Add a "1 <currency> to Sats" field

Shows the inverse of the existing "1 BTC to <currency>" rate — how
many Sats one unit of the default currency is worth — right below it.
Useful since many currencies are worth less than, or a low multiple
of, a single Sat.

CurrencyConverter.satsPerCurrencyUnit(rate) does the actual division;
ConverterUiState.oneCurrencyToSats() derives the display string from
the already-fetched rate the same way defaultCurrencyRate() does, so
no new ViewModel state was needed. Wired into both Compose and the
iOS bridge/SwiftUI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy

iosApp/iosApp/ContentView.swift

@@ -105,6 +105,17 @@ struct ContentView: View {
                         }
                     }
                 }
+
+                if !state.oneCurrencyToSats.isEmpty {
+                    HStack {
+                        Text(IosLocalizationKt.localizedFormattedString(
+                            resource: MR.strings.shared.currency_to_sats,
+                            args: [state.defaultCurrencyCode]
+                        ))
+                        Spacer()
+                        Text(NumberFormatKt.groupDigits(value: state.oneCurrencyToSats))
+                    }
+                }
             }
 
             if let error = state.errorMessage {

shared/src/appleMain/kotlin/xyz/tyiu/satsprice/IosPriceViewModel.kt

@@ -9,6 +9,7 @@ import xyz.tyiu.satsprice.ui.PriceViewModel
 import xyz.tyiu.satsprice.ui.currentCurrency
 import xyz.tyiu.satsprice.ui.defaultCurrencyRate
 import xyz.tyiu.satsprice.ui.exceedsMaxSupply
+import xyz.tyiu.satsprice.ui.oneCurrencyToSats
 import xyz.tyiu.satsprice.ui.selectedOtherCurrencies
 import xyz.tyiu.satsprice.ui.statusLine
 import xyz.tyiu.satsprice.ui.unselectedCurrencies
@@ -34,6 +35,7 @@ data class IosConverterState(
     val errorMessage: String?,
     val statusLine: String,
     val defaultCurrencyRate: String,
+    val oneCurrencyToSats: String,
 )
 
 /**
@@ -85,4 +87,5 @@ private fun ConverterUiState.toIosState(): IosConverterState = IosConverterState
     errorMessage = errorMessage,
     statusLine = statusLine(),
     defaultCurrencyRate = defaultCurrencyRate(),
+    oneCurrencyToSats = oneCurrencyToSats(),
 )

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/domain/CurrencyConverter.kt

@@ -44,4 +44,8 @@ object CurrencyConverter {
 
     fun fiatToSats(fiat: BigDecimal, rates: ExchangeRates, currency: String): BigDecimal? =
         fiatToBtc(fiat, rates, currency)?.let { btc -> btcToSats(btc) }
+
+    /** Sats worth 1 unit of a currency whose "1 BTC = ?" rate is [rate], or null if [rate] is zero. */
+    fun satsPerCurrencyUnit(rate: BigDecimal): BigDecimal? =
+        if (rate.isZero()) null else SATS_PER_BTC.divide(rate, DIVISION_MODE)
 }

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/ui/ConverterUiStateDisplay.kt

@@ -4,6 +4,7 @@ import kotlinx.datetime.TimeZone
 import kotlinx.datetime.toLocalDateTime
 import xyz.tyiu.satsprice.CurrencyInfo
 import xyz.tyiu.satsprice.domain.CurrencyConverter
+import xyz.tyiu.satsprice.domain.formatAmount
 import xyz.tyiu.satsprice.domain.toBigDecimalOrNull
 import kotlin.time.Instant
 
@@ -25,6 +26,12 @@ fun ConverterUiState.exceedsMaxSupply(): Boolean {
 fun ConverterUiState.defaultCurrencyRate(): String =
     rateDisplays[defaultCurrencyCode]?.takeIf { it.isNotEmpty() } ?: ""
 
+/** The current "1 [ConverterUiState.defaultCurrencyCode] = ? Sats" rate, as a plain number. */
+fun ConverterUiState.oneCurrencyToSats(): String {
+    val rate = rateDisplays[defaultCurrencyCode]?.toBigDecimalOrNull() ?: return ""
+    return CurrencyConverter.satsPerCurrencyUnit(rate)?.let { formatAmount(it, 2) } ?: ""
+}
+
 /** The pinned, non-removable "Current Currency" shown in the currency picker's own section. */
 fun ConverterUiState.currentCurrency(): CurrencyInfo =
     availableFiatCurrencies.find { it.code == defaultCurrencyCode } ?: CurrencyInfo(defaultCurrencyCode, defaultCurrencyCode)

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/ui/PriceScreen.kt

@@ -213,6 +213,18 @@ fun PriceScreen(
                             }
                         }
                     }
+
+                    val oneCurrencyToSats = state.oneCurrencyToSats()
+                    if (oneCurrencyToSats.isNotEmpty()) {
+                        Text(
+                            text = stringResource(MR.strings.currency_to_sats, state.defaultCurrencyCode),
+                            style = MaterialTheme.typography.bodyMedium,
+                        )
+                        Text(
+                            text = groupDigits(oneCurrencyToSats),
+                            style = MaterialTheme.typography.headlineSmall,
+                        )
+                    }
                 }
             }
 

shared/src/commonMain/moko-resources/base/strings.xml

@@ -11,6 +11,7 @@
     <string name="currency_option_label">%1$s - %2$s</string>
     <string name="currency_option_label_local">%1$s - %2$s</string>
     <string name="currency_options_content_description">Options for %1$s</string>
+    <string name="currency_to_sats">1 %1$s to Sats</string>
     <string name="done">Done</string>
     <string name="exceeds_max_supply">Exceeds the %1$s maximum supply</string>
     <string name="move_currency_down_content_description">Move down</string>

shared/src/commonTest/kotlin/xyz/tyiu/satsprice/domain/CurrencyConverterTest.kt

@@ -57,6 +57,17 @@ class CurrencyConverterTest {
         assertEquals(BigDecimal.fromLong(100_000_000), sats)
     }
 
+    @Test
+    fun satsPerCurrencyUnit_isInverseOfTheBtcRate() {
+        // 1 BTC = 50,000 USD, so 1 USD is worth 100,000,000 / 50,000 = 2,000 Sats.
+        assertEquals(BigDecimal.fromLong(2_000), CurrencyConverter.satsPerCurrencyUnit(BigDecimal.fromLong(50_000)))
+    }
+
+    @Test
+    fun satsPerCurrencyUnit_returnsNullForZeroRate() {
+        assertNull(CurrencyConverter.satsPerCurrencyUnit(BigDecimal.fromLong(0)))
+    }
+
     @Test
     fun handlesAmountsFarBeyondDoubleOrLongRange() {
         // 10^30 BTC: overflows both Double's exact-integer range and Long.MAX_VALUE.