Add locale-aware digit grouping to displayed amounts

Adds groupDigits(), a display-only formatter that inserts the current
locale's digit-grouping separators into an amount's integer part (e.g.
"1,000,000.5" in en-US vs "1.000.000,5" in de-DE, where the roles of ","
and "." are swapped, or hi-IN's irregular "12,34,567"). Delegates to each
platform's native locale APIs (java.text.NumberFormat/DecimalFormatSymbols,
NSNumberFormatter/NSLocale, Intl.NumberFormat with BigInt for arbitrary
precision) rather than reimplementing grouping rules, and never touches the
canonical '.'-decimal strings the rest of the app parses and stores.

Applied via a Compose VisualTransformation (BTC/Sats/fiat/manual-rate
fields, plus the read-only rate display) and, on the native SwiftUI screen,
by re-deriving the field's displayed text on every change. Also made input
parsing locale-aware: sanitizeDecimalInput now recognizes the locale's own
decimal separator (e.g. what a locale-aware decimal keypad sends), not just
"." — with a stricter rule on the SwiftUI side specifically, since its
single text buffer re-feeds the grouped display back through sanitize, so
a "." can be grouping noise there rather than a decimal point.

The exceeds-max-supply warning's "21,000,000 BTC" is no longer baked into
the localized string with hardcoded grouping; it's substituted in at
runtime via groupDigits so the link-detection text always matches exactly,
in every locale.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QpjMKWGoiT5aJBzhxvXkwp

shared/src/androidMain/kotlin/xyz/tyiu/satsprice/domain/NumberFormat.android.kt

@@ -0,0 +1,12 @@
+package xyz.tyiu.satsprice.domain
+
+import java.math.BigInteger
+import java.text.DecimalFormatSymbols
+import java.text.NumberFormat
+import java.util.Locale
+
+actual fun localizedGroupedInteger(digits: String): String =
+    NumberFormat.getIntegerInstance(Locale.getDefault()).format(BigInteger(digits))
+
+actual fun localizedDecimalSeparator(): String =
+    DecimalFormatSymbols.getInstance(Locale.getDefault()).decimalSeparator.toString()

shared/src/appleMain/kotlin/xyz/tyiu/satsprice/domain/NumberFormat.apple.kt

@@ -0,0 +1,15 @@
+package xyz.tyiu.satsprice.domain
+
+import platform.Foundation.*
+
+actual fun localizedGroupedInteger(digits: String): String {
+    val formatter = NSNumberFormatter().apply {
+        numberStyle = NSNumberFormatterDecimalStyle
+        locale = NSLocale.currentLocale
+        usesGroupingSeparator = true
+    }
+    val number = NSDecimalNumber(string = digits)
+    return formatter.stringFromNumber(number) ?: digits
+}
+
+actual fun localizedDecimalSeparator(): String = NSLocale.currentLocale.decimalSeparator

