Fix handling of output offset in SkippableComposition classes

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));
+        }
+    }
 }