defmodule Mv.MembershipFees.CycleGenerator do
  @moduledoc """
  Module for generating membership fee cycles for members.

  This module provides functions to automatically generate membership fee cycles
  based on a member's fee type, start date, and exit date.

  ## Algorithm

  1. Load member with relationships (membership_fee_type, membership_fee_cycles)
  2. Determine the generation start point:
     - If NO cycles exist: Start from `membership_fee_start_date` (or calculated from `join_date`)
     - If cycles exist: Start from the cycle AFTER the last existing one
  3. Generate all cycle starts from the determined start point to today (or `exit_date`)
  4. Create new cycles with the current amount from `membership_fee_type`

  ## Important: Gap Handling

  **Gaps are NOT filled.** If a cycle was explicitly deleted (e.g., 2023 was deleted
  but 2022 and 2024 exist), the generator will NOT recreate the deleted cycle.
  It always continues from the LAST existing cycle, regardless of any gaps.

  This behavior ensures that manually deleted cycles remain deleted and prevents
  unwanted automatic recreation of intentionally removed cycles.

  ## Concurrency

  Uses PostgreSQL advisory locks to prevent race conditions when generating
  cycles for the same member concurrently.

  ## Examples

      # Generate cycles for a single member
      {:ok, cycles} = CycleGenerator.generate_cycles_for_member(member)

      # Generate cycles for all active members
      {:ok, results} = CycleGenerator.generate_cycles_for_all_members()

  """

  alias Mv.MembershipFees.CalendarCycles
  alias Mv.MembershipFees.MembershipFeeCycle
  alias Mv.MembershipFees.Changes.SetMembershipFeeStartDate
  alias Mv.Membership.Member
  alias Mv.Repo

  require Ash.Query
  require Logger

  @type generate_result :: {:ok, [MembershipFeeCycle.t()]} | {:error, term()}

  @doc """
  Generates membership fee cycles for a single member.

  Uses an advisory lock to prevent concurrent generation for the same member.

  ## Parameters

    - `member` - The member struct or member ID
    - `opts` - Options:
      - `:today` - Override today's date (useful for testing)

  ## Returns

    - `{:ok, cycles}` - List of newly created cycles
    - `{:error, reason}` - Error with reason

  ## Examples

      {:ok, cycles} = CycleGenerator.generate_cycles_for_member(member)
      {:ok, cycles} = CycleGenerator.generate_cycles_for_member(member_id)
      {:ok, cycles} = CycleGenerator.generate_cycles_for_member(member, today: ~D[2024-12-31])

  """
  @spec generate_cycles_for_member(Member.t() | String.t(), keyword()) :: generate_result()
  def generate_cycles_for_member(member_or_id, opts \\ [])

  def generate_cycles_for_member(member_id, opts) when is_binary(member_id) do
    case load_member(member_id) do
      {:ok, member} -> generate_cycles_for_member(member, opts)
      {:error, reason} -> {:error, reason}
    end
  end

  def generate_cycles_for_member(%Member{} = member, opts) do
    today = Keyword.get(opts, :today, Date.utc_today())

    # Use advisory lock to prevent concurrent generation
    # Notifications are handled inside with_advisory_lock after transaction commits
    with_advisory_lock(member.id, fn ->
      do_generate_cycles(member, today)
    end)
  end

  @doc """
  Generates membership fee cycles for all members with a fee type assigned.

  This includes both active and inactive (left) members. Inactive members
  will have cycles generated up to their exit_date if they don't have cycles
  for that period yet. This allows for catch-up generation of missing cycles.

  Members processed are those who:
  - Have a membership_fee_type assigned
  - Have a join_date set

  The exit_date boundary is respected during generation (not in the query),
  so inactive members will get cycles up to their exit date.

  ## Parameters

    - `opts` - Options:
      - `:today` - Override today's date (useful for testing)
      - `:batch_size` - Number of members to process in parallel (default: 10)

  ## Returns

    - `{:ok, results}` - Map with :success and :failed counts
    - `{:error, reason}` - Error with reason

  """
  @spec generate_cycles_for_all_members(keyword()) :: {:ok, map()} | {:error, term()}
  def generate_cycles_for_all_members(opts \\ []) do
    today = Keyword.get(opts, :today, Date.utc_today())
    batch_size = Keyword.get(opts, :batch_size, 10)

    # Query ALL members with fee type assigned (including inactive/left members)
    # The exit_date boundary is applied during cycle generation, not here.
    # This allows catch-up generation for members who left but are missing cycles.
    query =
      Member
      |> Ash.Query.filter(not is_nil(membership_fee_type_id))
      |> Ash.Query.filter(not is_nil(join_date))

    case Ash.read(query) do
      {:ok, members} ->
        results = process_members_in_batches(members, batch_size, today)
        {:ok, build_results_summary(results)}

