2020 day 10

2020/10/part2.js

@@ -0,0 +1,69 @@
+const fs = require('fs');
+const data = fs.readFileSync('input.txt', 'utf-8');
+const lines = data.split('\n').map(num => parseInt(num)).sort((a,b)=>(a-b));
+lines.unshift(0);
+lines.push(lines[lines.length-1]+3);
+
+
+// convert to linked list
+let last = {};
+let i = 0
+do {
+  let entry = {
+    value: lines[i],
+    prev: last
+  };
+  last.next = entry;
+  last = entry;
+  i++;
+} while (i < lines.length)
+
+// find removeable adaptors
+let removeableAdaptors = [];
+const lastVal = last
+last = last.prev
+while (last.prev) {
+  if (last.next.value - last.prev.value <= 3){
+    removeableAdaptors.push(last)
+  }
+  last = last.prev
+}
+
+// divide removeable adaptors into groups that don't interfere with one another when removed
+// this changes the runtime with the given input from a over a month to less than a millisecond
+// however, in a worst case scenareo, where every adaptor is removeable, this would still take trillions of years.
+let adaptorGroups = []
+function makeGroups(removeableAdaptors) {
+  for (let i = removeableAdaptors.length-1; i>-1; i--) {
+    // if the gap between two removeable adaptors > 2 then you know they will not intefere with each other
+    if (i == 0 || removeableAdaptors[i-1].value - removeableAdaptors[i].value > 2) {
+      adaptorGroups.push(removeableAdaptors.slice(i))
+      makeGroups(removeableAdaptors.slice(0, i))
+      break;
+    }
+  }
+}
+makeGroups(removeableAdaptors)
+
+function findArrangements (removeableAdaptors, s = 0) {
+  total = 1
+  // for each adaptor in the group
+  for (let i = s; i < removeableAdaptors.length; i++){
+    // if you can remove this adaptor
+    if (removeableAdaptors[i].next.value-removeableAdaptors[i].prev.value <= 3) {
+      // remove the adaptor and find arrangements with it gone
+      removeableAdaptors[i].prev.next = removeableAdaptors[i].next
+      total += findArrangements (removeableAdaptors, i+1)
+      // add the adaptor back for following tests
+      removeableAdaptors[i].prev.next = removeableAdaptors[i]
+    }
+  }
+  return total
+}
+
+console.time('time')
+// multipy the amount of arrangements in each group to get the total amount of arrangements.
+answer = adaptorGroups.reduce((a, g) => {return a*findArrangements(g)},1)
+console.timeEnd('time')
+
+console.log(answer)