2020 day 13

2020/13/part2.js

@@ -0,0 +1,19 @@
+// this was a tough one. I still don't fully understand what the puzzle was asking for. but i played around in excel and found this method to find the solution. 
+const fs = require('fs');
+const input = fs.readFileSync('input.txt', 'utf-8').split('\n');
+const busses = input[1].split(',').map(n => Number(n)).map((v,i)=>[v,i]).filter(v => !isNaN(v[0]));
+console.log(busses)
+// start with a timestamp of 0
+let time = 0;
+// if you incrment by the ID of the first bus, the first bus will always meet the pattern described in the puzzle.
+let inc = busses[0][0]
+for (let bus = 1; bus < busses.length; bus++){
+  // continue incrementing until the next bus meets the pattern described in the puzzle
+  while (((time+busses[bus][1]) % busses[bus][0]) != 0) {
+    time += inc
+  }
+  // now you can increment by the new bus that met the pattern, and it will continue to meet the pattern
+  inc*=busses[bus][0]
+}
+// since we looped through all the busses, we know that all busses meet the pattern.
+console.log(time)