defmodule Aprsme.Performance.InsertOptimizer do
  @moduledoc """
  Optimizations specifically for INSERT performance.

  This module contains strategies to improve packet insertion performance:
  - Optimized batch sizing based on system load
  - Reduced index maintenance overhead
  - Streamlined packet preparation
  - Connection pool optimizations
  """

  use GenServer

  require Logger

  # Configuration for INSERT optimization
  @base_batch_size 200
  @max_batch_size 800
  @min_batch_size 100
  # 30 seconds
  @optimization_check_interval 30_000

  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  @doc """
  Get the optimal batch size for current system conditions.
  """
  def get_optimal_batch_size do
    GenServer.call(__MODULE__, :get_batch_size)
  catch
    :exit, {:noproc, _} -> @base_batch_size
  end

  @doc """
  Get INSERT optimization settings.
  """
  def get_insert_options do
    GenServer.call(__MODULE__, :get_insert_options)
  catch
    :exit, {:noproc, _} -> default_insert_options()
  end

  @doc """
  Record INSERT performance metrics for optimization.
  """
  def record_insert_metrics(batch_size, duration_ms, success_count) do
    GenServer.cast(__MODULE__, {:record_metrics, batch_size, duration_ms, success_count})
  end

  @impl true
  def init(_opts) do
    schedule_optimization_check()

    state = %{
      current_batch_size: @base_batch_size,
      insert_options: default_insert_options(),
      performance_history: [],
      last_optimization: System.monotonic_time(:millisecond)
    }

    {:ok, state}
  end

  @impl true
  def handle_call(:get_batch_size, _from, state) do
    {:reply, state.current_batch_size, state}
  end

  @impl true
  def handle_call(:get_insert_options, _from, state) do
    {:reply, state.insert_options, state}
  end

  @impl true
  def handle_cast({:record_metrics, batch_size, duration_ms, success_count}, state) do
    metric = %{
      batch_size: batch_size,
      duration_ms: duration_ms,
      success_count: success_count,
      throughput: throughput(success_count, duration_ms),
      timestamp: System.monotonic_time(:millisecond)
    }

    # Keep last 20 metrics for analysis
    new_history = Enum.take([metric | state.performance_history], 20)

    {:noreply, %{state | performance_history: new_history}}
  end

  # Packets-per-second, or 0 when the batch completed instantly (avoid div-by-0).
  defp throughput(_count, 0), do: 0
  defp throughput(_count, duration_ms) when duration_ms < 0, do: 0
  defp throughput(count, duration_ms), do: count / (duration_ms / 1000)

  @impl true
  def handle_info(:optimize_settings, state) do
    new_state = optimize_based_on_performance(state)
    schedule_optimization_check()
    {:noreply, new_state}
  end

  defp schedule_optimization_check do
    Process.send_after(self(), :optimize_settings, @optimization_check_interval)
  end

  # Require at least three samples before doing anything — match directly on the
  # list prefix instead of calling length/1 so we short-circuit for short lists.
  defp optimize_based_on_performance(%{performance_history: [_, _, _ | _] = history} = state) do
    recent_metrics = Enum.take(history, 5)
    sample_size = length(recent_metrics)
    avg_throughput = average(recent_metrics, & &1.throughput, sample_size)
    avg_duration = average(recent_metrics, & &1.duration_ms, sample_size)

    new_batch_size = calculate_optimal_batch_size(avg_throughput, avg_duration, state.current_batch_size)
    new_insert_options = calculate_insert_options(avg_duration)

    Logger.debug(
      "INSERT optimization: batch_size=#{new_batch_size}, avg_throughput=#{Float.round(avg_throughput, 2)} pps"
    )

    :telemetry.execute([:aprsme, :insert_optimizer, :batch_size], %{value: new_batch_size}, %{})
    :telemetry.execute([:aprsme, :insert_optimizer, :throughput], %{value: avg_throughput}, %{})
    :telemetry.execute([:aprsme, :insert_optimizer, :duration], %{value: avg_duration}, %{})
    maybe_emit_optimization(new_batch_size, state.current_batch_size)

    %{
      state
      | current_batch_size: new_batch_size,
        insert_options: new_insert_options,
        last_optimization: System.monotonic_time(:millisecond)
    }
  end

  defp optimize_based_on_performance(state), do: state

  defp average(items, getter, count) do
    items |> Enum.map(getter) |> Enum.sum() |> Kernel./(count)
  end

  defp maybe_emit_optimization(same, same), do: :ok

  defp maybe_emit_optimization(_new, _old) do
    :telemetry.execute([:aprsme, :insert_optimizer, :optimizations], %{count: 1}, %{})
  end

  defp calculate_optimal_batch_size(avg_throughput, avg_duration, current_batch_size) do
    cond do
      # If throughput is low and duration is high, reduce batch size
      avg_throughput < 50 and avg_duration > 5000 ->
        max(@min_batch_size, round(current_batch_size * 0.8))

      # If throughput is good and duration is acceptable, increase batch size
      avg_throughput > 200 and avg_duration < 2000 ->
        min(@max_batch_size, round(current_batch_size * 1.2))

      # Otherwise keep current size
      true ->
        current_batch_size
    end
  end

  # When INSERTs are slow (>3s avg), use a longer timeout; otherwise stick with defaults.
  defp calculate_insert_options(avg_duration) when avg_duration > 3_000 do
    Keyword.merge(default_insert_options(),
      returning: false,
      on_conflict: :nothing,
      timeout: 30_000
    )
  end

  defp calculate_insert_options(_avg_duration), do: default_insert_options()

  defp default_insert_options do
    [
      # Don't return IDs unless needed
      returning: false,
      # Skip conflicts instead of raising
      on_conflict: :nothing,
      # 15 second timeout
      timeout: 15_000
    ]
  end
end