Fix issues in expiry date visual transformation

payments-ui-core/src/test/java/com/stripe/android/ui/core/elements/ExpiryDateVisualTransformationTest.kt

@@ -1,34 +1,64 @@
 package com.stripe.android.ui.core.elements
 
 import androidx.compose.ui.text.AnnotatedString
-import com.google.common.truth.Truth
+import androidx.compose.ui.text.input.TransformedText
+import com.google.common.truth.Truth.assertThat
 import com.stripe.android.uicore.elements.ExpiryDateVisualTransformation
 import org.junit.Test
 
 internal class ExpiryDateVisualTransformationTest {
+
     private val transform = ExpiryDateVisualTransformation()
 
     @Test
     fun `verify 19 get separated between 1 and 9`() {
-        Truth.assertThat(transform.filter(AnnotatedString("19")).text.text)
-            .isEqualTo("1 / 9")
+        val result = transform.filter(AnnotatedString("19"))
+        assertThat(result.text.text).isEqualTo("1 / 9")
+        assertCorrectMapping(original = "19", result)
     }
 
     @Test
     fun `verify 123 get separated between 2 and 3`() {
-        Truth.assertThat(transform.filter(AnnotatedString("123")).text.text)
-            .isEqualTo("12 / 3")
+        val result = transform.filter(AnnotatedString("123"))
+        assertThat(result.text.text).isEqualTo("12 / 3")
+        assertCorrectMapping(original = "123", result)
+    }
+
+    @Test
+    fun `verify 143 get separated between 1 and 4`() {
+        val result = transform.filter(AnnotatedString("143"))
+        assertThat(result.text.text).isEqualTo("1 / 43")
+        assertCorrectMapping(original = "143", result)
     }
 
     @Test
     fun `verify 093 get separated between 9 and 3`() {
-        Truth.assertThat(transform.filter(AnnotatedString("093")).text.text)
-            .isEqualTo("09 / 3")
+        val result = transform.filter(AnnotatedString("093"))
+        assertThat(result.text.text).isEqualTo("09 / 3")
+        assertCorrectMapping(original = "093", result)
     }
 
     @Test
     fun `verify 53 get separated between 5 and 3`() {
-        Truth.assertThat(transform.filter(AnnotatedString("53")).text.text)
-            .isEqualTo("5 / 3")
+        val result = transform.filter(AnnotatedString("53"))
+        assertThat(result.text.text).isEqualTo("5 / 3")
+        assertCorrectMapping(original = "53", result)
+    }
+
+    private fun assertCorrectMapping(
+        original: String,
+        result: TransformedText,
+    ) {
+        val transformed = result.text.text
+
+        for (offset in 0..original.length) {
+            val transformedOffset = result.offsetMapping.originalToTransformed(offset)
+            assertThat(transformedOffset).isIn(0..transformed.length)
+        }
+
+        for (offset in 0..result.text.text.length) {
+            val originalOffset = result.offsetMapping.transformedToOriginal(offset)
+            assertThat(originalOffset).isIn(0..original.length)
+        }
     }
 }

stripe-ui-core/detekt-baseline.xml

@@ -23,6 +23,7 @@
     <ID>MagicNumber:DropdownFieldUI.kt$.8f</ID>
     <ID>MagicNumber:DropdownFieldUI.kt$.9f</ID>
     <ID>MagicNumber:DropdownFieldUI.kt$8.9f</ID>
+    <ID>MagicNumber:ExpiryDateVisualTransformation.kt$ExpiryDateVisualTransformation$12</ID>
     <ID>MagicNumber:Html.kt$0.1f</ID>
     <ID>MagicNumber:PostalCodeVisualTransformation.kt$PostalCodeVisualTransformation.&lt;no name provided>$3</ID>
     <ID>MagicNumber:PostalCodeVisualTransformation.kt$PostalCodeVisualTransformation.&lt;no name provided>$5</ID>

stripe-ui-core/src/main/java/com/stripe/android/uicore/elements/ExpiryDateVisualTransformation.kt

@@ -17,39 +17,54 @@ class ExpiryDateVisualTransformation : VisualTransformation {
          * 2, if the first number is 11 or 12 it will be after the second digit,
          * if the number is 01 it will be after the second digit.
          */
-        var separatorAfterIndex = 1
-        if (text.isNotBlank() && !(text[0] == '0' || text[0] == '1')) {
-            separatorAfterIndex = 0
-        } else if (text.length > 1 &&
-            (text[0] == '1' && requireNotNull(text[1].digitToInt()) > 2)
-        ) {
-            separatorAfterIndex = 0
-        }
+        val canOnlyBeSingleDigitMonth = text.isNotBlank() && !(text[0] == '0' || text[0] == '1')
+        val canOnlyBeJanuary = text.length > 1 && text.text.take(2).toInt() > 12
+        val isSingleDigitMonth = canOnlyBeSingleDigitMonth || canOnlyBeJanuary
+
+        val lastIndexOfMonth = if (isSingleDigitMonth) 0 else 1
 
-        var out = ""
-        for (i in text.indices) {
-            out += text[i]
-            if (i == separatorAfterIndex) {
-                out += separator
+        val output = buildString {
+            for ((index, char) in text.withIndex()) {
+                append(char)
+                if (index == lastIndexOfMonth) {
+                    append(separator)
+                }
             }
         }
 
+        val outputOffsets = calculateOutputOffsets(output)
+        val separatorIndices = calculateSeparatorOffsets(output)
+
         val offsetTranslator = object : OffsetMapping {
-            override fun originalToTransformed(offset: Int) =
-                if (offset <= separatorAfterIndex) {
-                    offset
-                } else {
-                    offset + separator.length
-                }
 
-            override fun transformedToOriginal(offset: Int) =
-                if (offset <= separatorAfterIndex + 1) {
-                    offset
-                } else {
-                    offset - separator.length
-                }
+            override fun originalToTransformed(offset: Int): Int {
+                return outputOffsets[offset]
+            }
+
+            override fun transformedToOriginal(offset: Int): Int {
+                val separatorCharactersBeforeOffset = separatorIndices.count { it < offset }
+                return offset - separatorCharactersBeforeOffset
+            }
+        }
+
+        return TransformedText(AnnotatedString(output), offsetTranslator)
+    }
+
+    private fun calculateOutputOffsets(output: String): List<Int> {
+        val digitOffsets = output.mapIndexedNotNull { index, char ->
+            // +1 because we're looking for offsets, not indices
+            index.takeIf { char.isDigit() }?.plus(1)
         }
 
-        return TransformedText(AnnotatedString(out), offsetTranslator)
+        // We're adding 0 so that the cursor can be placed at the start of the text,
+        // and replace the last digit offset with the length of the output. The latter
+        // is so that the offsets are set correctly for text such as "4 / ".
+        return listOf(0) + digitOffsets.dropLast(1) + output.length
+    }
+
+    private fun calculateSeparatorOffsets(output: String): List<Int> {
+        return output.mapIndexedNotNull { index, c ->
+            index.takeUnless { c.isDigit() }
+        }
     }
 }