package base62

import (
	"bytes"
	"errors"

	"github.com/samber/lo"
)

type Base62 struct {
	characters    []byte       // base62 编码规则
	charactersMap map[byte]int // base62 编码规则
}

func GetInstance() *Base62 {
	b62 := &Base62{}
	b62.SetCharacters([]byte("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"))
	return b62
}

func (base62 *Base62) SetCharacters(characters []byte) (*Base62, error) {
	characters = lo.FindUniques(characters)
	charactersMap := make(map[byte]int)

	if len(characters) != 62 {
		return nil, errors.New("Invalid characters")
	}

	for i, v := range characters {
		charactersMap[v] = i
	}

	base62.characters = characters
	base62.charactersMap = charactersMap

	return base62, nil
}

// Encode function receives a byte slice and encodes it to a string using the alphabet provided
func (base62 *Base62) Encode(source []byte) string {
	if len(source) == 0 {
		return ""
	}

	digits := []int{0}

	for i := 0; i < len(source); i++ {
		carry := int(source[i])

		for j := 0; j < len(digits); j++ {
			carry += digits[j] << 8
			digits[j] = carry % 62
			carry = carry / 62
		}

		for carry > 0 {
			digits = append(digits, carry%62)
			carry = carry / 62
		}
	}

	var res bytes.Buffer

	for k := 0; source[k] == 0 && k < len(source)-1; k++ {
		res.WriteByte(base62.characters[0])
	}

	for q := len(digits) - 1; q >= 0; q-- {
		res.WriteByte(base62.characters[digits[q]])
	}

	return res.String()
}

// Decode function decodes a string previously obtained from Encode, using the same alphabet and returns a byte slice
// In case the input is not valid an arror will be returned
func (base62 *Base62) Decode(source string) ([]byte, error) {
	if len(source) == 0 {
		return []byte{}, nil
	}

	runes := []byte(source)

	bytes := []byte{0}
	for i := 0; i < len(runes); i++ {
		value, ok := base62.charactersMap[runes[i]]

		if !ok {
			return nil, errors.New("Non Base Character")
		}

		carry := int(value)

		for j := 0; j < len(bytes); j++ {
			carry += int(bytes[j]) * 62
			bytes[j] = byte(carry & 0xff)
			carry >>= 8
		}

		for carry > 0 {
			bytes = append(bytes, byte(carry&0xff))
			carry >>= 8
		}
	}

	for k := 0; runes[k] == base62.characters[0] && k < len(runes)-1; k++ {
		bytes = append(bytes, 0)
	}

	// Reverse bytes
	for i, j := 0, len(bytes)-1; i < j; i, j = i+1, j-1 {
		bytes[i], bytes[j] = bytes[j], bytes[i]
	}

	return bytes, nil
}