ids

//// A module for generating UUIDs (Universally Unique Identifiers).
////
//// The module currently supports UUID versions:
//// - Version 4 (random)
////

import gleam/bit_string

/// Generates a version 4 (random) UUID. The version 4 UUID produced
/// by this function is generated using a cryptographically secure 
/// random number generator.
///
/// ### Usage
/// ```gleam
/// import ids/uuid
///
/// assert Ok(id) = uuid.generate_v4()
/// ```
///
pub fn generate_v4() -> Result(String, String) {
  let <<u0:size(48), _:size(4), u1:size(12), _:size(2), u2:size(62)>> =
    crypto_strong_rand_bytes(16)

  let <<
    a1:size(4),
    a2:size(4),
    a3:size(4),
    a4:size(4),
    a5:size(4),
    a6:size(4),
    a7:size(4),
    a8:size(4),
    b1:size(4),
    b2:size(4),
    b3:size(4),
    b4:size(4),
    c1:size(4),
    c2:size(4),
    c3:size(4),
    c4:size(4),
    d1:size(4),
    d2:size(4),
    d3:size(4),
    d4:size(4),
    e1:size(4),
    e2:size(4),
    e3:size(4),
    e4:size(4),
    e5:size(4),
    e6:size(4),
    e7:size(4),
    e8:size(4),
    e9:size(4),
    e10:size(4),
    e11:size(4),
    e12:size(4),
  >> = <<u0:size(48), 4:size(4), u1:size(12), 2:size(2), u2:size(62)>>

  let bitstr_id = <<
    e(a1),
    e(a2),
    e(a3),
    e(a4),
    e(a5),
    e(a6),
    e(a7),
    e(a8),
    45,
    e(b1),
    e(b2),
    e(b3),
    e(b4),
    45,
    e(c1),
    e(c2),
    e(c3),
    e(c4),
    45,
    e(d1),
    e(d2),
    e(d3),
    e(d4),
    45,
    e(e1),
    e(e2),
    e(e3),
    e(e4),
    e(e5),
    e(e6),
    e(e7),
    e(e8),
    e(e9),
    e(e10),
    e(e11),
    e(e12),
  >>

  case bit_string.to_string(bitstr_id) {
    Ok(str_id) ->
      str_id
      |> Ok
    Error(_) -> {
      let error: String = "Error: BitString could not be converted to String."
      error
      |> Error
    }
  }
}

fn e(n: Int) -> Int {
  case n {
    0 -> 48
    1 -> 49
    2 -> 50
    3 -> 51
    4 -> 52
    5 -> 53
    6 -> 54
    7 -> 55
    8 -> 56
    9 -> 57
    10 -> 97
    11 -> 98
    12 -> 99
    13 -> 100
    14 -> 101
    15 -> 102
  }
}

external fn crypto_strong_rand_bytes(Int) -> BitString =
  "crypto" "strong_rand_bytes"