2020 day 13

2020/13/input.txt

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+1001796
+37,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,x,457,x,x,x,x,x,x,x,x,x,x,x,x,13,17,x,x,x,x,x,x,x,x,23,x,x,x,x,x,29,x,431,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19

2020/13/part1.js

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+// this was fairly simple; just keep increasing the wait until you find a bus that leaves. there is probably a more efficient solution but this works
+const fs = require('fs');
+const input = fs.readFileSync('input.txt', 'utf-8').split('\n');
+const time = Number(input[0]);
+const busses = input[1].split(',').filter(v=>v!='x').map(n => Number(n));
+
+let isBus = false;
+let timeDif = 0;
+let busID;
+for (; !isBus; timeDif++) {
+  for(bus of busses) {
+    if ((time + timeDif) % bus == 0){
+      busID = bus;
+      isBus = true;
+      break;
+    }
+  }
+}
+// the for loop does an extra increment at the end oops
+timeDif--
+console.log(timeDif*busID)

2020/13/part2.js

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+// 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)

2020/13/test.txt

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+939
+7,13,x,x,59,x,31,19

2020/13/test2.txt

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+
+67,7,59,61

2020/13/test3.txt

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+
+67,x,7,59,61

2020/13/test4.txt

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+
+67,7,x,59,61

2020/13/test5.txt

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+
+1789,37,47,1889

README.md

@@ -53,15 +53,4 @@ if (advent) {
 10. [:star: :star:](https://adventofcode.com/2020/day/10 "see puzzle")
 11. [:star: :star:](https://adventofcode.com/2020/day/11 "see puzzle")
 12. [:star: :star:](https://adventofcode.com/2020/day/12 "see puzzle")
-
-## Languages Used
-* Python
-	- py
-* Node JS
- 	- js
-* Java (Processing)
-	- pde
-* HTML, CSS, JavaScript
-	- html
-  - css
-  - js
+13. [:star: :star:](https://adventofcode.com/2020/day/13 "see puzzle")