/**
 * Source of truth for prepaid balances, CS credits, usage, and payments.
 */
export class AccountBooks {
  constructor() {
    /** @type {Record<string, number>} */
    this.prepaid = {};
    /** @type {Record<string, string>} */
    this.veraeUserIds = {};
    this.credits = [];
    this.usage = [];
    this.payments = [];
  }

  remember(customerId, veraeUserId) {
    if (customerId && veraeUserId) this.veraeUserIds[customerId] = veraeUserId;
  }

  prepaidCents(customerId) {
    return this.prepaid[customerId] ?? 0;
  }

  adjust({ customerId, cents, reason, agent, kind = 'credit', veraeUserId }) {
    this.remember(customerId, veraeUserId);
    const delta = Math.trunc(Number(cents) || 0);
    const next = this.prepaidCents(customerId) + delta;
    this.prepaid[customerId] = next;
    const row = {
      id: `adj_${Date.now().toString(36)}_${Math.random().toString(36).slice(2, 6)}`,
      customerId,
      cents: delta,
      reason: reason || kind,
      agent: agent || 'system',
      kind,
      at: new Date().toISOString(),
      prepaidCents: next,
    };
    if (kind === 'payment' || kind === 'reload') this.payments.unshift(row);
    else this.credits.unshift(row);
    return row;
  }

  recordUsage(entry) {
    this.remember(entry.customerId, entry.veraeUserId);
    const cents = Number(entry.cents) || 0;
    const customerId = entry.customerId;
    const next = this.prepaidCents(customerId) - cents;
    this.prepaid[customerId] = next;
    const row = {
      customerId,
      endpointId: entry.endpointId || 'unknown',
      cents,
      at: entry.at || new Date().toISOString(),
      prepaidCents: next,
    };
    this.usage.unshift(row);
    return row;
  }

  recordPayment(entry) {
    return this.adjust({
      customerId: entry.customerId,
      cents: Number(entry.cents) || 0,
      reason: entry.reason || 'payment',
      agent: entry.agent || 'payments',
      kind: 'payment',
    });
  }

  statement(customerId) {
    const match = (rows) => rows.filter((r) => r.customerId === customerId).slice(0, 100);
    return {
      customerId,
      veraeUserId: this.veraeUserIds[customerId],
      prepaidCents: this.prepaidCents(customerId),
      credits: match(this.credits),
      usage: match(this.usage),
      payments: match(this.payments),
    };
  }

  dump() {
    return {
      prepaid: this.prepaid,
      veraeUserIds: this.veraeUserIds,
      credits: this.credits,
      usage: this.usage,
      payments: this.payments,
    };
  }

  load(raw) {
    if (!raw || typeof raw !== 'object') return this;
    this.prepaid = raw.prepaid || {};
    this.veraeUserIds = raw.veraeUserIds || {};
    this.credits = Array.isArray(raw.credits) ? raw.credits : [];
    this.usage = Array.isArray(raw.usage) ? raw.usage : [];
    this.payments = Array.isArray(raw.payments) ? raw.payments : [];
    return this;
  }
}

export function handle(subject, payload, books) {
  const p = payload || {};
  if (subject.endsWith('balance.get') || subject.endsWith('statement.get')) {
    books.remember(p.customerId, p.veraeUserId);
    return books.statement(p.customerId);
  }
  if (subject.endsWith('balance.adjust') || subject.endsWith('credit.applied')) {
    return books.adjust(p);
  }
  if (subject.endsWith('usage.recorded')) {
    return books.recordUsage(p);
  }
  if (subject.endsWith('payment.recorded')) {
    return books.recordPayment(p);
  }
  throw new Error(`unknown billing subject ${subject}`);
}