defmodule UXID do
  @moduledoc """
  Generates UXIDs and acts as an Ecto ParameterizedType

  **U**ser e**X**perience focused **ID**entifiers (UXIDs) are identifiers which:

  * Describe the resource (aid in debugging and investigation)
  * Work well with copy and paste (double clicking selects the entire ID)
  * Can be shortened for low cardinality resources
  * Are secure against enumeration attacks
  * Can be generated by application code (not tied to the datastore)
  * Are K-sortable (lexicographically sortable by time - works well with datastore indexing)
  * Do not require any coordination (human or automated) at startup, or generation
  * Are very unlikely to collide (more likely with less randomness)
  * Are easily and accurately transmitted to another human using a telephone

  Many of the concepts of Stripe IDs have been used in this library.
  """

  @typedoc "Options for generating a UXID"
  @type option ::
          {:case, atom()}
          | {:time, integer()}
          | {:size, atom() | nil}
          | {:rand_size, integer() | nil}
          | {:prefix, String.t() | nil}
          | {:delimiter, String.t() | nil}
          | {:compact_time, boolean() | nil}
          | {:monotonic, boolean() | [atom()] | nil}

  @type options :: [option()]

  @typedoc "A UXID represented as a String"
  @type t() :: String.t()

  alias UXID.{Codec, Encoder}
  alias UXID.Decoder

  # Crockford Base32 alphabet (excludes I, L, O, U), in both cases. A UXID body
  # is made up entirely of these characters.
  @crockford_body ~r/\A[0-9ABCDEFGHJKMNPQRSTVWXYZabcdefghjkmnpqrstvwxyz]+\z/

  # Canonical 36-character hyphenated UUID, any case. Identifiers that predate
  # UXID adoption are stored as UUID strings and must remain castable.
  @uuid_format ~r/\A[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}\z/

  # Shortest legitimate encoded body: a compact 40-bit timestamp with no
  # randomness encodes to 8 characters.
  @min_body_length 8

  @spec generate(opts :: options()) :: {:ok, __MODULE__.t()}
  @doc """
  Returns an encoded UXID string along with response status.
  """
  def generate(opts \\ []) do
    {:ok, %Codec{string: string}} = new(opts)

    {:ok, string}
  end

  @spec generate!(opts :: options()) :: __MODULE__.t()
  @doc """
  Returns an unwrapped encoded UXID string.
  """
  def generate!(opts \\ []) do
    {:ok, uxid} = generate(opts)
    uxid
  end

  @spec new(opts :: options()) :: {:ok, Codec.t()}
  @doc """
  Returns a new UXID.Codec struct. This is useful for development.
  """
  def new(opts \\ []) do
    case = Keyword.get(opts, :case, encode_case())
    prefix = Keyword.get(opts, :prefix)
    rand_size = Keyword.get(opts, :rand_size)
    size = Keyword.get(opts, :size)
    delimiter = Keyword.get(opts, :delimiter)
    compact_time = Keyword.get(opts, :compact_time)
    monotonic = Keyword.get(opts, :monotonic)
    timestamp = Keyword.get(opts, :time, System.system_time(:millisecond))

    %Codec{
      case: case,
      compact_time: compact_time,
      monotonic: monotonic,
      prefix: prefix,
      rand_size: rand_size,
      size: size,
      time: timestamp,
      delimiter: delimiter
    }
    |> Encoder.process()
  end

  def encode_case(), do: Application.get_env(:uxid, :case, :lower)

  @doc """
  Returns the delimiter used to separate a prefix from the encoded body.
  Defaults to `"_"` and can be overridden globally with the `:delimiter`
  application env, or per-call/per-field with the `:delimiter` option.
  """
  def default_delimiter(), do: Application.get_env(:uxid, :delimiter, "_")

  @doc """
  Returns the minimum size configuration.
  When set, any requested size smaller than this will be upgraded.
  """
  def min_size(), do: Application.get_env(:uxid, :min_size, nil)

  @doc """
  Returns whether compact time encoding is enabled for small sizes.
  When true, :xs/:xsmall and :s/:small use 40 bits for timestamp instead of 48,
  adding 8 bits to randomness. This is a global policy that can be overridden
  per-call with the `compact_time` option.
  This provides perfect 5-bit Crockford Base32 alignment (8 chars vs 10 chars).
  K-sortability is maintained until ~September 2039.
  """
  def compact_small_times(), do: Application.get_env(:uxid, :compact_small_times, false)

  @doc """
  Returns the global monotonic generation policy.

  When active, the random field is treated as a big-endian counter: the first
  ID in a millisecond is seeded from the CSPRNG and each subsequent same-ms ID
  advances it by a random positive step (per process, per prefix). This
  guarantees uniqueness and K-sortability within a burst while keeping
  consecutive IDs within a bounded window ahead — a mitigation, not cryptographic
  unpredictability — which is why it is off by default and opt-in per resource.

  Accepts `true`/`false`, or a list of sizes (alias-aware, e.g. `[:small]`
  matches both `:small` and `:s`). Overridable per-call/per-field with the
  `monotonic` option. See `UXID.Monotonic`.
  """
  def monotonic(), do: Application.get_env(:uxid, :monotonic, false)

  @spec decode(String.t()) :: {:ok, Codec.t()}
  @doc """
  Decodes a UXID string and returns a Codec struct with extracted components.
  """
  def decode(uxid) do
    %Codec{
      string: uxid
    }
    |> Decoder.process()
  end

  @spec valid?(term(), keyword()) :: boolean()
  @doc """
  Returns `true` if the given value is a structurally valid UXID.

