2024: day 5. part 1 and 2. golang

Author: Tsovak

src/2024/day05/1/main.go

@@ -0,0 +1,80 @@
+package main
+
+import (
+	_ "embed"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+//go:embed data.txt
+var fileContents string
+
+type rule struct {
+	before, after int
+}
+
+func parse(input string) ([]rule, [][]int) {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+
+	separatorIdx := -1
+	for i, line := range lines {
+		if line == "" {
+			separatorIdx = i
+			break
+		}
+	}
+
+	rules := make([]rule, len(lines[:separatorIdx]))
+	for i, line := range lines[:separatorIdx] {
+		parts := strings.Split(line, "|")
+		before, _ := strconv.Atoi(parts[0])
+		after, _ := strconv.Atoi(parts[1])
+		rules[i] = rule{before, after}
+	}
+
+	// parse updates
+	var updates [][]int
+	for _, line := range lines[separatorIdx+1:] {
+		var update []int
+		for _, num := range strings.Split(line, ",") {
+			n, _ := strconv.Atoi(num)
+			update = append(update, n)
+		}
+		updates = append(updates, update)
+	}
+
+	return rules, updates
+}
+
+func isValidUpdate(update []int, rules []rule) bool {
+	// for quick lookups
+	indexMap := make(map[int]int)
+	for i, num := range update {
+		indexMap[num] = i
+	}
+
+	for _, rule := range rules {
+		beforeIdx, beforeExists := indexMap[rule.before]
+		afterIdx, afterExists := indexMap[rule.after]
+
+		if beforeExists && afterExists && beforeIdx > afterIdx {
+			return false
+		}
+	}
+	return true
+}
+
+func main() {
+	rules, updates := parse(fileContents)
+
+	sum := 0
+	for _, update := range updates {
+		if isValidUpdate(update, rules) {
+			middleIdx := len(update) / 2
+			sum += update[middleIdx]
+		}
+	}
+
+	fmt.Println(sum)
+}

src/2024/day05/2/main.go

@@ -0,0 +1,141 @@
+package main
+
+import (
+	_ "embed"
+	"fmt"
+	"slices"
+	"strconv"
+	"strings"
+)
+
+//go:embed data.txt
+var fileContents string
+
+type rule struct {
+	before, after int
+}
+
+func parse(input string) ([]rule, [][]int) {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+
+	separatorIdx := -1
+	for i, line := range lines {
+		if line == "" {
+			separatorIdx = i
+			break
+		}
+	}
+
+	rules := make([]rule, len(lines[:separatorIdx]))
+	for i, line := range lines[:separatorIdx] {
+		parts := strings.Split(line, "|")
+		before, _ := strconv.Atoi(parts[0])
+		after, _ := strconv.Atoi(parts[1])
+		rules[i] = rule{before, after}
+	}
+
+	// parse updates
+	var updates [][]int
+	for _, line := range lines[separatorIdx+1:] {
+		var update []int
+		for _, num := range strings.Split(line, ",") {
+			n, _ := strconv.Atoi(num)
+			update = append(update, n)
+		}
+		updates = append(updates, update)
+	}
+
+	return rules, updates
+}
+
+func isValidUpdate(update []int, rules []rule) bool {
+	// for quick lookups
+	indexMap := make(map[int]int)
+	for i, num := range update {
+		indexMap[num] = i
+	}
+
+	for _, rule := range rules {
+		beforeIdx, beforeExists := indexMap[rule.before]
+		afterIdx, afterExists := indexMap[rule.after]
+
+		if beforeExists && afterExists && beforeIdx > afterIdx {
+			return false
+		}
+	}
+	return true
+}
+
+func buildGraph(update []int, rules []rule) map[int][]int {
+	graph := make(map[int][]int)
+	inDegree := make(map[int]int)
+
+	for _, num := range update {
+		if _, exists := graph[num]; !exists {
+			graph[num] = []int{}
+		}
+	}
+
+	for _, rule := range rules {
+		if slices.Contains(update, rule.before) && slices.Contains(update, rule.after) {
+			graph[rule.before] = append(graph[rule.before], rule.after)
+			inDegree[rule.after]++
+		}
+	}
+
+	return graph
+}
+
+func topologicalSort(update []int, rules []rule) []int {
+	graph := buildGraph(update, rules)
+	inDegree := make(map[int]int)
+	result := make([]int, 0, len(update))
+
+	// initial in-degrees
+	for _, num := range update {
+		inDegree[num] = 0
+	}
+	for _, rule := range rules {
+		if slices.Contains(update, rule.before) && slices.Contains(update, rule.after) {
+			inDegree[rule.after]++
+		}
+	}
+
+	// find nodes with no incoming edges
+	var queue []int
+	for _, num := range update {
+		if inDegree[num] == 0 {
+			queue = append(queue, num)
+		}
+	}
+
+	for len(queue) > 0 {
+		current := queue[0]
+		queue = queue[1:]
+		result = append(result, current)
+
+		for _, neighbor := range graph[current] {
+			inDegree[neighbor]--
+			if inDegree[neighbor] == 0 {
+				queue = append(queue, neighbor)
+			}
+		}
+	}
+
+	return result
+}
+
+func main() {
+	rules, updates := parse(fileContents)
+
+	sum := 0
+	for _, update := range updates {
+		if !isValidUpdate(update, rules) {
+			sortedUpdate := topologicalSort(update, rules)
+			middleIdx := len(sortedUpdate) / 2
+			sum += sortedUpdate[middleIdx]
+		}
+	}
+
+	fmt.Println(sum)
+}