Make StdIn report available bytes (#725 / #728)

Before reading from `System.in`, it is necessary to check
`System.in.available() != 0` to prevent blocking on empty input, but
`StdIn` subclasses `InputStream`, where `available()` always returns 0.

This change makes `StdIn` override `available()`, so it returns the
number of bytes from the input string that weren't yet read.

Closes: #725
PR: #728

src/main/java/org/junitpioneer/jupiter/StdIn.java

@@ -27,14 +27,27 @@ public class StdIn extends InputStream {
 	private final StringReader reader;
 	private final StringWriter writer = new StringWriter();
 
+	/**
+	 * Keeps track of number of bytes that are still available to {@link InputStream#read() read()}.
+	 */
+	private int availableBytes;
+
 	public StdIn(String[] values) {
-		reader = new StringReader(String.join(StdIoExtension.SEPARATOR, values));
+		var mockedInput = String.join(StdIoExtension.SEPARATOR, values);
+		reader = new StringReader(mockedInput);
+		availableBytes = mockedInput.getBytes().length;
+	}
+
+	@Override
+	public int available() throws IOException {
+		return availableBytes;
 	}
 
 	@Override
 	public int read() throws IOException {
 		int reading = reader.read();
 		if (reading != -1) {
+			availableBytes--;
 			writer.write(reading);
 		}
 		return reading;

src/test/java/org/junitpioneer/jupiter/StdIoExtensionTests.java

@@ -106,6 +106,34 @@ void catchesNothing(StdIn in) {
 			assertThat(in.capturedLines()).containsExactly("");
 		}
 
+		@Test
+		@StdIo({ "Doth homage to his new-appearing sight", "Serving with looks his sacred majesty;" })
+		@DisplayName("for non-empty input, available() reports input's number of bytes")
+		void everythingAvailableBeforeRead(StdIn in) throws IOException {
+			int inputLength = ("Doth homage to his new-appearing sight" + System.getProperty("line.separator")
+					+ "Serving with looks his sacred majesty;").getBytes().length;
+
+			assertThat(in.available()).isEqualTo(inputLength);
+		}
+
+		@Test
+		@StdIo({ "Doth homage to his new-appearing sight" })
+		@DisplayName("for non-empty input after partial read, available() reports correctly reduced number of bytes")
+		void somethingAvailableAfterRead(StdIn in) throws IOException {
+			int bytesToRead = 16;
+			app.read(bytesToRead);
+			int remainingLength = "Doth homage to his new-appearing sight".getBytes().length - bytesToRead;
+
+			assertThat(in.available()).isEqualTo(remainingLength);
+		}
+
+		@Test
+		@StdIo("")
+		@DisplayName("for empty input, available() returns 0 available bytes")
+		void nothingAvailableWhenEmpty(StdIn in) throws IOException {
+			assertThat(in.available()).isEqualTo(0);
+		}
+
 		@Test
 		@StdIo({ "And having climbed the steep-up heavenly hill,", "Resembling strong youth in his middle age," })
 		@DisplayName("catches the input from the standard in and the output on the standard out")
@@ -357,6 +385,12 @@ public void read() throws IOException {
 			lines.add(reader.readLine());
 		}
 
+		public void read(int byteCount) throws IOException {
+			for (int i = 0; i < byteCount; i++) {
+				System.in.read();
+			}
+		}
+
 		public void readAndWrite() throws IOException {
 			BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
 			lines.add(reader.readLine());