# =============================================================================
# lib/super_cache/queue.ex
#
# Unified FIFO queue — absorbs SuperCache.Distributed.Queue.
#
# Local mode:  Storage.take (atomic destructive read) for the counter/tail.
# Distributed: Storage.get (non-destructive) + ops list so that 3PC can
#              apply the full batch atomically (primary applies last).
# =============================================================================
defmodule SuperCache.Queue do
  @moduledoc """
  Named FIFO queues backed by SuperCache ETS partitions.

  Works transparently in both **local** and **distributed** modes — the
  mode is determined by the `:cluster` option passed to `SuperCache.start!/1`.

  In distributed mode, structural mutations (`add`, `out`, `get_all`) are
  routed to the partition's primary node.  Reads (`peak`, `count`) default
  to the local replica and accept `:read_mode` for stronger consistency.

  ## Example

      alias SuperCache.Queue

      Queue.add("jobs", :compress)
      Queue.add("jobs", :upload)
      Queue.count("jobs")          # => 2
      Queue.peak("jobs")           # => :compress
      Queue.out("jobs")            # => :compress
      Queue.get_all("jobs")        # => [:upload]
  """

  require Logger
  require SuperCache.Log

  alias SuperCache.{Storage, Partition, Config}
  alias SuperCache.Cluster.DistributedHelpers

  @max_count_retries 50
  @max_spin_retries 100

  ## ── Public API ──────────────────────────────────────────────────────────────

  @doc "Enqueue `value` into `queue_name`. Creates the queue if it does not exist."
  @spec add(any, any) :: true
  def add(queue_name, value) do
    if Config.distributed?() do
      DistributedHelpers.route_write(
        __MODULE__,
        :dist_enqueue,
        [queue_name, value],
        idx(queue_name)
      )
    else
      local_enqueue(Partition.get_partition(queue_name), queue_name, value)
    end
  end

  @doc "Dequeue and return the front value. Returns `default` (`nil`) when empty."
  @spec out(any, any) :: any
  def out(queue_name, default \\ nil) do
    if Config.distributed?() do
      DistributedHelpers.route_write(
        __MODULE__,
        :dist_dequeue,
        [queue_name, default],
        idx(queue_name)
      )
    else
      local_dequeue(Partition.get_partition(queue_name), queue_name, default)
    end
  end

  @doc """
  Peek at the front value without removing it.

  ## Options

  - `:read_mode` — `:local` (default), `:primary`, or `:quorum`.
  """
  @spec peak(any, any, keyword) :: any
  def peak(queue_name, default \\ nil, opts \\ []) do
    if Config.distributed?() do
      DistributedHelpers.route_read(
        __MODULE__,
        :dist_peek,
        [queue_name, default],
        idx(queue_name),
        opts
      )
    else
      local_peek(Partition.get_partition(queue_name), queue_name, default)
    end
  end

  @doc """
  Return the number of items.

  ## Options

  - `:read_mode` — `:local` (default), `:primary`, or `:quorum`.
  """
  @spec count(any, keyword) :: non_neg_integer
  def count(queue_name, opts \\ []) do
    if Config.distributed?() do
      DistributedHelpers.route_read(__MODULE__, :dist_count, [queue_name], idx(queue_name), opts)
    else
      count_safe(Partition.get_partition(queue_name), queue_name, @max_count_retries)
    end
  end

  @doc "Drain all items (oldest first). Returns `[]` for an empty queue."
  @spec get_all(any) :: list
  def get_all(queue_name) do
    if Config.distributed?() do
      DistributedHelpers.route_write(__MODULE__, :dist_drain, [queue_name], idx(queue_name))
    else
      local_drain(Partition.get_partition(queue_name), queue_name)
    end
  end

  ## ── Remote entry points — called via :erpc on primary (distributed mode) ────

  @doc false
  def dist_enqueue(queue_name, value) do
    dist_do_enqueue(Partition.get_partition(queue_name), queue_name, value)
  end

