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
}