defmodule ArcadiaCloud.LlmUsage do
  @moduledoc """
  Stores `llm_usage_recorded` events pushed by arcadia-llm-gateway and
  rolls them into tenant invoices.

  The gateway is the LLM-pricing authority — it sends the already-priced
  `customer_charge_cents` per request. arcadia-cloud does not re-price;
  it sums received events per deployment into invoice lines.

  Note on `plan_inclusive` LLM pricing: the per-request charge the
  gateway sends is the *marginal* (overage-rate) price. For
  `flat_percent` / `fixed_per_token` policies the sum equals the bill
  exactly. If a tenant is put on a `plan_inclusive` LLM policy, a
  period true-up against the gateway's authoritative `period_charge`
  would be needed — none are in use today.
  """

  import Ecto.Query, warn: false

  alias ArcadiaCloud.Repo
  alias ArcadiaCloud.Billing.LlmUsageEvent

  @doc """
  Idempotently records an event. A repeated `gateway_request_id` (a
  gateway retry) is a no-op. Returns `{:ok, :recorded | :duplicate}`.
  """
  def record_event(attrs) do
    gateway_request_id = attrs[:gateway_request_id] || attrs["gateway_request_id"]

    if gateway_request_id && Repo.exists?(seen(gateway_request_id)) do
      {:ok, :duplicate}
    else
      insert_event(attrs)
    end
  end

  defp insert_event(attrs) do
    case %LlmUsageEvent{} |> LlmUsageEvent.changeset(attrs) |> Repo.insert() do
      {:ok, _event} ->
        {:ok, :recorded}

      {:error, %{errors: errors} = changeset} ->
        # A concurrent insert of the same event tripped the unique index.
        if Keyword.has_key?(errors, :gateway_request_id),
          do: {:ok, :duplicate},
          else: {:error, changeset}
    end
  end

  defp seen(gateway_request_id) do
    from(e in LlmUsageEvent, where: e.gateway_request_id == ^gateway_request_id)
  end

  @doc """
  Invoice lines for a tenant's LLM usage in `[period_start, period_end]`,
  one per deployment. Returns `[]` when there was no usage.
  """
  def period_lines(tenant_id, %Date{} = period_start, %Date{} = period_end) do
    from = DateTime.new!(period_start, ~T[00:00:00], "Etc/UTC")
    to = DateTime.new!(Date.add(period_end, 1), ~T[00:00:00], "Etc/UTC")

    from(e in LlmUsageEvent,
      where: e.tenant_id == ^tenant_id and e.occurred_at >= ^from and e.occurred_at < ^to,
      group_by: e.deployment_id,
      select: %{
        deployment_id: e.deployment_id,
        requests: count(e.id),
        input_tokens: coalesce(sum(e.input_tokens), 0),
        output_tokens: coalesce(sum(e.output_tokens), 0),
        # Sum the exact decimal charge, round to cents once — summing the
        # gateway's pre-rounded per-request cents would drift on high
        # request counts.
        charge: coalesce(sum(e.customer_charge), 0)
      }
    )
    |> Repo.all()
    |> Enum.map(&to_line/1)
    |> Enum.reject(&(&1.amount_cents == 0))
  end

  defp to_line(row) do
    amount_cents =
      row.charge
      |> to_decimal()
      |> Decimal.mult(Decimal.new(100))
      |> Decimal.round(0, :half_up)
      |> Decimal.to_integer()

    %{
      deployment_id: row.deployment_id,
      kind: "llm_usage",
      resource_kind: "llm_tokens",
      description:
        "LLM tokens — #{row.input_tokens} in / #{row.output_tokens} out (#{row.requests} requests)",
      qty: row.input_tokens + row.output_tokens,
      unit: "token",
      unit_price_cents: nil,
      amount_cents: amount_cents,
      meta: %{
        "input_tokens" => row.input_tokens,
        "output_tokens" => row.output_tokens,
        "requests" => row.requests
      }
    }
  end

  defp to_decimal(%Decimal{} = d), do: d
  defp to_decimal(n) when is_integer(n), do: Decimal.new(n)
  defp to_decimal(_), do: Decimal.new(0)
end