Add methods for detecting retriable and fatal Ably exceptions

common/src/main/java/com/ably/tracking/common/AblyHelpers.kt

@@ -221,3 +221,21 @@ private fun TokenAuthException.toAblyException(): AblyException =
         is CouldNotFetchTokenException ->
             AblyException.fromErrorInfo(ErrorInfo(message, 401, 100_002))
     }
+
+/**
+ * Indicates whether the exception from Ably is fatal and we should not attempt to retry it.
+ * Fatal errors have codes like 4xx (e.g. 400).
+ */
+fun ConnectionException.isFatal(): Boolean {
+    val firstErrorCodeDigit = errorInformation.code.toString().first()
+    return firstErrorCodeDigit == '4'
+}
+
+/**
+ * Indicates whether the exception from Ably is retriable and we can attempt to retry it.
+ * Non-fatal errors have codes like 5xx (e.g. 500).
+ */
+fun ConnectionException.isRetriable(): Boolean {
+    val firstErrorCodeDigit = errorInformation.code.toString().first()
+    return firstErrorCodeDigit == '5'
+}

common/src/test/java/com/ably/tracking/common/ConnectionExceptionTests.kt

@@ -0,0 +1,59 @@
+package com.ably.tracking.common
+
+import com.ably.tracking.ConnectionException
+import com.ably.tracking.ErrorInformation
+import org.junit.Assert
+import org.junit.Test
+
+class ConnectionExceptionTests {
+    @Test
+    fun `exception with error code starting with 4 should be treated as fatal`() {
+        // given
+        val exception = createConnectionException(400, 40000, "Fatal error")
+
+        // when
+        val result = exception.isFatal()
+
+        // then
+        Assert.assertTrue(result)
+    }
+
+    @Test
+    fun `exception with error code starting with 4 should not be treated as retriable`() {
+        // given
+        val exception = createConnectionException(400, 40000, "Fatal error")
+
+        // when
+        val result = exception.isRetriable()
+
+        // then
+        Assert.assertFalse(result)
+    }
+
+    @Test
+    fun `exception with error code starting with 5 should not be treated as fatal`() {
+        // given
+        val exception = createConnectionException(500, 50000, "Retriable error")
+
+        // when
+        val result = exception.isFatal()
+
+        // then
+        Assert.assertFalse(result)
+    }
+
+    @Test
+    fun `exception with error code starting with 5 should be treated as retriable`() {
+        // given
+        val exception = createConnectionException(500, 50000, "Retriable error")
+
+        // when
+        val result = exception.isRetriable()
+
+        // then
+        Assert.assertTrue(result)
+    }
+
+    private fun createConnectionException(code: Int, statusCode: Int, message: String): ConnectionException =
+        ConnectionException(ErrorInformation(code, statusCode, message, null, null))
+}