package jwt

import (
	"encoding/base64"
	"fmt"
)

// Base64EncodedBufferLen return the size needed to encode the
// Base64Encode (padding '=' would be added, can be stripped with
// Base64DecodedStrippedLen
func Base64EncodedBufferLen(n int) int {
	return base64.URLEncoding.EncodedLen(n)
}

// Base64EncodedStrippedLen return the size of the encoded slice with no
// padding.
func Base64EncodedStrippedLen(n int) int {
	eN := Base64EncodedBufferLen(n)
	mod := n % 3
	if mod == 0 {
		return eN
	}
	return eN - (3 - mod)
}

// Base64DecodedStrippedLen returns the size of the data, without 0
// bit padding, from the stripped length n.
func Base64DecodedStrippedLen(n int) int {
	return Base64DecodedLenFromStripped(n) + n - ((n+3)/4)*4
}

// Base64DecodedLenFromStripped returns the size of the data, with 0
// bit padding, from the stripped length n.
func Base64DecodedLenFromStripped(n int) int {
	return base64.URLEncoding.DecodedLen(4 * ((n + 3) / 4))
}

// Base64Decode decodes a payload of data as a []byte, from a stripped
// '=' encoded string.
func Base64Decode(dst, src []byte) error {
	switch len(src) % 4 {
	// in this case, we have to copy the src, in order to allow decode part of the input
	case 2:
		oldSrc := src
		src = make([]byte, len(src)+2)
		copy(src, oldSrc)
		src[len(src)-2] = '='
		src[len(src)-1] = '='
	case 3:
		oldSrc := src
		src = make([]byte, len(src)+1)
		copy(src, oldSrc)
		src[len(src)-1] = '='
	case 1:
		return fmt.Errorf("jwt: Invalid base64 string (length:%d %% 4 == 1): '%s'", len(src), src)
	}
	_, err := base64.URLEncoding.Decode(dst, src)
	if err != nil {
		return fmt.Errorf("jwt: %s", err)
	}

	return nil
}