fits: fix PCOUNT bug in fits-var writer

In the VariableFitsTableSerializer, which uses P and Q output descriptors
to write variable-length arrays into the FITS HDU heap area,
I was writing the wrong value for the PCOUNT header card.
It was using the size of the heap, but it must include also the size
of any gap between the end of the data area and the start of the heap.
Fixed.

fits/src/main/uk/ac/starlink/fits/VariableFitsTableSerializer.java

@@ -47,7 +47,7 @@ public VariableFitsTableSerializer( StarTable table,
         storagePolicy_ = storagePolicy;
         allowSignedByte_ = allowSignedByte;
         init( table );
-        set64BitMode( getPCount() > Integer.MAX_VALUE );
+        set64BitMode( getHeapSize() > Integer.MAX_VALUE );
     }
 
     /**
@@ -70,9 +70,11 @@ public void set64BitMode( boolean useQ ) {
 
     public Header getHeader() throws HeaderCardException {
         Header hdr = super.getHeader();
-        long pcount = getPCount();
-        long theap = hdr.getIntValue( "NAXIS1" ) * hdr.getIntValue( "NAXIS2" );
-        theap = ( ( theap + 2880 - 1 ) / 2880 ) * 2880;
+        long heapSize = getHeapSize();
+        long datEnd = hdr.getIntValue( "NAXIS1" ) * hdr.getIntValue( "NAXIS2" );
+        long theap = ( ( datEnd + 2880 - 1 ) / 2880 ) * 2880;
+        long gap = theap - datEnd;
+        long pcount = gap + heapSize;
 
         /* Header manipulation methods leave a bit to be desired.
          * It is a bit fiddly to make sure these cards go in the right place. */
@@ -85,7 +87,8 @@ public Header getHeader() throws HeaderCardException {
             HeaderCard card = (HeaderCard) it.next();
             String key = card.getKey();
             if ( "PCOUNT".equals( key ) ) {
-                cardList.add( new HeaderCard( "PCOUNT", pcount, "heap size" ) );
+                cardList.add( new HeaderCard( "PCOUNT", pcount,
+                                              "heap size + gap" ) );
             }
             else if ( "TFIELDS".equals( key ) ) {
                 cardList.add( card );
@@ -129,14 +132,14 @@ private VariableArrayColumnWriter[] getVariableArrayColumnWriters() {
      *
      * @return   heap size
      */
-    private long getPCount() {
-        long pcount = 0L;
+    private final long getHeapSize() {
+        long count = 0L;
         VariableArrayColumnWriter[] vcws = getVariableArrayColumnWriters();
         for ( int iv = 0; iv < vcws.length; iv++ ) {
             VariableArrayColumnWriter vcw = vcws[ iv ];
-            pcount += vcw.totalElements_ * vcw.arrayWriter_.getByteCount();
+            count += vcw.totalElements_ * vcw.arrayWriter_.getByteCount();
         }
-        return pcount;
+        return count;
     }
 
     public void writeData( DataOutput out ) throws IOException {
@@ -161,7 +164,8 @@ public void writeData( DataOutput out ) throws IOException {
         finally {
             byteStore.close();
         }
-        int over = (int) ( getPCount() % 2880 );
+        assert byteStore.getLength() == getHeapSize();
+        int over = (int) ( getHeapSize() % 2880 );
         if ( over > 0 ) {
             out.write( new byte[ 2880 - over ] );
         }

topcat/src/docs/sun253.xml

@@ -20577,6 +20577,8 @@ introduced since the last version:
         <ref id="HistogramPlotWindow">Histogram window</ref>
         allow you to export histogram bin data as a new table
         (this reinstates behaviour available from old plot windows).</li>
+    <li>Fixed error in fits-var output
+        (PCOUNT header card did not include block alignment gap).</li>
     </ul>
     </p></dd>
 

ttools/src/docs/sun256.xml

@@ -10055,6 +10055,8 @@ eds. C. Gabriel et al., ASP Conf. Ser. 351, p. 666 (2006)
         <code>plot2plane</code>.</li>
     <li>Fix NullPointerException bug when null value was supplied to
         multi-word parameter (e.g. <code>tcube</code>).</li>
+    <li>Fixed error in fits-var output
+        (PCOUNT header card did not include block alignment gap).</li>
     </ul>
     </p></dd>