Fix handling of output offset in SkippableComposition classes

Previously, when outpos passed to headlessUncompress was greater
than zero, the second scheme would receive an incorrect number of
remaining integers to decode.

Author: piotrrzysko

src/main/java/me/lemire/integercompression/SkippableComposition.java

@@ -52,12 +52,14 @@ public void headlessCompress(int[] in, IntWrapper inpos, int inlength, int[] out
     public void headlessUncompress(int[] in, IntWrapper inpos, int inlength, int[] out,
             IntWrapper outpos, int num) {
         int init = inpos.get();
+        int outposInit = outpos.get();
+
         F1.headlessUncompress(in, inpos, inlength, out, outpos, num);
         if (inpos.get() == init) {
               inpos.increment();
         }
         inlength -= inpos.get() - init;
-        num -= outpos.get();
+        num -= outpos.get() - outposInit;
         F2.headlessUncompress(in, inpos, inlength, out, outpos, num);
     }
 

src/main/java/me/lemire/integercompression/differential/SkippableIntegratedComposition.java

@@ -66,13 +66,15 @@ public void headlessUncompress(int[] in, IntWrapper inpos, int inlength,
         if (inlength == 0)
             return;
         int init = inpos.get();
+        int outposInit = outpos.get();
+
         F1.headlessUncompress(in, inpos, inlength, out, outpos,num,initvalue);
         if (inpos.get() == init) {
             inpos.increment();
         }
         inlength -= inpos.get() - init;
 
-        num -= outpos.get();
+        num -= outpos.get() - outposInit;
         F2.headlessUncompress(in, inpos, inlength, out, outpos,num,initvalue);
     }
 

src/main/java/me/lemire/longcompression/SkippableLongComposition.java

@@ -53,12 +53,14 @@ public void headlessCompress(long[] in, IntWrapper inpos, int inlength, long[] o
     public void headlessUncompress(long[] in, IntWrapper inpos, int inlength, long[] out,
             IntWrapper outpos, int num) {
         int init = inpos.get();
+        int outposInit = outpos.get();
+
         F1.headlessUncompress(in, inpos, inlength, out, outpos, num);
         if (inpos.get() == init) {
               inpos.increment();
         }
         inlength -= inpos.get() - init;
-        num -= outpos.get();
+        num -= outpos.get() - outposInit;
         F2.headlessUncompress(in, inpos, inlength, out, outpos, num);
     }
 

src/test/java/me/lemire/integercompression/SkippableBasicTest.java

@@ -15,6 +15,7 @@
 import me.lemire.integercompression.differential.SkippableIntegratedIntegerCODEC;
 import org.junit.Test;
 
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertTrue;
 
 /**
@@ -236,4 +237,52 @@ private static void testMaxHeadlessCompressedLength(SkippableIntegerCODEC codec,
             }
         }
     }
+
+    @Test
+    public void testUncompressOutputOffset_SkippableComposition() {
+        for (int offset : new int[] {0, 1, 6}) {
+            SkippableComposition codec = new SkippableComposition(new BinaryPacking(), new VariableByte());
+
+            int[] input = { 2, 3, 4, 5 };
+            int[] compressed = new int[codec.maxHeadlessCompressedLength(new IntWrapper(0), input.length)];
+            int[] uncompressed = new int[offset + input.length];
+
+            IntWrapper inputOffset = new IntWrapper(0);
+            IntWrapper compressedOffset = new IntWrapper(0);
+
+            codec.headlessCompress(input, inputOffset, input.length, compressed, compressedOffset);
+
+            int compressedLength = compressedOffset.get();
+            IntWrapper uncompressedOffset = new IntWrapper(offset);
+            compressedOffset = new IntWrapper(0);
+            codec.headlessUncompress(compressed, compressedOffset, compressedLength, uncompressed, uncompressedOffset, input.length);
+
+            assertArrayEquals(input, Arrays.copyOfRange(uncompressed, offset, offset + input.length));
+        }
+    }
+
+    @Test
+    public void testUncompressOutputOffset_SkippableIntegratedComposition() {
+        for (int offset : new int[] {0, 1, 6}) {
+            SkippableIntegratedComposition codec = new SkippableIntegratedComposition(new IntegratedBinaryPacking(), new IntegratedVariableByte());
+
+            int[] input = { 2, 3, 4, 5 };
+            int[] compressed = new int[codec.maxHeadlessCompressedLength(new IntWrapper(0), input.length)];
+            int[] uncompressed = new int[offset + input.length];
+
+            IntWrapper inputOffset = new IntWrapper(0);
+            IntWrapper compressedOffset = new IntWrapper(0);
+            IntWrapper initValue = new IntWrapper(0);
+
+            codec.headlessCompress(input, inputOffset, input.length, compressed, compressedOffset, initValue);
+
+            int compressedLength = compressedOffset.get();
+            IntWrapper uncompressedOffset = new IntWrapper(offset);
+            compressedOffset = new IntWrapper(0);
+            initValue = new IntWrapper(0);
+            codec.headlessUncompress(compressed, compressedOffset, compressedLength, uncompressed, uncompressedOffset, input.length, initValue);
+
+            assertArrayEquals(input, Arrays.copyOfRange(uncompressed, offset, offset + input.length));
+        }
+    }
 }

src/test/java/me/lemire/longcompression/SkippableLongBasicTest.java

@@ -15,6 +15,7 @@
 import me.lemire.integercompression.TestUtils;
 import me.lemire.integercompression.VariableByte;
 
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertTrue;
 
 /**
@@ -167,4 +168,27 @@ private static void testMaxHeadlessCompressedLength(SkippableLongCODEC codec, in
             assertTrue(maxOutputLength <= outPos.get() + 1); // +1 because SkippableLongComposition always adds one extra integer for the potential header
         }
     }
+
+    @Test
+    public void testUncompressOutputOffset_SkippableLongComposition() {
+        for (int offset : new int[] {0, 1, 6}) {
+            SkippableLongComposition codec = new SkippableLongComposition(new LongBinaryPacking(), new LongVariableByte());
+
+            long[] input = { 2, 3, 4, 5 };
+            long[] compressed = new long[codec.maxHeadlessCompressedLength(new IntWrapper(0), input.length)];
+            long[] uncompressed = new long[offset + input.length];
+
+            IntWrapper inputOffset = new IntWrapper(0);
+            IntWrapper compressedOffset = new IntWrapper(0);
+
+            codec.headlessCompress(input, inputOffset, input.length, compressed, compressedOffset);
+
+            int compressedLength = compressedOffset.get();
+            IntWrapper uncompressedOffset = new IntWrapper(offset);
+            compressedOffset = new IntWrapper(0);
+            codec.headlessUncompress(compressed, compressedOffset, compressedLength, uncompressed, uncompressedOffset, input.length);
+
+            assertArrayEquals(input, Arrays.copyOfRange(uncompressed, offset, offset + input.length));
+        }
+    }
 }