  A valid UXID is a binary made up of an optional prefix, the delimiter, and a
  Crockford Base32 body of at least #{@min_body_length} characters. This checks
  *structure*, not authenticity: it cannot distinguish a generated UXID from an
  arbitrary Base32 string of the same shape, and it deliberately does **not**
  accept a bare UUID (see `cast/2`'s `:validate` mode for UUID coexistence).

  ## Options

    * `:prefix` - when present, the value must carry exactly this prefix. When
      omitted, any prefix (or none) is accepted.
    * `:delimiter` - the prefix delimiter (defaults to the configured
      delimiter, see `default_delimiter/0`).

  ## Examples

      iex> UXID.valid?("cus_01emdgjf0dqxqj8fm78xe97y3h")
      true

      iex> UXID.valid?("cus_01emdgjf0dqxqj8fm78xe97y3h", prefix: "cus")
      true

      iex> UXID.valid?("cus_01emdgjf0dqxqj8fm78xe97y3h", prefix: "usr")
      false

      iex> UXID.valid?("nope!")
      false
  """
  def valid?(term, opts \\ [])

  def valid?(term, opts) when is_binary(term) do
    delimiter = Keyword.get(opts, :delimiter, default_delimiter())
    {prefix, body} = split_prefix(term, delimiter)

    body_valid?(body) and prefix_ok?(prefix, Keyword.get(opts, :prefix))
  end

  def valid?(_term, _opts), do: false

  defp body_valid?(body) do
    String.length(body) >= @min_body_length and Regex.match?(@crockford_body, body)
  end

  defp prefix_ok?(_actual, nil), do: true
  defp prefix_ok?(actual, expected), do: actual == expected

  defp split_prefix(string, delimiter) do
    case String.split(string, delimiter) do
      [body] ->
        {nil, body}

      parts ->
        {body, prefix_parts} = List.pop_at(parts, -1)
        {Enum.join(prefix_parts, delimiter), body}
    end
  end

  # Only ever called on a binary (cast_strict/2 guards is_binary before calling),
  # so no non-binary clause is needed — the guard documents that contract.
  defp uuid_string?(term) when is_binary(term), do: Regex.match?(@uuid_format, term)

  # Define additional functions for custom Ecto type if Ecto is loaded
  if Code.ensure_loaded?(Ecto.ParameterizedType) do
    @behaviour Ecto.ParameterizedType

    @doc """
    Generates a loaded version of the UXID.

    Field options are threaded through to `generate!/1`, including `monotonic`
    for opt-in monotonic generation:

        field :id, UXID, autogenerate: true, prefix: "evt", size: :small, monotonic: true
    """
    def autogenerate(opts) do
      case = Map.get(opts, :case, encode_case())
      prefix = Map.get(opts, :prefix)
      size = Map.get(opts, :size)
      rand_size = Map.get(opts, :rand_size)
      delimiter = Map.get(opts, :delimiter)
      compact_time = Map.get(opts, :compact_time)
      monotonic = Map.get(opts, :monotonic)

      __MODULE__.generate!(
        case: case,
        prefix: prefix,
        size: size,
        rand_size: rand_size,
        delimiter: delimiter,
        compact_time: compact_time,
        monotonic: monotonic
      )
    end

    @doc """
    Returns the underlying schema type for a UXID.
    """
    def type(_opts), do: :string

    @doc """
    Converts the options specified in the field macro into parameters to be used in other callbacks.
    """
    def init(opts) do
      # validate_opts(opts)
      Enum.into(opts, %{})
    end

    @doc """
    Casts the given input to the UXID ParameterizedType with the given parameters.

    By default any binary is accepted unchanged (backwards compatible). When the
    field opts in with `validate: true`, the value must be either a structurally
    valid UXID carrying the field's configured `:prefix` (see `valid?/2`) or a
    legacy bare UUID string; anything else casts to `:error`. UUID coexistence
    can be turned off with `allow_uuid: false`.

        field :owner_org_id, UXID, prefix: "org", validate: true
    """
    def cast(data, params) do
      if Map.get(params, :validate, false) do
        cast_strict(data, params)
      else
        cast_binary(data)
      end
    end

    defp cast_binary(nil), do: {:ok, nil}
    defp cast_binary(term) when is_binary(term), do: {:ok, term}
    defp cast_binary(_), do: :error

    defp cast_strict(nil, _params), do: {:ok, nil}

    defp cast_strict(term, params) when is_binary(term) do
      opts = [
        prefix: Map.get(params, :prefix),
        delimiter: Map.get(params, :delimiter, default_delimiter())
      ]

      cond do
        valid?(term, opts) -> {:ok, term}
        Map.get(params, :allow_uuid, true) and uuid_string?(term) -> {:ok, term}
        true -> :error
      end
    end

    defp cast_strict(_term, _params), do: :error

    @doc """
    Loads the given term into a UXID.
    """
    def load(data, _loader, _params), do: {:ok, data}

    @doc """
    Dumps the given term into an Ecto native type.
    """
    def dump(data, _dumper, _params), do: {:ok, data}

    @doc """
    Dictates how the type should be treated if embedded.
    For UXIDs, we use :self since they're already strings.
    """
    def embed_as(_format, _params), do: :self

    @doc """
    Checks if two UXIDs are equal.
    """
    def equal?(left, right, _params), do: left == right
  end
end