  @doc false
  def dist_dequeue(queue_name, default) do
    dist_do_dequeue(Partition.get_partition(queue_name), queue_name, default)
  end

  @doc false
  def dist_peek(queue_name, default) do
    local_peek(Partition.get_partition(queue_name), queue_name, default)
  end

  @doc false
  def dist_count(queue_name) do
    count_safe(Partition.get_partition(queue_name), queue_name, @max_count_retries)
  end

  @doc false
  def dist_drain(queue_name) do
    dist_do_drain(Partition.get_partition(queue_name), queue_name)
  end

  ## ── Private — local mode (Storage.take for atomicity) ───────────────────────

  defp local_enqueue(partition, queue_name, value) do
    local_enqueue(partition, queue_name, value, 0)
  end

  defp local_enqueue(_partition, queue_name, _value, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, local_enqueue spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    false
  end

  defp local_enqueue(partition, queue_name, value, retries) do
    case Storage.take({:queue, :tail, queue_name}, partition) do
      [] ->
        case Storage.get({:queue, :updating, queue_name}, partition) do
          [] ->
            local_init(queue_name)
            local_enqueue(partition, queue_name, value, retries)

          _ ->
            :erlang.yield()
            local_enqueue(partition, queue_name, value, retries + 1)
        end

      [{_, 0}] ->
        lock(partition, queue_name)
        Storage.delete({:queue, :head, queue_name}, partition)
        Storage.put({{:queue, queue_name, 1}, value}, partition)
        Storage.put({{:queue, :head, queue_name}, 1}, partition)
        Storage.put({{:queue, :tail, queue_name}, 1}, partition)
        unlock(partition, queue_name)
        true

      [{_, counter}] ->
        next = counter + 1
        lock(partition, queue_name)
        Storage.put({{:queue, queue_name, next}, value}, partition)
        Storage.put({{:queue, :tail, queue_name}, next}, partition)
        unlock(partition, queue_name)
        true
    end
  end

  defp local_dequeue(partition, queue_name, default) do
    local_dequeue(partition, queue_name, default, 0)
  end

  defp local_dequeue(_partition, queue_name, default, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, local_dequeue spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    default
  end

  defp local_dequeue(partition, queue_name, default, retries) do
    case Storage.get({:queue, :updating, queue_name}, partition) do
      [_] ->
        :erlang.yield()
        local_dequeue(partition, queue_name, default, retries + 1)

      [] ->
        case Storage.get({:queue, :head, queue_name}, partition) do
          [] ->
            default

          [{_, 0}] ->
            default

          [{_, counter}] ->
            lock(partition, queue_name)

            value =
              case Storage.take({:queue, queue_name, counter}, partition) do
                [] ->
                  reset_queue(partition, queue_name)
                  default

                [{_, v}] ->
                  next = counter + 1

                  case Storage.get({:queue, :tail, queue_name}, partition) do
                    [{_, tail}] when next > tail ->
                      reset_queue(partition, queue_name)

                    _ ->
                      Storage.delete({:queue, :head, queue_name}, partition)
                      Storage.put({{:queue, :head, queue_name}, next}, partition)
                  end

                  v
              end

            unlock(partition, queue_name)
            value
        end
    end
  end

  defp local_drain(partition, queue_name) do
    local_drain(partition, queue_name, 0)
  end

  defp local_drain(_partition, queue_name, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, local_drain spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    []
  end

  defp local_drain(partition, queue_name, retries) do
    case Storage.get({:queue, :updating, queue_name}, partition) do
      [_] ->
        :erlang.yield()
        local_drain(partition, queue_name, retries + 1)

      [] ->
        case Storage.get({:queue, :head, queue_name}, partition) do
          [] ->
            []

          [{_, 0}] ->
            []

