/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */

package org.elasticsearch.search.suggest.phrase;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Terms;
import org.apache.lucene.util.BytesRef;
import org.elasticsearch.common.ParseField;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.common.xcontent.XContentBuilder;
import org.elasticsearch.common.xcontent.XContentParser;
import org.elasticsearch.common.xcontent.XContentParser.Token;
import org.elasticsearch.search.suggest.phrase.WordScorer.WordScorerFactory;

import java.io.IOException;
import java.util.Objects;

/**
 * An <a href="http://en.wikipedia.org/wiki/Additive_smoothing">additive
 * smoothing</a> model.
 * <p>
 * See <a
 * href="http://en.wikipedia.org/wiki/N-gram#Smoothing_techniques">N-Gram
 * Smoothing</a> for details.
 * </p>
 */
public final class Laplace extends SmoothingModel {
    public static final String NAME = "laplace";
    private static final ParseField ALPHA_FIELD = new ParseField("alpha");
    static final ParseField PARSE_FIELD = new ParseField(NAME);
    /**
     * Default alpha parameter for laplace smoothing
     */
    public static final double DEFAULT_LAPLACE_ALPHA = 0.5;

    private double alpha = DEFAULT_LAPLACE_ALPHA;

    /**
     * Creates a Laplace smoothing model.
     *
     */
    public Laplace(double alpha) {
        this.alpha = alpha;
    }

    /**
     * Read from a stream.
     */
    public Laplace(StreamInput in) throws IOException {
        alpha = in.readDouble();
    }

    @Override
    public void writeTo(StreamOutput out) throws IOException {
        out.writeDouble(alpha);
    }

    /**
     * @return the laplace model alpha parameter
     */
    public double getAlpha() {
        return this.alpha;
    }

    @Override
    protected XContentBuilder innerToXContent(XContentBuilder builder, Params params) throws IOException {
        builder.field(ALPHA_FIELD.getPreferredName(), alpha);
        return builder;
    }

    @Override
    public String getWriteableName() {
        return NAME;
    }

    @Override
    protected boolean doEquals(SmoothingModel other) {
        Laplace otherModel = (Laplace) other;
        return Objects.equals(alpha, otherModel.alpha);
    }

    @Override
    protected int doHashCode() {
        return Objects.hash(alpha);
    }

    public static SmoothingModel fromXContent(XContentParser parser) throws IOException {
        XContentParser.Token token;
        String fieldName = null;
        double alpha = DEFAULT_LAPLACE_ALPHA;
        while ((token = parser.nextToken()) != Token.END_OBJECT) {
            if (token == XContentParser.Token.FIELD_NAME) {
                fieldName = parser.currentName();
            }
            if (token.isValue() && ALPHA_FIELD.match(fieldName, parser.getDeprecationHandler())) {
                alpha = parser.doubleValue();
            }
        }
        return new Laplace(alpha);
    }

    @Override
    public WordScorerFactory buildWordScorerFactory() {
        return (IndexReader reader, Terms terms, String field, double realWordLikelihood, BytesRef separator)
                -> new LaplaceScorer(reader, terms,  field, realWordLikelihood, separator, alpha);
    }
}
