fix(costs): keep convertBooked at five parameters and update the tests to as-typed display rows

The display-currency check is its own helper (inDisplayCurrency), used by the Costs model and the shared trip page. The tests that pinned #2525's reading of a display-currency bill through the trip currency now pin the as-typed reading, which the balances use too; read in the trip currency, such a bill and its repayment may stay a euro cent apart.

client/src/hooks/useExchangeRates.test.ts

@@ -2,7 +2,7 @@ import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { renderHook, waitFor } from '@testing-library/react'
 import { http, HttpResponse } from 'msw'
 import { server } from '../../tests/helpers/msw/server'
-import { bookedInTrip, convertBooked, convertedLine, crossRate, fetchExchangeRates, fetchFreshRates, freshRates, clearExchangeRateCache, tripAmountOf, useExchangeRates, withFallbackFx } from './useExchangeRates'
+import { bookedInTrip, convertBooked, convertedLine, inDisplayCurrency, crossRate, fetchExchangeRates, fetchFreshRates, freshRates, clearExchangeRateCache, tripAmountOf, useExchangeRates, withFallbackFx } from './useExchangeRates'
 
 const FX_URL = 'https://api.frankfurter.dev/v2/rates'
 
@@ -198,16 +198,11 @@ describe('convertBooked', () => {
   })
 
   // #2965: read in dollars, a bill entered in dollars counts as typed, as the balances net it.
-  it('shows a row in the display currency as typed, whatever rate it was frozen at', () => {
-    const liveUsd = (amount: number, from: string | null | undefined): number => {
-      const rates: Record<string, number> = { USD: 1, EUR: 1 / 1.1398 }
-      const r = rates[(from || 'USD').toUpperCase()]
-      return r && r > 0 ? amount / r : amount
-    }
-    expect(convertBooked(801.76, 'USD', 1.17, 'EUR', liveUsd, 'USD')).toBe(801.76)
-    expect(convertBooked(801.76, 'usd', 1.17, 'EUR', liveUsd, 'USD')).toBe(801.76)
-    // Another currency still goes through the trip currency.
-    expect(convertBooked(801.76, 'USD', 1.17, 'EUR', liveUsd, 'GBP')).toBeCloseTo((801.76 / 1.17) * 1.1398, 6)
+  it('knows a row entered in the display currency, a NULL currency being the trip one', () => {
+    expect(inDisplayCurrency('usd', 'EUR', 'USD')).toBe(true)
+    expect(inDisplayCurrency(null, 'EUR', 'eur')).toBe(true)
+    expect(inDisplayCurrency(null, 'EUR', 'USD')).toBe(false)
+    expect(inDisplayCurrency('USD', 'EUR', 'GBP')).toBe(false)
   })
 })
 

client/src/hooks/useExchangeRates.ts

@@ -133,19 +133,27 @@ export function convertBooked(
   frozenRate: number | null | undefined,
   tripCurrency: string,
   convertLive: (amount: number, from: string | null | undefined) => number,
-  displayCurrency?: string,
 ): number {
   const trip = (tripCurrency || 'EUR').toUpperCase()
-  // A row entered in the display currency is shown as typed (#2965): through the
-  // trip currency at its frozen rate and back at today's, it drifted off the
-  // amount entered, and the balances (which net such rows as typed) disagreed.
-  if (displayCurrency && (rowCurrency || trip).toUpperCase() === displayCurrency.toUpperCase()) return amount
   const inTrip = bookedInTrip(amount, rowCurrency, frozenRate, trip)
   if (inTrip != null) return convertLive(inTrip, trip)
   // A NULL currency means the trip's own, and then there was never anything to freeze.
   return convertLive(amount, (rowCurrency || trip).toUpperCase())
 }
 
+/**
+ * Whether a row was entered in the display currency (#2965). Such a row counts as typed:
+ * through the trip currency at its frozen rate and back at today's it drifted off the
+ * amount entered, and the balances, which net such rows as typed, would disagree.
+ */
+export function inDisplayCurrency(
+  rowCurrency: string | null | undefined,
+  tripCurrency: string,
+  displayCurrency: string,
+): boolean {
+  return ((rowCurrency || tripCurrency) || '').toUpperCase() === (displayCurrency || '').toUpperCase()
+}
+
 /**
  * The amount a row was booked at in the trip currency, when it was booked through one: a
  * foreign currency with a frozen rate. Null for a row in the trip currency, and for one