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High-performance Java Dataframe with integrated columnar storage (fork of tablesaw)
There is a newer version: 0.9.1
package org.datavec.dataframe.index;

import org.datavec.dataframe.api.IntColumn;
import org.datavec.dataframe.util.BitmapBackedSelection;
import org.datavec.dataframe.util.Selection;
import it.unimi.dsi.fastutil.ints.Int2ObjectAVLTreeMap;
import it.unimi.dsi.fastutil.ints.Int2ObjectOpenHashMap;
import it.unimi.dsi.fastutil.ints.Int2ObjectSortedMap;
import it.unimi.dsi.fastutil.ints.IntArrayList;

import java.util.Comparator;

/**
 * An index for four-byte integer and integer backed columns (date, category, time)
 */
public class IntIndex {

    private final Int2ObjectAVLTreeMap index;

    public IntIndex(IntColumn column) {
        int sizeEstimate = Integer.min(1_000_000, column.size() / 100);
        Int2ObjectOpenHashMap tempMap = new Int2ObjectOpenHashMap<>(sizeEstimate);
        for (int i = 0; i < column.size(); i++) {
            int value = column.get(i);
            IntArrayList recordIds = tempMap.get(value);
            if (recordIds == null) {
                recordIds = new IntArrayList();
                recordIds.add(i);
                tempMap.trim();
                tempMap.put(value, recordIds);
            } else {
                recordIds.add(i);
            }
        }
        index = new Int2ObjectAVLTreeMap<>(tempMap);
    }

    private final static Comparator intArrayComparator = new Comparator() {
        public int compare(int[] a, int[] b) {
            return Integer.compare(a[1], b[1]);
        }
    };

    /**
     * Returns a bitmap containing row numbers of all cells matching the given int
     *
     * @param value This is a 'key' from the index perspective, meaning it is a value from the standpoint of the column
     */
    public Selection get(int value) {
        Selection selection = new BitmapBackedSelection();
        IntArrayList list = index.get(value);
        addAllToSelection(list, selection);
        return selection;
    }

    public Selection atLeast(int value) {
        Selection selection = new BitmapBackedSelection();
        Int2ObjectSortedMap tail = index.tailMap(value);
        for (IntArrayList keys : tail.values()) {
            addAllToSelection(keys, selection);
        }
        return selection;
    }

    public Selection greaterThan(int value) {
        Selection selection = new BitmapBackedSelection();
        Int2ObjectSortedMap tail = index.tailMap(value + 1);
        for (IntArrayList keys : tail.values()) {
            addAllToSelection(keys, selection);
        }
        return selection;
    }

    public Selection atMost(int value) {
        Selection selection = new BitmapBackedSelection();
        Int2ObjectSortedMap head = index.headMap(value + 1); // we add 1 to get values equal to the arg
        for (IntArrayList keys : head.values()) {
            addAllToSelection(keys, selection);
        }
        return selection;
    }

    public Selection lessThan(int value) {
        Selection selection = new BitmapBackedSelection();
        Int2ObjectSortedMap head = index.headMap(value); // we add 1 to get values equal to the arg
        for (IntArrayList keys : head.values()) {
            addAllToSelection(keys, selection);
        }
        return selection;
    }

    private static void addAllToSelection(IntArrayList tableKeys, Selection selection) {
        for (int i : tableKeys) {
            selection.add(i);
        }
    }
}