
cursor = 0

while(candidates.length > 1){
  // given a cursor index, decide which bit is more common
  let column = candidates.map(each => each[cursor])
  let sum = column.reduce((a,b) => a + b)
  let result = Number(sum >= (column.length / 2))
  // then filter the candidates 
  candidates = candidates.filter(each => each[cursor] == result)
  cursor++
}

o2 = parseInt(candidates.pop().join(''), 2)
console.log("O2", o2)

candidates = input
cursor = 0

while(candidates.length > 1){
  // given a cursor index, decide which bit is more common
  let column = candidates.map(each => each[cursor])
  let sum = column.reduce((a,b) => a + b)
  let result = Number(sum < (column.length / 2))
  // then filter the candidates 
  candidates = candidates.filter(each => each[cursor] == result)
  cursor++
}

co2 = parseInt(candidates.pop().join(''), 2)
console.log("CO2", co2)

day4.js

parseInput = input => {
  let [maybes, ...boards] = input.split('\n\n')
  maybes = maybes.split(',').map(Number)
  boards = boards.map(board => board.split('\n').map(e => e.trim().split(/\s+/).map(Number)))
  return {maybes, boards}
}

let transpose = m => m[0].map((x,i) => m.map(x => x[i]))
let flatten = m => m.reduce((prev, curr) => prev.concat(curr))

let {maybes, boards} = parseInput(input)
let bingo = new Map()
boards.forEach(board => {
  board.map(row => bingo.set(row, board))
  transpose(board).map(col => bingo.set(col, board))
})
// 
let fives = []
bingo.forEach((_, five) => fives.push(five))

var score = 0
var winner = null
var cursor = 5
while(boards.length > 1 && cursor < maybes.length){
  let test = maybes.slice(0, cursor++)
  fives.forEach(five => {
    if(five.every(one => test.includes(one))){
      winner = bingo.get(five)
      console.log("WINNER", winner)
      console.log("SCORE", flatten(winner).filter(n => !test.includes(n)).reduce((a,b)=>a+b))
      console.log("LAST", test[test.length - 1])
      console.log("INDEX", boards.indexOf(winner))
      boards.splice(boards.indexOf(winner), 1)
    }
  })
}
console.log(boards)
// given the winning board, fl