// Reference JavaScript port of the Odoo bundle engine
// (reference/odoo-c2c_bundle_offers/models/bundle_engine.py).
//
// Pure functions, no Shopify imports: drop this into the Discount Function (JavaScript) or port it
// line by line to Rust. It must keep passing spec/test-vectors.json (run: node run-vectors.mjs).
//
// Behaviour that MUST be preserved (it decides which items are discounted):
//  * One unit per item; pool order = cart line order, then unit order.
//  * Offers are tried in priority order; every unit an offer uses leaves the pool.
//  * An offer repeats while it still matches, up to maxUses (0 = unlimited).
//  * _take sorts matching units by price with a STABLE sort (ties keep pool order).
//  * fixed: per bundle line, cheapest matching units first; all-or-nothing (pool restored on failure).
//  * mix: cheapest matching units first.
//  * bxgy: "buy" = MOST expensive matching units, "get" = cheapest remaining matching units.
//  * A use that gives 0 discount puts its units back at the END of the pool and stops that offer.
//  * The discount of a use is spread over ALL its units in proportion to price (bxgy included).

export function bundleDiscount(regular, discountType, value) {
  regular = Math.max(regular, 0);
  value = Math.max(value || 0, 0);
  if (discountType === 'percent') return (regular * Math.min(value, 100)) / 100;
  if (discountType === 'amount') return Math.min(value, regular);
  if (discountType === 'fixed_price') return Math.max(regular - value, 0);
  return 0;
}

function matches(unit, scope) {
  // scope = { variantIds: Set, productIds: Set, collectionIds: Set }
  if (scope.variantIds && scope.variantIds.has(unit.variantId)) return true;
  if (scope.productIds && scope.productIds.has(unit.productId)) return true;
  if (scope.collectionIds) for (const c of unit.collectionIds) if (scope.collectionIds.has(c)) return true;
  return false;
}

function take(pool, predicate, qty, cheapestFirst = true) {
  const found = pool.filter(predicate);                        // keeps pool order
  found.sort((a, b) => (cheapestFirst ? a.price - b.price : b.price - a.price)); // stable (ES2019+)
  if (found.length < qty) return null;
  const taken = found.slice(0, qty);
  for (const unit of taken) pool.splice(pool.indexOf(unit), 1);
  return taken;
}

function applyOnce(pool, offer) {
  let used;
  let discount;
  if (offer.type === 'fixed') {
    const lines = (offer.lines || []).filter((l) => l.quantity > 0);
    if (!lines.length) return null;
    const backup = pool.slice();
    used = [];
    for (const line of lines) {
      const pred = line.variantId
        ? (u) => u.variantId === line.variantId
        : (u) => u.productId === line.productId;                 // product-level line: any variant
      const taken = take(pool, pred, line.quantity);
      if (!taken) { pool.splice(0, pool.length, ...backup); return null; }
      used.push(...taken);
    }
    discount = bundleDiscount(sum(used), offer.discountType, offer.value);
  } else if (offer.type === 'mix') {
    if (!(offer.qty > 0)) return null;
    used = take(pool, (u) => matches(u, offer.scope), offer.qty, true);
    if (!used) return null;
    discount = bundleDiscount(sum(used), offer.discountType, offer.value);
  } else if (offer.type === 'bxgy') {
    if (!(offer.buyQty > 0) || !(offer.getQty > 0)) return null;
    const backup = pool.slice();
    const bought = take(pool, (u) => matches(u, offer.buyScope), offer.buyQty, false);
    if (!bought) return null;
    const got = take(pool, (u) => matches(u, offer.getScope), offer.getQty, true);
    if (!got) { pool.splice(0, pool.length, ...backup); return null; }
    used = bought.concat(got);
    const pct = Math.min(Math.max(offer.getPercent || 0, 0), 100);
    discount = (sum(got) * pct) / 100;
  } else {
    return null;
  }
  if (discount <= 0) { pool.push(...used); return null; }
  return { offerId: offer.id, discount, units: used };
}

function sum(units) { return units.reduce((s, u) => s + u.price, 0); }

/** units: [{lineId, variantId, productId, collectionIds: string[], price}] ; offers in priority order */
export function applyOffers(units, offers) {
  const pool = units.slice();
  const applications = [];
  for (const offer of offers) {
    let uses = 0;
    const maxUses = offer.maxUses || 0;
    while (!maxUses || uses < maxUses) {
      const app = applyOnce(pool, offer);
      if (!app) break;
      applications.push(app);
      uses += 1;
    }
  }
  return applications;
}

/** Discount per (offerId, lineId), spread by price exactly like the Odoo split_by(). */
export function allocateByLine(applications) {
  const totals = new Map();
  for (const app of applications) {
    const base = sum(app.units);
    for (const unit of app.units) {
      const share = base ? app.discount * (unit.price / base) : app.discount / app.units.length;
      const key = app.offerId + '\u0000' + unit.lineId;
      totals.set(key, (totals.get(key) || 0) + share);
    }
  }
  return totals;
}

/** Round amounts to cents so they add up to round(total): largest remainder, ties by input order. */
export function largestRemainderCents(amounts) {
  const exact = amounts.map((a) => a * 100);
  const target = Math.round(amounts.reduce((s, a) => s + a, 0) * 100 + 1e-9);
  const floors = exact.map((c) => Math.floor(c + 1e-9));
  let remainder = target - floors.reduce((s, f) => s + f, 0);
  const order = exact.map((c, i) => i).sort((i, j) => (exact[j] - floors[j]) - (exact[i] - floors[i]) || i - j);
  for (const i of order) { if (remainder <= 0) break; floors[i] += 1; remainder -= 1; }
  return floors.map((f) => f / 100);
}

/** cart lines -> units, one per item, in cart order. */
export function toUnits(lines) {
  const units = [];
  for (const l of lines) {
    for (let i = 0; i < l.quantity; i += 1) {
      units.push({ lineId: l.id, variantId: l.variantId, productId: l.productId,
                   collectionIds: l.collectionIds || [], price: l.unitPrice });
    }
  }
  return units;
}