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

@@ -15,15 +15,21 @@ fun String.toBigDecimalOrNull(): BigDecimal? {
     }
 }
 
-/** Strips a raw text-field edit down to digits and at most one decimal point. */
+/**
+ * Strips a raw text-field edit down to digits and at most one decimal point. Recognizes either
+ * "." or the current locale's own decimal separator (e.g. a locale-aware decimal keypad's key,
+ * which shows "," in de-DE) as that decimal point — either way, the output always uses ".", the
+ * canonical form [toBigDecimalOrNull] and the rest of this file expect.
+ */
 fun sanitizeDecimalInput(raw: String): String {
+    val localizedSeparator = localizedDecimalSeparator()
     val sb = StringBuilder()
     var seenDot = false
     for (c in raw) {
         when {
             c.isDigit() -> sb.append(c)
-            c == '.' && !seenDot -> {
-                sb.append(c)
+            !seenDot && (c == '.' || c.toString() == localizedSeparator) -> {
+                sb.append('.')
                 seenDot = true
             }
         }
@@ -64,3 +70,36 @@ fun formatAmountFixed(value: BigDecimal, decimals: Int): String {
     val fraction = text.substringAfter('.', "")
     return "$whole.${fraction.padEnd(decimals, '0')}"
 }
+
+/**
+ * Inserts locale-appropriate digit-grouping separators into [value]'s integer part (never the
+ * fractional part), e.g. "1000000.5" -> "1,000,000.5" in en-US, or "1.000.000,5" in de-DE where
+ * the roles of "," and "." are swapped — and not necessarily every 3 digits, since some locales
+ * group differently (e.g. hi-IN's "12,34,567"). Display-only: [value] itself stays plain-digit
+ * and '.'-decimal, parseable by [toBigDecimalOrNull], so this is applied on top of already-
+ * formatted/stored amounts rather than baked into them — a partial value being typed (like "1."
+ * or a bare "-") passes through with its structure intact.
+ */
+fun groupDigits(value: String): String {
+    val negative = value.startsWith("-")
+    val unsigned = if (negative) value.substring(1) else value
+    val dotIndex = unsigned.indexOf('.')
+    val integerPart = if (dotIndex >= 0) unsigned.substring(0, dotIndex) else unsigned
+    val fractionPart = if (dotIndex >= 0) unsigned.substring(dotIndex + 1) else null
+    if (integerPart.isEmpty()) return value
+
+    return buildString {
+        if (negative) append('-')
+        append(localizedGroupedInteger(integerPart))
+        if (fractionPart != null) {
+            append(localizedDecimalSeparator())
+            append(fractionPart)
+        }
+    }
+}
+
+/** Formats a plain non-negative integer digit string with the current locale's grouping. */
+expect fun localizedGroupedInteger(digits: String): String
+
+/** The current locale's decimal-point character, e.g. "." in en-US or "," in de-DE. */
+expect fun localizedDecimalSeparator(): String

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

@@ -52,6 +52,9 @@ import androidx.compose.ui.text.SpanStyle
 import androidx.compose.ui.text.TextLinkStyles
 import androidx.compose.ui.text.buildAnnotatedString
 import androidx.compose.ui.text.input.KeyboardType
+import androidx.compose.ui.text.input.OffsetMapping
+import androidx.compose.ui.text.input.TransformedText
+import androidx.compose.ui.text.input.VisualTransformation
 import androidx.compose.ui.text.style.TextDecoration
 import androidx.compose.ui.text.withLink
 import androidx.compose.ui.unit.dp
@@ -62,6 +65,8 @@ import dev.icerock.moko.resources.compose.stringResource
 import xyz.tyiu.satsprice.CurrencyInfo
 import xyz.tyiu.satsprice.domain.CurrencyConverter
 import xyz.tyiu.satsprice.domain.formatAmount
+import xyz.tyiu.satsprice.domain.groupDigits
+import xyz.tyiu.satsprice.domain.localizedDecimalSeparator
 import xyz.tyiu.satsprice.shared.MR
 
 private val SectionColors
@@ -73,6 +78,33 @@ private val SectionHeaderStyle
         letterSpacing = 1.2.sp,
     )
 
+/** Displays a plain-digit amount field's value with locale-appropriate digit grouping, without touching it. */
+private object DigitGroupingTransformation : VisualTransformation {
+    override fun filter(text: AnnotatedString): TransformedText {
+        val grouped = groupDigits(text.text)
+        val decimalSeparator = localizedDecimalSeparator()
+        // Everything grouping inserts is neither a digit, the sign, nor the (possibly localized,
+        // e.g. "," in de-DE) decimal point — so anything else is an inserted grouping separator
+        // to skip, whatever character the locale actually uses for it.
+        fun isGroupingSeparator(char: Char) = !char.isDigit() && char != '-' && char.toString() != decimalSeparator
+
+        val offsetMapping = object : OffsetMapping {
+            override fun originalToTransformed(offset: Int): Int {
+                var originalSeen = 0
+                for ((index, char) in grouped.withIndex()) {
+                    if (originalSeen == offset) return index
+                    if (!isGroupingSeparator(char)) originalSeen++
+                }
+                return grouped.length
+            }
+
+            override fun transformedToOriginal(offset: Int): Int =
+                grouped.take(offset.coerceIn(0, grouped.length)).count { !isGroupingSeparator(it) }
+        }
+        return TransformedText(AnnotatedString(grouped), offsetMapping)
+    }
+}
+
 @Composable
 fun PriceScreen(
     viewModel: PriceViewModel = viewModel { PriceViewModel() },
@@ -145,13 +177,14 @@ fun PriceScreen(
                                 label = { Text(stringResource(MR.strings.rate_label)) },
                                 keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
                                 singleLine = true,
+                                visualTransformation = DigitGroupingTransformation,
                                 modifier = Modifier.weight(1f),
                             )
                         } else {
                             val rate = state.defaultCurrencyRate()
                             if (rate.isNotEmpty()) {
                                 Text(
-                                    text = rate,
+                                    text = groupDigits(rate),
                                     style = MaterialTheme.typography.headlineSmall,
                                     modifier = Modifier.weight(1f),
                                 )
@@ -202,6 +235,7 @@ fun PriceScreen(
                         keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number),
                         singleLine = true,
                         isError = exceedsMaxSupply,
+                        visualTransformation = DigitGroupingTransformation,
                         modifier = Modifier.fillMaxWidth(),
                     )
 
@@ -212,14 +246,16 @@ fun PriceScreen(
                         keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
                         singleLine = true,
                         isError = exceedsMaxSupply,
+                        visualTransformation = DigitGroupingTransformation,
                         modifier = Modifier.fillMaxWidth(),
                     )
 
                     if (exceedsMaxSupply) {
-                        val warning = stringResource(MR.strings.exceeds_max_supply)
-                        // Matches the literal text of MR.strings.exceeds_max_supply so it can be
-                        // turned into a link; falls back to plain text if that ever drifts apart.
-                        val maxSupplyText = "21,000,000 BTC"
+                        // Locale-grouped (e.g. "21,000,000 BTC" in en-US, "21.000.000 BTC" in
+                        // de-DE) since it's substituted into the string, then searched for
+                        // verbatim below to turn it into a link — the two always match exactly.
+                        val maxSupplyText = groupDigits(formatAmount(CurrencyConverter.MAX_BTC_SUPPLY, 0)) + " BTC"
+                        val warning = stringResource(MR.strings.exceeds_max_supply, maxSupplyText)
                         val linkStart = warning.indexOf(maxSupplyText)
                         val annotatedWarning = if (linkStart < 0) {
                             AnnotatedString(warning)
@@ -304,6 +340,7 @@ fun PriceScreen(
                                     } else {
                                         { Text(stringResource(MR.strings.currency_not_priced, state.sourceName)) }
                                     },
+                                    visualTransformation = DigitGroupingTransformation,
                                     modifier = Modifier.weight(1f),
                                 )
                                 CurrencyRowMenu(

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

@@ -11,7 +11,7 @@
     <string name="currency_option_label_local">%1$s - %2$s</string>
     <string name="currency_options_content_description">Options for %1$s</string>
     <string name="done">Done</string>
-    <string name="exceeds_max_supply">Exceeds the 21,000,000 BTC maximum supply</string>
+    <string name="exceeds_max_supply">Exceeds the %1$s maximum supply</string>
     <string name="move_currency_down_content_description">Move %1$s down</string>
     <string name="move_currency_up_content_description">Move %1$s up</string>
     <string name="price_source">Price Source</string>

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

@@ -4,6 +4,7 @@ import com.ionspin.kotlin.bignum.decimal.BigDecimal
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNull
+import kotlin.test.assertTrue
 
 class NumberFormatTest {
 
@@ -76,4 +77,33 @@ class NumberFormatTest {
     fun formatAmountFixed_dropsDecimalPointWhenZeroDecimals() {
         assertEquals("100", formatAmountFixed(BigDecimal.fromLong(100), 0))
     }
+
+    // groupDigits' separator characters are locale-dependent (see localizedGroupedInteger /
+    // localizedDecimalSeparator), so asserting a specific separator belongs in a JVM-only test
+    // that can pin Locale.getDefault() — see NumberFormatJvmTest.
+
+    @Test
+    fun groupDigits_passesThroughInputWithNoIntegerPartUnchanged() {
+        // No integer part to group, so these bail out before consulting the locale at all.
+        assertEquals("", groupDigits(""))
+        assertEquals("-", groupDigits("-"))
+        assertEquals(".5", groupDigits(".5"))
+    }
+
+    @Test
+    fun groupDigits_usesLocaleDecimalSeparatorForATrailingDot() {
+        // "1." has an integer part, so unlike the cases above, this one *does* consult the
+        // locale — the trailing "." becomes whatever that locale's decimal separator is.
+        assertEquals("1" + localizedDecimalSeparator(), groupDigits("1."))
+    }
+
+    @Test
+    fun groupDigits_groupsLargeIntegersRegardlessOfLocale() {
+        val grouped = groupDigits("1000000")
+
+        // The separator character is locale-dependent, but the digits and the fact that
+        // grouping happened at all aren't.
+        assertEquals("1000000", grouped.filter { it.isDigit() })
+        assertTrue(grouped.length > "1000000".length)
+    }
 }

