Day 2

day2/example.txt

@@ -0,0 +1,6 @@
+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9

day2/go.mod

@@ -0,0 +1,3 @@
+module stevenlr.com/aoc2024/day2
+
+go 1.22.2

day2/main.go

@@ -0,0 +1,95 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func IntAbs(a int) int {
+	if a >= 0 {
+		return a
+	}
+	return -a
+}
+
+func main() {
+	lists := readData("data.txt")
+	fmt.Println(doTheThing(lists, 0))
+	fmt.Println(doTheThing(lists, 1))
+}
+
+func readData(fileName string) (list [][]int) {
+	fp, err := os.Open(fileName)
+	if err != nil {
+		panic(err)
+	}
+
+	scanner := bufio.NewScanner(fp)
+
+	list = make([][]int, 0)
+
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		sublist := make([]int, 0)
+
+		for _, n := range strings.Split(line, " ") {
+			value, err := strconv.Atoi(n)
+			if err != nil {
+				panic(err)
+			}
+
+			sublist = append(sublist, value)
+		}
+
+		list = append(list, sublist)
+	}
+
+	return
+}
+
+func doTheThing(list [][]int, canSkip int) (valid int) {
+	for _, l := range list {
+		if isValid(l, canSkip, -1) {
+			valid += 1
+		}
+	}
+
+	return
+}
+
+func isValid(l []int, canSkip int, toSkip int) bool {
+	sumDiff := 0
+	sumAbsDiff := 0
+
+	for i := 0; i < len(l)-1; i++ {
+		if i == toSkip {
+			continue
+		}
+
+		j := i + 1
+		if j == toSkip {
+			j += 1
+			if j >= len(l) {
+				continue
+			}
+		}
+
+		diff := l[i] - l[j]
+
+		if diff == 0 || IntAbs(diff) > 3 || IntAbs(sumDiff+diff) != sumAbsDiff+IntAbs(diff) {
+			if canSkip > 0 {
+				return isValid(l, canSkip-1, i-1) || isValid(l, canSkip-1, i) || isValid(l, canSkip-1, j)
+			} else {
+				return false
+			}
+		}
+
+		sumDiff += diff
+		sumAbsDiff += IntAbs(diff)
+	}
+
+	return true
+}