      {:error, reason} ->
        {:error, reason}
    end
  end

  defp process_members_in_batches(members, batch_size, today) do
    members
    |> Enum.chunk_every(batch_size)
    |> Enum.flat_map(&process_batch(&1, today))
  end

  defp process_batch(batch, today) do
    batch
    |> Task.async_stream(fn member ->
      {member.id, generate_cycles_for_member(member, today: today)}
    end)
    |> Enum.map(fn
      {:ok, result} ->
        result

      {:exit, reason} ->
        # Task crashed - log and return error tuple
        Logger.error("Task crashed during cycle generation: #{inspect(reason)}")
        {nil, {:error, {:task_exit, reason}}}
    end)
  end

  defp build_results_summary(results) do
    success_count = Enum.count(results, fn {_id, result} -> match?({:ok, _}, result) end)
    failed_count = Enum.count(results, fn {_id, result} -> match?({:error, _}, result) end)

    %{success: success_count, failed: failed_count, total: length(results)}
  end

  # Private functions

  defp load_member(member_id) do
    Member
    |> Ash.Query.filter(id == ^member_id)
    |> Ash.Query.load([:membership_fee_type, :membership_fee_cycles])
    |> Ash.read_one()
    |> case do
      {:ok, nil} -> {:error, :member_not_found}
      {:ok, member} -> {:ok, member}
      {:error, reason} -> {:error, reason}
    end
  end

  defp with_advisory_lock(member_id, fun) do
    # Convert UUID to integer for advisory lock (use hash)
    lock_key = :erlang.phash2(member_id)

    # Check if we're already in a transaction (e.g., called from Ash action)
    if Repo.in_transaction?() do
      with_advisory_lock_in_transaction(lock_key, fun)
    else
      with_advisory_lock_new_transaction(lock_key, fun)
    end
  end

  # Already in transaction: use advisory lock directly without starting new transaction
  # This prevents nested transactions which can cause deadlocks
  defp with_advisory_lock_in_transaction(lock_key, fun) do
    Ecto.Adapters.SQL.query!(Repo, "SELECT pg_advisory_xact_lock($1)", [lock_key])
    normalize_fun_result(fun.())
  end

  # Not in transaction: start new transaction with advisory lock
  defp with_advisory_lock_new_transaction(lock_key, fun) do
    result =
      Repo.transaction(fn ->
        # Acquire advisory lock for this transaction
        Ecto.Adapters.SQL.query!(Repo, "SELECT pg_advisory_xact_lock($1)", [lock_key])
        execute_within_transaction(fun)
      end)

    handle_transaction_result(result)
  end

  # Execute function within transaction and return normalized result
  # When in transaction, create_cycles returns {:ok, cycles, notifications}
  # When not in transaction, create_cycles returns {:ok, cycles}
  defp execute_within_transaction(fun) do
    case fun.() do
      {:ok, result, notifications} when is_list(notifications) ->
        # In transaction case: return result and notifications separately
        {result, notifications}

      {:ok, result} ->
        # Not in transaction case: notifications handled by Ash automatically
        {result, []}

      {:error, reason} ->
        Repo.rollback(reason)
    end
  end

  # Normalize function result to consistent format
  # When in transaction, create_cycles returns {:ok, cycles, notifications}
  # When not in transaction, create_cycles returns {:ok, cycles}
  defp normalize_fun_result({:ok, result, _notifications}) do
    # In transaction case: notifications will be sent after outer transaction commits
    # Return in same format as non-transaction case for consistency
    {:ok, result}
  end

  defp normalize_fun_result({:ok, result}), do: {:ok, result}
  defp normalize_fun_result({:error, reason}), do: {:error, reason}

  # Handle transaction result and send notifications if needed
  defp handle_transaction_result({:ok, {cycles, notifications}}) do
    if Enum.any?(notifications) do
      Ash.Notifier.notify(notifications)
    end

    {:ok, cycles}
  end

  defp handle_transaction_result({:error, reason}), do: {:error, reason}

  defp do_generate_cycles(member, today) do
    # Reload member with relationships to ensure fresh data
    case load_member(member.id) do
      {:ok, member} ->
        cond do
          is_nil(member.membership_fee_type_id) ->
            {:error, :no_membership_fee_type}

          is_nil(member.join_date) ->
            {:error, :no_join_date}

          true ->
            generate_missing_cycles(member, today)
        end

      {:error, reason} ->
        {:error, reason}
    end
  end

  defp generate_missing_cycles(member, today) do
    fee_type = member.membership_fee_type
    interval = fee_type.interval
    amount = fee_type.amount
    existing_cycles = member.membership_fee_cycles || []

    # Determine start point based on existing cycles
    # Note: We do NOT fill gaps - only generate from the last existing cycle onwards
    start_date = determine_generation_start(member, existing_cycles, interval)

