From 40fd20224473cccc49cafee598593b4fb9ace8ec Mon Sep 17 00:00:00 2001
From: Nico <nico@astolfo.org>
Date: Tue, 2 Dec 2025 22:42:06 +1100
Subject: [PATCH] add aoc 2025 day 2 solution

---
 2025/2/README.md |  8 ++++++
 2025/2/input.txt |  1 +
 2025/2/main.go   | 75 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+)
 create mode 100644 2025/2/README.md
 create mode 100644 2025/2/input.txt
 create mode 100644 2025/2/main.go

diff --git a/2025/2/README.md b/2025/2/README.md
new file mode 100644
index 0000000..e780b8b
--- /dev/null
+++ b/2025/2/README.md
@@ -0,0 +1,8 @@
+# aoc 2025 day 2
+
+run with:
+```sh
+go run main.go
+```
+
+you will need to put your input into input.txt first though. replace input.txt with your puzzle input
diff --git a/2025/2/input.txt b/2025/2/input.txt
new file mode 100644
index 0000000..a6d92df
--- /dev/null
+++ b/2025/2/input.txt
@@ -0,0 +1 @@
+197-407,262128-339499,557930-573266,25-57,92856246-93001520,2-12,1919108745-1919268183,48414903-48538379,38342224-38444598,483824-534754,1056-1771,4603696-4688732,75712519-75792205,20124-44038,714164-782292,4429019-4570680,9648251-9913729,6812551522-6812585188,58-134,881574-897488,648613-673853,5261723647-5261785283,60035-128980,9944818-10047126,857821365-857927915,206885-246173,1922-9652,424942-446151,408-1000
diff --git a/2025/2/main.go b/2025/2/main.go
new file mode 100644
index 0000000..a3db9aa
--- /dev/null
+++ b/2025/2/main.go
@@ -0,0 +1,75 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+type Range struct {
+	From int
+	To int
+}
+
+func main() {
+	// iterate over all IDs
+	var invalidsum int = 0
+	ranges := Decode("input.txt")
+
+	for _, i := range ranges {
+		fmt.Printf("\niterating over range: %d to %d\n", i.From, i.To)
+
+	  // +1 makes the range inclusive
+		for x := range i.To-i.From+1 {
+			// to get the "real" number as we took it away initially
+			num := x+i.From
+
+			// extract half of the number
+			numstring := strconv.Itoa(num)
+			mid := len(numstring)/2
+
+			// if both halves are the same, it is an invalid number
+			if numstring[:mid] == numstring[mid:] {
+				fmt.Printf("found invalid id %d\n", num)
+				invalidsum = invalidsum + num
+			}
+		}
+	}
+
+	fmt.Println("sum of invalid ids:", invalidsum)
+}
+
+func Decode(filename string) []Range {
+	f, err := os.ReadFile(filename)
+
+	if err != nil {
+		panic(err)
+	}
+
+	fstring := string(f)
+	fromrangeregex := regexp.MustCompile("^[0-9]*")
+	torangeregex := regexp.MustCompile("[0-9]*$")
+
+	var ranges []Range
+
+	for _, i := range strings.Split(fstring, ",") {
+		// get range from IDs to iterate over
+		fromrangestr := fromrangeregex.FindAllString(i, 1)
+		torangestr := torangeregex.FindAllString(i, 1)
+
+		var a Range
+
+		fromrange, _ := strconv.Atoi(fromrangestr[0])
+		torange, _ := strconv.Atoi(torangestr[0])
+
+		a.From = fromrange
+		a.To = torange
+
+		fmt.Printf("\nextracted ranges to iterate over: %d to %d\n", fromrange, torange)
+		ranges = append(ranges, a)
+	}
+
+	return ranges
+}