defmodule TelegramMiniappValidation do
  @moduledoc """
  Provides functionality to validate Telegram Mini App initialization data.

  This module implements the validation algorithm described in the Telegram Mini Apps documentation:
  https://docs.telegram-mini-apps.com/platform/init-data#validating
  """

  @doc """
  Validates the initialization data from a Telegram Mini App.

  The validation process follows these steps:
  1. Parse the query string into key-value pairs
  2. Extract the hash value and remove it from the pairs
  3. Sort the remaining pairs alphabetically
  4. Create a data check string by joining the pairs with newlines
  5. Create an HMAC-SHA256 signature of the bot token using "WebAppData" as the key
  6. Create an HMAC-SHA256 signature of the data check string using the result from step 5 as the key
  7. Compare the calculated hash with the provided hash

  ## Parameters

  - `init_data`: The raw initialization data string from the Telegram Mini App
  - `bot_token`: The Telegram Bot token used to sign the data
  - `max_age_seconds`: Optional maximum age in seconds for the auth_date parameter (default: 86400 - 24 hours)

  ## Returns

  - `{:ok, parsed_data}`: If validation is successful, returns the parsed data as a map
  - `{:error, reason}`: If validation fails, returns the reason for failure

  ## Examples

  This example shows the structure of the returned data, but in a real scenario,
  you would need to use a valid hash and bot token:

      # Using 0 for max_age_seconds to skip auth_date validation in this example
      iex> init_data = "query_id=AAHdF6IQAAAAAN0XohDhrOrc&user=%7B%22id%22%3A279058397%7D&auth_date=1662771648&hash=mock_hash"
      iex> TelegramMiniappValidation.parse_and_extract_hash(init_data)
      {:ok, {%{"auth_date" => "1662771648", "query_id" => "AAHdF6IQAAAAAN0XohDhrOrc", "user" => %{"id" => 279058397}}, "mock_hash"}}

  """
  @spec validate(String.t(), String.t(), non_neg_integer()) ::
          {:ok, map()} | {:error, String.t()}
  def validate(init_data, bot_token, max_age_seconds \\ 86400) do
    with {:ok, {data_map, hash}} <- parse_and_extract_hash(init_data),
         :ok <- validate_auth_date(data_map, max_age_seconds),
         true <- valid_hash?(init_data, hash, bot_token) do
      {:ok, data_map}
    else
      {:error, reason} -> {:error, reason}
      false -> {:error, "Invalid hash"}
    end
  end

  @doc """
  Parses the initialization data and extracts the hash value.

  ## Parameters

  - `init_data`: The raw initialization data string from the Telegram Mini App

  ## Returns

  - `{:ok, {data_map, hash}}`: If parsing is successful, returns the data map and hash
  - `{:error, reason}`: If parsing fails, returns the reason for failure
  """
  @spec parse_and_extract_hash(String.t()) ::
          {:ok, {map(), String.t()}} | {:error, String.t()}
  def parse_and_extract_hash(init_data) do
    try do
      # Parse the query string into key-value pairs
      pairs =
        init_data
        |> String.split("&")
        |> Enum.map(fn pair ->
          case String.split(pair, "=", parts: 2) do
            [key, value] -> {key, URI.decode_www_form(value)}
            _ -> nil
          end
        end)
        |> Enum.reject(&is_nil/1)
        |> Enum.into(%{})

      # Extract the hash value
      hash = Map.get(pairs, "hash")

      if is_nil(hash) do
        {:error, "Hash parameter is missing"}
      else
        # Remove the hash from the data map
        data_map = Map.delete(pairs, "hash")

        # Parse the user field if it exists and is valid JSON
        data_map =
          if Map.has_key?(data_map, "user") do
            case Jason.decode(data_map["user"]) do
              {:ok, user_map} -> Map.put(data_map, "user", user_map)
              _ -> data_map
            end
          else
            data_map
          end

        {:ok, {data_map, hash}}
      end
    rescue
      e -> {:error, "Failed to parse init data: #{inspect(e)}"}
    end
  end

  @doc """
  Validates the auth_date parameter to ensure it's not too old.

  ## Parameters

  - `data_map`: The parsed data map
  - `max_age_seconds`: Maximum age in seconds for the auth_date parameter

  ## Returns

  - `:ok`: If the auth_date is valid
  - `{:error, reason}`: If the auth_date is invalid or missing
  """
  @spec validate_auth_date(map(), non_neg_integer()) :: :ok | {:error, String.t()}
  def validate_auth_date(data_map, max_age_seconds) do
    # Skip validation if max_age_seconds is 0
    if max_age_seconds == 0 do
      :ok
    else
      case Map.get(data_map, "auth_date") do
        nil ->
          {:error, "Auth date is missing"}

        auth_date_str ->
          try do
            auth_date = String.to_integer(auth_date_str)
            current_time = :os.system_time(:second)

            if current_time - auth_date > max_age_seconds do
              {:error, "Auth date is too old"}
            else
              :ok
            end
          rescue
            _ -> {:error, "Invalid auth date format"}
          end
      end
    end
  end

  @doc """
  Checks if the provided hash is valid for the given data and bot token.

  ## Parameters

  - `data_map`: The parsed data map
  - `hash`: The hash value to validate
  - `bot_token`: The Telegram Bot token used to sign the data

  ## Returns

  - `true`: If the hash is valid
  - `false`: If the hash is invalid
  """
  @spec valid_hash?(map(), String.t(), String.t()) :: boolean()
  def valid_hash?(init_data, hash, bot_token) do
    # Parse the query string
    init_data = URI.decode_query(init_data)

    # Sort key-value pairs alphabetically, excluding the hash
    data_to_check =
      init_data
      |> Map.drop(["hash"])
      |> Enum.sort()
      |> Enum.map(fn {key, value} -> "#{key}=#{value}" end)
      |> Enum.join("\n")

    # HMAC Calculation
    secret =
      :crypto.mac(:hmac, :sha256, "WebAppData", bot_token)

    calculated_hash =
      :crypto.mac(:hmac, :sha256, secret, data_to_check)
      |> Base.encode16(case: :lower)

    calculated_hash == hash
  end
end