/*
 * Java Genetic Algorithm Library (jenetics-8.1.0).
 * Copyright (c) 2007-2024 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.ext.grammar;

import static java.lang.String.format;
import static java.util.Objects.requireNonNull;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toCollection;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Represents a <em>context-free</em> grammar
 * (<a href="https://en.wikipedia.org/wiki/Context-free_grammar"><b>CFG</b></a>).
 * <p>
 * <b>Formal definition</b>
 * <p>
 * A context-free grammar {@code G} is defined by the 4-tuple
 * {@code G = (N, T, R, S)}, where
 * <ul>
 *     <li>{@code N} is a finite set; each element {@code n ∈ N} is called a
 *     non-terminal ({@link NonTerminal}) character or a variable. Each
 *     variable represents a different type of phrase or clause in the sentence.
 *     Variables are also sometimes called syntactic categories. Each variable
 *     defines a sub-language of the language defined by {@code G}.
 *     </li>
 *     <li>{@code T} is a finite set of terminals ({@link Terminal}) disjoint
 *     from {@code N}, which make up the actual content of the sentence. The set
 *     of terminals is the alphabet of the language defined by the grammar
 *     {@code G}.
 *     </li>
 *     <li>{@code R} is a finite relation in {@code N × (N ∪ T)∗}, where the
 *     asterisk represents the <a href="https://en.wikipedia.org/wiki/Kleene_star">
 *     Kleene star</a> operation. The members of {@code R} are called the
 *     (rewrite) rules ({@link Rule}) or productions of the grammar.
 *     </li>
 *     <li>{@code S} is the start variable (or start symbol), used to represent
 *     the whole sentence (or program). It must be an element of {@code N}
 *     ({@link NonTerminal})
 *     .</li>
 * </ul>
 *
 * You can easily create a <em>Cfg</em> object from a given BNF grammar.
 * {@snippet lang="java":
 * final Cfg<String> grammar = Bnf.parse("""
 *     <expr> ::= <num> | <var> | '(' <expr> <op> <expr> ')'
 *     <op>   ::= + | - | * | /
 *     <var>  ::= x | y
 *     <num>  ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
 *     """
 * );
 * }
 *
 * It is also possible to create the grammar above programmatically.
 * {@snippet lang="java":
 * final Cfg<String> grammar = Cfg.of(
 *     R("expr",
 *         E(NT("num")),
 *         E(NT("var")),
 *         E(T("("), NT("expr"), NT("op"), NT("expr"), T(")"))
 *     ),
 *     R("op", E(T("+")), E(T("-")), E(T("*")), E(T("/"))),
 *     R("var", E(T("x")), E(T("y"))),
 *     R("num",
 *         E(T("0")), E(T("1")), E(T("2")), E(T("3")),
 *         E(T("4")), E(T("5")), E(T("6")), E(T("7")),
 *         E(T("8")), E(T("9"))
 *     )
 * );
 * }
 *
 * @see Bnf#parse(String)
 *
 * @param <T> the terminal symbol value type
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
 * @since 7.1
 * @version 7.1
 */
public record Cfg<T>(
        List<NonTerminal<T>> nonTerminals,
        List<Terminal<T>> terminals,
        List<Rule<T>> rules,
        NonTerminal<T> start
) {

        /**
         * Represents the <em>symbols</em> the BNF grammar consists.
         *
         * @param <T> the terminal symbol value type
         */
        public sealed interface Symbol<T> {

                /**
                 * Return the name of the symbol.
                 *
                 * @return the name of the symbol
                 */
                String name();

        }

        /**
         * Represents the non-terminal symbols of the grammar ({@code NT}).
         *
         * @param <T> the terminal symbol value type
         */
        public record NonTerminal<T>(String name) implements Symbol<T> {

                /**
                 * @param name the name of the non-terminal symbol
                 * @throws IllegalArgumentException if the given {@code name} is not
                 *         a valid <em>non-terminal</em> name
                 * @throws NullPointerException if one of the arguments is {@code null}
                 */
                public NonTerminal {
                        if (name.isEmpty()) {
                                throw new IllegalArgumentException(
                                        "Non-terminal value must not be empty."
                                );
                        }
                }
        }

        /**
         * Represents a terminal symbols of the grammar ({@code T}).
         *
         * @param <T> the terminal symbol value type
         */
        public record Terminal<T>(String name, T value) implements Symbol<T> {

                /**
                 * @param name the name of the terminal symbol
                 * @param value the value of the terminal symbol
                 * @throws IllegalArgumentException if the given terminal {@code name}
                 *         is empty
                 */
                public Terminal {
                        if (name.isEmpty()) {
                                throw new IllegalArgumentException(
                                        "Terminal value must not be empty."
                                );
                        }
                }

                /**
                 * Return a new terminal symbol where the name of the symbol is equal
                 * to its value.
                 *
                 * @param name the name (and value) of the terminal symbol
                 * @return a new terminal symbol with the given {@code name}
                 * @throws IllegalArgumentException if the given terminal {@code name}
                 *         is empty
                 */
                public static Terminal<String> of(final String name) {
                        return new Terminal<>(name, name);
                }

        }

        /**
         * Represents one <em>expression</em> (list of alternative symbols) a
         * production rule consists of.
         *
         * @param <T> the terminal symbol value type
         */
        public record Expression<T>(List<Symbol<T>> symbols) {

                /**
                 * @param symbols the list of symbols of the expression
                 * @throws IllegalArgumentException if the list of {@code symbols} is
                 *         empty
                 */
                public Expression {
                        if (symbols.isEmpty()) {
                                throw new IllegalArgumentException(
                                        "The list of symbols must not be empty."
                                );
                        }
                        symbols = List.copyOf(symbols);
                }

