2020 - Day 15 - Part II - Cleanup

Author: xiel

src/Day 15 - Rambunctious Recitation/Day15.test.ts

@@ -0,0 +1,69 @@
+describe("Day 15 - Rambunctious Recitation", () => {
+  describe("Part I", () => {
+    test("Example", () => {
+      expect(playMemory("0,3,6", 2020)).toEqual(436);
+      expect(playMemory("3,1,2", 2020)).toEqual(1836);
+    });
+
+    test("Input", () => {
+      expect(playMemory("0,20,7,16,1,18,15", 2020)).toEqual(1025);
+    });
+  });
+
+  describe("Part II", () => {
+    test("Example", () => {
+      expect(playMemory("0,3,6", 30_000_000)).toEqual(175594);
+      expect(playMemory("3,1,2", 30_000_000)).toEqual(362);
+    });
+
+    test("Input", () => {
+      expect(playMemory("0,20,7,16,1,18,15", 30_000_000)).toEqual(129262);
+    });
+  });
+});
+
+function playMemory(listStr: string, turnToReturn: number) {
+  const initialList = listStr.split(",").map((n) => Number(n));
+  let mostRecentNum = NaN;
+  let turnNo = initialList.length;
+
+  const mostRecentPlayedAt = new Map<number, number>();
+  const prevPlayedAt = new Map<number, number>();
+
+  initialList.forEach((num, i) => {
+    mostRecentPlayedAt.set(num, i + 1); // turn is a 1-based index
+    mostRecentNum = num;
+  });
+
+  while (turnNo < turnToReturn) {
+    turnNo++;
+
+    const mostRecent = mostRecentPlayedAt.get(mostRecentNum);
+    const prev = prevPlayedAt.get(mostRecentNum);
+
+    if (mostRecent !== undefined && prev !== undefined) {
+      const newNum = mostRecent - prev;
+      addNum(newNum);
+      mostRecentNum = newNum;
+    } else {
+      addNum(0);
+      mostRecentNum = 0;
+    }
+
+    if (turnNo === turnToReturn) {
+      return mostRecentNum;
+    }
+  }
+
+  function addNum(num: number, atTurn = turnNo) {
+    const mostRecent = mostRecentPlayedAt.get(num);
+    if (mostRecent !== undefined) {
+      prevPlayedAt.set(num, mostRecent);
+      mostRecentPlayedAt.set(num, atTurn);
+    } else {
+      mostRecentPlayedAt.set(num, atTurn);
+    }
+  }
+
+  return mostRecentNum;
+}