defmodule CCXT.Spec do
  @moduledoc """
  Compile-time JSON spec loader for exchange specifications.

  Reads exchange specs from `priv/specs/json/output/` and decodes them for use
  by the generator macro (`CCXT.Exchange`). Specs are loaded at compile time
  inside the generator — this module provides the loading functions but does
  not store specs as module attributes.

  ## Usage

      # In generator macro (compile time):
      spec = CCXT.Spec.load!("bybit")

      # List available exchanges:
      CCXT.Spec.exchanges()
      #=> ["aftermath", "alpaca", "apex", ...]

  ## Schema Version

  `load!/1` and `load_manifest!/0` enforce `schema_version` major `3`
  (ccxt_extract Schema 3.0.0+). Per-exchange specs must also carry a
  non-empty top-level `_provenance` map (RFC 6901 pointer keys →
  `"raw"` / `"derived"` / `"override"`); the manifest does not.

  A mismatch raises with a specific message naming the spec and the
  offending field, so a stale `priv/specs/json/output/` surfaces as a
  clear compile-time failure instead of a downstream `MatchError`.

  ## Dead Weight Stripping

  `load!/1` removes extraction-only data (`structure.interface_signatures`
  and `structure.methods`) that is only useful for static analysis, not
  runtime code generation. This reduces memory for `Macro.escape/1`.
  """

  @spec_dir "priv/specs/json/output"

  # Only lowercase alphanumeric and underscore — rejects path traversal ("../mix")
  @valid_id_pattern ~r/^[a-z0-9_]+$/

  # Keys removed from structure — extraction artifacts, not needed at runtime
  @stripped_keys ["interface_signatures", "methods"]

  # Accepted major version of the upstream spec schema. Bumped 1 → 2 for the
  # `_provenance` sidecar (ccxt_extract Task 61c/61d), then 2 → 3 when Task 117
  # pruned `runtime.markets` (→ compact `runtime.symbols_index`) and dropped
  # `structure.parse_methods` + `structure.ws_methods` to fit the Hex cap.
  @schema_major 3

  @doc """
  Loads and decodes an exchange spec by ID.

  Reads the JSON file, decodes it, and strips extraction-only data from
  `structure`. Returns a map with string keys.

  Raises `File.Error` if the spec file doesn't exist, or `Jason.DecodeError`
  on invalid JSON.

  ## Examples

      spec = CCXT.Spec.load!("bybit")
      spec["exchange"]["id"]
      #=> "bybit"

  """
  @spec load!(String.t()) :: map()
  def load!(exchange_id) when is_binary(exchange_id) do
    validate_id!(exchange_id)

    exchange_id
    |> spec_path()
    |> File.read!()
    |> Jason.decode!()
    |> validate_schema!(exchange_id)
    |> strip()
  end

  @doc """
  Loads and decodes the manifest file.

  Returns the decoded `_manifest.json` containing exchange list,
  CCXT version, and extraction metadata.

  ## Examples

      manifest = CCXT.Spec.load_manifest!()
      manifest["ccxt_version"]
      #=> "4.5.46"

      manifest["exchange_count"]
      #=> 23

  """
  @spec load_manifest!() :: map()
  def load_manifest! do
    manifest_path()
    |> File.read!()
    |> Jason.decode!()
    |> validate_manifest_schema!()
  end

  @doc """
  Returns the list of available exchange IDs from the manifest.

  ## Examples

      CCXT.Spec.exchanges()
      #=> ["aftermath", "alpaca", "apex", "arkham", ...]

  """
  @spec exchanges() :: [String.t()]
  def exchanges do
    load_manifest!()["exchanges"]
  end

  @doc """
  Returns the absolute path to a spec file for the given exchange ID.

  ## Examples

      CCXT.Spec.spec_path("bybit")
      #=> "/path/to/priv/specs/json/output/bybit.json"

  """
  @spec spec_path(String.t()) :: String.t()
  def spec_path(exchange_id) when is_binary(exchange_id) do
    validate_id!(exchange_id)
    Path.join(spec_dir(), "#{exchange_id}.json")
  end

  @doc """
  Returns the absolute path to the manifest file.

