defmodule CCXT.Dispatch do
  @moduledoc """
  Shared request dispatcher for generated exchange endpoint functions.

  All generated endpoint functions delegate to `call/4`, which handles:

  1. **Path interpolation** — replaces `{param}` templates with values from params
  2. **Base URL resolution** — navigates `exchange.base_urls` using endpoint sections
  3. **Signing** — authenticates private endpoint requests via `CCXT.Signing.sign/4`
  4. **HTTP delegation** — calls `CCXT.HTTP.request/4` or `CCXT.HTTP.signed_request/4`

  ## Future phases

  - **Phase 5**: Response parsing (field mapping to unified structs)
  - **Task 17**: Symbol denormalization (unified → exchange-specific format)
  """

  alias CCXT.Error
  alias CCXT.Exchange
  alias CCXT.HTTP
  alias CCXT.Signing

  require Logger

  @typedoc "Compile-time endpoint configuration from spec"
  @type endpoint_config :: %{
          name: atom(),
          method: atom(),
          path: String.t(),
          sections: [String.t()],
          weight: number(),
          url_prefix: String.t(),
          authenticated: boolean()
        }

  @doc """
  Dispatches a request to an exchange endpoint.

  Resolves the base URL from the endpoint's sections, interpolates path
  templates, and delegates to `CCXT.HTTP.request/4`.

  ## Parameters

  - `exchange` — `%CCXT.Exchange{}` runtime configuration
  - `endpoint_config` — compile-time endpoint map with `:name`, `:method`, `:path`, `:sections`, `:weight`
  - `params` — request parameters (query for GET/HEAD/DELETE, body for POST/PUT/PATCH)
  - `opts` — passed through to `CCXT.HTTP.request/4` (`:timeout`, `:headers`, etc.)

  ## Examples

      config = %{name: :public_get_v5_market_tickers, method: :get,
        path: "v5/market/tickers", sections: ["public"], weight: 5}

      CCXT.Dispatch.call(exchange, config, %{"category" => "spot"})

  """
  @spec call(Exchange.t(), endpoint_config(), map(), keyword()) ::
          {:ok, HTTP.response()} | {:error, Error.t()}
  def call(%Exchange{} = exchange, %{} = endpoint_config, params \\ %{}, opts \\ []) do
    %{method: method, path: path_template, sections: sections} = endpoint_config

    {path, remaining_params} = interpolate_path(path_template, params)
    base_url = resolve_base_url(sections, exchange.base_urls)
    url_prefix = Map.get(endpoint_config, :url_prefix, "/")

    weight = Map.get(endpoint_config, :weight, 1)

    if Map.get(endpoint_config, :authenticated, false) do
      sign_and_request(exchange, method, url_prefix <> path, remaining_params, base_url, opts, weight)
    else
      http_opts = Keyword.merge(opts, base_url: base_url, params: remaining_params, endpoint_weight: weight)
      HTTP.request(exchange, method, url_prefix <> path, http_opts)
    end
  end

  # ---------------------------------------------------------------------------
  # Signing Integration
  #
  # Private endpoints are signed via CCXT.Signing.sign/4 before HTTP execution.
  # Visibility is spec-driven via `structure.authenticated_sections` (schema 1.7.1+),
  # materialized onto each endpoint config as `:authenticated` at build time.
  # Signing pattern/config come from the exchange struct, populated at compile time
  # (generator) and runtime (Exchange.new/2) by the Classifier.
  # ---------------------------------------------------------------------------

  # Validates credentials and signing pattern, signs the request, delegates to HTTP.
  defp sign_and_request(exchange, method, path, params, base_url, opts, weight) do
    with {:ok, credentials} <- require_credentials(exchange),
         {:ok, {pattern, config}} <- require_signing_pattern(exchange) do
      signing_request = %{
        method: method,
        path: path,
        body: nil,
        params: params
      }

      signed = Signing.sign(pattern, signing_request, credentials, config)
      HTTP.signed_request(exchange, signed, base_url, Keyword.put(opts, :endpoint_weight, weight))
    end
  end

  defp require_credentials(%Exchange{credentials: nil, id: id}) do
    {:error,
     Error.authentication_error(
       message: "Credentials required for private endpoint",
       exchange: id
     )}
  end

  defp require_credentials(%Exchange{credentials: credentials}), do: {:ok, credentials}

  defp require_signing_pattern(%Exchange{signing_pattern: nil, id: id}) do
    {:error,
     Error.authentication_error(
       message: "No signing pattern configured for exchange",
       exchange: id
     )}
  end

  # TODO: signing_config should always be a map via Exchange.new/2 and the generator macro,
  # but manual struct construction can leave it nil. Consider raising here instead of defaulting.
  defp require_signing_pattern(%Exchange{signing_pattern: pattern, signing_config: config}) do
    {:ok, {pattern, config || %{}}}
  end

