fix(folio): reject amounts parseCents cannot represent

examples/folio/app/shared/src/commonMain/kotlin/app/folio/util/Format.kt

@@ -54,9 +54,8 @@ fun parseCents(input: String): Long? {
     val fracPadded = (frac + "00").substring(0, 2)
     val wholeLong = whole.toLongOrNull() ?: return null
     val fracLong = fracPadded.toLongOrNull() ?: return null
-    val total = wholeLong * 100 + fracLong
+    if (wholeLong > (Long.MAX_VALUE - fracLong) / 100) return null
-    if (total < 0) return null
+    return wholeLong * 100 + fracLong
-    return total
 }
 
 fun signedAmount(t: Transaction): Long = if (t.type == TxnType.credit) t.amount else -t.amount

examples/folio/app/shared/src/commonTest/kotlin/app/folio/util/ParseCentsTest.kt

@@ -0,0 +1,38 @@
+package app.folio.util
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+class ParseCentsTest {
+    @Test
+    fun parsesEverydayAmountsExactly() {
+        assertEquals(1L, parseCents("0.01"))
+        assertEquals(1234L, parseCents("12.34"))
+        assertEquals(1250L, parseCents("12.5"))
+        assertEquals(1200L, parseCents("12.0"))
+        assertEquals(100000L, parseCents("1000"))
+        assertEquals(123456L, parseCents("1,234.56"))
+    }
+
+    @Test
+    fun rejectsEighteenDigitWholeThatWrapsToAPositiveLong() {
+        assertNull(parseCents("999999999999999999"))
+    }
+
+    @Test
+    fun rejectsSeventeenDigitWholeThatWrapsToANegativeLong() {
+        assertNull(parseCents("99999999999999999"))
+    }
+
+    @Test
+    fun rejectsNineteenDigitWholeThatNoLongerFitsALong() {
+        assertNull(parseCents("9999999999999999999"))
+    }
+
+    @Test
+    fun acceptsTheLargestRepresentableAmountAndRejectsOneCentMore() {
+        assertEquals(Long.MAX_VALUE, parseCents("92233720368547758.07"))
+        assertNull(parseCents("92233720368547758.08"))
+    }
+}