defmodule GcpCompute do
  @moduledoc """
  Spawn and manage Google Compute Engine instances over the REST API.

  > #### Why REST? {: .info}
  >
  > The Compute Engine API is **REST/JSON only** — it has no gRPC endpoint.
  > This library talks to it with [`Req`](https://hexdocs.pm/req); there is no
  > connection pool or supervised process to run. A `GcpCompute.Config` is the
  > only handle you pass around.

  ## Quickstart

      # 1. Mint tokens with Goth (or any GcpCompute.TokenProvider)
      children = [{Goth, name: MyApp.Goth}]

      # 2. Build a config (do this once, reuse it)
      {:ok, config} =
        GcpCompute.Config.production(
          project: "my-project",
          zone: "us-central1-a",
          goth: MyApp.Goth
        )

      # 3. Spawn a cheap, self-deleting Spot VM and wait for it
      {:ok, instance} =
        GcpCompute.insert_instance_and_wait(config,
          name: "worker-1",
          machine_type: "e2-micro",
          spot: true,
          max_run_duration: 3600,
          startup_script: "#!/bin/bash\\necho ready > /tmp/ready"
        )

      GcpCompute.Instance.external_ip(instance)
      #=> "34.x.x.x"

      # 4. Tear it down
      {:ok, _op} = GcpCompute.delete_instance_and_wait(config, "worker-1")

  The functions here are thin wrappers over `GcpCompute.Instances` and
  `GcpCompute.Operations`; reach for those modules directly for the full surface.
  """

  alias GcpCompute.{Config, Instances, Operations}

  @doc "See `GcpCompute.Config.new/1`."
  defdelegate new(opts), to: Config

  @doc "See `GcpCompute.Config.production/1`."
  defdelegate production(opts), to: Config

  @doc "See `GcpCompute.Config.local/1`."
  defdelegate local(opts), to: Config

  @doc "See `GcpCompute.Config.from_env/2`."
  defdelegate from_env(app, key), to: Config

  @call_keys [:zone, :request_id, :timeout]

  @doc """
  Load (and cache) a config from application env.

      # config :my_app, :gcp_compute, project: "p", zone: "...", token_provider: {...}
      {:ok, config} = GcpCompute.config(:my_app)

  The result is memoized in `:persistent_term` on the first successful load and
  reused on every later call. Caching is **first-write-wins with no
  invalidation**: once cached, later changes to the application env are ignored.
  Only successful loads are cached — an invalid env returns its `{:error, _}`
  without caching, so it is re-read next call. To force a reload after
  reconfiguring at runtime, call `clear_config/2` first. (`:persistent_term`
  writes trigger a global GC, so treat clearing as a rare operation, not a
  per-request one.)
  """
  def config(otp_app, key \\ :gcp_compute) do
    pt_key = {__MODULE__, :config, otp_app, key}

    case :persistent_term.get(pt_key, nil) do
      nil ->
        case Config.from_env(otp_app, key) do
          {:ok, config} = ok ->
            :persistent_term.put(pt_key, config)
            ok

          error ->
            error
        end

      config ->
        {:ok, config}
    end
  end

  @doc """
  Drop the cached config for `{otp_app, key}` so the next `config/2` re-reads env.

  Returns `:ok` whether or not an entry was cached. See `config/2` for the
  caching caveat (`:persistent_term` writes trigger a global GC).
  """
  def clear_config(otp_app, key \\ :gcp_compute) do
    :persistent_term.erase({__MODULE__, :config, otp_app, key})
    :ok
  end

  @doc """
  Spawn an instance and wait until it's running.

  Sugar over `insert_instance_and_wait/3`: the name is lifted out, and spec
  options (`machine_type`, `spot`, …) are auto-separated from call options
  (`:zone`, `:request_id`, `:timeout`).

      {:ok, vm} = GcpCompute.launch(config, "worker-1", machine_type: "e2-micro", spot: true)
  """
  def launch(config, name, opts \\ []) do
    {call_opts, spec_opts} = Keyword.split(opts, @call_keys)
    Instances.insert_and_wait(config, [name: name] ++ spec_opts, call_opts)
  end

  @doc "Delete an instance and wait for it to be gone. Returns `:ok`."
  def terminate(config, name, opts \\ []) do
    case Instances.delete_and_wait(config, name, opts) do
      {:ok, _op} -> :ok
      {:error, _} = error -> error
    end
  end

  @doc "See `GcpCompute.Instances.insert/3`."
  def insert_instance(config, instance, opts \\ []), do: Instances.insert(config, instance, opts)

  @doc "See `GcpCompute.Instances.insert_and_wait/3`."
  def insert_instance_and_wait(config, instance, opts \\ []),
    do: Instances.insert_and_wait(config, instance, opts)

  @doc "See `GcpCompute.Instances.get/3`."
  def get_instance(config, name, opts \\ []), do: Instances.get(config, name, opts)

  @doc "See `GcpCompute.Instances.list/2`."
  def list_instances(config, opts \\ []), do: Instances.list(config, opts)

  @doc "See `GcpCompute.Instances.list_page/2`."
  def list_instances_page(config, opts \\ []), do: Instances.list_page(config, opts)

  @doc "See `GcpCompute.Instances.delete/3`."
  def delete_instance(config, name, opts \\ []), do: Instances.delete(config, name, opts)

  @doc "See `GcpCompute.Instances.delete_and_wait/3`."
  def delete_instance_and_wait(config, name, opts \\ []),
    do: Instances.delete_and_wait(config, name, opts)

  @doc "See `GcpCompute.Instances.start/3`."
  def start_instance(config, name, opts \\ []), do: Instances.start(config, name, opts)

  @doc "See `GcpCompute.Instances.stop/3`."
  def stop_instance(config, name, opts \\ []), do: Instances.stop(config, name, opts)

  @doc "See `GcpCompute.Operations.poll_until_done/3`."
  def wait_for_operation(config, operation, opts \\ []),
    do: Operations.poll_until_done(config, operation, opts)
end