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package org.apache.lucene.queries.function.valuesource;

import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.queries.function.FunctionValues;
import org.apache.lucene.queries.function.ValueSource;
import org.apache.lucene.queries.function.docvalues.FloatDocValues;
import org.apache.lucene.search.IndexSearcher;

import java.io.IOException;
import java.util.Map;

ReciprocalFloatFunction implements a reciprocal function f(x) = a/(mx+b), based on the float value of a field or function as exported by ValueSource.
When a and b are equal, and x>=0, this function has a maximum value of 1 that drops as x increases. Increasing the value of a and b together results in a movement of the entire function to a flatter part of the curve.

These properties make this an idea function for boosting more recent documents.

Example: recip(ms(NOW,mydatefield),3.16e-11,1,1)

A multiplier of 3.16e-11 changes the units from milliseconds to years (since there are about 3.16e10 milliseconds per year). Thus, a very recent date will yield a value close to 1/(0+1) or 1, a date a year in the past will get a multiplier of about 1/(1+1) or 1/2, and date two years old will yield 1/(2+1) or 1/3.

See Also:
/** * <code>ReciprocalFloatFunction</code> implements a reciprocal function f(x) = a/(mx+b), based on * the float value of a field or function as exported by {@link org.apache.lucene.queries.function.ValueSource}. * <br> * * When a and b are equal, and x&gt;=0, this function has a maximum value of 1 that drops as x increases. * Increasing the value of a and b together results in a movement of the entire function to a flatter part of the curve. * <p>These properties make this an idea function for boosting more recent documents. * <p>Example:<code> recip(ms(NOW,mydatefield),3.16e-11,1,1)</code> * <p>A multiplier of 3.16e-11 changes the units from milliseconds to years (since there are about 3.16e10 milliseconds * per year). Thus, a very recent date will yield a value close to 1/(0+1) or 1, * a date a year in the past will get a multiplier of about 1/(1+1) or 1/2, * and date two years old will yield 1/(2+1) or 1/3. * * @see org.apache.lucene.queries.function.FunctionQuery * * */
public class ReciprocalFloatFunction extends ValueSource { protected final ValueSource source; protected final float m; protected final float a; protected final float b;
f(source) = a/(m*float(source)+b)
/** * f(source) = a/(m*float(source)+b) */
public ReciprocalFloatFunction(ValueSource source, float m, float a, float b) { this.source=source; this.m=m; this.a=a; this.b=b; } @Override public FunctionValues getValues(Map context, LeafReaderContext readerContext) throws IOException { final FunctionValues vals = source.getValues(context, readerContext); return new FloatDocValues(this) { @Override public float floatVal(int doc) throws IOException { return a/(m*vals.floatVal(doc) + b); } @Override public boolean exists(int doc) throws IOException { return vals.exists(doc); } @Override public String toString(int doc) throws IOException { return Float.toString(a) + "/(" + m + "*float(" + vals.toString(doc) + ')' + '+' + b + ')'; } }; } @Override public void createWeight(Map context, IndexSearcher searcher) throws IOException { source.createWeight(context, searcher); } @Override public String description() { return Float.toString(a) + "/(" + m + "*float(" + source.description() + ")" + "+" + b + ')'; } @Override public int hashCode() { int h = Float.floatToIntBits(a) + Float.floatToIntBits(m); h ^= (h << 13) | (h >>> 20); return h + (Float.floatToIntBits(b)) + source.hashCode(); } @Override public boolean equals(Object o) { if (ReciprocalFloatFunction.class != o.getClass()) return false; ReciprocalFloatFunction other = (ReciprocalFloatFunction)o; return this.m == other.m && this.a == other.a && this.b == other.b && this.source.equals(other.source); } }