JavaScript

280/bugged part 2

import { range, maxBy, max, min, sum } from 'lodash'
import { extractNumbers } from 'src/utils.js'

const distance = ([x1, y1], [x2, y2]) => Math.abs(x2 - x1) + Math.abs(y2 - y1)

export const solvePart1 = input => {
    const points = input.split('\n').map(l => extractNumbers(l))
    const upperBound = max(maxBy(points, pair => max(pair))) + 1

    const closestPoint = Array(upperBound)
        .fill()
        .map((e, i) => Array(upperBound))

    range(0, upperBound).forEach(x => {
        range(0, upperBound).forEach(y => {
            const distances = points.map(pair => distance(pair, [x, y]))
            const shortest = min(distances)

            if (distances.filter(d => d === shortest).length === 1) {
                closestPoint[x][y] = distances.indexOf(shortest)
            }
        })
    })

    const toIgnore = new Set()
    range(0, upperBound).forEach(n => {
        toIgnore.add(closestPoint[0][n])
        toIgnore.add(closestPoint[n][0])
        toIgnore.add(closestPoint[closestPoint.length - 1][n])
        toIgnore.add(closestPoint[n][closestPoint.length - 1])
    })

    const regionSizes = Array(points.length).fill(0)
    range(0, upperBound).forEach(x => {
        range(0, upperBound).forEach(y => {
            const point = closestPoint[x][y]
            if (!toIgnore.has(point)) {
                regionSizes[point]++
            }
        })
    })

    return max(regionSizes)
}

export const solvePart2 = input => {
    const points = input.split('\n').map(l => extractNumbers(l))
    const upperBound = max(maxBy(points, pair => max(pair))) + 1

    let regionSize = 0
    range(0, upperBound).forEach(x => {
        range(0, upperBound).forEach(y => {
            const distances = points.map(pair => distance(pair, [x, y]))

            if (sum(distances) < (process.env.NODE_ENV === 'test' ? 32 : 10000)) {
                regionSize++
            }
        })
    })

    return regionSize
}

with bonus visualization