2019/day4/main.go

package main

import (
	"log"
	"math"
	"strconv"
)

func main() {
	countPasswords := CountPossiblePasswords(245318, 765747, false)
	countAdditionalPasswords := CountPossiblePasswords(245318, 765747, true)

	log.Println("Part1: " + strconv.Itoa(countPasswords))
	log.Println("Part2: " + strconv.Itoa(countAdditionalPasswords))
}

func CountPossiblePasswords(start int, end int, additional bool) int {
	sum := 0
	for i := start; i < end; i++ {
		if MeetPasswordCriteria(i, additional) {
			sum++
		}
	}
	return sum
}

func MeetPasswordCriteria(number int, additional bool) bool {

	if CountDigits(number) != 6 {
		return false
	}

	if !AreDigitsNotDecreasing(number) {
		return false
	}

	if !HaveDoubleInputs(number) {
		return false
	}

	if additional && !HasOneDouble(number) {
		return false
	}

	return true
}

func CountDigits(i int) int {
	count := 0
	for i != 0 {
		i /= 10
		count = count + 1
	}
	return count
}

func AreDigitsNotDecreasing(i int) bool {
	iStr := strconv.Itoa(i)
	prevInt := math.MinInt32

	for _, a := range iStr {
		aInt, _ := strconv.Atoi(string(a))

		if prevInt > aInt {
			return false
		}
		prevInt = aInt
	}
	return true
}

func HaveDoubleInputs(i int) bool {
	iStr := strconv.Itoa(i)
	hasDouble := false
	for i := 0; i < len(iStr)-1; i++ {
		if iStr[i] == iStr[i+1] {
			hasDouble = true
		}
	}
	return hasDouble
}

func HasOneDouble(i int) bool {
	iStr := strconv.Itoa(i)

	for len(iStr) >= 2 {
		if iStr[0] != iStr[1] {
			iStr = iStr[1:]
			continue
		}

		i := 1
		for len(iStr) > i && iStr[0] == iStr[i] {
			i++
		}

		if i == 2 {
			return true
		}

		if len(iStr) <= i {
			return false
		}

		iStr = iStr[i:]
	}
	return false
}