diff --git a/components/net/sf/briar/transport/ErasureDecoder.java b/components/net/sf/briar/transport/ErasureDecoder.java
new file mode 100644
index 0000000000000000000000000000000000000000..679c26ab20f19ffa35a17bafdd510488c8485b11
--- /dev/null
+++ b/components/net/sf/briar/transport/ErasureDecoder.java
@@ -0,0 +1,13 @@
+package net.sf.briar.transport;
+
+import net.sf.briar.api.FormatException;
+import net.sf.briar.api.transport.Segment;
+
+interface ErasureDecoder {
+
+ /**
+ * Decodes the given set of segments into the given frame, or returns false
+ * if the segments cannot be decoded. The segment set may contain nulls.
+ */
+ public boolean decodeFrame(Frame f, Segment[] set) throws FormatException;
+}
diff --git a/components/net/sf/briar/transport/FrameWindowImpl.java b/components/net/sf/briar/transport/FrameWindowImpl.java
index ddf3e71cf65793dca4c97199dba7bcbfc09afd9f..c3ff67a75a789c42d15fa8528f0f682d7c391c5a 100644
--- a/components/net/sf/briar/transport/FrameWindowImpl.java
+++ b/components/net/sf/briar/transport/FrameWindowImpl.java
@@ -6,6 +6,7 @@ import static net.sf.briar.util.ByteUtils.MAX_32_BIT_UNSIGNED;
import java.util.Collection;
import java.util.HashSet;
+/** A frame window that allows a limited amount of reordering. */
class FrameWindowImpl implements FrameWindow {
private final Collection<Long> window;
diff --git a/components/net/sf/briar/transport/IncomingErrorCorrectionLayerImpl.java b/components/net/sf/briar/transport/IncomingErrorCorrectionLayerImpl.java
new file mode 100644
index 0000000000000000000000000000000000000000..db3ab691d2713d6470bf11550a6fbf387a182a00
--- /dev/null
+++ b/components/net/sf/briar/transport/IncomingErrorCorrectionLayerImpl.java
@@ -0,0 +1,68 @@
+package net.sf.briar.transport;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+
+import net.sf.briar.api.FormatException;
+import net.sf.briar.api.transport.Segment;
+
+class IncomingErrorCorrectionLayerImpl implements IncomingErrorCorrectionLayer {
+
+ private final IncomingEncryptionLayer in;
+ private final ErasureDecoder decoder;
+ private final int n, k;
+ private final Map<Long, Integer> discardCounts;
+ private final Map<Long, Segment[]> segmentSets;
+
+ IncomingErrorCorrectionLayerImpl(IncomingEncryptionLayer in,
+ ErasureDecoder decoder, int n, int k) {
+ this.in = in;
+ this.decoder = decoder;
+ this.n = n;
+ this.k = k;
+ discardCounts = new HashMap<Long, Integer>();
+ segmentSets = new HashMap<Long, Segment[]>();
+ }
+
+ public boolean readFrame(Frame f, FrameWindow window) throws IOException,
+ InvalidDataException {
+ // Free any segment sets that have been removed from the window
+ Iterator<Long> it = segmentSets.keySet().iterator();
+ while(it.hasNext()) if(!window.contains(it.next())) it.remove();
+ // Free any discard counts that are no longer too high for the window
+ Iterator<Long> it1 = discardCounts.keySet().iterator();
+ while(it1.hasNext()) if(!window.isTooHigh(it1.next())) it1.remove();
+ // Allocate a segment
+ Segment s = new SegmentImpl();
+ // Read segments until a frame can be decoded
+ while(true) {
+ // Read segments until a segment in the window is returned
+ long frameNumber;
+ while(true) {
+ if(!in.readSegment(s)) return false;
+ frameNumber = s.getSegmentNumber() / n;
+ if(window.contains(frameNumber)) break;
+ if(window.isTooHigh(frameNumber)) countDiscard(frameNumber);
+ }
+ // Add the segment to its segment set, or create one if necessary
+ Segment[] set = segmentSets.get(frameNumber);
+ if(set == null) {
+ set = new Segment[n];
+ segmentSets.put(frameNumber, set);
+ } else {
+ set[(int) (frameNumber % n)] = s;
+ }
+ // Try to decode the frame
+ if(decoder.decodeFrame(f, set)) return true;
+ }
+ }
+
+ private void countDiscard(long frameNumber) throws FormatException {
+ Integer count = discardCounts.get(frameNumber);
