/*
 * Java Genetic Algorithm Library (jenetics-7.2.0).
 * Copyright (c) 2007-2023 Franz Wilhelmstötter
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Author:
 *    Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
 */
package io.jenetics.prog.op;

import static java.lang.String.format;
import static io.jenetics.ext.internal.util.Names.isIdentifier;

import java.io.Serial;
import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.SortedSet;
import java.util.TreeSet;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.Collectors;

import io.jenetics.ext.util.Tree;
import io.jenetics.ext.util.TreeNode;

/**
 * Represents the program variables. The {@code Var} operation is a
 * <em>terminal</em> operation, which just returns the value with the defined
 * index of the input variable array. It is essentially an orthogonal projection
 * of the <em>n</em>-dimensional input space to the <em>1</em>-dimensional
 * result space.
 *
 * <pre>{@code
 * final ISeq<? extends Op<Double>> operations = ISeq.of(...);
 * final ISeq<? extends Op<Double>> terminals = ISeq.of(
 *     Var.of("x", 0), Var.of("y", 1)
 * );
 * }</pre>
 *
 * The example above shows how to define the terminal operations for a GP, which
 * tries to optimize a 2-dimensional function.
 *
 * <pre>{@code
 * static double error(final ProgramChromosome<Double> program) {
 *     final double x = ...;
 *     final double y = ...;
 *     final double result = program.eval(x, y);
 *     ...
 *     return ...;
 * }
 * }</pre>
 *
 * @implNote
 * The {@code Var} object is comparable, according its name.
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
 * @version 7.0
 * @since 3.9
 */
public final class Var<T> implements Op<T>, Comparable<Var<T>>, Serializable {

        @Serial
        private static final long serialVersionUID = 1L;

        private final String _name;
        private final int _index;

        /**
         * Create a new variable with the given {@code name} and projection
         * {@code index}.
         *
         * @param name the variable name. Used when printing the operation tree
         *        (program)
         * @param index the projection index
         * @throws IllegalArgumentException if the given {@code name} is not a valid
         *         Java identifier
         * @throws IndexOutOfBoundsException if the projection {@code index} is
         *         smaller than zero
         * @throws NullPointerException if the given variable {@code name} is
         *         {@code null}
         */
        private Var(final String name, final int index) {
                if (!isIdentifier(name)) {
                        throw new IllegalArgumentException(format(
                                "'%s' is not a valid identifier.", name
                        ));
                }
                if (index < 0) {
                        throw new IndexOutOfBoundsException(
                                "Index smaller than zero: " + index
                        );
                }

                _name = name;
                _index = index;
        }

        /**
         * The projection index of the variable.
         *
         * @return the projection index of the variable
         */
        public int index() {
                return _index;
        }

        @Override
        public String name() {
                return _name;
        }

        @Override
        public int arity() {
                return 0;
        }

        @Override
        public T apply(final T[] variables) {
                if (_index >= variables.length) {
                        throw new IllegalArgumentException(format(
                                "No value for variable '%s' given.", this
                        ));
                }
                return variables[_index];
        }

        @Override
        public int compareTo(final Var<T> o) {
                return _name.compareTo(o._name);
        }

        @Override
        public int hashCode() {
                return Objects.hashCode(_name);
        }

        @Override
        public boolean equals(final Object obj) {
                return obj == this ||
                        obj instanceof Var<?> other &&
                        Objects.equals(other._name, _name);
        }

        @Override
        public String toString() {
                return _name;
        }

        /**
         * Create a new variable with the given {@code name} and projection
         * {@code index}.
         *
         * @see #parse(String)
         *
         * @param name the variable name. Used when printing the operation tree
         *        (program)
         * @param index the projection index
         * @param <T> the variable type
         * @return a new variable with the given {@code name} and projection
         *         {@code index}
         * @throws IllegalArgumentException if the given {@code name} is not a valid
         *         Java identifier
         * @throws IndexOutOfBoundsException if the projection {@code index} is
         *         smaller than zero
         * @throws NullPointerException if the given variable {@code name} is
         *         {@code null}
         */
        public static <T> Var<T> of(final String name, final int index) {
                return new Var<>(name, index);
        }

        /**
         * Create a new variable with the given {@code name}. The projection index
         * is set to zero. Always prefer to create new variables with
         * {@link #of(String, int)}, especially when you define your terminal
         * operation for your GP problem.
         *
         * @see #parse(String)
         *
         * @param name the variable name. Used when printing the operation tree
         *        (program)
         * @param <T> the variable type
         * @return a new variable with the given {@code name} and projection index
         *         zero
         * @throws IllegalArgumentException if the given {@code name} is not a valid
         *         Java identifier
         * @throws NullPointerException if the given variable {@code name} is
         *         {@code null}
         */
        public static <T> Var<T> of(final String name) {
                return new Var<>(name, 0);
        }

        private static final Pattern VAR_INDEX = Pattern.compile("(.+)\\[\\s*(\\d+)\\s*]");

