Clean up indexing of ByteBufferView

Motivation:

ByteBufferView is not zero indexed, but parseSimpleString assumes it is.

Modifications:

- Correctly compute on the distance between two indices.
- New, somewhat contrived, test case.

Result:

No functional change: because RediStack assumes the remote peer will
always correctly terminate with /r/n, there is no point at which this
code could misbehave in the current implementation. However, with small
changes it is possible to trigger it, as the new test demonstrates.

Author: Lukasa

Sources/RediStack/RESP/RESPTranslator.swift

@@ -167,7 +167,7 @@ extension RESPTranslator {
         let bytes = buffer.readableBytesView
         guard
             let newlineIndex = bytes.firstIndex(of: .newline),
-            newlineIndex >= 2 // strings should at least have a CRLF ending
+            newlineIndex - bytes.startIndex >= 1 // strings should at least have a CRLF ending
         else { return nil }
 
         // grab the bytes that we've determined is the full simple string,

Tests/RediStackTests/RESPTranslatorTests.swift

@@ -299,6 +299,18 @@ extension RESPTranslatorTests {
 
         return RESPValue.simpleString(stringBuffer).string
     }
+
+    func testSimpleStringWithoutRemovingToken() {
+        var buffer = allocator.buffer(capacity: 2)
+        buffer.writeString("\r\n")
+
+        guard let stringBuffer = parser.parseSimpleString(from: &buffer) else {
+            XCTFail("Did not return")
+            return
+        }
+
+        XCTAssertEqual(RESPValue.simpleString(stringBuffer).string, "")
+    }
 }
 
 // MARK: Integers

Tests/RediStackTests/XCTestManifests.swift

@@ -39,6 +39,7 @@ extension RESPTranslatorTests {
         ("testParsing_simpleString_withContent", testParsing_simpleString_withContent),
         ("testParsing_simpleString_withNoContent", testParsing_simpleString_withNoContent),
         ("testParsing_simpleString", testParsing_simpleString),
+        ("testSimpleStringWithoutRemovingToken", testSimpleStringWithoutRemovingToken),
         ("testWriting_arrays", testWriting_arrays),
         ("testWriting_bulkStrings", testWriting_bulkStrings),
         ("testWriting_errors", testWriting_errors),