2021 Day 21 Complete

2021/day21/input

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+Player 1 starting position: 4
+Player 2 starting position: 2

2021/day21/main.go

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+package main
+
+import (
+	"fmt"
+
+	h "git.bullercodeworks.com/brian/adventofcode/helpers"
+)
+
+func main() {
+	inp := h.StdinToStringSlice()
+	var starting []int
+	for i := range inp {
+		starting = append(starting, int(inp[i][len(inp[i])-1]-'0'))
+	}
+	part1(starting)
+	fmt.Println()
+	part2(starting)
+}
+
+type player struct {
+	number          int
+	position, score int
+}
+
+func (p *player) move(spaces int) {
+	p.position = (p.position + spaces) % 10
+	if p.position == 0 {
+		p.position = 10
+	}
+	p.score += p.position
+}
+
+type die struct {
+	value int
+	rolls int
+}
+
+func (d *die) roll() int {
+	d.rolls++
+	d.value++
+	if d.value > 100 {
+		d.value = 1
+	}
+	return d.value
+}
+
+func part1(pos []int) {
+	var players []player
+	for i := range pos {
+		players = append(players, player{number: i + 1, position: pos[i]})
+	}
+	die := die{value: 0}
+	pTurn := 0
+	for players[0].score < 1000 && players[1].score < 1000 {
+		turn := die.roll() + die.roll() + die.roll()
+		players[pTurn].move(turn)
+		pTurn = (pTurn + 1) % len(players)
+	}
+	var loser player
+	for i := range players {
+		if loser.number == 0 || players[i].score < loser.score {
+			loser = players[i]
+		}
+	}
+	fmt.Println("# Part 1")
+	fmt.Printf("Player %d loses, score %d\n", (loser.number), loser.score*die.rolls)
+}
+
+func part2(pos []int) {
+	var players []player
+	for i := range pos {
+		players = append(players, player{number: i + 1, position: pos[i]})
+	}
+
+	diracIt(pos[0], pos[1])
+}
+
+type gameState struct {
+	p1, p2 int
+	s1, s2 int
+}
+
+// Run the games with the dirac die.
+func diracIt(p1, p2 int) {
+	move := func(pos, roll int) int {
+		pos += roll
+		pos = pos % 10
+		if pos == 0 {
+			return 10
+		}
+		return pos
+	}
+
+	states := map[gameState]int64{{p1, p2, 0, 0}: 1}
+	rolls := map[int]int64{}
+	for r1 := 1; r1 <= 3; r1++ {
+		for r2 := 1; r2 <= 3; r2++ {
+			for r3 := 1; r3 <= 3; r3++ {
+				rolls[r1+r2+r3] += 1
+			}
+		}
+	}
+	var p1w, p2w int64 = 0, 0
+	for len(states) != 0 {
+		for turn := 1; turn <= 2; turn++ {
+			nextStates := map[gameState]int64{}
+			for state, count := range states {
+				for roll, rCount := range rolls {
+					p1, p2 := state.p1, state.p2
+					s1, s2 := state.s1, state.s2
+					n := count * rCount
+
+					if turn == 1 {
+						p1 = move(p1, roll)
+						s1 += p1
+						if s1 > 20 {
+							p1w += n
+						} else {
+							nextStates[gameState{p1, p2, s1, s2}] += n
+						}
+					} else {
+						p2 = move(p2, roll)
+						s2 += p2
+						if s2 > 20 {
+							p2w += n
+						} else {
+							nextStates[gameState{p1, p2, s1, s2}] += n
+						}
+					}
+				}
+			}
+			states = nextStates
+		}
+	}
+	fmt.Println("# Part 2")
+	fmt.Printf("Player 1 Wins: %d\nPlayer 2 Wins: %d\n", p1w, p2w)
+}

2021/day21/testinput

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+Player 1 starting position: 4
+Player 2 starting position: 8