PARQUET-623: Fix DeltaByteArrayReader#skip.

Previously, this passed the skip to the underlying readers, but would
not update previous and would corrupt values or cause exceptions.

Author: Ryan Blue <blue@apache.org>

Closes #366 from rdblue/PARQUET-623-fix-delta-byte-array-skip and squashes the following commits:

f85800c [Ryan Blue] PARQUET-623: Fix DeltaByteArrayReader#skip.

Author: rdblue

parquet-column/src/main/java/org/apache/parquet/column/values/deltastrings/DeltaByteArrayReader.java

@@ -55,8 +55,8 @@ public void initFromPage(int valueCount, ByteBuffer page, int offset)
 
   @Override
   public void skip() {
-    prefixLengthReader.skip();
-    suffixReader.skip();
+    // read the next value to skip so that previous is correct.
+    readBytes();
   }
 
   @Override

parquet-column/src/test/java/org/apache/parquet/column/values/deltastrings/TestDeltaByteArray.java

@@ -19,6 +19,7 @@
 package org.apache.parquet.column.values.deltastrings;
 
 import java.io.IOException;
+import java.nio.ByteBuffer;
 
 import org.junit.Test;
 import org.junit.Assert;
@@ -49,6 +50,13 @@ public void testRandomStrings() throws Exception {
     assertReadWrite(writer, reader, randvalues);
   }
 
+  @Test
+  public void testRandomStringsWithSkip() throws Exception {
+    DeltaByteArrayWriter writer = new DeltaByteArrayWriter(64 * 1024, 64 * 1024, new DirectByteBufferAllocator());
+    DeltaByteArrayReader reader = new DeltaByteArrayReader();
+    assertReadWriteWithSkip(writer, reader, randvalues);
+  }
+
   @Test
   public void testLengths() throws IOException {
     DeltaByteArrayWriter writer = new DeltaByteArrayWriter(64 * 1024, 64 * 1024, new DirectByteBufferAllocator());
@@ -81,6 +89,16 @@ private void assertReadWrite(DeltaByteArrayWriter writer, DeltaByteArrayReader r
     }
   }
 
+  private void assertReadWriteWithSkip(DeltaByteArrayWriter writer, DeltaByteArrayReader reader, String[] vals) throws Exception {
+    Utils.writeData(writer, vals);
+
+    reader.initFromPage(vals.length, writer.getBytes().toByteBuffer(), 0);
+    for (int i = 0; i < vals.length; i += 2) {
+      Assert.assertEquals(Binary.fromString(vals[i]), reader.readBytes());
+      reader.skip();
+    }
+  }
+
   @Test
   public void testWriterReset() throws Exception {
     DeltaByteArrayWriter writer = new DeltaByteArrayWriter(64 * 1024, 64 * 1024, new DirectByteBufferAllocator());