/**
 * Coupon codes — percentage, fixed-amount and comp (100%) discounts on registrations.
 *
 * Rules:
 * - A registration can carry one active coupon. Its discount is stored as a
 *   CouponRedemption (amount locked at apply time) and mirrored as the sum of active
 *   redemptions on Registration.discountTotal, which computeRegistrationTotalDue
 *   subtracts. Early-bird pricing refreshes never touch it.
 * - Editing a registration's items recalculates percentage and comp codes over the new
 *   items; fixed-amount codes keep their amount (capped at what's in scope).
 * - Cancelling a registration releases its redemption, freeing the use.
 * - Scope: no CouponOption rows = the whole event; otherwise only matching options
 *   (or one variant of an option).
 * - stackEarlyBird=false means "best price wins": the coupon is worked out against the
 *   standard price, and only the part that beats the early-bird price is discounted.
 * - Restricted codes (users, roles, email domain) need a known account, so guests
 *   can't use them; a match on any one restriction is enough.
 */

const prisma = require('../config/db');

function normalizeCode(code) {
  return String(code || '').trim().toUpperCase();
}

function couponError(message) {
  const err = new Error(message);
  err.isCouponError = true;
  err.statusCode = 400;
  return err;
}

const round2 = (n) => Math.round(Number(n || 0) * 100) / 100;

/**
 * Discountable lines from a registration — one per price tranche, so tickets bought at
 * different prices are each discounted at what was actually charged.
 * `basePrice` is the standard (non-early-bird) price, used for "best price wins".
 */
function buildCouponLines(registration) {
  const lines = [];
  for (const ro of registration?.registrationOptions || []) {
    const eo = ro.eventOption || {};
    const variant = ro.variantId ? (eo.variants || []).find(v => v.id === ro.variantId) : null;
    const standard = variant && variant.price !== null && variant.price !== undefined ? Number(variant.price) : Number(eo.price || 0);
    const tranches = ro.tranches && ro.tranches.length > 0
      ? ro.tranches
      : [{ quantity: ro.quantity, priceSnapshot: ro.priceSnapshot ?? standard, appliedTierId: ro.appliedTierId }];
    for (const t of tranches) {
      const unitPrice = Number(t.priceSnapshot ?? standard);
      lines.push({
        eventOptionId: ro.eventOptionId,
        variantId: ro.variantId || null,
        quantity: Number(t.quantity || 0),
        unitPrice,
        // Only an early-bird tranche has a "standard" price above what was charged; a
        // staff price or plain price is already the base the coupon works from.
        basePrice: t.appliedTierId ? Math.max(standard, unitPrice) : unitPrice,
      });
    }
  }
  return lines;
}

function lineInScope(line, scopes) {
  if (!scopes || scopes.length === 0) return true;
  return scopes.some(s => s.eventOptionId === line.eventOptionId && (!s.variantId || s.variantId === line.variantId));
}

function scopedLines(coupon, lines) {
  return (lines || []).filter(l => lineInScope(l, coupon.options));
}

/**
 * The discount a coupon gives on these lines, in rand, never more than what the in-scope
 * lines cost. Pure — used for previews, applying, and recalculating after edits.
 */
function computeCouponDiscount(coupon, lines) {
  const scoped = scopedLines(coupon, lines);
  const charged = scoped.reduce((s, l) => s + l.unitPrice * l.quantity, 0);
  if (charged <= 0) return 0;

  let discount = 0;
  if (coupon.type === 'comp') {
    discount = charged;
  } else if (coupon.type === 'percent') {
    const pct = Math.min(100, Math.max(0, Number(coupon.value || 0))) / 100;
    if (coupon.stackEarlyBird) {
      discount = charged * pct;
    } else {
      discount = scoped.reduce((s, l) => s + (l.unitPrice - Math.min(l.unitPrice, l.basePrice * (1 - pct))) * l.quantity, 0);
    }
  } else if (coupon.type === 'fixed') {
    const amount = Math.max(0, Number(coupon.value || 0));
    if (coupon.stackEarlyBird) {
      discount = amount;
    } else {
      const base = scoped.reduce((s, l) => s + l.basePrice * l.quantity, 0);
      discount = charged - Math.max(0, base - amount);
    }
  }
  return round2(Math.min(charged, Math.max(0, discount)));
}

