package invited_code

import (
	"fmt"
	"math"
	"strings"
)

type Converter struct {
	Base         int
	Length       int
	ConvertTable []string
}

func (c *Converter) EncodeFromNum(id int64) (string, error) {
	code, err := generateCode(id, c.Base, c.Length, c.ConvertTable)
	if err != nil {
		return "", err
	}

	return code, nil
}

func (c *Converter) DecodeFromCode(code string) (int64, error) {
	var result int64 = 0
	length := len(code)

	for i := 0; i < length; i++ {
		char := string(code[i])
		index := -1
		for j, v := range c.ConvertTable {
			if v == char {
				index = j
				break
			}
		}

		if index >= 0 {
			result = result*int64(c.Base) + int64(index)
		} else {
			return 0, fmt.Errorf("character not found in convert table")
		}
	}
	return result, nil
}

// generateCode 從 UID 生成 referralCode
func generateCode(id int64, base int, length int, convertTable []string) (string, error) {
	maxReferralUIDBoundary := int64(math.Pow(float64(len(ConvertTable)), float64(DefaultCodeLen)))
	if id > maxReferralUIDBoundary {
		return "", fmt.Errorf("encode out of range")
	}

	encoded := encodeToBase(id, base, length, convertTable)

	return encoded, nil
}

func encodeToBase(num int64, base int, length int, convertTable []string) string {
	result := ""
	for num > 0 {
		index := num % int64(base)
		result = convertTable[index] + result
		num /= int64(base)
	}

	if len(result) < length {
		result = strings.Repeat(convertTable[0], length-len(result)) + result
	}

	return result
}

func MustConverter(base int, length int, convertTable []string) ConvertUseCase {
	return &Converter{
		Base:         base,
		Length:       length,
		ConvertTable: convertTable,
	}
}