Day 9.1 solution.

2018/day09-1.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/day09"
+)
+
+func main() {
+	numPlayers, finNum := day09.ParseInput()
+	_, score := day09.PlayGame(numPlayers, finNum)
+	fmt.Println(score)
+}

2018/internal/day09/game.go

@@ -0,0 +1,39 @@
+package day09
+
+// Returns the winning player's index and score
+func PlayGame(numPlayers, finNum int) (int, int) {
+	players := make([]int, numPlayers)
+
+	// initialize the 'board'
+	currentMarble := &Marble{Number: 0}
+	currentMarble.CW = currentMarble
+	currentMarble.CCW = currentMarble
+
+	currentPlayer := 0
+	for i := 1; currentMarble.Number < finNum; i++ {
+		var score int
+		currentMarble, score = currentMarble.PlaceNewMarble(i)
+		players[currentPlayer] += score
+
+		currentPlayer++
+		if currentPlayer >= len(players) {
+			currentPlayer = 0
+		}
+	}
+
+	return findWinnerData(players)
+}
+
+func findWinnerData(players []int) (int, int) {
+	topScore := -1
+	winner := -1
+
+	for player, score := range players {
+		if score > topScore {
+			topScore = score
+			winner = player
+		}
+	}
+
+	return winner, topScore
+}

2018/internal/day09/marbles.go

@@ -0,0 +1,55 @@
+package day09
+
+type Marble struct {
+	Number int
+	CW     *Marble
+	CCW    *Marble
+}
+
+// Returns the new current marble and the score to add.
+func (m *Marble) PlaceNewMarble(number int) (*Marble, int) {
+	// if the marble that is about to be placed has a number which is a multiple
+	// of 23, the current player keeps the marble they would have placed, adding
+	// it to their score. In addition, the marble 7 marbles counter-clockwise
+	// from the current marble is removed from the circle and also added to the
+	// current player's score. The marble located immediately clockwise of the
+	// marble that was removed becomes the new current marble.
+	if number%23 == 0 {
+		score := number
+		cursor := m
+		for i := 0; i < 7; i++ {
+			cursor = cursor.CCW
+		}
+		score += cursor.Number
+		cursor = cursor.Remove()
+		return cursor, score
+	}
+
+	// [Insert the marble] between the marbles that are 1 and 2
+	// marbles clockwise of the current marble.
+	// The marble that was just placed then becomes the current marble.
+
+	// first create the marble and populate its fields correctly
+	newMarble := &Marble{Number: number}
+	cursor := m.CW
+	newMarble.CCW = cursor
+	newMarble.CW = cursor.CW
+
+	// now sync up the pointers in the adjacent marbles
+	newMarble.CCW.CW = newMarble
+	newMarble.CW.CCW = newMarble
+	return newMarble, 0
+}
+
+func (m *Marble) Remove() *Marble {
+	// removes the marble from the list, repairing links. Returns the
+	// marble immediately CW of this one.
+
+	// link the two newly-adjacent marbles.
+	ccw := m.CCW
+	cw := m.CW
+	ccw.CW = cw
+	cw.CCW = ccw
+
+	return cw
+}

2018/internal/day09/util.go

@@ -0,0 +1,25 @@
+package day09
+
+import (
+	"strconv"
+	"strings"
+
+	"internal/util"
+)
+
+func ParseInput() (int, int) {
+	rawData := strings.TrimSpace(util.ReadInput()[0])
+	data := strings.Split(rawData, " ")
+
+	players, err := strconv.Atoi(data[0])
+	if err != nil {
+		panic(err)
+	}
+
+	lastNum, err := strconv.Atoi(data[6])
+	if err != nil {
+		panic(err)
+	}
+
+	return players, lastNum
+}