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 membership_fee_start_date (calculate if nil)
  3. Find the last existing cycle start date (or use membership_fee_start_date)
  4. Generate all cycle starts from last to today (or left_at)
  5. Filter out existing cycles (idempotency)
  6. Create new cycles with the current amount from membership_fee_type

  ## 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
    with_advisory_lock(member.id, fn ->
      do_generate_cycles(member, today)
    end)
  end

  @doc """
  Generates membership fee cycles for all active members.

  Active members are those who:
  - Have a membership_fee_type assigned
  - Have a join_date set
  - Either have no exit_date or exit_date >= today

  ## 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 active members with fee type assigned
    query =
      Member
      |> Ash.Query.filter(not is_nil(membership_fee_type_id))
      |> Ash.Query.filter(not is_nil(join_date))
      |> Ash.Query.filter(is_nil(exit_date) or exit_date >= ^today)

    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 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)

    Repo.transaction(fn ->
      # Acquire advisory lock for this transaction
      Ecto.Adapters.SQL.query!(Repo, "SELECT pg_advisory_xact_lock($1)", [lock_key])

      case fun.() do
        {:ok, result} -> result
        {:error, reason} -> Repo.rollback(reason)
      end
    end)
  end

  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 date
    start_date = determine_start_date(member, interval)

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

    # Generate all cycle starts from start_date to end_date
    all_cycle_starts = generate_cycle_starts(start_date, end_date, interval)

    # Filter out existing cycles
    existing_starts = MapSet.new(existing_cycles, & &1.cycle_start)
    missing_starts = Enum.reject(all_cycle_starts, &MapSet.member?(existing_starts, &1))

    # Create missing cycles
    create_cycles(missing_starts, member.id, fee_type.id, amount)
  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 cycle that contains the exit date
      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
    cycles =
      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) do
          {:ok, cycle} -> {:ok, cycle}
          {:error, reason} -> {:error, {cycle_start, reason}}
        end
      end)

    errors = Enum.filter(cycles, &match?({:error, _}, &1))

    if Enum.empty?(errors) do
      {:ok, Enum.map(cycles, fn {:ok, cycle} -> cycle end)}
    else
      Logger.warning("Some cycles failed to create: #{inspect(errors)}")
      # Return successfully created cycles anyway
      successful = Enum.filter(cycles, &match?({:ok, _}, &1)) |> Enum.map(fn {:ok, c} -> c end)
      {:ok, successful}
    end
  end
end