    # Determine end date (today or exit_date, whichever is earlier)
    end_date = determine_end_date(member, today)

    # Only generate if start_date <= end_date
    if start_date && Date.compare(start_date, end_date) != :gt do
      cycle_starts = generate_cycle_starts(start_date, end_date, interval)
      create_cycles(cycle_starts, member.id, fee_type.id, amount)
    else
      {:ok, [], []}
    end
  end

  # No existing cycles: start from membership_fee_start_date
  defp determine_generation_start(member, [], interval) do
    determine_start_date(member, interval)
  end

  # Has existing cycles: start from the cycle AFTER the last one
  # This ensures gaps (deleted cycles) are NOT filled
  defp determine_generation_start(_member, existing_cycles, interval) do
    last_cycle_start =
      existing_cycles
      |> Enum.map(& &1.cycle_start)
      |> Enum.max(Date)

    CalendarCycles.next_cycle_start(last_cycle_start, interval)
  end

  defp determine_start_date(member, interval) do
    if member.membership_fee_start_date do
      member.membership_fee_start_date
    else
      # Calculate from join_date using global settings
      include_joining_cycle = get_include_joining_cycle()

      SetMembershipFeeStartDate.calculate_start_date(
        member.join_date,
        interval,
        include_joining_cycle
      )
    end
  end

  defp determine_end_date(member, today) do
    if member.exit_date && Date.compare(member.exit_date, today) == :lt do
      # Member has left - use the exit date as boundary
      # Note: If exit_date == cycle_start, the cycle IS still generated.
      # This means the member is considered a member on the first day of that cycle.
      # Example: exit_date = 2025-01-01, yearly interval
      #   -> The 2025 cycle (starting 2025-01-01) WILL be generated
      member.exit_date
    else
      today
    end
  end

  defp get_include_joining_cycle do
    case Mv.Membership.get_settings() do
      {:ok, %{include_joining_cycle: include}} -> include
      {:error, _} -> true
    end
  end

  @doc """
  Generates all cycle start dates from a start date to an end date.

  ## Parameters

    - `start_date` - The first cycle start date
    - `end_date` - The date up to which cycles should be generated
    - `interval` - The billing interval

  ## Returns

    List of cycle start dates.

  ## Examples

      iex> generate_cycle_starts(~D[2024-01-01], ~D[2024-12-31], :quarterly)
      [~D[2024-01-01], ~D[2024-04-01], ~D[2024-07-01], ~D[2024-10-01]]

  """
  @spec generate_cycle_starts(Date.t(), Date.t(), atom()) :: [Date.t()]
  def generate_cycle_starts(start_date, end_date, interval) do
    # Ensure start_date is aligned to cycle boundary
    aligned_start = CalendarCycles.calculate_cycle_start(start_date, interval)

    generate_cycle_starts_acc(aligned_start, end_date, interval, [])
  end

  defp generate_cycle_starts_acc(current_start, end_date, interval, acc) do
    if Date.compare(current_start, end_date) == :gt do
      # Current cycle start is after end date - stop
      Enum.reverse(acc)
    else
      # Include this cycle and continue to next
      next_start = CalendarCycles.next_cycle_start(current_start, interval)
      generate_cycle_starts_acc(next_start, end_date, interval, [current_start | acc])
    end
  end

  defp create_cycles(cycle_starts, member_id, fee_type_id, amount) do
    # Always return notifications when in a transaction (required by Ash)
    # When not in transaction, Ash handles notifications automatically
    # When in transaction, we must return notifications and send them after commit
    return_notifications? = Repo.in_transaction?()

    results =
      Enum.map(cycle_starts, fn cycle_start ->
        attrs = %{
          cycle_start: cycle_start,
          member_id: member_id,
          membership_fee_type_id: fee_type_id,
          amount: amount,
          status: :unpaid
        }

        case Ash.create(MembershipFeeCycle, attrs, return_notifications?: return_notifications?) do
          {:ok, cycle, notifications} when is_list(notifications) ->
            {:ok, cycle, notifications}

          {:ok, cycle} ->
            {:ok, cycle, []}

          {:error, reason} ->
            {:error, {cycle_start, reason}}
        end
      end)

    {successes, errors} = Enum.split_with(results, &match?({:ok, _, _}, &1))

    all_notifications =
      Enum.flat_map(successes, fn {:ok, _cycle, notifications} -> notifications end)

    if Enum.empty?(errors) do
      successful_cycles = Enum.map(successes, fn {:ok, cycle, _notifications} -> cycle end)

      if return_notifications? do
        # Return cycles and notifications to be sent after transaction commits
        {:ok, successful_cycles, all_notifications}
      else
        # Not in transaction: Ash handles notifications automatically
        {:ok, successful_cycles}
      end
    else
      Logger.warning("Some cycles failed to create: #{inspect(errors)}")
      # Return partial failure with errors
      # Note: When this error occurs, the transaction will be rolled back,
      # so no cycles were actually persisted in the database
      {:error, {:partial_failure, errors}}
    end
  end
end