defmodule ArcadiaCloud.Invoicing do
  @moduledoc """
  Month-end invoice rollup — turns a period of metered usage into tenant
  invoices.

  For each tenant with active subscriptions, one tenant_invoice is
  produced. For each of that tenant's subscriptions, the quote engine
  runs with the deployment's ACTUAL metered usage; the resulting
  recurring + overage lines become tenant_invoice_lines tagged with the
  deployment. A GST line (AU 10%) is appended at the tenant level.

  Because the same quote engine serves projection and invoicing, the
  price a tenant was quoted at provisioning time and the price they're
  invoiced are computed identically.

  Idempotent: unique (tenant_id, period_start). Re-running skips tenants
  that already have an invoice for the period unless `force: true`.
  """

  import Ecto.Query, warn: false

  alias ArcadiaCloud.Repo
  alias ArcadiaCloud.{Catalog, Metering, Quoting, Subscriptions}
  alias ArcadiaCloud.Billing.{Subscription, TenantInvoice, TenantInvoiceLine}
  alias ArcadiaCloud.Deployments.CloudDeployment

  # AU GST. Sky AI is Australian (project_skyai_australia memory).
  @gst_rate 0.10

  @doc """
  Roll up invoices for a calendar month. `month` is any Date in that
  month; defaults to the previous month.
  """
  def roll_up_month(month \\ default_month(), opts \\ []) do
    period_start = Date.beginning_of_month(month)
    period_end = Date.end_of_month(month)
    roll_up_period(period_start, period_end, opts)
  end

  def roll_up_period(%Date{} = period_start, %Date{} = period_end, opts \\ []) do
    force = Keyword.get(opts, :force, false)

    subscriptions_by_tenant()
    |> Enum.map(fn {tenant_id, subs} ->
      roll_up_tenant(tenant_id, subs, period_start, period_end, force)
    end)
  end

  # ---- per-tenant -----------------------------------------------------------

  defp roll_up_tenant(tenant_id, subscriptions, period_start, period_end, force) do
    existing = Repo.get_by(TenantInvoice, tenant_id: tenant_id, period_start: period_start)

    cond do
      existing && not force ->
        {:skipped, tenant_id, existing.id}

      true ->
        if existing, do: Repo.delete!(existing)
        {:ok, invoice} = build_invoice(tenant_id, subscriptions, period_start, period_end)
        {:rolled, tenant_id, invoice.id}
    end
  end

  defp build_invoice(tenant_id, subscriptions, period_start, period_end) do
    now = DateTime.utc_now() |> DateTime.truncate(:second)

    Repo.transaction(fn ->
      {:ok, invoice} =
        %TenantInvoice{}
        |> TenantInvoice.changeset(%{
          tenant_id: tenant_id,
          period_start: period_start,
          period_end: period_end,
          status: "issued",
          issued_at: now
        })
        |> Repo.insert()

      lines = Enum.flat_map(subscriptions, &subscription_lines(&1, period_start, period_end))
      Enum.each(lines, &insert_line(invoice.id, &1))

      subtotal = Enum.reduce(lines, 0, &(&1.amount_cents + &2))
      tax = round(subtotal * @gst_rate)

      if tax > 0 do
        insert_line(invoice.id, %{
          deployment_id: nil,
          kind: "tax",
          resource_kind: nil,
          description: "GST (#{round(@gst_rate * 100)}%)",
          qty: nil,
          unit: nil,
          unit_price_cents: nil,
          amount_cents: tax,
          meta: %{"rate" => @gst_rate}
        })
      end

      invoice
      |> TenantInvoice.changeset(%{
        subtotal_cents: subtotal,
        tax_cents: tax,
        total_cents: subtotal + tax
      })
      |> Repo.update!()
    end)
  end

  # quote one subscription -> invoice lines tagged with its deployment
  defp subscription_lines(%Subscription{} = sub, period_start, period_end) do
    deployment_id = sub.deployment_id
    plan_version = Catalog.get_version(sub.plan_version_id)
    addons = Subscriptions.list_addons(sub.id)
    usage = Metering.usage_for_period(deployment_id, period_start, period_end)

    quote = Quoting.quote(plan_version, addons, usage: usage)

    (quote.recurring.lines ++ quote.overage.lines)
    |> Enum.map(fn line -> Map.put(line, :deployment_id, deployment_id) end)
  end

  defp insert_line(invoice_id, line) do
    %TenantInvoiceLine{}
    |> TenantInvoiceLine.changeset(%{
      invoice_id: invoice_id,
      deployment_id: line[:deployment_id],
      kind: line.kind,
      resource_kind: line[:resource_kind],
      description: line[:description],
      qty: to_decimal(line[:qty]),
      unit: line[:unit],
      unit_price_cents: line[:unit_price_cents],
      amount_cents: line.amount_cents,
      meta: stringify(line[:meta] || %{})
    })
    |> Repo.insert!()
  end

  # ---- queries --------------------------------------------------------------

  def list_invoices(opts \\ []) do
    from(i in TenantInvoice, order_by: [desc: i.period_start])
    |> filter(:tenant_id, opts[:tenant_id])
    |> filter(:status, opts[:status])
    |> Repo.all()
  end

  def get_invoice(id) do
    case Repo.get(TenantInvoice, id) do
      nil -> nil
      inv -> Repo.preload(inv, :lines)
    end
  end

  # ---- helpers --------------------------------------------------------------

  defp subscriptions_by_tenant do
    from(s in Subscription,
      where: s.status == "active",
      join: d in CloudDeployment,
      on: d.id == s.deployment_id,
      preload: [:addons],
      select: {d.tenant_id, s}
    )
    |> Repo.all()
    |> Enum.group_by(fn {tenant_id, _} -> tenant_id end, fn {_, sub} -> sub end)
  end

  defp default_month do
    today = Date.utc_today()
    Date.add(Date.beginning_of_month(today), -1)
  end

  defp filter(q, _f, nil), do: q
  defp filter(q, field, value), do: from(i in q, where: field(i, ^field) == ^value)

  defp to_decimal(nil), do: nil
  defp to_decimal(%Decimal{} = d), do: d
  defp to_decimal(n) when is_integer(n), do: Decimal.new(n)
  defp to_decimal(n) when is_float(n), do: Decimal.from_float(Float.round(n, 4))

  # invoice_lines.meta is a jsonb column — keys must be strings.
  defp stringify(map) when is_map(map) do
    Map.new(map, fn {k, v} -> {to_string(k), v} end)
  end
end