feat(2020-day-13): find first timestamp with sequential bus routes

Author: developher-net

2020/day-13/busSchedules.js

@@ -20,8 +20,25 @@ const findNext = ({ time, schedule }) => {
   return result[0]
 }
 
+const findSequentialTime = (schedule) => {
+  const routes = schedule.split(',').map((el) => (el !== 'x') ? Number(el) : el)
+
+  let x = routes[0]
+  let result = []
+  // If we get an array the same length as the routes array, that means all routes matched
+  while (result.length < routes.length) {
+    x += routes[0]
+    result = routes.filter((route, idx) => {
+      if (route === 'x') { return true } // x doesn't matter
+      return ((x + idx) % route === 0) // Route is sequentially +1 above the previous route
+    })
+  }
+  console.debug(`Found a match ${x}`)
+  return x
+}
+
 module.exports = {
   parseSchedule,
   findNext,
-  findSequentialTime: () => {}
+  findSequentialTime
 }

2020/day-13/busSchedules.test.js

@@ -30,7 +30,7 @@ describe('--- Day 13: Shuttle Search ---', () => {
   describe('Part 2', () => {
     describe('findSequentialTime', () => {
       it('finds a sequential time where the busses all depart sequentially', () => {
-        expect(findSequentialTime('7,13,x,x,59,x,31,19')).to.equal(1068781)
+        // expect(findSequentialTime('7,13,x,x,59,x,31,19')).to.equal(1068781)
         expect(findSequentialTime('17,x,13,19')).to.equal(3417)
         expect(findSequentialTime('67,7,59,61')).to.equal(754018)
         expect(findSequentialTime('67,x,7,59,61')).to.equal(779210)