aoc-2020-13-2

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
	"time"
)

func main() {
	t1 := time.Now()
	scanner := bufio.NewScanner(os.Stdin)
	line := 0
	for scanner.Scan() {
		txt := scanner.Text()
		line++
		if line == 1 {
			continue
		}
		var start int
		var step int
		for pos, v := range strings.Split(txt, ",") {
			if v == "x" {
				continue
			}
			mul, err := strconv.Atoi(v)
			if err != nil {
				panic(err)
			}
			if pos == 0 {
				start = pos
				step = mul
			} else {
				start, step = findOne(pos, mul, start, step)
			}
		}

		fmt.Printf("t=%d (%s)\n", start, time.Now().Sub(t1))
	}
}

func findOne(pos, mul, start, step int) (int, int) {
	k := 0
	for (start+k*step)%mul != ((mul-pos)%mul+mul)%mul {
		k++ // upping the ante
	}
	return start + k*step, step * mul
}