# Advent of Code 2021 - Day 5

I spent hours on day 5 building complex things that didn’t work. That left me in a mood to hack and golf more, idealize less.

## Day 5

interface Point {
x: number
y: number
}

interface Segment {
a: Point
b: Point
}

export const pointsOf = (g: Segment): Point[] => {
let {a, b} = g
const dx = b.x < a.x ? -1 : b.x > a.x ? 1 : 0
const dy = b.y < a.y ? -1 : b.y > a.y ? 1 : 0
const points = []
for (let x = a.x, y = a.y; x !== b.x || y !== b.y; x += dx, y += dy) {
points.push({x, y})
}
points.push(b)
return points
}

export const d5 = ({
input = inputs.d5,
part,
}: {
input?: string
part: 'a' | 'b'
}) => {
const horiz = ({a, b}: Segment) => a.y === b.y
const vert = ({a, b}: Segment) => a.x === b.x
const lines = ranks(ints(input), 4)
.map(([ax, ay, bx, by]) => ({a: {x: ax, y: ay}, b: {x: bx, y: by}}))
.filter(g => part === 'b' || horiz(g) || vert(g))
const seen = reduce(
lines,
(seen, line) =>
reduce(
pointsOf(line).map(({x, y}) => `\${x},\${y}`),
(seen, p) => ((seen[p] = (seen[p] || 0) + 1), seen),
seen,
),
{},
)
return Object.values(seen).filter(n => n > 1).length
}

Day 5a sample:

Day 5a mine:

Day 5b sample:

Day 5b mine: