edr-platform/packages/api-common/src/services/exchange/exchange.service.ts

import { Inject, Injectable } from "@nestjs/common";

import { CbeExchangeProvider, CbeProviderStatus } from "./cbe.provider";
import { EXCHANGE_OPTIONS, ExchangeOptions } from "./exchange.options";
import { CurrencyCode } from "./exchange.types";

/**
 * Currency exchange service. Resolves the rate between any supported currency
 * pair and converts amounts, backed by a rate provider (currently CBE).
 *
 * Resolution order for `getRate(from, to)`:
 *  1. `from === to` → `1`.
 *  2. Provider supplies the pair directly (e.g. CBE → USD→ETB, DJF→ETB).
 *  3. Provider supplies the inverse → return `1 / inverse` (e.g. ETB→USD).
 *  4. Both sides quote against ETB → cross them (e.g. DJF→USD via ETB).
 *
 * Configure via {@link ExchangeModule.forRoot} / `forRootAsync`.
 */
@Injectable()
export class ExchangeService {
  private readonly provider: CbeExchangeProvider;

  constructor(@Inject(EXCHANGE_OPTIONS) options: ExchangeOptions) {
    this.provider = new CbeExchangeProvider(options);
  }

  /**
   * Returns the rate to convert 1 unit of `from` into `to`
   * (i.e. `amountInTo = amountInFrom * getRate(from, to)`).
   */
  async getRate(from: CurrencyCode, to: CurrencyCode): Promise<number> {
    if (from === to) {
      return 1;
    }

    const direct = await this.provider.getBaseRate({ from, to });
    if (direct !== null) {
      return direct;
    }

    const inverse = await this.provider.getBaseRate({ from: to, to: from });
    if (inverse !== null && inverse > 0) {
      return 1 / inverse;
    }

    // Neither leg touches ETB directly — e.g. DJF→USD. The provider quotes everything
    // against ETB, so cross the two ETB legs rather than declaring the pair unavailable.
    const cross = await this.crossViaEtb(from, to);
    if (cross !== null) {
      return cross;
    }

    throw new Error(
      `No exchange rate available for ${from}→${to} from provider ${this.provider.name}`,
    );
  }

  /**
   * `from→to` derived from the two ETB-quoted legs. Returns `null` when either leg is
   * missing or unusable, so the caller raises rather than pricing off a bad number.
   */
  private async crossViaEtb(
    from: CurrencyCode,
    to: CurrencyCode,
  ): Promise<number | null> {
    const [fromEtb, toEtb] = await Promise.all([
      this.provider.getBaseRate({ from, to: "ETB" }),
      this.provider.getBaseRate({ from: to, to: "ETB" }),
    ]);

    if (fromEtb === null || toEtb === null || toEtb <= 0 || fromEtb <= 0) {
      return null;
    }

    return fromEtb / toEtb;
  }

  /**
   * Every `USD→code` rate in one object, so a pricing pass resolves its conversions once up
   * front instead of awaiting inside each line builder. `USD` is always `1`.
   *
   * The provider caches a whole CBE payload, so the extra codes cost no extra HTTP request.
   */
  async getRatesFromUsd(
    codes: readonly CurrencyCode[],
  ): Promise<Record<CurrencyCode, number>> {
    const unique = Array.from(new Set(codes));
    const rates = await Promise.all(
      unique.map((code) => this.getRate("USD", code)),
    );

    return Object.fromEntries(
      unique.map((code, i) => [code, rates[i]]),
    ) as Record<CurrencyCode, number>;
  }

  /**
   * Health of the underlying rate feed — what was served last and whether it
   * is currently failing. For operator-facing status displays.
   */
  getProviderStatus(): CbeProviderStatus {
    return this.provider.getStatus();
  }

  /** Converts `amount` from one currency to another using {@link getRate}. */
  async convert(
    amount: number,
    from: CurrencyCode,
    to: CurrencyCode,
  ): Promise<number> {
    const rate = await this.getRate(from, to);
    return amount * rate;
  }

  /**
   * Convenience alias for `getRate('USD', 'ETB')`.
   * @deprecated Prefer {@link getRate}; kept for existing callers.
   */
  getUsdToEtbRate(): Promise<number> {
    return this.getRate("USD", "ETB");
  }

  /** Convenience alias for `getRate('ETB', 'USD')`. */
  getEtbToUsdRate(): Promise<number> {
    return this.getRate("ETB", "USD");
  }
}