          [{_, first}] ->
            lock(partition, queue_name)
            [{_, last}] = Storage.get({:queue, :tail, queue_name}, partition)

            values =
              Enum.reduce(first..last, [], fn i, acc ->
                case Storage.take({:queue, queue_name, i}, partition) do
                  [] -> acc
                  [{_, v}] -> [v | acc]
                end
              end)
              |> Enum.reverse()

            reset_queue(partition, queue_name)
            unlock(partition, queue_name)
            values
        end
    end
  end

  defp local_init(queue_name) do
    part = Partition.get_partition(queue_name)

    if Storage.insert_new({{:queue, :updating, queue_name}, true}, part) do
      Storage.put({{:queue, :head, queue_name}, 0}, part)
      Storage.put({{:queue, :tail, queue_name}, 0}, part)
      Storage.delete({:queue, :updating, queue_name}, part)
    end
  end

  ## ── Private — distributed mode (Storage.get + ops list for 3PC) ─────────────

  defp dist_do_enqueue(partition, queue_name, value) do
    dist_do_enqueue(partition, queue_name, value, 0)
  end

  defp dist_do_enqueue(_partition, queue_name, _value, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, dist_do_enqueue spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    false
  end

  defp dist_do_enqueue(partition, queue_name, value, retries) do
    case Storage.get({:queue, :tail, queue_name}, partition) do
      [] ->
        case Storage.get({:queue, :updating, queue_name}, partition) do
          [] ->
            dist_init(queue_name)
            dist_do_enqueue(partition, queue_name, value, retries)

          _ ->
            :erlang.yield()
            dist_do_enqueue(partition, queue_name, value, retries + 1)
        end

      [{_, 0}] ->
        lock(partition, queue_name)

        ops = [
          {:delete, {:queue, :head, queue_name}},
          {:put, {{:queue, queue_name, 1}, value}},
          {:put, {{:queue, :head, queue_name}, 1}},
          {:put, {{:queue, :tail, queue_name}, 1}}
        ]

        DistributedHelpers.apply_write(idx(queue_name), partition, ops)
        unlock(partition, queue_name)
        true

      [{_, counter}] ->
        next = counter + 1
        lock(partition, queue_name)

        ops = [
          {:put, {{:queue, queue_name, next}, value}},
          {:put, {{:queue, :tail, queue_name}, next}}
        ]

        DistributedHelpers.apply_write(idx(queue_name), partition, ops)
        unlock(partition, queue_name)
        true
    end
  end

  defp dist_do_dequeue(partition, queue_name, default) do
    dist_do_dequeue(partition, queue_name, default, 0)
  end

  defp dist_do_dequeue(_partition, queue_name, default, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, dist_do_dequeue spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    default
  end

  defp dist_do_dequeue(partition, queue_name, default, retries) do
    case Storage.get({:queue, :updating, queue_name}, partition) do
      [_] ->
        :erlang.yield()
        dist_do_dequeue(partition, queue_name, default, retries + 1)

      [] ->
        case Storage.get({:queue, :head, queue_name}, partition) do
          [] ->
            default

          [{_, 0}] ->
            default

          [{_, counter}] ->
            lock(partition, queue_name)

            {value, ops} =
              case Storage.get({:queue, queue_name, counter}, partition) do
                [] ->
                  {default,
                   [
                     {:put, {{:queue, :head, queue_name}, 0}},
                     {:put, {{:queue, :tail, queue_name}, 0}}
                   ]}

                [{_, v}] ->
                  next = counter + 1

                  ops =
                    case Storage.get({:queue, :tail, queue_name}, partition) do
                      [{_, tail}] when next > tail ->
                        [
                          {:delete, {:queue, queue_name, counter}},
                          {:put, {{:queue, :head, queue_name}, 0}},
                          {:put, {{:queue, :tail, queue_name}, 0}}
                        ]

                      _ ->
                        [
                          {:delete, {:queue, queue_name, counter}},
                          {:put, {{:queue, :head, queue_name}, next}}
                        ]
                    end

