feat: ( payments ) convert ETB to method currency before charging

apps/edr-passenger-api/src/modules/currency/currency.service.ts

@@ -1,13 +1,69 @@
-import { Injectable, Logger, NotFoundException } from '@nestjs/common';
+import {
+  Injectable,
+  Logger,
+  NotFoundException,
+  BadRequestException,
+} from '@nestjs/common';
 import { PrismaService } from '../../common/prisma.service';
 import { Currency } from '@prisma/client';
 
+/**
+ * Minor-unit decimal places per currency, used to round the CHARGE amount sent to the payment
+ * microservice. DJF has no minor unit (whole francs only); ETB and USD use 2 decimals.
+ */
+const CHARGE_CURRENCY_DECIMALS: Record<string, number> = {
+  ETB: 2,
+  USD: 2,
+  DJF: 0,
+};
+
 @Injectable()
 export class CurrencyService {
   private readonly logger = new Logger(CurrencyService.name);
 
   constructor(private readonly prisma: PrismaService) {}
 
+  async convertEtbMinorToChargeMajor(
+    amountMinorEtb: number,
+    targetCurrency: string,
+  ): Promise<number> {
+    const target = targetCurrency.toUpperCase();
+    const decimals = CHARGE_CURRENCY_DECIMALS[target];
+    if (decimals === undefined) {
+      throw new BadRequestException(`Unsupported charge currency: ${targetCurrency}`);
+    }
+
+    const sourceMajor = amountMinorEtb / 100;
+    if (target === Currency.ETB) {
+      return this.roundTo(sourceMajor, decimals);
+    }
+
+    const rate = await this.getRateOrThrow(Currency.ETB, target as Currency);
+    return this.roundTo(sourceMajor * rate, decimals);
+  }
+
+  async getRateOrThrow(
+    fromCurrency: Currency,
+    toCurrency: Currency,
+  ): Promise<number> {
+    if (fromCurrency === toCurrency) return 1;
+    const exchangeRate = await this.prisma.currencyExchangeRate.findFirst({
+      where: { fromCurrency, toCurrency },
+      orderBy: { effectiveDate: 'desc' },
+    });
+    if (!exchangeRate) {
+      throw new BadRequestException(
+        `No exchange rate configured for ${fromCurrency}->${toCurrency}`,
+      );
+    }
+    return Number(exchangeRate.rate);
+  }
+
+  private roundTo(value: number, decimals: number): number {
+    const factor = 10 ** decimals;
+    return Math.round(value * factor) / factor;
+  }
+
   async convertAmount(
     amountMinor: number,
     fromCurrency: Currency,