Solve Day 9

Signed-off-by: Alexander Kurbatov <sir.alkurbatov@yandex.ru>

Author: alkurbatov

2023/9/main.go

@@ -0,0 +1,80 @@
+// --- Day 9: Mirage Maintenance ---
+// https://adventofcode.com/2023/day/9
+package main
+
+import (
+	"bufio"
+	"log"
+	"os"
+
+	"github.com/alkurbatov/adventofcode/internal/parsers"
+)
+
+func readInput(path string) ([][]int, error) {
+	input, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer input.Close()
+
+	scanner := bufio.NewScanner(input)
+	result := make([][]int, 0)
+
+	for scanner.Scan() {
+		sequence, err := parsers.ReadNumbers(scanner.Text())
+		if err != nil {
+			return nil, err
+		}
+
+		result = append(result, sequence)
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}
+
+func calcPredictions(values []int) (left, right int) {
+	stepDiff := make([]int, 0)
+	allZeroes := true
+
+	for i := 1; i < len(values); i++ {
+		nextValue := values[i] - values[i-1]
+
+		if nextValue != 0 {
+			allZeroes = false
+		}
+
+		stepDiff = append(stepDiff, nextValue)
+	}
+
+	if allZeroes {
+		return values[0], values[len(values)-1]
+	}
+
+	left, right = calcPredictions(stepDiff)
+
+	return values[0] - left, right + values[len(values)-1]
+}
+
+func main() {
+	input, err := readInput("2023/9/input.txt")
+	if err != nil {
+		log.Printf("Failed to read data from input.txt: %v", err)
+		return
+	}
+
+	var firstPartResult, secondPartResult int
+
+	for _, history := range input {
+		left, right := calcPredictions(history)
+
+		firstPartResult += right
+		secondPartResult += left
+	}
+
+	log.Printf("Part 1 result: %d", firstPartResult)
+	log.Printf("Part 2 result: %d", secondPartResult)
+}

internal/parsers/numbers.go

@@ -0,0 +1,22 @@
+package parsers
+
+import (
+	"strconv"
+	"strings"
+)
+
+// Read sequence of numbers from string.
+func ReadNumbers(src string) ([]int, error) {
+	result := make([]int, 0)
+
+	for _, field := range strings.Fields(src) {
+		num, err := strconv.Atoi(field)
+		if err != nil {
+			return nil, err
+		}
+
+		result = append(result, num)
+	}
+
+	return result, nil
+}