# SPDX-License-Identifier: MIT
defmodule Testcontainers.Docker.Auth do
  @moduledoc """
  Resolves Docker registry credentials from the user's Docker config file
  (typically `~/.docker/config.json`) and returns a ready-to-send
  `X-Registry-Auth` header value.

  Scope:

    * Only the `auths` map in `config.json` is supported.
    * Credential helpers (`credsStore`, `credHelpers`) are intentionally out
      of scope — if encountered, a debug log is emitted and `nil` is returned
      so the caller can fall back to anonymous access.

  The `DOCKER_CONFIG` environment variable is honoured: when set, the config
  file is read from `$DOCKER_CONFIG/config.json`; otherwise the default path
  `~/.docker/config.json` is used.

  The header value is a URL-safe base64 encoding (without padding) of a JSON
  document describing the credentials, as specified by the Docker Engine API.
  """

  require Logger

  @docker_hub_key "https://index.docker.io/v1/"

  @doc """
  Resolves registry credentials for the given `image` and returns the
  ready-to-send `X-Registry-Auth` header value, or `nil` if no matching
  credentials can be found.

  `config_path` may be `nil`, in which case the default lookup logic is used
  (respecting the `DOCKER_CONFIG` environment variable).
  """
  @spec resolve(String.t(), String.t() | nil) :: String.t() | nil
  def resolve(image, config_path \\ nil) when is_binary(image) do
    case read_config(config_path) do
      {:ok, config} ->
        registry = registry_for_image(image)
        resolve_from_config(config, registry)

      :error ->
        nil
    end
  end

  @doc """
  Returns the registry key that should be used for looking up credentials
  for the given `image` (Docker config convention).

  Unnamespaced or explicitly `docker.io`-hosted images resolve to
  `https://index.docker.io/v1/`; everything else resolves to the registry
  host component of the image reference.
  """
  @spec registry_for_image(String.t()) :: String.t()
  def registry_for_image(image) when is_binary(image) do
    case String.split(image, "/", parts: 2) do
      [_single] -> @docker_hub_key
      [maybe_host, _rest] -> registry_from_host_component(maybe_host)
    end
  end

  defp registry_from_host_component(component) do
    cond do
      not host?(component) -> @docker_hub_key
      component in ["docker.io", "index.docker.io"] -> @docker_hub_key
      true -> component
    end
  end

  # A registry host contains a "." or ":" or is exactly "localhost".
  defp host?(component) do
    component == "localhost" or String.contains?(component, ".") or
      String.contains?(component, ":")
  end

  defp read_config(nil), do: read_config(default_config_path())

  defp read_config(path) when is_binary(path) do
    with {:ok, contents} <- File.read(path),
         {:ok, decoded} <- Jason.decode(contents) do
      {:ok, decoded}
    else
      {:error, reason} ->
        Logger.debug(
          "Testcontainers.Docker.Auth: could not read Docker config at #{path}: #{inspect(reason)}"
        )

        :error
    end
  end

  defp default_config_path do
    case System.get_env("DOCKER_CONFIG") do
      nil -> Path.join(System.user_home() || "", ".docker/config.json")
      "" -> Path.join(System.user_home() || "", ".docker/config.json")
      dir -> Path.join(dir, "config.json")
    end
  end

  defp resolve_from_config(config, registry) do
    auths = Map.get(config, "auths", %{})

    case find_auth_entry(auths, registry) do
      {matched_key, %{"auth" => encoded}} when is_binary(encoded) and encoded != "" ->
        build_header(encoded, matched_key)

      {_matched_key, _entry} ->
        maybe_warn_cred_helper(config, registry)
        nil

      :not_found ->
        maybe_warn_cred_helper(config, registry)
        nil
    end
  end

  defp find_auth_entry(auths, registry) when is_map(auths) do
    candidates = candidate_keys(registry)

    Enum.find_value(candidates, :not_found, fn key ->
      case Map.get(auths, key) do
        nil -> nil
        entry when is_map(entry) -> {key, entry}
        _ -> nil
      end
    end)
  end

  defp find_auth_entry(_auths, _registry), do: :not_found

  # Docker config.json keys are sometimes stored with scheme/path prefixes and
  # sometimes as plain hostnames. We try the most specific form first and fall
  # back to the bare host.
  defp candidate_keys(@docker_hub_key) do
    [
      @docker_hub_key,
      "index.docker.io",
      "docker.io",
      "https://index.docker.io/v1",
      "https://index.docker.io",
      "https://docker.io"
    ]
  end

  defp candidate_keys(registry) do
    [
      registry,
      "https://" <> registry,
      "http://" <> registry,
      "https://" <> registry <> "/",
      "http://" <> registry <> "/"
    ]
  end

  defp build_header(encoded, server_address) do
    with {:ok, decoded} <- Base.decode64(encoded),
         [username, password] <- String.split(decoded, ":", parts: 2) do
      payload = %{
        "username" => username,
        "password" => password,
        "serveraddress" => normalize_server_address(server_address)
      }

      payload
      |> Jason.encode!()
      |> Base.url_encode64(padding: false)
    else
      _ ->
        Logger.debug(
          "Testcontainers.Docker.Auth: could not decode auth entry for #{server_address}"
        )

        nil
    end
  end

  # Docker config.json keys are often stored as URLs (e.g.
  # "https://index.docker.io/v1/") but the Docker Engine API's
  # `serveraddress` field expects a bare domain/IP (optionally with port) —
  # podman in particular rejects a full URL with HTTP 400.
  @doc false
  @spec normalize_server_address(String.t()) :: String.t()
  def normalize_server_address(address) when is_binary(address) do
    address
    |> strip_scheme()
    |> strip_trailing_path()
    |> canonicalize_docker_hub()
  end

  defp strip_scheme(address) do
    case String.split(address, "://", parts: 2) do
      [_scheme, rest] -> rest
      [single] -> single
    end
  end

  defp strip_trailing_path(address) do
    address
    |> String.split("/", parts: 2)
    |> List.first()
  end

  defp canonicalize_docker_hub("index.docker.io"), do: "docker.io"
  defp canonicalize_docker_hub(host), do: host

  defp maybe_warn_cred_helper(config, registry) do
    cond do
      Map.has_key?(config, "credsStore") ->
        Logger.debug(
          "Testcontainers.Docker.Auth: credsStore present in Docker config; " <>
            "credential helpers are not supported, returning nil for #{registry}"
        )

      is_map(Map.get(config, "credHelpers")) and
          Map.has_key?(config["credHelpers"], registry) ->
        Logger.debug(
          "Testcontainers.Docker.Auth: credHelpers entry for #{registry} present; " <>
            "credential helpers are not supported, returning nil"
        )

      true ->
        :ok
    end
  end
end