feat(2020-day-05): find the first available seat

Sort the tickets, add a bunch of fake ones for the non-existent rows
at the front of the plane, then find the gap where IDs stop matching
the array position

Author: developher-net

2020/day-05/seats.js

@@ -7,24 +7,54 @@ const seatCodeToBitmap = (seatCode) => {
     .replace(/R/g, 1)
 }
 
-const calcSeatId = ({ row, column }) => row * 8 + column
+// This is just binary to int
+// const calcSeatId = ({ row, column }) => row * 8 + column
 
 const getSeat = (seatCode) => {
   // Seat codes are basically bitmaps, so convert them
   const seatBitMap = seatCodeToBitmap(seatCode)
 
-  // First octect is row
+  // First septuplet is row
   const row = parseInt(seatBitMap.slice(0, 7), 2)
   // Last triplet is column
   const column = parseInt(seatBitMap.slice(7), 2)
 
   return {
     row,
     column,
-    id: calcSeatId({ row, column })
+    id: parseInt(seatBitMap, 2)
   }
 }
 
+const findAvailableSeat = (seats) => {
+  // We could do some complicated logic to loop through a sorted
+  // list of seats comparing indexes to find the first one where there's
+  // a gap by looking for an off-by-one situation while excluding first
+  // and last rows:
+  //
+  // if (seat.row === 0 || seat.row === 127) return false
+  // // Find gap between this seat and previous
+  // return (
+  //   (seat.id - 1) !== arr[idx - 1].id
+  // )
+  //
+  // Or we can just and a fake first row, and then look for any
+  // seat where the ID doesn't match the array index
+  //
+  const occupied = seats.map(getSeat)
+    .sort((a, b) => a.id - b.id)
+  // The first seat is not in row 0, so add a bunch of
+  // fake rows at the front of the plane
+  occupied.unshift(...Array(occupied[0].id))
+  // Find the gap, ignoring the fake seats
+  return occupied.find((seat, idx, arr) => {
+    // !!seat filters out the fake ones since they're undefined
+    return !!seat && seat.id !== idx
+    // we can ignore rows at the back even if missing
+  }).id - 1 // The open seat will be immediately before the mismatched seat
+}
+
 module.exports = {
-  getSeat
+  getSeat,
+  findAvailableSeat
 }

2020/day-05/seats.test.js

@@ -1,6 +1,6 @@
 /* eslint-env mocha */
 const { expect } = require('chai')
-const { getSeat } = require('./seats')
+const { getSeat, findAvailableSeat } = require('./seats')
 
 describe('--- Day 5: Binary Boarding ---', () => {
   describe('Part 1', () => {
@@ -23,5 +23,20 @@ describe('--- Day 5: Binary Boarding ---', () => {
         })
       })
     })
+    describe('findAvailableSeat()', () => {
+      it('finds the first available seat', () => {
+        const secondRow = [
+          'FFFFFFBLLL',
+          'FFFFFFBLLR',
+          'FFFFFFBLRL',
+          'FFFFFFBLRR',
+          // 'FFFFFFBRLL', row:1, col:4, id: 12
+          'FFFFFFBRLR',
+          'FFFFFFBRRL',
+          'FFFFFFBRRR'
+        ]
+        expect(findAvailableSeat(secondRow)).to.equal(12)
+      })
+    })
   })
 })

2020/day-05/solution.js

@@ -2,7 +2,7 @@ const fs = require('fs')
 const path = require('path')
 const filePath = path.join(__dirname, 'input.txt')
 const { inputToArray } = require('../../2018/inputParser')
-const { getSeat } = require('./seats')
+const { getSeat, findAvailableSeat } = require('./seats')
 
 fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
   if (err) throw err
@@ -24,9 +24,8 @@ fs.readFile(filePath, { encoding: 'utf8' }, (err, initData) => {
   }
 
   const part2 = () => {
-    const data = resetInput()
-    console.debug(data)
-    return 'No answer yet'
+    const tickets = resetInput()
+    return findAvailableSeat(tickets)
   }
   const answers = []
   answers.push(part1())