For #19065: Add extension functions to be used with credit cards.

app/src/main/java/org/mozilla/fenix/settings/creditcards/String.kt

@@ -0,0 +1,51 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.settings.creditcards
+
+import androidx.annotation.VisibleForTesting
+
+private const val MAX_CREDIT_CARD_NUMBER_LENGTH = 19
+private const val MIN_CREDIT_CARD_NUMBER_LENGTH = 12
+
+/**
+ * Strips characters other than digits from a string.
+ * Used to strip a credit card number user input of spaces and separators.
+ */
+fun String.toCreditCardNumber(): String {
+    return this.filter { it.isDigit() }
+}
+
+/**
+ * Uses string size and Luhn Algorithm validation to validate a credit card number.
+ */
+fun String.validateCreditCardNumber(): Boolean {
+    val creditCardNumber = this.toCreditCardNumber()
+
+    if (creditCardNumber != this) return false
+
+    // credit card numbers have at least 12 digits and at most 19 digits
+    if (creditCardNumber.length < MIN_CREDIT_CARD_NUMBER_LENGTH ||
+        creditCardNumber.length > MAX_CREDIT_CARD_NUMBER_LENGTH
+    ) return false
+
+    return luhnAlgorithmValidation(creditCardNumber)
+}
+
+/**
+ * Implementation of Luhn Algorithm validation (https://en.wikipedia.org/wiki/Luhn_algorithm)
+ */
+@Suppress("MagicNumber")
+@VisibleForTesting
+internal fun luhnAlgorithmValidation(creditCardNumber: String): Boolean {
+    var checksum = 0
+    val reversedCardNumber = creditCardNumber.reversed()
+
+    for (index in reversedCardNumber.indices) {
+        val digit = Character.getNumericValue(reversedCardNumber[index])
+        checksum += if (index % 2 == 0) digit else (digit * 2).let { (it / 10) + (it % 10) }
+    }
+
+    return (checksum % 10) == 0
+}

app/src/test/java/org/mozilla/fenix/settings/creditcards/StringTest.kt

@@ -0,0 +1,102 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.settings.creditcards
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mozilla.fenix.helpers.FenixRobolectricTestRunner
+
+@RunWith(FenixRobolectricTestRunner::class)
+class StringTest {
+
+    @Test
+    fun `toCreditCardNumber returns a string with only digits `() {
+        assertEquals("123456789", "1 234 5678 9".toCreditCardNumber())
+        assertEquals("123456789", "1.23.4+5678/9".toCreditCardNumber())
+        assertEquals("123456789", ",12r34t5678&9".toCreditCardNumber())
+        assertEquals("123456789", " 1 234 5678 9 ".toCreditCardNumber())
+        assertEquals("123456789", " abc 1 234  abc 5678 9".toCreditCardNumber())
+        assertEquals("123456789", "1-234-5678-9".toCreditCardNumber())
+    }
+
+    @Test
+    fun `validateCreditCardNumber returns true for valid credit card numbers `() {
+        val americanExpressCard = "371449635398431"
+        val dinnersClubCard = "30569309025904"
+        val discoverCard = "6011111111111117"
+        val jcbCard = "3530111333300000"
+        val masterCardCard = "5555555555554444"
+        val visaCard = "4111111111111111"
+        val voyagerCard = "869941728035895"
+
+        assertTrue(americanExpressCard.validateCreditCardNumber())
+        assertTrue(dinnersClubCard.validateCreditCardNumber())
+        assertTrue(discoverCard.validateCreditCardNumber())
+        assertTrue(jcbCard.validateCreditCardNumber())
+        assertTrue(masterCardCard.validateCreditCardNumber())
+        assertTrue(visaCard.validateCreditCardNumber())
+        assertTrue(voyagerCard.validateCreditCardNumber())
+    }
+
+    @Test
+    fun `validateCreditCardNumber returns false got invalid credit card numbers `() {
+        val shortCardNumber = "12345678901"
+        val longCardNumber = "12345678901234567890"
+
+        val americanExpressCardInvalid = "371449635398432"
+        val dinnersClubCardInvalid = "30569309025905"
+        val discoverCardInvalid = "6011111111111118"
+        val jcbCardInvalid = "3530111333300001"
+        val masterCardCardInvalid = "5555555555554445"
+        val visaCardInvalid = "4111111111111112"
+        val voyagerCardInvalid = "869941728035896"
+
+        assertFalse(shortCardNumber.validateCreditCardNumber())
+        assertFalse(longCardNumber.validateCreditCardNumber())
+
+        assertFalse(americanExpressCardInvalid.validateCreditCardNumber())
+        assertFalse(dinnersClubCardInvalid.validateCreditCardNumber())
+        assertFalse(discoverCardInvalid.validateCreditCardNumber())
+        assertFalse(jcbCardInvalid.validateCreditCardNumber())
+        assertFalse(masterCardCardInvalid.validateCreditCardNumber())
+        assertFalse(visaCardInvalid.validateCreditCardNumber())
+        assertFalse(voyagerCardInvalid.validateCreditCardNumber())
+    }
+
+    @Test
+    fun `luhnAlgorithmValidation returns false for invalid identification numbers `() {
+        // "4242424242424242" is a valid identification number
+        assertFalse(luhnAlgorithmValidation("4242424242424240"))
+        assertFalse(luhnAlgorithmValidation("4242424242424241"))
+        assertFalse(luhnAlgorithmValidation("4242424242424243"))
+        assertFalse(luhnAlgorithmValidation("4242424242424244"))
+        assertFalse(luhnAlgorithmValidation("4242424242424245"))
+        assertFalse(luhnAlgorithmValidation("4242424242424246"))
+        assertFalse(luhnAlgorithmValidation("4242424242424247"))
+        assertFalse(luhnAlgorithmValidation("4242424242424248"))
+        assertFalse(luhnAlgorithmValidation("4242424242424249"))
+        assertFalse(luhnAlgorithmValidation("1"))
+        assertFalse(luhnAlgorithmValidation("12"))
+        assertFalse(luhnAlgorithmValidation("123"))
+    }
+
+    @Test
+    fun `luhnAlgorithmValidation returns true for valid identification numbers `() {
+        assertTrue(luhnAlgorithmValidation("0"))
+        assertTrue(luhnAlgorithmValidation("00"))
+        assertTrue(luhnAlgorithmValidation("18"))
+        assertTrue(luhnAlgorithmValidation("0000000000000000"))
+        assertTrue(luhnAlgorithmValidation("4242424242424242"))
+        assertTrue(luhnAlgorithmValidation("42424242424242426"))
+        assertTrue(luhnAlgorithmValidation("424242424242424267"))
+        assertTrue(luhnAlgorithmValidation("4242424242424242675"))
+        assertTrue(luhnAlgorithmValidation("000000018"))
+        assertTrue(luhnAlgorithmValidation("99999999999999999999"))
+        assertTrue(luhnAlgorithmValidation("1234567812345670"))
+    }
+}