From 7b31fa1cbe48a27f9a7ce64aa7127a35c03da108 Mon Sep 17 00:00:00 2001
From: Dreaded_X <tim.huizinga@gmail.com>
Date: Wed, 22 Dec 2021 00:32:09 +0100
Subject: [PATCH] 2021 - Day 21

---
 2021/21/go.mod  |   9 +++
 2021/21/go.sum  |   2 +
 2021/21/main.go | 173 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 184 insertions(+)
 create mode 100644 2021/21/go.mod
 create mode 100644 2021/21/go.sum
 create mode 100644 2021/21/main.go

diff --git a/2021/21/go.mod b/2021/21/go.mod
new file mode 100644
index 0000000..a1dc25e
--- /dev/null
+++ b/2021/21/go.mod
@@ -0,0 +1,9 @@
+module AoC/2021/21
+
+require AoC/2021/common v0.0.0
+
+require github.com/joho/godotenv v1.4.0 // indirect
+
+replace AoC/2021/common v0.0.0 => ../common
+
+go 1.17
diff --git a/2021/21/go.sum b/2021/21/go.sum
new file mode 100644
index 0000000..8c9f290
--- /dev/null
+++ b/2021/21/go.sum
@@ -0,0 +1,2 @@
+github.com/joho/godotenv v1.4.0 h1:3l4+N6zfMWnkbPEXKng2o2/MR5mSwTrBih4ZEkkz1lg=
+github.com/joho/godotenv v1.4.0/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
diff --git a/2021/21/main.go b/2021/21/main.go
new file mode 100644
index 0000000..11ff5f3
--- /dev/null
+++ b/2021/21/main.go
@@ -0,0 +1,173 @@
+package main
+
+import (
+	aoc "AoC/2021/common"
+	"bufio"
+	"fmt"
+	"regexp"
+	"strconv"
+)
+
+type DeterministicDie struct {
+	counter int
+}
+
+func (d DeterministicDie) Roll(times int) (DeterministicDie, int) {
+	s := 0
+
+	for i := 0; i < times; i++ {
+		s += d.counter + 1
+		d.counter++
+		d.counter %= 100
+	}
+
+	return d, s
+}
+
+type Player struct {
+	position int
+	score int
+	rolls int
+}
+
+func NewPlayer(position int) Player {
+	var p Player
+	p.position = position - 1
+	return p
+}
+
+func (p Player) Move(spaces int) Player {
+	p.position += spaces
+	p.position %= 10
+
+	p.score += p.Position()
+
+	p.rolls += 3
+
+	return p
+}
+
+func (p Player) Position() int {
+	// We use a postion from 0 to 9 internally as it is easier to work with
+	return p.position + 1
+}
+
+func (p Player) Score() int {
+	return p.score
+}
+
+func (p Player) Rolls() int {
+	return p.rolls
+}
+
+type GameState struct {
+	pos [2]int
+	score [2]int
+	turn int
+}
+
+func main() {
+	challenge := aoc.New(2021, 21)
+
+	challenge.Test(`Player 1 starting position: 4
+Player 2 starting position: 8`, []int{739785, 444356092776315})
+
+	challenge.Solution(1, func (input *bufio.Scanner) int {
+		r := regexp.MustCompile("[0-9]+")
+
+		var players [2]Player
+		for i := range players {
+			input.Scan()
+			position, _ := strconv.Atoi(r.FindAllString(input.Text(), 2)[1])
+			players[i] = NewPlayer(position)
+		}
+
+		var die DeterministicDie
+		game:
+		for i := 0; true; i++ {
+			for j, p := range players {
+				var rolled int
+				die, rolled = die.Roll(3)
+				players[j] = p.Move(rolled)
+
+				if players[j].Score() >= 1000 {
+					break game
+				}
+			}
+		}
+
+		score := 0
+		rolls := 0
+		for _, p := range players {
+			rolls += p.Rolls()
+			if p.Score() < 1000 {
+				score = p.Score()
+			}
+		}
+
+		fmt.Println(score, rolls)
+
+		return score*rolls
+	})
+
+	challenge.Solution(2, func (input *bufio.Scanner) int {
+		r := regexp.MustCompile("[0-9]+")
+
+		var players [2]int
+		for i := range players {
+			input.Scan()
+			position, _ := strconv.Atoi(r.FindAllString(input.Text(), 2)[1])
+			players[i] = position-1
+		}
+
+		// List of all current states
+		states := make(map[GameState]int)
+		states[GameState{players, [2]int{}, 0}] = 1
+
+		// Pre compute all possible outcomes and their frequency
+		outcomes := make(map[int]int)
+		maxNum := 3
+		for r := 1; r <= maxNum; r++ {
+			for s := 1; s <= maxNum; s++ {
+				for t := 1; t <= maxNum; t++ {
+					outcomes[r + s + t]++
+				}
+			}
+		}
+
+		var victories [2]int
+		// Keep going as long as there are unfinished games
+		for i := 0; len(states) > 0; i++ {
+			statesNext := make(map[GameState]int)
+
+			// Update all states
+			for state, occurs := range states {
+				// Create all possible outcomes
+				for outcome, num := range outcomes {
+					stateNext := state
+
+					stateNext.pos[state.turn] += outcome
+					stateNext.pos[state.turn] %= 10
+					stateNext.score[state.turn] += stateNext.pos[state.turn] + 1
+
+					if stateNext.score[state.turn] >= 21 {
+						// Player has won
+						victories[state.turn] += occurs*num
+					} else {
+						stateNext.turn = 1 - state.turn
+						statesNext[stateNext] += occurs*num
+					}
+				} 
+
+			}
+			states = statesNext
+		}
+		
+		fmt.Println(victories)
+		if victories[1] > victories[0] {
+			return victories[1]
+		}
+
+		return victories[0]
+	})
+}