Add voucher code domain model

Author: Rawa

android/lib/model/src/main/kotlin/net/mullvad/mullvadvpn/lib/model/VoucherCode.kt

@@ -0,0 +1,31 @@
+package net.mullvad.mullvadvpn.lib.model
+
+import arrow.core.Either
+import arrow.core.raise.either
+import arrow.core.raise.ensure
+
+@JvmInline
+value class VoucherCode private constructor(val value: String) {
+
+    companion object {
+        // Parsing reference:
+        // <services-repository>/services/docs/adr/0018-distinguish-voucher-codes-from-account-numbers.md
+        fun fromString(value: String): Either<ParseVoucherCodeError, VoucherCode> = either {
+            val trimmedValue = value.trim()
+            ensure(trimmedValue.length >= MIN_VOUCHER_LENGTH) {
+                ParseVoucherCodeError.TooShort(trimmedValue)
+            }
+            ensure(!value.all { it.isDigit() }) { ParseVoucherCodeError.AllDigit(trimmedValue) }
+            VoucherCode(trimmedValue)
+        }
+
+        const val MIN_VOUCHER_LENGTH = 16
+    }
+}
+
+sealed interface ParseVoucherCodeError {
+
+    data class AllDigit(val value: String) : ParseVoucherCodeError
+
+    data class TooShort(val value: String) : ParseVoucherCodeError
+}

android/lib/model/src/test/kotlin/net/mullvad/mullvadvpn/lib/model/VoucherCodeTest.kt

@@ -0,0 +1,34 @@
+package net.mullvad.mullvadvpn.lib.model
+
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+import org.junit.jupiter.api.Test
+
+class VoucherCodeTest {
+    @Test
+    fun `parsing a too short voucher code should return TooShort`() {
+        val input = "mycode"
+        val result = VoucherCode.fromString(input)
+
+        assertTrue(result.isLeft())
+        assertEquals(ParseVoucherCodeError.TooShort(input), result.leftOrNull())
+    }
+
+    @Test
+    fun `numbers only should not be allowed`() {
+        val input = "1234123412341234"
+        val result = VoucherCode.fromString(input)
+
+        assertTrue(result.isLeft())
+        assertEquals(ParseVoucherCodeError.AllDigit(input), result.leftOrNull())
+    }
+
+    @Test
+    fun `number only input when too short should return TooShort`() {
+        val input = "123412341234"
+        val result = VoucherCode.fromString(input)
+
+        assertTrue(result.isLeft())
+        assertEquals(ParseVoucherCodeError.TooShort(input), result.leftOrNull())
+    }
+}