Source code interpretation of docValue in lucene (9) - reading of SortedNumericDocValue

Before looking at the specific reading process, let's take a look at the functions of the SortedNumericDocValue interface.

public abstract class SortedNumericDocValues {
  
  /** Sole constructor. (For invocation by subclass  constructors, typically implicit.) */
  protected SortedNumericDocValues() {}

  /** locate a doc */
  public abstract void setDocument(int doc);
  
  /** Returns the number of the current doc */
  public abstract long valueAt(int index);
  
  /** The number of current doc values ​​*/
  public abstract int count();
}

 It can be found that, in fact, the interface provided by SortedNumericDocValue has very few functions, only to get all the numbers of a doc. Through the previous blog, I can also guess the principle of his implementation.

Let's take a look at the real read operation, the method is in Lucene410DocValuesProducer.getSortedNumeric(FieldInfo):

public SortedNumericDocValues getSortedNumeric(FieldInfo field) throws IOException {
	SortedSetEntry ss = sortedNumerics.get(field.number);
	NumericEntry numericEntry = numerics.get(field.number);
	final LongValues values = getNumeric(numericEntry);
	if (ss.format == SORTED_SINGLE_VALUED) {//All doc values ​​are only one case. neglect
		final Bits docsWithField = getMissingBits(numericEntry.missingOffset);
		return DocValues.singleton(values, docsWithField);
	} else if (ss.format == SORTED_WITH_ADDRESSES) {//
		//Read the second part, that is, the first value of each doc is the number of all the values.
		final MonotonicBlockPackedReader ordIndex = getOrdIndexInstance(field, ordIndexes.get(field.number));
		return new SortedNumericDocValues() {
			
			long startOffset;
			long endOffset;

			@Override
			public void setDocument(int doc) {
				startOffset = ordIndex.get(doc);//The offset of the value of the doc to be found
				endOffset = ordIndex.get(doc + 1L);//The start position of the next doc of the doc to be found, which is the end position of this doc
			}

			@Override
			public long valueAt(int index) {//Read the nth value of the current doc.
				return values.get(startOffset + index);
			}

			@Override
			public int count() {//How many values ​​are there in the current doc
				return (int) (endOffset - startOffset);
			}
		};
	} else {
		throw new AssertionError();
	}
}

 It can be found that reading SortedNumericDocValue is actually very simple. Use the second part to determine the start position and end position of the value of a doc, that is, the offset, and then read the value of a doc according to the offset.