package apg

import (
	"crypto/rand"
	"encoding/binary"
	"errors"
	"fmt"
	"math/big"
	"strings"
)

const (
	// 7 bits to represent a letter index
	letterIdxBits = 7
	// All 1-bits, as many as letterIdxBits
	letterIdxMask = 1<<letterIdxBits - 1
	// # of letter indices fitting in 63 bits)
	letterIdxMax = 63 / letterIdxBits
)

var (
	// ErrInvalidLength is returned if the provided maximum number is equal or less than zero
	ErrInvalidLength = errors.New("provided length value cannot be zero or less")
	// ErrLengthMismatch is returned if the number of generated bytes does not match the expected length
	ErrLengthMismatch = errors.New("number of generated random bytes does not match the expected length")
	// ErrInvalidCharRange is returned if the given range of characters is not valid
	ErrInvalidCharRange = errors.New("provided character range is not valid or empty")
)

// RandomBytes returns a byte slice of random bytes with length n that got generated by
// the crypto/rand generator
func (g *Generator) RandomBytes(n int64) ([]byte, error) {
	if n < 1 {
		return nil, ErrInvalidLength
	}
	b := make([]byte, n)
	l, err := rand.Read(b)
	if int64(l) != n {
		return nil, ErrLengthMismatch
	}
	if err != nil {
		return nil, err
	}

	return b, nil
}

// RandomStringFromCharRange returns a random string of length l based of the range of characters given.
// The method makes use of the crypto/random package and therfore is
// cryptographically secure
func (g *Generator) RandomStringFromCharRange(l int, cr string) (string, error) {
	if l < 1 {
		return "", ErrInvalidLength
	}
	if len(cr) < 1 {
		return "", ErrInvalidCharRange
	}
	rs := strings.Builder{}
	rs.Grow(l)
	crl := len(cr)

	rp := make([]byte, 8)
	_, err := rand.Read(rp)
	if err != nil {
		return rs.String(), err
	}
	for i, c, r := l-1, binary.BigEndian.Uint64(rp), letterIdxMax; i >= 0; {
		if r == 0 {
			_, err = rand.Read(rp)
			if err != nil {
				return rs.String(), err
			}
			c, r = binary.BigEndian.Uint64(rp), letterIdxMax
		}
		if idx := int(c & letterIdxMask); idx < crl {
			rs.WriteByte(cr[idx])
			i--
		}
		c >>= letterIdxBits
		r--
	}

	return rs.String(), nil
}

// RandNum generates a random, non-negative number with given maximum value
func (g *Generator) RandNum(m int64) (int64, error) {
	if m < 1 {
		return 0, ErrInvalidLength
	}
	mbi := big.NewInt(m)
	rn, err := rand.Int(rand.Reader, mbi)
	if err != nil {
		return 0, fmt.Errorf("random number generation failed: %w", err)
	}
	return rn.Int64(), nil
}

// CoinFlip performs a simple coinflip based on the rand library and returns 1 or 0
func (g *Generator) CoinFlip() int64 {
	cf, _ := g.RandNum(2)
	return cf
}

// CoinFlipBool performs a simple coinflip based on the rand library and returns true or false
func (g *Generator) CoinFlipBool() bool {
	return g.CoinFlip() == 1
}