feat: refactor exchange rate handling by integrating new ExchangeService and removing CbeExchangeService

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

@@ -0,0 +1,72 @@
+import { Inject, Injectable } from "@nestjs/common";
+
+import { CbeExchangeProvider } 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).
+ *  3. Provider supplies the inverse → return `1 / inverse` (e.g. ETB→USD).
+ *
+ * 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;
+    }
+
+    throw new Error(
+      `No exchange rate available for ${from}→${to} from provider ${this.provider.name}`,
+    );
+  }
+
+  /** 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");
+  }
+}