import { METERS, meterDef } from "~/lib/meters";
import type { MeterId } from "~/lib/meters";
import { MeteringError } from "./record";
import type { BillingSettings } from "./quota";

/** Numbers from plan config and the price table, inlined into SQL; never user input. */
function sqlNumber(value: number): string {
  if (!Number.isFinite(value)) throw new MeteringError("Non-finite billing constant");
  return String(value);
}

const MICRO_PER_CENT = 1e6;

/** Overage microcents of every other priced counter meter this period, holds included. */
function otherCountersSql(settings: BillingSettings): string {
  const cases = METERS.filter((meter) => meter.kind !== "gauge" && meter.microCentsPerUnit > 0)
    .map((meter) => {
      const included = sqlNumber(settings.plan.included[meter.id] ?? 0);
      const price = sqlNumber(meter.microCentsPerUnit);
      return `WHEN '${meter.id}' THEN MAX(0, p.quantity - p.credit_covered + p.reserved - ${included}) * ${price}`;
    })
    .join(" ");
  return `COALESCE((SELECT SUM(CASE p.meter ${cases} ELSE 0 END)
    FROM usage_periods p WHERE p.account_id = ?2 AND p.period = ?4 AND p.meter != ?3), 0)`;
}

/** Overage microcents of the held meter once this hold (?5) lands, less the prepaid credit still available. */
function heldMeterSql(settings: BillingSettings, meter: MeterId, headroom: string): string {
  const included = sqlNumber(settings.plan.included[meter] ?? 0);
  const price = sqlNumber(meterDef(meter)?.microCentsPerUnit ?? 0);
  return `COALESCE((SELECT MAX(0, p.quantity - p.credit_covered + p.reserved + ?5 - (${headroom}) - ${included}) * ${price}
    FROM usage_periods p WHERE p.account_id = ?2 AND p.period = ?4 AND p.meter = ?3), 0)`;
}

function gaugeSpendSql(settings: BillingSettings): string {
  const cases = METERS.filter((meter) => meter.kind === "gauge" && meter.microCentsPerUnit > 0)
    .map((meter) => {
      const included = sqlNumber(settings.plan.included[meter.id] ?? 0);
      return `WHEN '${meter.id}' THEN MAX(0, g.peak - ${included}) * ${sqlNumber(meter.microCentsPerUnit)}`;
    })
    .join(" ");
  if (!cases) return "0";
  return `COALESCE((SELECT SUM(CASE g.meter ${cases} ELSE 0 END)
    FROM usage_gauges g WHERE g.account_id = ?2 AND g.period = ?4), 0)`;
}

/**
 * The spending-limit guard for reserveUsage's insert (BILL-P1): the period's overage across every priced
 * meter, with this hold added to its own meter, must stay within the limit. It runs inside the same D1
 * transaction that adds the hold, so concurrent holds on different meters see each other and cannot
 * jointly overshoot. Prepaid credit offsets only the held meter; other meters' holds count in full,
 * which errs on the strict side. Compared in microcents so the boundary is exact.
 */
export function spendGuard(settings: BillingSettings, meter: MeterId, headroom = "0"): string {
  if (!settings.plan.billsOverage || settings.spendingLimitCents === null) return "";
  if ((meterDef(meter)?.microCentsPerUnit ?? 0) === 0) return "";
  const limit = sqlNumber(settings.spendingLimitCents * MICRO_PER_CENT);
  return ` AND ${otherCountersSql(settings)} + ${heldMeterSql(settings, meter, headroom)} + ${gaugeSpendSql(settings)} <= ${limit}`;
}