import gleam/bit_string
import gleam/string

/// Encodes a BitString into a base 64 encoded string.
///
pub fn encode64(input: BitString, padding: Bool) -> String {
  let encoded = do_encode64(input)
  case padding {
    True -> encoded
    False -> string.replace(encoded, "=", "")
  }
}

@external(erlang, "base64", "encode")
@external(javascript, "../gleam_stdlib.mjs", "encode64")
fn do_encode64(a: BitString) -> String

/// Decodes a base 64 encoded string into a `BitString`.
///
pub fn decode64(encoded: String) -> Result(BitString, Nil) {
  let padded = case bit_string.byte_size(bit_string.from_string(encoded)) % 4 {
    0 -> encoded
    n -> string.append(encoded, string.repeat("=", 4 - n))
  }
  do_decode64(padded)
}

@external(erlang, "gleam_stdlib", "base_decode64")
@external(javascript, "../gleam_stdlib.mjs", "decode64")
fn do_decode64(a: String) -> Result(BitString, Nil)

/// Encodes a `BitString` into a base 64 encoded string with URL and filename safe alphabet.
///
pub fn url_encode64(input: BitString, padding: Bool) -> String {
  encode64(input, padding)
  |> string.replace("+", "-")
  |> string.replace("/", "_")
}

/// Decodes a base 64 encoded string with URL and filename safe alphabet into a `BitString`.
///
pub fn url_decode64(encoded: String) -> Result(BitString, Nil) {
  encoded
  |> string.replace("-", "+")
  |> string.replace("_", "/")
  |> decode64()
}