  # ---------------------------------------------------------------------------
  # Path Interpolation
  #
  # Replaces `{param}` placeholders in endpoint paths with values from the
  # params map. Consumed params are removed to avoid duplication as query/body.
  # Common pattern across the CCXT universe (e.g., "orders/{id}", "{settle}/candlesticks").
  # ---------------------------------------------------------------------------

  @doc "Replaces `{param}` placeholders in path with values from params, returning remaining params."
  @spec interpolate_path(String.t(), map()) :: {String.t(), map()}
  def interpolate_path(path, params) when map_size(params) == 0, do: {path, params}

  def interpolate_path(path, params) do
    # Fast path: no templates
    if String.contains?(path, "{") do
      do_interpolate(path, params, %{}, %{}, path)
    else
      {path, params}
    end
  end

  # Scans for {param} placeholders, replaces with param values, tracks consumed keys.
  # Missing path params are preserved as-is so the exchange returns a descriptive error.
  # Explicit nil values raise ArgumentError — nil is a caller bug, not "missing".
  # `original_path` is threaded through recursion so warnings always reference the full
  # input template, not a recursed suffix after `is_map_key(skipped, ...)` splits.
  # Note: Duplicate placeholders in one path (e.g., "{id}/copy/{id}") would leave the
  # second un-interpolated after the first consumes the param. No known spec does this.
  defp do_interpolate(path, params, consumed, skipped, original_path) do
    case Regex.run(~r/\{([\w-]+)\}/, path) do
      [_placeholder, param_name] when is_map_key(skipped, param_name) ->
        [before, rest] = String.split(path, "{#{param_name}}", parts: 2)
        {interpolated_rest, remaining} = do_interpolate(rest, params, consumed, skipped, original_path)
        {before <> "{#{param_name}}" <> interpolated_rest, remaining}

      [placeholder, param_name] ->
        case lookup_param(params, param_name) do
          :not_found ->
            Logger.warning(
              "CCXT.Dispatch: path param '#{param_name}' missing for path '#{original_path}' — " <>
                "placeholder '#{placeholder}' preserved; exchange will likely return a descriptive error"
            )

            do_interpolate(path, params, consumed, Map.put(skipped, param_name, true), original_path)

          nil ->
            raise ArgumentError,
                  "path param '#{param_name}' is nil — pass a value or omit the key"

          value ->
            new_path = String.replace(path, placeholder, to_string(value), global: false)
            do_interpolate(new_path, params, Map.put(consumed, param_name, true), skipped, original_path)
        end

      nil ->
        remaining = remove_consumed(params, consumed)
        {path, remaining}
    end
  end

  # Looks up a param by string key, falling back to atom key.
  # Returns :not_found when the param is absent from the map.
  defp lookup_param(params, name) do
    case Map.fetch(params, name) do
      {:ok, value} ->
        value

      :error ->
        atom_key = String.to_existing_atom(name)

        case Map.fetch(params, atom_key) do
          {:ok, value} -> value
          :error -> :not_found
        end
    end
  rescue
    ArgumentError -> :not_found
  end

  # Removes consumed path params from the params map (handles both string and atom keys)
  defp remove_consumed(params, consumed) when map_size(consumed) == 0, do: params

  defp remove_consumed(params, consumed) do
    Map.reject(params, fn {key, _value} ->
      key_str = if is_atom(key), do: Atom.to_string(key), else: key
      is_map_key(consumed, key_str)
    end)
  end

  # ---------------------------------------------------------------------------
  # Base URL Resolution
  #
  # Navigates exchange.base_urls using the endpoint's sections list.
  # Handles 4 URL patterns across the CCXT universe:
  #
  #   Flat:     %{"public" => "https://api.bybit.com"} — direct key lookup
  #   Distinct: %{"sapi" => "https://api.binance.com/sapi/v1"} — section IS the key
  #   Nested:   %{"public" => %{"delivery" => "https://..."}} — walk sections
  #   Mixed:    %{"spot" => "https://..."} — early-stop when string found
  # ---------------------------------------------------------------------------

  @doc "Navigates `base_urls` using endpoint sections to find the appropriate base URL."
  @spec resolve_base_url([String.t()], map()) :: String.t()
  def resolve_base_url(sections, base_urls) do
    navigate_sections(sections, base_urls) || find_any_url(base_urls) || ""
  end

  # Walks sections into the URL map. Stops when a string (URL) is found.
  defp navigate_sections([], _urls), do: nil

  defp navigate_sections([section | rest], urls) when is_map(urls) do
    case Map.get(urls, section) do
      url when is_binary(url) -> url
      nested when is_map(nested) and rest != [] -> navigate_sections(rest, nested)
      nested when is_map(nested) -> find_any_url(nested)
      nil -> navigate_sections(rest, urls)
    end
  end

  defp navigate_sections(_sections, _urls), do: nil

  # Fallback: find any string URL value in the map (recursive)
  defp find_any_url(urls) when is_map(urls) do
    Enum.find_value(urls, fn
      {_key, value} when is_binary(value) -> value
      {_key, value} when is_map(value) -> find_any_url(value)
      _ -> nil
    end)
  end

  defp find_any_url(_), do: nil
end