        /**
         * Parses the given variable string to its name and index. The expected
         * format is <em>var_name</em>[<em>index</em>].
         *
         * <pre> {@code
         * x[0]
         * y[3]
         * my_var[4]
         * }</pre>
         *
         * If no variable <em>index</em> is encoded in the name, a variable with
         * index 0 is created.
         *
         * @see #of(String, int)
         *
         * @param name the variable name + index
         * @param <T> the operation type
         * @return a new variable parsed from the input string
         * @throws IllegalArgumentException if the given variable couldn't be parsed
         *         and the given {@code name} is not a valid Java identifier
         * @throws NullPointerException if the given variable {@code name} is
         *         {@code null}
         */
        public static <T> Var<T> parse(final String name) {
                final Matcher matcher = VAR_INDEX.matcher(name);

                return matcher.matches()
                        ? of(matcher.group(1), Integer.parseInt(matcher.group(2)))
                        : of(name, 0);
        }

        /**
         * Re-indexes the variables of the given operation {@code tree}. If the
         * operation tree is created from its string representation, the indices
         * of the variables ({@link Var}), are all set to zero, since it needs the
         * whole tree for setting the indices correctly. The mapping from the node
         * string to the {@link Op} object, on the other hand, is a <em>local</em>
         * operation. This method gives you the possibility to fix the indices of
         * the variables. The indices of the variables are assigned according it's
         * <em>natural</em> order.
         *
         * <pre>{@code
         * final TreeNode<Op<Double>> tree = TreeNode.parse(
         *     "add(mul(x,y),sub(y,x))",
         *     MathOp::toMathOp
         * );
         *
         * assert Program.eval(tree, 10.0, 5.0) == 100.0; // wrong result
         * Var.reindex(tree);
         * assert Program.eval(tree, 10.0, 5.0) == 45.0; // correct result
         * }</pre>
         * The example above shows a use-case of this method. If you parse a tree
         * string and convert it to an operation tree, you have to re-index the
         * variables first. If not, you will get the wrong result when evaluating
         * the tree. After the re-indexing, you will get the correct result of 45.0.
         *
         * @since 5.0
         *
         * @see MathOp#toMathOp(String)
         * @see Program#eval(Tree, Object[])
         *
         * @param tree the tree where the variable indices need to be fixed
         * @param <V> the operation value type
         */
        public static <V> void reindex(final TreeNode<Op<V>> tree) {
                final SortedSet<Var<V>> vars = tree.stream()
                        .filter(node -> node.value() instanceof Var)
                        .map(node -> (Var<V>)node.value())
                        .collect(Collectors.toCollection(TreeSet::new));

                int index = 0;
                final Map<Var<V>, Integer> indexes = new HashMap<>();
                for (Var<V> var : vars) {
                        indexes.put(var, index++);
                }

                reindex(tree, indexes);
        }

        /**
         * Re-indexes the variables of the given operation {@code tree}. If the
         * operation tree is created from its string representation, the indices
         * of the variables ({@link Var}), are all set to zero, since it needs the
         * whole tree for setting the indices correctly.
         *
         * <pre>{@code
         * final TreeNode<Op<Double>> tree = TreeNode.parse(
         *     "add(mul(x,y),sub(y,x))",
         *     MathOp::toMathOp
         * );
         *
         * assert Program.eval(tree, 10.0, 5.0) == 100.0; // wrong result
         * final Map<Var<Double>, Integer> indexes = new HashMap<>();
         * indexes.put(Var.of("x"), 0);
         * indexes.put(Var.of("y"), 1);
         * Var.reindex(tree, indexes);
         * assert Program.eval(tree, 10.0, 5.0) == 45.0; // correct result
         * }</pre>
         * The example above shows a use-case of this method. If you parse a tree
         * string and convert it to an operation tree, you have to re-index the
         * variables first. If not, you will get the wrong result when evaluating
         * the tree. After the re-indexing, you will get the correct result of 45.0.
         *
         * @since 5.0
         *
         * @see MathOp#toMathOp(String)
         * @see BoolOp#toBoolOp(String)
         * @see Program#eval(Tree, Object[])
         *
         * @param tree the tree where the variable indices need to be fixed
         * @param indexes the variable to index mapping
         * @param <V> the operation value type
         */
        public static <V> void reindex(
                final TreeNode<Op<V>> tree,
                final Map<Var<V>, Integer> indexes
        ) {
                for (TreeNode<Op<V>> node : tree) {
                        final Op<V> op = node.value();
                        if (op instanceof Var) {
                                node.value(Var.of(op.name(), indexes.get(op)));
                        }
                }
        }

}