[IO-427] Add TrailerInputStream

src/main/java/org/apache/commons/io/input/TrailerInputStream.java

@@ -0,0 +1,168 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.io.input;
+
+import java.io.IOException;
+import java.io.InputStream;
+import org.apache.commons.io.IOUtils;
+
+/**
+ * Reads the underlying input stream while holding back the trailer.
+ * 
+ * <p>
+ * "Normal" read calls read the underlying stream except the last few bytes (the trailer). The
+ * trailer is updated with each read call. The trailer can be gotten by one of the copyTrailer
+ * overloads.
+ * </p>
+ * 
+ * <p>
+ * It is safe to fetch the trailer at any time but the trailer will change with each read call
+ * until the underlying stream is EOF.
+ * </p>
+ * 
+ * <p>
+ * Useful, e.g., for handling checksums: payload is followed by a fixed size hash, so while
+ * streaming the payload the trailer finally contains the expected hash (this example needs
+ * extra caution to revert actions when the final checksum match fails).
+ * </p>
+ */
+public final class TrailerInputStream extends InputStream {
+
+    private final  InputStream source;
+    /**
+     * Invariant: After every method call which exited without exception, the trailer has to be
+     * completely filled.
+     */
+    private final byte[] trailer;
+
+    /**
+     * Constructs the TrailerInputStream and initializes the trailer buffer.
+     * 
+     * <p>
+     * Reads exactly {@code trailerLength} bytes from {@code source}.
+     * </p>
+     * 
+     * @param source underlying stream from which is read.
+     * @param trailerLength the length of the trailer which is hold back (must be &gt;= 0).
+     * @throws IOException initializing the trailer buffer failed.
+     */
+    public TrailerInputStream(final InputStream source, final int trailerLength)
+            throws IOException {
+        if (trailerLength < 0) {
+            throw new IllegalArgumentException("Trailer length must be >= 0: " + trailerLength);
+        }
+        this.source = source;
+        this.trailer = trailerLength == 0 ? IOUtils.EMPTY_BYTE_ARRAY : new byte[trailerLength];
+        IOUtils.readFully(this.source, this.trailer);
+    }
+
+    @Override
+    public int read() throws IOException {
+        // Does exactly on source read call.
+        // Copies this.trailer.length bytes if source is not EOF.
+        final int newByte = this.source.read();
+        if (newByte == IOUtils.EOF || this.trailer.length == 0) {
+            return newByte;
+        }
+        final int ret = this.trailer[0];
+        System.arraycopy(this.trailer, 1, this.trailer, 0, this.trailer.length - 1);
+        this.trailer[this.trailer.length - 1] = (byte) newByte;
+        return ret;
+    }
+
+    @Override
+    public int read(final byte[] b, final int off, final int len) throws IOException {
+        // Does at most 2 IOUtils.read calls to source.
+        // Copies at most 2 * this.trailer.length bytes.
+        // All other bytes are directly written to the target buffer.
+        if (off < 0 || len < 0 || b.length - off < len) {
+            throw new IndexOutOfBoundsException();
+        }
+        if (len == 0) {
+            return 0;
+        }
+        final int readIntoBuffer;
+        int read;
+        // fist step: move trailer + read data
+        // overview - b: [---------], t: [1234] --> b: [1234abcde], t: [fghi]
+        if (len <= this.trailer.length) {
+            // 1 IOUtils.read calls to source, copies this.trailer.length bytes
+            // trailer can fill b, so only read into trailer needed
+            // b: [----], trailer: [123456789] --> b: [1234], trailer: [----56789]
+            System.arraycopy(this.trailer, 0, b, off, len);
+            readIntoBuffer = len;
+            // b: [1234], trailer: [----56789] --> b: [1234], trailer: [56789----]
+            System.arraycopy(this.trailer, len, this.trailer, 0, this.trailer.length - len);
+            // b: [1234], trailer: [56789----] --> b: [1234], trailer: [56789abcd]
+            read = IOUtils.read(this.source, this.trailer, this.trailer.length - len, len);
+        } else {
+            // 1 or 2 IOUtils.read calls to source, copies this.trailer.length bytes
+            // trailer smaller than b, so need to read into b and trailer
+            // b: [---------], t: [1234] --> b: [1234-----], t: [----]
+            System.arraycopy(this.trailer, 0, b, off, this.trailer.length);
+            // b: [1234-----], t: [----] --> b: [1234abcde], t: [----]
+            read = IOUtils.read(
+                            this.source, b, off + this.trailer.length, len - this.trailer.length);
+            readIntoBuffer = this.trailer.length + read;
+            // b: [1234abcde], t: [----] --> b: [1234abcde], t: [fghi]
+            if (read == len - this.trailer.length) { // don't try reading data when stream source EOF
+                read += IOUtils.read(this.source, this.trailer);
+            }
+        }
+        // if less data than requested has been read, the trailer buffer is not full
+        // --> need to fill the trailer with the last bytes from b
+        // (only possible if we reached EOF)
+        // second step: ensure that trailer is completely filled with data
+        // overview - b: [abcdefghi], t: [jk--] --> b: [abcdefg--], t: [hijk]
+        final int underflow = Math.min(len - read, this.trailer.length);
+        if (underflow > 0) {
+            // at most this.trailer.length are copied to fill the trailer buffer
+            if (underflow < this.trailer.length) {
+                // trailer not completely empty, so move data to the end
+                // b: [abcdefghi], t: [jk--] --> b: [abcdefghi], t: [--jk]
+                System.arraycopy(
+                        this.trailer, 0, this.trailer, underflow, this.trailer.length - underflow);
+            }
+            // fill trailer with last bytes from b
+            // b: [abcdefghi], t: [--jk] --> b: [abcdefg--], t: [hijk]
+            System.arraycopy(b, off + readIntoBuffer - underflow, this.trailer, 0, underflow);
+        }
+        // IOUtils.read reads as many bytes as possible, so reading 0 bytes means EOF.
+        // Then, we have to mark this.
+        return read == 0 && len != 0 ? IOUtils.EOF : read;
+    }
+
+    @Override
+    public int available() throws IOException {
+        return this.source.available();
+    }
+
+    @Override
+    public void close() throws IOException {
+        try {
+            this.source.close();
+        } finally {
+            super.close();
+        }
+    }
+
+    public int getTrailerLength() {
+        return this.trailer.length;
+    }
+
+    public byte[] copyTrailer() {
+        return this.trailer.clone();
+    }
+
+}