nearle_pos/lib/domain/services/promo_engine.dart

import '../../core/utils/extensions.dart';
import '../entities/cart.dart';
import '../entities/promo.dart';

/// Decides which campaigns fire on a bill, and for how much.
///
/// Kept out of [Cart] on purpose. Cart owns arithmetic that must never be
/// wrong; this owns policy that a shop changes weekly. Mixing them would put a
/// marketing decision in the same class as the GST calculation.
class PromoEngine {
  const PromoEngine._();

  /// Evaluates [promos] against [cart] and returns what actually fires.
  ///
  /// ### Stacking
  ///
  /// All eligible **stackable** promos apply together. Of the **exclusive**
  /// ones, only the single best applies — the one worth most to the shopper,
  /// with [Promo.priority] breaking ties.
  ///
  /// This is the conservative reading, and deliberately so. Letting two
  /// percentages compound produces a discount nobody costed, and a shop finds
  /// out at the end of the month rather than at the till.
  ///
  /// The total is capped at the cart subtotal: no combination of campaigns can
  /// make a bill negative, or turn a sale into a payout.
  static List<AppliedPromo> evaluate({
    required Cart cart,
    required List<Promo> promos,
    required DateTime at,
  }) {
    if (cart.isEmpty || promos.isEmpty) return const [];

    final subtotal = cart.subtotal;

    final eligible = <AppliedPromo>[];
    for (final promo in promos) {
      if (!promo.isLiveAt(at)) continue;
      if (subtotal < promo.minBillValue) continue;

      final amount = amountFor(promo: promo, cart: cart);
      if (amount <= 0) continue;

      eligible.add(AppliedPromo(promo: promo, amount: amount));
    }

    if (eligible.isEmpty) return const [];

    final stackable = eligible.where((a) => a.promo.stackable).toList()
      ..sort((a, b) => a.promo.priority.compareTo(b.promo.priority));

    final exclusive = eligible.where((a) => !a.promo.stackable).toList()
      ..sort((a, b) {
        // Best for the shopper first; priority only breaks a genuine tie.
        final byAmount = b.amount.compareTo(a.amount);
        if (byAmount != 0) return byAmount;
        return a.promo.priority.compareTo(b.promo.priority);
      });

    final chosen = <AppliedPromo>[
      ...stackable,
      if (exclusive.isNotEmpty) exclusive.first,
    ]..sort((a, b) => a.promo.priority.compareTo(b.promo.priority));

    return _capped(chosen, subtotal);
  }

  /// Trims the applied set so it can never exceed the bill.
  ///
  /// Trimming the last one rather than scaling all of them keeps every other
  /// figure on the receipt exactly what the campaign promised.
  static List<AppliedPromo> _capped(List<AppliedPromo> applied, double ceiling) {
    final result = <AppliedPromo>[];
    var running = 0.0;

    for (final entry in applied) {
      final headroom = (ceiling - running).asMoney;
      if (headroom <= 0) break;

      final amount = entry.amount <= headroom ? entry.amount : headroom;
      result.add(AppliedPromo(promo: entry.promo, amount: amount));
      running = (running + amount).asMoney;
    }

    return result;
  }

  /// What one promo is worth on this cart, ignoring stacking rules.
  static double amountFor({required Promo promo, required Cart cart}) {
    final raw = switch (promo.type) {
      PromoType.percentOffBill => cart.subtotal * (promo.value / 100),
      PromoType.flatOffBill => promo.value,
      // Both delegate the "does this line count?" question to the promo
      // itself, because [Cart] asks the same question when it decides which
      // lines carry the GST reduction. Answering it twice invites the two to
      // drift apart.
      PromoType.percentOffCategory =>
        _percentOfMatching(cart, promo.value, promo),
      PromoType.percentOffProduct =>
        _percentOfMatching(cart, promo.value, promo),
      PromoType.buyXGetY => _buyXGetY(cart, promo),
    };

    final capped = promo.maxDiscount == null
        ? raw
        : (raw < promo.maxDiscount! ? raw : promo.maxDiscount!);

    return capped.clamp(0, cart.subtotal).toDouble().asMoney;
  }

  static double _percentOfMatching(Cart cart, double percent, Promo promo) {
    final base = cart.lines
        .where((line) => promo.targets(line.product))
        .fold(0.0, (sum, line) => sum + line.payable);
    return base * (percent / 100);
  }

  /// Free units are the cheapest way to price this: for every group of
  /// (buy + free), the shopper pays for `buy` of them.
  ///
  /// Deliberately counts whole groups only. A "buy 2 get 1" on three items
  /// gives one free; on five it still gives one, because the fifth has not
  /// earned the second group.
  static double _buyXGetY(Cart cart, Promo promo) {
    if (promo.buyQuantity <= 0 || promo.freeQuantity <= 0) return 0;

    final line = cart.lines.firstWhereOrNull(
      (l) => l.product.id == promo.targetId,
    );
    if (line == null) return 0;

    final groupSize = promo.buyQuantity + promo.freeQuantity;
    final groups = line.quantity ~/ groupSize;
    if (groups <= 0) return 0;

    // Priced at the unit rate actually being charged, so a line that already
    // carries a manual discount does not refund more than it took.
    final unitPrice =
        line.quantity <= 0 ? 0.0 : line.payable / line.quantity;

    return groups * promo.freeQuantity * unitPrice;
  }
}