jpountz commented on a change in pull request #980: LUCENE-8920: Reduce the
memory used by direct addressing of arcs
URL: https://github.com/apache/lucene-solr/pull/980#discussion_r339956145
##########
File path: lucene/core/src/java/org/apache/lucene/util/fst/FST.java
##########
@@ -864,6 +1012,68 @@ private long readUnpackedNodeTarget(BytesReader in)
throws IOException {
return readNextRealArc(arc, in);
}
+ /**
+ * Reads the presence bits of a direct-addressing node, store them in the
provided arc {@link Arc#bitTable()} and returns the number of presence bytes.
+ */
+ private int readPresenceBytes(Arc<T> arc, BytesReader in) throws IOException
{
+ int numPresenceBytes = getNumPresenceBytes(arc.numArcs());
+ long[] presenceBits = new long[numPresenceBytes + 7 >>> 3];
+ int longIndex = 0;
+ int byteIndex = 0;
+ for (int i = 0; i < numPresenceBytes; i++) {
+ presenceBits[longIndex] |= (long) (in.readByte() & 0xFF) << (i << 3);
+ if (++byteIndex == 8) {
+ byteIndex = 0;
+ longIndex++;
+ }
+ }
+ arc.bitTable = presenceBits;
+ assert checkPresenceBytesAreValid(arc);
+ return numPresenceBytes;
+ }
+
+ static boolean checkPresenceBytesAreValid(Arc arc) {
+ assert (arc.bitTable()[0] & 1L) != 0; // First bit must be set.
+ assert (arc.bitTable()[arc.bitTable().length - 1] & (1L << (arc.numArcs()
- 1))) != 0; // Last bit must be set.
+ assert countBits(arc.bitTable()) <= arc.numArcs(); // Total bit set must
be <= label range.
+ return true;
+ }
+
+ /**
+ * Counts all bits set in the provided longs.
+ */
+ static int countBits(long[] bits) {
+ int bitCount = 0;
+ for (long l : bits) {
+ bitCount += Long.bitCount(l);
+ }
+ return bitCount;
+ }
+
+ /**
+ * Returns whether the bit at given index is set.
+ */
+ static boolean isBitSet(long[] bits, int bitIndex) {
+ return (bits[bitIndex >> 6] & (1L << (bitIndex & 63))) != 0;
+ }
+
+ /**
+ * Counts the bits set up to the given bit index, exclusive.
+ */
+ static int countBitsUpTo(long[] bits, int bitIndex) {
+ int bitCount = 0;
+ int lastLong = bitIndex >>> 6; // index / 64
+ for (int i = 0; i < lastLong; i++) {
+ bitCount += Long.bitCount(bits[i]);
+ }
+ int indexInLastLong = bitIndex & 63; // index % 64
+ if (indexInLastLong != 0) {
+ long mask = 0xFFFFFFFFFFFFFFFFL >>> (Long.SIZE - indexInLastLong);
+ bitCount += Long.bitCount(bits[lastLong] & mask);
+ }
Review comment:
we should be able to remove the conditional? e.g. something like this:
```
long mask = (1L << bitIndex) - 1L; // shifts are mode 64
bitCount += Long.bitCount(bits[lastLong] & mask);
```
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