  ## Examples

      CCXT.Spec.manifest_path()
      #=> "/path/to/priv/specs/json/output/_manifest.json"

  """
  @spec manifest_path() :: String.t()
  def manifest_path do
    Path.join(spec_dir(), "_manifest.json")
  end

  # Resolves the spec directory path.
  # Uses :code.priv_dir at runtime, falls back to compile-time path.
  defp spec_dir do
    case :code.priv_dir(:ccxt_client) do
      {:error, :bad_name} -> @spec_dir
      priv_dir -> Path.join(to_string(priv_dir), "specs/json/output")
    end
  end

  # Removes extraction-only keys from the structure section.
  # interface_signatures and methods are analysis artifacts from ccxt_extract,
  # not needed for runtime code generation.
  defp strip(%{"structure" => structure} = spec) do
    %{spec | "structure" => Map.drop(structure, @stripped_keys)}
  end

  defp strip(spec), do: spec

  @doc """
  Validates a decoded per-exchange spec map against the Schema 3.0.0+ contract:
  `schema_version` must be a string whose numeric major equals `#{@schema_major}`,
  and `_provenance` must be a non-empty top-level map. On mismatch, raises with
  a message naming the exchange and the offending field.

  Returns the spec unchanged on success. Called internally from `load!/1`;
  exposed publicly so test suites can exercise every branch against in-memory
  maps without writing fixture files into the live spec directory.
  """
  @spec validate_schema!(map(), String.t()) :: map()
  def validate_schema!(%{"schema_version" => v, "_provenance" => prov} = spec, exchange_id)
      when is_binary(v) and is_map(prov) and map_size(prov) > 0 do
    assert_major!(v, "spec #{inspect(exchange_id)}")
    spec
  end

  def validate_schema!(%{"schema_version" => v, "_provenance" => prov}, exchange_id) when is_binary(v) do
    raise "spec #{inspect(exchange_id)} has invalid _provenance: expected non-empty map, got #{inspect(prov)}"
  end

  def validate_schema!(%{"_provenance" => _}, exchange_id) do
    raise "spec #{inspect(exchange_id)} missing schema_version (expected major #{@schema_major})"
  end

  def validate_schema!(%{"schema_version" => v}, exchange_id) when is_binary(v) do
    assert_major!(v, "spec #{inspect(exchange_id)}")

    raise "spec #{inspect(exchange_id)} missing required _provenance map (schema 3.0.0+)"
  end

  def validate_schema!(_spec, exchange_id) do
    raise "spec #{inspect(exchange_id)} missing schema_version and/or _provenance"
  end

  @doc """
  Validates a decoded manifest map against the Schema 3.0.0+ contract.

  Only the `schema_version` major is enforced — manifests have no `_provenance`
  by design (different document type from per-exchange specs). Raises on
  mismatch; returns the manifest unchanged on success. Exposed for test
  coverage of the guard branches.
  """
  @spec validate_manifest_schema!(map()) :: map()
  def validate_manifest_schema!(%{"schema_version" => v} = manifest) when is_binary(v) do
    assert_major!(v, "manifest")
    manifest
  end

  def validate_manifest_schema!(_manifest) do
    raise "manifest missing schema_version (expected major #{@schema_major})"
  end

  # Accepts the configured @schema_major either alone (e.g. "3") or followed by
  # "." and anything ("3.0.1"). Rejects trailing garbage like "3malformed" —
  # Integer.parse("3malformed") returns {3, "malformed"}, so the raw major
  # check alone would otherwise pass.
  defp assert_major!(version, label) do
    case Integer.parse(version) do
      {@schema_major, ""} ->
        :ok

      {@schema_major, "." <> _} ->
        :ok

      _ ->
        raise "#{label} has unsupported schema_version #{inspect(version)} (expected major #{@schema_major})"
    end
  end

  # Validates exchange_id contains only safe characters (lowercase alphanumeric + underscore).
  # Rejects path traversal attempts like "../mix" or "foo/bar".
  defp validate_id!(exchange_id) do
    if !Regex.match?(@valid_id_pattern, exchange_id) do
      raise ArgumentError, "invalid exchange ID: #{inspect(exchange_id)} (must match #{inspect(@valid_id_pattern)})"
    end
  end
end