day09 solutions

pkg/ints/ints.go

@@ -43,3 +43,12 @@ func Min(numbers []int) int {
 	}
 	return m
 }
+
+func AllSame(numbers []int) bool {
+	for i := 1; i < len(numbers); i++ {
+		if numbers[i] != numbers[i-1] {
+			return false
+		}
+	}
+	return true
+}

pkg/ints/ints_test.go

@@ -59,3 +59,23 @@ func TestAbs(t *testing.T) {
 		assert.Equal(t, expected, actual)
 	})
 }
+
+func TestAllSame(t *testing.T) {
+
+	t.Run("Should return true if all values in a slice are the same", func(t *testing.T) {
+		assert.True(t, AllSame([]int{1, 1, 1}))
+	})
+
+	t.Run("Should return false if all values in a slice are not the same", func(t *testing.T) {
+		assert.False(t, AllSame([]int{1, 2, 1}))
+	})
+}
+
+func TestMin(t *testing.T) {
+
+	t.Run("Should return the minimum value in a slice of ints", func(t *testing.T) {
+		expected := 1
+		actual := Min([]int{1, 2, 3})
+		assert.Equal(t, expected, actual)
+	})
+}

puzzles/day09/main.go

@@ -0,0 +1,83 @@
+package main
+
+import (
+	"fmt"
+	"github.com/kylehoehns/aoc-2023-go/pkg/files"
+	"github.com/kylehoehns/aoc-2023-go/pkg/ints"
+	"slices"
+	"strings"
+)
+
+func main() {
+	fmt.Println("Part 1: ", performMirageMaintenance("input.txt", false))
+	fmt.Println("Part 2: ", performMirageMaintenance("input.txt", true))
+}
+
+func performMirageMaintenance(name string, reverse bool) int {
+	lines := files.ReadLines(name)
+	report := createEnvironmentalReport(lines, reverse)
+
+	sumOfPredicatedValues := 0
+	for _, history := range report.histories {
+		sumOfPredicatedValues += history.predictNextValue()
+	}
+
+	return sumOfPredicatedValues
+}
+
+type EnvironmentalReport struct {
+	histories []History
+}
+
+func createEnvironmentalReport(lines []string, reverse bool) EnvironmentalReport {
+	histories := make([]History, 0)
+	for _, line := range lines {
+		values := ints.FromStringSlice(strings.Fields(line))
+		if reverse {
+			slices.Reverse(values)
+		}
+		histories = append(histories, History{values})
+	}
+	return EnvironmentalReport{histories}
+}
+
+type History struct {
+	values []int
+}
+
+func (h History) predictNextValue() int {
+	allHistoryValues := h.extrapolateAndBuildAllHistories()
+	updatedHistories := make([][]int, len(allHistoryValues))
+
+	for i := len(allHistoryValues) - 1; i >= 0; i-- {
+		historyCopy := make([]int, len(allHistoryValues[i]))
+		copy(historyCopy, allHistoryValues[i])
+
+		if ints.AllSame(historyCopy) {
+			historyCopy = append(historyCopy, historyCopy[0])
+		} else {
+			lastValue := historyCopy[len(historyCopy)-1]
+			lastValueInRowBelow := updatedHistories[i+1][len(updatedHistories[i+1])-1]
+			historyCopy = append(historyCopy, lastValue+lastValueInRowBelow)
+		}
+		updatedHistories[i] = historyCopy
+	}
+	return updatedHistories[0][len(updatedHistories[0])-1]
+}
+
+func (h History) extrapolateAndBuildAllHistories() [][]int {
+	var allHistoryValues [][]int
+	allHistoryValues = append(allHistoryValues, h.values)
+	currentHistory := h
+
+	for !ints.AllSame(currentHistory.values) {
+		newValues := make([]int, len(currentHistory.values)-1)
+		for i := 1; i < len(currentHistory.values); i++ {
+			newValues[i-1] = currentHistory.values[i] - currentHistory.values[i-1]
+		}
+		currentHistory = History{newValues}
+		allHistoryValues = append(allHistoryValues, currentHistory.values)
+	}
+
+	return allHistoryValues
+}

puzzles/day09/main_test.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPart1(t *testing.T) {
+
+	t.Run("Part 1", func(t *testing.T) {
+		expected := 114
+		actual := performMirageMaintenance("test-input.txt", false)
+		assert.Equal(t, expected, actual)
+	})
+
+}
+
+func TestPart2(t *testing.T) {
+
+	t.Run("Part 2", func(t *testing.T) {
+		expected := 2
+		actual := performMirageMaintenance("test-input.txt", true)
+		assert.Equal(t, expected, actual)
+	})
+
+}

puzzles/day09/test-input.txt

@@ -0,0 +1,3 @@
+0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45