// Package domain provides the domain logic for the extended FizzBuzz computation.
package domain

import (
	"errors"
	"fmt"
	"math"
	"regexp"
	"strconv"
)

type (
	Empty     struct{}
	modulo    int64
	moduloMap map[modulo]struct{}
)

const (
	Fizz = "fizz"
	Buzz = "buzz"
	Both = Fizz + Buzz
)

var (
	// RFC3986GenDelims matches the reserved characters in RFC3986 §2.2
	//
	// See https://www.rfc-editor.org/rfc/rfc3986#section-2.2
	RFC3986GenDelims = regexp.MustCompile(`[:|/\?#\[\]@]`)
	Void             Empty
)

// IsValid returns true if the received is in the open interval ]1,limit[,
// and is not already present in the existing map.
func (m modulo) IsValid(limit modulo, existing moduloMap) bool {
	if m <= 1 || m > limit {
		return false
	}
	if _, found := existing[m]; found {
		return false
	}
	return true
}

// FizzBuzzes is an ordered list of FizzBuzz strings;
type FizzBuzzes []string

// validateNumbers fails if either of the numbers fails validation.
//
// A successful return means that limit is >= 1 and int1 and int2 are different and within [1,limit[.
func validateNumbers(int1, int2, limit int) error {
	existing := make(moduloMap, 2)
	l := modulo(limit)
	if !l.IsValid(math.MaxInt, existing) {
		return errors.New("limit is invalid")
	}
	m1 := modulo(int1)
	if !m1.IsValid(l, existing) {
		return errors.New("int1 is invalid")
	}
	existing[m1] = Void
	m2 := modulo(int2)
	if !m2.IsValid(l, existing) {
		return errors.New("int2 is invalid")
	}
	return nil
}

func validateStrings(str1, str2 string) error {
	validateString := func(s string) error {
		if len(s) == 0 {
			return fmt.Errorf("invalid string parameter: empty")
		}
		if RFC3986GenDelims.MatchString(s) {
			return fmt.Errorf("contains a RFC-3986 reserved character")
		}
		return nil
	}
	ss := [...]string{str1, str2}
	for i, s := range ss {
		if err := validateString(s); err != nil {
			return fmt.Errorf("validateString(%d): %w", i, err)
		}
	}
	return nil
}

// FizzBuzz builds a list of strings with numbers in the closed interval [1,limit], where:
//
//   - all multiples of int1 are replaced by str1, except those that are also multiples of int2
//   - all multiples of int2 are replaced by str2, except those that are also multiples of int1
//   - all multiples of int1 and int2 are replaced by str1str2.
func FizzBuzz(int1, int2, limit int, str1, str2 string) (FizzBuzzes, error) {
	if err := validateNumbers(int1, int2, limit); err != nil {
		return nil, fmt.Errorf("invalid modulo parameters: %w", err)
	}
	if err := validateStrings(str1, str2); err != nil {
		return nil, fmt.Errorf("invalid string parameters: %w", err)
	}
	both := str1 + str2

	fbs := make(FizzBuzzes, 0, limit)

	// Arithmetic for loop on append to a preallocated slice is usually faster than
	// indexed access as in fbs := make(FizzBuzzes, limit); loop on fbs[i] = <value>.
	for i := 1; i <= limit; i++ {
		// The switch is more readable than the usual nested "if" solution.
		switch {
		case i%int1 == 0 && i%int2 == 0:
			fbs = append(fbs, both)
		case i%int1 == 0:
			fbs = append(fbs, str1)
		case i%int2 == 0:
			fbs = append(fbs, str2)
		default:
			fbs = append(fbs, strconv.Itoa(i))
		}
	}
	return fbs, nil
}