                  {v, ops}
              end

            DistributedHelpers.apply_write(idx(queue_name), partition, ops)
            unlock(partition, queue_name)
            value
        end
    end
  end

  defp dist_do_drain(partition, queue_name) do
    dist_do_drain(partition, queue_name, 0)
  end

  defp dist_do_drain(_partition, queue_name, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, dist_do_drain spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    []
  end

  defp dist_do_drain(partition, queue_name, retries) do
    case Storage.get({:queue, :updating, queue_name}, partition) do
      [_] ->
        :erlang.yield()
        dist_do_drain(partition, queue_name, retries + 1)

      [] ->
        case Storage.get({:queue, :head, queue_name}, partition) do
          [] ->
            []

          [{_, 0}] ->
            []

          [{_, first}] ->
            lock(partition, queue_name)
            [{_, last}] = Storage.get({:queue, :tail, queue_name}, partition)

            {values, del_ops} =
              Enum.reduce(first..last, {[], []}, fn i, {vs, ops} ->
                case Storage.get({:queue, queue_name, i}, partition) do
                  [] -> {vs, ops}
                  [{_, v}] -> {[v | vs], [{:delete, {:queue, queue_name, i}} | ops]}
                end
              end)

            reset_ops = [
              {:put, {{:queue, :head, queue_name}, 0}},
              {:put, {{:queue, :tail, queue_name}, 0}}
            ]

            DistributedHelpers.apply_write(idx(queue_name), partition, del_ops ++ reset_ops)
            unlock(partition, queue_name)
            Enum.reverse(values)
        end
    end
  end

  defp dist_init(queue_name) do
    part = Partition.get_partition(queue_name)

    if Storage.insert_new({{:queue, :updating, queue_name}, true}, part) do
      DistributedHelpers.apply_write(idx(queue_name), part, [
        {:put, {{:queue, :head, queue_name}, 0}},
        {:put, {{:queue, :tail, queue_name}, 0}}
      ])

      Storage.delete({:queue, :updating, queue_name}, part)
    end
  end

  ## ── Private — shared helpers ─────────────────────────────────────────────────

  defp local_peek(partition, queue_name, default) do
    local_peek(partition, queue_name, default, 0)
  end

  defp local_peek(_partition, queue_name, default, retries)
       when retries >= @max_spin_retries do
    Logger.error(
      "super_cache, queue, local_peek spin-wait exceeded #{@max_spin_retries} retries for #{inspect(queue_name)}"
    )

    default
  end

  defp local_peek(partition, queue_name, default, retries) do
    case Storage.get({:queue, :head, queue_name}, partition) do
      [] ->
        case Storage.get({:queue, :updating, queue_name}, partition) do
          [] ->
            default

          _ ->
            :erlang.yield()
            local_peek(partition, queue_name, default, retries + 1)
        end

      [{_, 0}] ->
        default

      [{_, counter}] ->
        case Storage.get({:queue, queue_name, counter}, partition) do
          [] -> default
          [{_, v}] -> v
        end
    end
  end

  defp count_safe(_part, _name, 0), do: 0

  defp count_safe(part, name, retries) do
    case Storage.get({:queue, :updating, name}, part) do
      [_] ->
        :erlang.yield()
        count_safe(part, name, retries - 1)

      [] ->
        head = Storage.get({:queue, :head, name}, part)
        tail = Storage.get({:queue, :tail, name}, part)

        case {head, tail} do
          {[], []} ->
            0

          {[{_, 0}], _} ->
            0

          {_, [{_, 0}]} ->
            0

          {[], _} ->
            :erlang.yield()
            count_safe(part, name, retries - 1)

          {[{_, h}], [{_, t}]} ->
            max(0, t - h + 1)
        end
    end
  end

  defp lock(partition, name), do: Storage.put({{:queue, :updating, name}, true}, partition)
  defp unlock(partition, name), do: Storage.delete({:queue, :updating, name}, partition)

  defp reset_queue(partition, name) do
    Storage.put({{:queue, :head, name}, 0}, partition)
    Storage.put({{:queue, :tail, name}, 0}, partition)
  end

  defp idx(name), do: Partition.get_partition_order(name)
end