/**
 * Checks whether a coupon can be used. Pure — the caller supplies the use counts.
 *
 * @param {object} coupon   - with events[], options[], allowedUsers[]
 * @param {object} ctx      - { eventId, user: {id, role, email}|null, now, usesTotal, usesByUser, lines,
 *                              deferRestrictions } — deferRestrictions lets staff preview a
 *                              restricted code before the attendee's account exists; it's
 *                              checked against the attendee when the registration is saved.
 * @returns {{ ok: true } | { ok: false, message: string }}
 */
function validateCoupon(coupon, ctx) {
  const no = (message) => ({ ok: false, message });
  if (!coupon) return no('That code isn\'t valid.');
  if (!coupon.active) return no('This code is no longer active.');
  const now = ctx.now ? new Date(ctx.now) : new Date();
  if (coupon.validFrom && now < new Date(coupon.validFrom)) return no('This code isn\'t valid yet.');
  if (coupon.validUntil && now > new Date(coupon.validUntil)) return no('This code has expired.');
  if ((coupon.events || []).length > 0 && !coupon.events.some(e => e.eventId === ctx.eventId)) {
    return no('This code isn\'t valid for this event.');
  }

  const restrictedUsers = (coupon.allowedUsers || []).map(u => u.userId);
  const restrictedRoles = coupon.restrictRoles || [];
  const domain = String(coupon.restrictDomain || '').trim().toLowerCase().replace(/^@/, '');
  if ((restrictedUsers.length || restrictedRoles.length || domain) && !(ctx.deferRestrictions && !ctx.user)) {
    const user = ctx.user;
    if (!user) return no('Log in to use this code.');
    const email = String(user.email || '').toLowerCase();
    const allowed = restrictedUsers.includes(user.id)
      || restrictedRoles.includes(user.role)
      || (domain && email.endsWith(`@${domain}`));
    if (!allowed) return no('This code isn\'t available for your account.');
  }

  if (coupon.maxUses !== null && coupon.maxUses !== undefined && (ctx.usesTotal || 0) >= coupon.maxUses) {
    return no('This code has been fully used.');
  }
  if (coupon.maxUsesPerUser !== null && coupon.maxUsesPerUser !== undefined && ctx.user && (ctx.usesByUser || 0) >= coupon.maxUsesPerUser) {
    return no('You\'ve already used this code.');
  }

  const scopedQty = scopedLines(coupon, ctx.lines).reduce((s, l) => s + l.quantity, 0);
  if (scopedQty === 0) return no('This code doesn\'t apply to the items you\'ve chosen.');
  if (coupon.minQuantity && scopedQty < coupon.minQuantity) {
    return no(`This code needs at least ${coupon.minQuantity} qualifying ticket${coupon.minQuantity === 1 ? '' : 's'}.`);
  }
  return { ok: true };
}

/** Short human description, e.g. "20% off", "R50 off", "Free". */
function describeCoupon(coupon) {
  if (!coupon) return '';
  if (coupon.type === 'comp') return 'Free';
  if (coupon.type === 'percent') return `${Number(coupon.value)}% off`;
  return `R${Number(coupon.value).toFixed(2)} off`;
}

// ─── Database helpers ─────────────────────────────────────────────────────────

const COUPON_INCLUDE = { events: true, options: true, allowedUsers: true };

const REGISTRATION_INCLUDE = {
  registrationOptions: { include: { tranches: true, eventOption: { include: { variants: true } } } },
  couponRedemptions: { where: { releasedAt: null }, include: { coupon: { include: COUPON_INCLUDE } } },
  user: { select: { id: true, role: true, email: true } },
};

async function findCouponByCode(code, db = prisma) {
  const normalized = normalizeCode(code);
  if (!normalized) return null;
  return db.coupon.findUnique({ where: { code: normalized }, include: COUPON_INCLUDE });
}

/** Active uses: unreleased redemptions on registrations that aren't cancelled. */
async function countCouponUses(couponId, userId, db = prisma) {
  const base = { couponId, releasedAt: null, registration: { status: { not: 'cancelled' } } };
  const [usesTotal, usesByUser] = await Promise.all([
    db.couponRedemption.count({ where: base }),
    userId ? db.couponRedemption.count({ where: { ...base, userId } }) : Promise.resolve(0),
  ]);
  return { usesTotal, usesByUser };
}

