day 9, part 2

day9.js

@@ -1,6 +1,5 @@
 class Circle {
   constructor (numPlayers) {
-    this.marbles = []
     this.numPlayers = numPlayers
     this.scores = {}
     for (let i = 0; i < numPlayers; i++) {
@@ -10,35 +9,52 @@ class Circle {
 
   placeMarble () {
     if (this.currentMarble === undefined) {
-      this.currentMarble = 0
+      const marble = {n: 0}
+      marble.next = marble
+      marble.prev = marble
+      this.currentMarble = marble
+      this.firstMarble = marble
       this.placements = 0
-      this.marbles.push(this.currentMarble)
     } else {
-      const marbleToBePlaced = ++this.placements
+      const marbleToBePlacedValue = ++this.placements
       const currentPlayer = this.placements % this.numPlayers
-      if (marbleToBePlaced % 23 === 0) {
+      if (marbleToBePlacedValue % 23 === 0) {
         // However, if the marble that is about to be placed has a number which is a multiple of 23, something entirely different happens:
         // First, the current player keeps the marble they would have placed, adding it to their score.
-        this.scores[currentPlayer] += marbleToBePlaced
+        this.scores[currentPlayer] += marbleToBePlacedValue
         // In addition, the marble 7 marbles counter-clockwise from the current marble is removed from the circle
-        let idx = (this.marbles.indexOf(this.currentMarble) - 7) % this.marbles.length
-        // Correct negative offset
-        if (idx < 0) {
-          idx = this.marbles.length + idx
+        let marbleToRemove = this.currentMarble
+        for (let i = 0; i < 7; i++) {
+          marbleToRemove = marbleToRemove.prev
         }
+        marbleToRemove.prev.next = marbleToRemove.next
         // The marble located immediately clockwise of the marble that was removed becomes the new current marble.
-        this.currentMarble = this.marbles[(idx + 1) % this.marbles.length]
+        this.currentMarble = marbleToRemove.next
         // and also added to the current player's score.
-        this.scores[currentPlayer] += this.marbles[idx]
-        this.marbles.splice(idx, 1)
+        this.scores[currentPlayer] += marbleToRemove.n
       } else {
-        const idx = (this.marbles.indexOf(this.currentMarble) + 1) % this.marbles.length
-        this.marbles.splice(idx + 1, 0, marbleToBePlaced)
-        this.currentMarble = marbleToBePlaced
+        const marble = {
+          n: marbleToBePlacedValue,
+          prev: this.currentMarble.next,
+          next: this.currentMarble.next.next
+        }
+        this.currentMarble.next.next = marble
+        this.currentMarble.next.next.next.prev = marble
+        this.currentMarble = marble
       }
     }
   }
 
+  marbles () {
+    const marbles = []
+    let marble = this.firstMarble
+    do {
+      marbles.push(marble.n)
+      marble = marble.next
+    } while (marble !== this.firstMarble)
+    return marbles
+  }
+
   winningScore () {
     return Object.keys(this.scores).reduce((max, player) => this.scores[player] > max ? this.scores[player] : max, 0)
   }

day9.spec.js

@@ -9,41 +9,41 @@ describe('marble mania', () => {
   it('should calculate the winning score', () => {
     const c = new Circle(5)
     c.placeMarble()
-    expect(c.currentMarble).toEqual(0)
-    expect(c.marbles).toEqual([0])
+    expect(c.currentMarble.n).toEqual(0)
+    expect(c.marbles()).toEqual([0])
     c.placeMarble()
-    expect(c.currentMarble).toEqual(1)
-    expect(c.marbles).toEqual([0, 1])
+    expect(c.currentMarble.n).toEqual(1)
+    expect(c.marbles()).toEqual([0, 1])
     c.placeMarble()
-    expect(c.currentMarble).toEqual(2)
-    expect(c.marbles).toEqual([0, 2, 1])
+    expect(c.currentMarble.n).toEqual(2)
+    expect(c.marbles()).toEqual([0, 2, 1])
     c.placeMarble()
-    expect(c.currentMarble).toEqual(3)
-    expect(c.marbles).toEqual([0, 2, 1, 3])
+    expect(c.currentMarble.n).toEqual(3)
+    expect(c.marbles()).toEqual([0, 2, 1, 3])
     c.placeMarble()
-    expect(c.currentMarble).toEqual(4)
-    expect(c.marbles).toEqual([0, 4, 2, 1, 3])
+    expect(c.currentMarble.n).toEqual(4)
+    expect(c.marbles()).toEqual([0, 4, 2, 1, 3])
     for (let i = 0; i < 18; i++) {
       c.placeMarble()
     }
-    expect(c.currentMarble).toEqual(22)
-    expect(c.marbles).toEqual([
+    expect(c.currentMarble.n).toEqual(22)
+    expect(c.marbles()).toEqual([
       0, 16, 8, 17, 4, 18, 9, 19, 2, 20, 10, 21, 5, 22, 11, 1, 12, 6, 13, 3, 14, 7, 15
     ])
     // Rule 23
     c.placeMarble()
-    expect(c.currentMarble).toEqual(19)
-    expect(c.marbles).toEqual([
+    expect(c.currentMarble.n).toEqual(19)
+    expect(c.marbles()).toEqual([
       0, 16, 8, 17, 4, 18, 19, 2, 20, 10, 21, 5, 22, 11, 1, 12, 6, 13, 3, 14, 7, 15
     ])
     c.placeMarble()
-    expect(c.currentMarble).toEqual(24)
-    expect(c.marbles).toEqual([
+    expect(c.currentMarble.n).toEqual(24)
+    expect(c.marbles()).toEqual([
       0, 16, 8, 17, 4, 18, 19, 2, 24, 20, 10, 21, 5, 22, 11, 1, 12, 6, 13, 3, 14, 7, 15
     ])
     c.placeMarble()
-    expect(c.currentMarble).toEqual(25)
-    expect(c.marbles).toEqual([
+    expect(c.currentMarble.n).toEqual(25)
+    expect(c.marbles()).toEqual([
       0, 16, 8, 17, 4, 18, 19, 2, 24, 20, 25, 10, 21, 5, 22, 11, 1, 12, 6, 13, 3, 14, 7, 15
     ])
     expect(c.winningScore()).toEqual(32)
@@ -57,5 +57,6 @@ describe('marble mania', () => {
   })
   it('should calculate the puzzle', () => {
     expect(getWinningScore(413, 71082)).toEqual(416424)
+    expect(getWinningScore(413, 7108200)).toEqual(3498287922)
   })
 })