From 996b8bffd67e9fbd1a0fd4359dce10ea5c80c8de Mon Sep 17 00:00:00 2001
From: RPJosh <RPJosh@rpjosh.de>
Date: Fri, 8 Dec 2023 13:28:54 +0100
Subject: [PATCH] 2023: day 8

---
 internal/2023/day_08/in.go | 119 ++++++++++++++++++++++++++++++++++++-
 pkg/utils/utils.go         |  23 +++++++
 2 files changed, 140 insertions(+), 2 deletions(-)

diff --git a/internal/2023/day_08/in.go b/internal/2023/day_08/in.go
index ba756a5..db85546 100644
--- a/internal/2023/day_08/in.go
+++ b/internal/2023/day_08/in.go
@@ -1,11 +1,126 @@
 package day_08
 
+import (
+	"fmt"
+	"regexp"
+	"strings"
+
+	"rpjosh.de/adventOfCode/pkg/utils"
+)
+
 type Day struct{}
 
 func (d *Day) Part1(in string) string {
-	return ""
+	inSplit := strings.Split(in, "\n")
+
+	// Initialize variables
+	instructions := inSplit[0]
+	line := 0
+	instructionCounter := 0
+	instructionRegex := regexp.MustCompile(`[ \(](?P<primary>\w+)[,\)]`)
+	steps := 0
+
+	// Build indexed map for better performance
+	instructionMap := make(map[string]int, 0)
+	for i, val := range inSplit {
+		if i > 1 && len(val) > 2 {
+			v := val[0:3]
+			instructionMap[v] = i
+
+			// Found start
+			if v == "AAA" {
+				line = i
+			}
+		}
+	}
+
+	// Find destination
+	for {
+		// Start by first instruction on end
+		if instructionCounter >= len(instructions) {
+			instructionCounter = 0
+		}
+
+		// Get left or right
+		instructionIndex := 0
+		if instructions[instructionCounter] == 'R' {
+			instructionIndex = 1
+		}
+
+		// Get the next instruction to move on
+		next := instructionRegex.FindAllStringSubmatch(inSplit[line], -1)[instructionIndex]
+		line = instructionMap[next[1]]
+		//logger.Debug("Going to %q on line %d", next[1], line)
+
+		steps++
+		instructionCounter++
+
+		// Did we reach the end?
+		if strings.HasPrefix(next[1], "ZZZ") {
+			break
+		}
+	}
+
+	return fmt.Sprintf("%d", steps)
 }
 
 func (d *Day) Part2(in string) string {
-	return ""
+	inSplit := strings.Split(in, "\n")
+
+	// Initialize variables
+	instructions := inSplit[0]
+	starts := make([]int, 0)
+	stepsByStarts := make([]int, 0)
+	instructionRegex := regexp.MustCompile(`[ \(](?P<primary>\w+)[,\)]`)
+
+	// Build map for better performance
+	instructionMap := make(map[string]int, 0)
+	for i, val := range inSplit {
+		if i > 1 && len(val) > 2 {
+			v := val[0:3]
+			instructionMap[v] = i
+
+			// Found start
+			if v[2] == 'A' {
+				starts = append(starts, i)
+			}
+		}
+	}
+
+	// Find for each starting point the amount of steps that we need to go to the
+	// first 'Z'.
+	// The input data makes sure that the way back from a Z Node to another Z Node
+	// is always the same as from start to the first Z node.
+	// How would you find out without looping the first time!
+	for _, val := range starts {
+		steps := 0
+		line := val
+		instructionCounter := 0
+		for {
+			// Start by first instruction on end
+			if instructionCounter >= len(instructions) {
+				instructionCounter = 0
+			}
+
+			// Get left or right
+			instructionIndex := 0
+			if instructions[instructionCounter] == 'R' {
+				instructionIndex = 1
+			}
+
+			next := instructionRegex.FindAllStringSubmatch(inSplit[line], -1)[instructionIndex]
+			line = instructionMap[next[1]]
+			//logger.Debug("%d. Going to %q on line %d", i, next[1], line)
+
+			steps++
+			instructionCounter++
+
+			if next[1][2] == 'Z' {
+				break
+			}
+		}
+		stepsByStarts = append(stepsByStarts, steps)
+	}
+
+	return fmt.Sprintf("%d", utils.CalculateLCM(stepsByStarts...))
 }
diff --git a/pkg/utils/utils.go b/pkg/utils/utils.go
index b15e907..6ed5e5b 100644
--- a/pkg/utils/utils.go
+++ b/pkg/utils/utils.go
@@ -150,3 +150,26 @@ func SortRunes(value string) string {
 func ReplaceCharacterInString(str, replace string, index int) string {
 	return str[:index] + replace + str[index+1:]
 }
+
+// CalculateGCD (greates common divisor) calculates the biggest
+// positive integer that is divisible by a and b
+func CalculateGCD(a, b int) int {
+	for b != 0 {
+		t := b
+		b = a % b
+		a = t
+	}
+	return a
+}
+
+// CalculateLCM (least common multiple) calculates the smallest
+// positive integer that is divisible by all numbers
+func CalculateLCM(ints ...int) int {
+	if len(ints) == 1 {
+		return ints[0]
+	} else if len(ints) == 2 {
+		return ints[0] * ints[1] / CalculateGCD(ints[0], ints[1])
+	}
+
+	return CalculateLCM(ints[0], CalculateLCM(ints[1:]...))
+}