Add solution for day 9 part 1

day9/main.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	if len(os.Args) != 2 && len(os.Args) != 3 {
+		fmt.Fprintf(os.Stderr, "Usage: %s inputfile\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	inputFilename := os.Args[1]
+	inputFile, err := os.Open(inputFilename)
+	if err != nil {
+		panic(fmt.Sprintf("could not open input file: %s", err))
+	}
+
+	defer inputFile.Close()
+
+	inputBytes, err := io.ReadAll(inputFile)
+	if err != nil {
+		panic(fmt.Sprintf("could not read input file: %s", err))
+	}
+
+	input := strings.TrimSpace(string(inputBytes))
+	inputLines := strings.Split(input, "\n")
+	histories, err := parseHistories(inputLines)
+	if err != nil {
+		panic(fmt.Sprintf("failed to parse histories: %s", err))
+	}
+
+	fmt.Printf("Part 1: %d\n", part1(histories))
+}
+
+func part1(histories [][]int) int {
+	total := 0
+	for _, history := range histories {
+		total += predictNextValue(history)
+	}
+
+	return total
+}
+
+// predictNextValue calculates the next item in the sequence by using the nth differences
+func predictNextValue(history []int) int {
+	if allEqual(history, 0) {
+		return 0
+	}
+
+	differences := make([]int, len(history)-1)
+	for i := range history[1:] {
+		differences[i] = history[i+1] - history[i]
+	}
+
+	return history[len(history)-1] + predictNextValue(differences)
+}
+
+func allEqual[T comparable, S ~[]T](values S, shouldEqual T) bool {
+	for _, value := range values {
+		if value != shouldEqual {
+			return false
+		}
+	}
+
+	return true
+}
+
+func parseHistories(lines []string) ([][]int, error) {
+	histories := make([][]int, len(lines))
+	for i, line := range lines {
+		history, err := parseHistory(line)
+		if err != nil {
+			return nil, fmt.Errorf("parse history %d: %s", i, err)
+		}
+
+		histories[i] = history
+	}
+
+	return histories, nil
+}
+
+func parseHistory(line string) ([]int, error) {
+	rawNums := strings.Split(line, " ")
+	history := make([]int, len(rawNums))
+	for i, rawNum := range rawNums {
+		n, err := strconv.Atoi(rawNum)
+		if err != nil {
+			return nil, fmt.Errorf("parse item %d: %w", i, err)
+		}
+
+		history[i] = n
+	}
+
+	return history, nil
+}