+ if(count == null) discardCounts.put(frameNumber, 1);
+ else if(count == n - k) throw new FormatException();
+ else discardCounts.put(frameNumber, count + 1);
+ }
+}
diff --git a/components/net/sf/briar/transport/NullFrameWindow.java b/components/net/sf/briar/transport/NullFrameWindow.java
index c56a78a37dea818d2c0f01109d39d52c378f4b3f..9f5fb19f7145b800fb871b31b5460472fd5c8621 100644
--- a/components/net/sf/briar/transport/NullFrameWindow.java
+++ b/components/net/sf/briar/transport/NullFrameWindow.java
@@ -2,6 +2,7 @@ package net.sf.briar.transport;
import static net.sf.briar.util.ByteUtils.MAX_32_BIT_UNSIGNED;
+/** A frame window that does not allow any reordering. */
class NullFrameWindow implements FrameWindow {
private long base = 0L;
diff --git a/components/net/sf/briar/transport/NullIncomingErrorCorrectionLayer.java b/components/net/sf/briar/transport/NullIncomingErrorCorrectionLayer.java
index 87ce8d4b358ee0bb4d2dc76b248fc8a4d9e37c69..54e1f04aec8821e127bc7ae4486da58cc6bed18b 100644
--- a/components/net/sf/briar/transport/NullIncomingErrorCorrectionLayer.java
+++ b/components/net/sf/briar/transport/NullIncomingErrorCorrectionLayer.java
@@ -14,12 +14,13 @@ class NullIncomingErrorCorrectionLayer implements IncomingErrorCorrectionLayer {
segment = new SegmentImpl();
}
- public boolean readFrame(Frame f, FrameWindow window)
- throws IOException, InvalidDataException {
+ public boolean readFrame(Frame f, FrameWindow window) throws IOException,
+ InvalidDataException {
while(true) {
if(!in.readSegment(segment)) return false;
byte[] buf = segment.getBuffer();
- if(window.contains(HeaderEncoder.getFrameNumber(buf))) break;
+ long frameNumber = HeaderEncoder.getFrameNumber(buf);
+ if(window.contains(frameNumber)) break;
}
int length = segment.getLength();
// FIXME: Unnecessary copy
diff --git a/components/net/sf/briar/transport/XorErasureDecoder.java b/components/net/sf/briar/transport/XorErasureDecoder.java
new file mode 100644
index 0000000000000000000000000000000000000000..61682185a1673f76f575e50707e6e2f86dd186f5
--- /dev/null
+++ b/components/net/sf/briar/transport/XorErasureDecoder.java
@@ -0,0 +1,60 @@
+package net.sf.briar.transport;
+
+import static net.sf.briar.api.transport.TransportConstants.MAX_FRAME_LENGTH;
+import net.sf.briar.api.FormatException;
+import net.sf.briar.api.transport.Segment;
+
+/** An erasure decoder that uses k data segments and one parity segment. */
+class XorErasureDecoder implements ErasureDecoder {
+
+ private final int n;
+
+ XorErasureDecoder(int n) {
+ this.n = n;
+ }
+
+ public boolean decodeFrame(Frame f, Segment[] set) throws FormatException {
+ // We need at least n - 1 pieces
+ int pieces = 0;
+ for(int i = 0; i < n; i++) if(set[i] != null) pieces++;
+ if(pieces < n - 1) return false;
+ // All the pieces must have the same length - take the minimum
+ int length = MAX_FRAME_LENGTH;
+ for(int i = 0; i < n; i++) {
+ if(set[i] == null) {
+ int len = set[i].getLength();
+ if(len < length) length = len;
+ }
+ }
+ if(length * (n - 1) > MAX_FRAME_LENGTH) throw new FormatException();
+ // Decode the frame
+ byte[] dest = f.getBuffer();
+ int offset = 0;
+ if(pieces == n || set[n - 1] == null) {
+ // We don't need no stinkin' parity segment
+ for(int i = 0; i < n - 1; i++) {
+ byte[] src = set[i].getBuffer();
+ System.arraycopy(src, 0, dest, offset, length);
+ offset += length;
+ }
+ } else {
+ // Reconstruct the missing segment
+ byte[] parity = new byte[length];
+ int missingOffset = -1;
+ for(int i = 0; i < n; i++) {
+ if(set[i] == null) {
+ missingOffset = offset;
+ } else {
+ byte[] src = set[i].getBuffer();
+ System.arraycopy(src, 0, dest, offset, length);
+ for(int j = 0; j < length; j++) parity[j] ^= src[j];
+ }
+ offset += length;
+ }
+ assert missingOffset != -1;
+ System.arraycopy(parity, 0, dest, missingOffset, length);
+ }
+ f.setLength(offset);
+ return true;
+ }
+}