shared/src/jvmMain/kotlin/xyz/tyiu/satsprice/domain/NumberFormat.jvm.kt

@@ -0,0 +1,12 @@
+package xyz.tyiu.satsprice.domain
+
+import java.math.BigInteger
+import java.text.DecimalFormatSymbols
+import java.text.NumberFormat
+import java.util.Locale
+
+actual fun localizedGroupedInteger(digits: String): String =
+    NumberFormat.getIntegerInstance(Locale.getDefault()).format(BigInteger(digits))
+
+actual fun localizedDecimalSeparator(): String =
+    DecimalFormatSymbols.getInstance(Locale.getDefault()).decimalSeparator.toString()

shared/src/jvmTest/kotlin/xyz/tyiu/satsprice/domain/NumberFormatJvmTest.kt

@@ -0,0 +1,50 @@
+package xyz.tyiu.satsprice.domain
+
+import java.util.Locale
+import kotlin.test.AfterTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+/** [groupDigits]'s separators follow [Locale.getDefault], not a hardcoded "," and ".". */
+class NumberFormatJvmTest {
+
+    private val originalLocale: Locale = Locale.getDefault()
+
+    @AfterTest
+    fun restoreLocale() {
+        Locale.setDefault(originalLocale)
+    }
+
+    @Test
+    fun groupDigits_usesCommaGroupingAndPeriodDecimalInUsEnglish() {
+        Locale.setDefault(Locale.US)
+
+        assertEquals("1,000,000", groupDigits("1000000"))
+        assertEquals("1,000,000.5", groupDigits("1000000.5"))
+        assertEquals("-1,234", groupDigits("-1234"))
+    }
+
+    @Test
+    fun groupDigits_usesPeriodGroupingAndCommaDecimalInGerman() {
+        Locale.setDefault(Locale.GERMANY)
+
+        assertEquals("1.000.000", groupDigits("1000000"))
+        assertEquals("1.000.000,5", groupDigits("1000000.5"))
+        assertEquals("-1.234", groupDigits("-1234"))
+    }
+
+    @Test
+    fun sanitizeDecimalInput_acceptsTheLocaleDecimalSeparatorInGerman() {
+        Locale.setDefault(Locale.GERMANY)
+
+        // A locale-aware decimal keypad's key sends ",", not ".", in German.
+        assertEquals("123.45", sanitizeDecimalInput("123,45"))
+    }
+
+    @Test
+    fun sanitizeDecimalInput_stillAcceptsPeriodEvenInGerman() {
+        Locale.setDefault(Locale.GERMANY)
+
+        assertEquals("123.45", sanitizeDecimalInput("123.45"))
+    }
+}

shared/src/webMain/kotlin/xyz/tyiu/satsprice/domain/NumberFormat.web.kt

@@ -0,0 +1,14 @@
+@file:OptIn(kotlin.js.ExperimentalWasmJsInterop::class)
+
+package xyz.tyiu.satsprice.domain
+
+// BigInt keeps this precise for arbitrarily large integer parts (a plain JS number would lose
+// precision past 2^53); binding each constructor result to a variable first, rather than
+// chaining `new Foo(x).bar()`, avoids a known js() misparse — see SystemCurrencies.web.kt.
+actual fun localizedGroupedInteger(digits: String): String = js(
+    """(function() { var big = BigInt(digits); var fmt = new Intl.NumberFormat(); return fmt.format(big); })()""",
+)
+
+actual fun localizedDecimalSeparator(): String = js(
+    """(function() { var fmt = new Intl.NumberFormat(); var parts = fmt.formatToParts(1.5); var found = "."; parts.forEach(function(p) { if (p.type === "decimal") { found = p.value; } }); return found; })()""",
+)