/**
 * Validates `code` against hypothetical lines (nothing saved) and returns the discount it
 * would give. Throws a coupon error with a user-facing message when it can't be used.
 */
async function previewCoupon({ code, eventId, user, lines, deferRestrictions = false }) {
  const coupon = await findCouponByCode(code);
  const uses = coupon ? await countCouponUses(coupon.id, user?.id) : {};
  const check = validateCoupon(coupon, { eventId, user, lines, deferRestrictions, ...uses });
  if (!check.ok) throw couponError(check.message);
  return { coupon, discount: computeCouponDiscount(coupon, lines) };
}

/**
 * Applies `code` to a saved registration: validates it against the registration's owner
 * and items, locks in the discount, and updates Registration.discountTotal. Runs under a
 * per-coupon advisory lock so the last available use can't be claimed twice.
 */
async function applyCouponToRegistration({ registrationId, code, appliedById = null }) {
  const normalized = normalizeCode(code);
  if (!normalized) throw couponError('Enter a code.');

  return prisma.$transaction(async (tx) => {
    const coupon = await findCouponByCode(normalized, tx);
    if (!coupon) throw couponError('That code isn\'t valid.');
    await tx.$executeRaw`SELECT pg_advisory_xact_lock(hashtext(${coupon.id}))`;

    const registration = await tx.registration.findUnique({ where: { id: registrationId }, include: REGISTRATION_INCLUDE });
    if (!registration) throw couponError('Registration not found.');
    if (registration.couponRedemptions.length > 0) {
      throw couponError(registration.couponRedemptions[0].couponId === coupon.id
        ? 'This code is already applied to this registration.'
        : 'A code has already been applied to this registration.');
    }

    const lines = buildCouponLines(registration);
    const uses = await countCouponUses(coupon.id, registration.userId, tx);
    const check = validateCoupon(coupon, { eventId: registration.eventId, user: registration.user, lines, ...uses });
    if (!check.ok) throw couponError(check.message);

    const discount = computeCouponDiscount(coupon, lines);
    const redemption = await tx.couponRedemption.create({
      data: { couponId: coupon.id, registrationId, userId: registration.userId, discountAmount: discount, appliedById },
    });
    await tx.registration.update({ where: { id: registrationId }, data: { discountTotal: discount, updatedAt: new Date() } });
    return { coupon, discount, redemption };
  });
}

/**
 * Re-works active discounts after a registration's items change: percentage and comp
 * codes are recalculated over the new items, fixed codes keep their amount (capped at
 * what's still in scope). Pass `tx` to run inside the caller's transaction.
 */
async function recalculateRegistrationDiscount(registrationId, db = prisma) {
  const registration = await db.registration.findUnique({ where: { id: registrationId }, include: REGISTRATION_INCLUDE });
  if (!registration) return 0;
  if (registration.couponRedemptions.length === 0) {
    if (registration.discountTotal !== 0) {
      await db.registration.update({ where: { id: registrationId }, data: { discountTotal: 0 } });
    }
    return 0;
  }

  const lines = buildCouponLines(registration);
  let total = 0;
  for (const r of registration.couponRedemptions) {
    let amount;
    if (r.coupon.type === 'fixed') {
      const inScope = scopedLines(r.coupon, lines).reduce((s, l) => s + l.unitPrice * l.quantity, 0);
      amount = round2(Math.min(r.discountAmount, inScope));
    } else {
      amount = computeCouponDiscount(r.coupon, lines);
    }
    if (Math.abs(amount - r.discountAmount) > 0.001) {
      await db.couponRedemption.update({ where: { id: r.id }, data: { discountAmount: amount } });
    }
    total += amount;
  }
  total = round2(total);
  if (Math.abs(total - registration.discountTotal) > 0.001) {
    await db.registration.update({ where: { id: registrationId }, data: { discountTotal: total } });
  }
  return total;
}

/** Releases a cancelled registration's coupon uses so they can be claimed again. */
async function releaseRegistrationCoupons(registrationId, db = prisma) {
  const released = await db.couponRedemption.updateMany({
    where: { registrationId, releasedAt: null },
    data: { releasedAt: new Date() },
  });
  if (released.count > 0) {
    await db.registration.update({ where: { id: registrationId }, data: { discountTotal: 0 } });
  }
  return released.count;
}

module.exports = {
  normalizeCode,
  couponError,
  buildCouponLines,
  computeCouponDiscount,
  validateCoupon,
  describeCoupon,
  findCouponByCode,
  countCouponUses,
  previewCoupon,
  applyCouponToRegistration,
  recalculateRegistrationDiscount,
  releaseRegistrationCoupons,
};