From 4a7c9082ba71c4bed73731ccd914f7f1bf9c412c Mon Sep 17 00:00:00 2001
From: onyx-and-iris <code@onyxandiris.online>
Date: Sat, 9 Dec 2023 17:25:53 +0000
Subject: [PATCH] day-9

---
 day-9/go.mod      |  3 +++
 day-9/one.go      | 25 +++++++++++++++++++++++++
 day-9/run.sh      |  5 +++++
 day-9/solution.go | 21 +++++++++++++++++++++
 day-9/two.go      | 25 +++++++++++++++++++++++++
 day-9/util.go     | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 6 files changed, 126 insertions(+)
 create mode 100644 day-9/go.mod
 create mode 100644 day-9/one.go
 create mode 100644 day-9/run.sh
 create mode 100644 day-9/solution.go
 create mode 100644 day-9/two.go
 create mode 100644 day-9/util.go

diff --git a/day-9/go.mod b/day-9/go.mod
new file mode 100644
index 0000000..2ec3aca
--- /dev/null
+++ b/day-9/go.mod
@@ -0,0 +1,3 @@
+module github.com/onyx-and-iris/aoc2023/day-9
+
+go 1.20
diff --git a/day-9/one.go b/day-9/one.go
new file mode 100644
index 0000000..a35fc56
--- /dev/null
+++ b/day-9/one.go
@@ -0,0 +1,25 @@
+package main
+
+// next returns the next value in the sequence
+func next(nums []int) int {
+	diffs := []int{}
+	for i := 1; i < len(nums); i++ {
+		diffs = append(diffs, nums[i]-nums[i-1])
+	}
+
+	if allEqual(diffs) {
+		return nums[len(nums)-1] + diffs[len(diffs)-1]
+	}
+	return nums[len(nums)-1] + next(diffs)
+}
+
+// one returns the sum of all next values
+func one(lines []string) int {
+	sum := 0
+	for _, line := range lines {
+		nums := convertToInts(line)
+		sum += next(nums)
+	}
+
+	return sum
+}
diff --git a/day-9/run.sh b/day-9/run.sh
new file mode 100644
index 0000000..3f6776a
--- /dev/null
+++ b/day-9/run.sh
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+INPUT="input.txt"
+
+cat $INPUT | go run .
\ No newline at end of file
diff --git a/day-9/solution.go b/day-9/solution.go
new file mode 100644
index 0000000..926f1d4
--- /dev/null
+++ b/day-9/solution.go
@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+
+	log "github.com/sirupsen/logrus"
+)
+
+func init() {
+	log.SetLevel(log.InfoLevel)
+}
+
+func main() {
+	lines := readlines()
+
+	ans := one(lines)
+	fmt.Printf("solution one: %d\n", ans)
+
+	ans = two(lines)
+	fmt.Printf("solution two: %d\n", ans)
+}
diff --git a/day-9/two.go b/day-9/two.go
new file mode 100644
index 0000000..b0921e5
--- /dev/null
+++ b/day-9/two.go
@@ -0,0 +1,25 @@
+package main
+
+// previous returns the previous value in the sequence
+func previous(nums []int) int {
+	diffs := []int{}
+	for i := 1; i < len(nums); i++ {
+		diffs = append(diffs, nums[i]-nums[i-1])
+	}
+
+	if allEqual(diffs) {
+		return nums[0] - diffs[0]
+	}
+	return nums[0] - previous(diffs)
+}
+
+// two returns the sum of all previous values
+func two(lines []string) int {
+	sum := 0
+	for _, line := range lines {
+		nums := convertToInts(line)
+		sum += previous(nums)
+	}
+
+	return sum
+}
diff --git a/day-9/util.go b/day-9/util.go
new file mode 100644
index 0000000..9e41e11
--- /dev/null
+++ b/day-9/util.go
@@ -0,0 +1,47 @@
+package main
+
+import (
+	"bufio"
+	"log"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// readlines reads lines from stdin.
+// returns input as an array of strings
+func readlines() []string {
+	lines := []string{}
+
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+
+	if err := scanner.Err(); err != nil {
+		log.Fatal(err)
+	}
+
+	return lines
+}
+
+// convertToInts converts a string representing ints to an array of ints
+func convertToInts(line string) []int {
+	numStr := strings.Split(line, " ")
+	nums := make([]int, 0)
+	for _, elem := range numStr {
+		n, _ := strconv.Atoi(elem)
+		nums = append(nums, n)
+	}
+	return nums
+}
+
+// allEqual returns true if all values in array are equal
+func allEqual(a []int) bool {
+	for i := 1; i < len(a); i++ {
+		if a[i] != a[0] {
+			return false
+		}
+	}
+	return true
+}