        }

        /**
         * Represents a production rule of the grammar ({@code R}).
         *
         * @param <T> the terminal symbol value type
         */
        public record Rule<T>(NonTerminal<T> start, List<Expression<T>> alternatives) {

                /**
                 * Creates a new rule object.
                 *
                 * @param start the start symbol of the rule
                 * @param alternatives the list of alternative rule expressions
                 * @throws IllegalArgumentException if the given list of
                 *         {@code alternatives} is empty
                 * @throws NullPointerException if one of the arguments is {@code null}
                 */
                public Rule {
                        requireNonNull(start);
                        if (alternatives.isEmpty()) {
                                throw new IllegalArgumentException(
                                        "Rule alternatives must not be empty."
                                );
                        }
                        alternatives = List.copyOf(alternatives);
                }

        }

        /**
         * Create a new <em>context-free</em> grammar object.
         *
         * @param nonTerminals the non-terminal symbols of {@code this} grammar
         * @param terminals the terminal symbols of {@code this} grammar
         * @param rules the <em>production</em> rules of {@code this} grammar
         * @param start the start symbol of {@code this} grammar
         * @throws NullPointerException if one of the arguments is {@code null}
         * @throws IllegalArgumentException if a rule is defined more than once, the
         *         start symbol points to a missing rule or the rules uses symbols
         *         not defined in the list of {@link #nonTerminals()} or
         *         {@link #terminals()}
         */
        public Cfg {
                if (rules.isEmpty()) {
                        throw new IllegalArgumentException(
                                "The given list of rules must not be empty."
                        );
                }

                // Check the uniqueness of the rules.
                final var duplicatedRules = rules.stream()
                        .collect(Collectors.groupingBy(Rule::start))
                        .values().stream()
                        .filter(values -> values.size() > 1)
                        .map(rule -> rule.get(0).start.name)
                        .toList();

                if (!duplicatedRules.isEmpty()) {
                        throw new IllegalArgumentException(
                                "Found duplicate rule(s), " + duplicatedRules + "."
                        );
                }

                // Check if start symbol points to an existing rule.
                final var startRule = rules.stream()
                        .filter(r -> start.equals(r.start))
                        .findFirst();
                if (startRule.isEmpty()) {
                        throw new IllegalArgumentException(
                                "No rule found for start symbol %s.".formatted(start)
                        );
                }

                // Check that all symbols used in the given rules are also defined
                // in the list of non-terminals and terminals.
                final Set<Symbol<T>> required = rules.stream()
                        .flatMap(Cfg::ruleSymbols)
                        .collect(Collectors.toUnmodifiableSet());

                final Set<Symbol<T>> available = Stream
                        .concat(nonTerminals.stream(), terminals.stream())
                        .collect(Collectors.toUnmodifiableSet());

                final var missing = new HashSet<>(required);
                missing.removeAll(available);

                if (!missing.isEmpty()) {
                        throw new IllegalArgumentException(
                                "Unknown symbols defined in rules: " + missing
                        );
                }

                // Check if the name of terminals and non-terminals are distinct.
                final var terminalNames = terminals.stream()
                        .map(Symbol::name)
                        .collect(Collectors.toSet());

                final var nonTerminalNames = nonTerminals.stream()
                        .map(Symbol::name)
                        .collect(Collectors.toSet());

                terminalNames.retainAll(nonTerminalNames);
                if (!terminalNames.isEmpty()) {
                        throw new IllegalArgumentException(format(
                                "Terminal and non-terminal symbols with same name: %s",
                                terminalNames.stream().sorted().toList()
                        ));
                }

                nonTerminals = List.copyOf(nonTerminals);
                terminals = List.copyOf(terminals);
                rules = List.copyOf(rules);
                requireNonNull(start);
        }

        /**
         * Return the rule for the given {@code start} symbol.
         *
         * @param start the start symbol of the rule
         * @return the rule for the given {@code start} symbol
         * @throws NullPointerException if the given {@code start} symbol is
         *         {@code null}
         */
        public Optional<Rule<T>> rule(final NonTerminal<?> start) {
                requireNonNull(start);
                for (var rule : rules) {
                        if (rule.start().name().equals(start.name())) {
                                return Optional.of(rule);
                        }
                }
                return Optional.empty();
        }

        /**
         * Maps the values of the terminal symbols from type {@code T} to type
         * {@code A}.
         *
         * @param mapper the mapper function
         * @param <A> the new value type of the terminal symbols
         * @return the mapped grammar
         * @throws NullPointerException if the given mapper is {@code null}
         */
        public <A> Cfg<A> map(final Function<? super Terminal<T>, ? extends A> mapper) {
                requireNonNull(mapper);

                final var cache = new HashMap<Terminal<T>, Terminal<A>>();
                final Function<Terminal<T>, Terminal<A>> mapping = t -> cache
                        .computeIfAbsent(t, t2 -> new Terminal<>(t2.name(), mapper.apply(t2)));

                @SuppressWarnings("unchecked")
                final List<Rule<A>> rules = rules().stream()
                        .map(rule -> new Rule<>(
                                (NonTerminal<A>)rule.start(),
                                rule.alternatives().stream()
                                        .map(expr -> new Expression<>(
                                                expr.symbols().stream()
                                                        .map(sym -> sym instanceof Cfg.Terminal<T> t
                                                                ? mapping.apply(t) : (Symbol<A>)sym)
                                                        .toList()
                                                ))
                                        .toList()
                                ))
                        .toList();

                return Cfg.of(rules);
        }

        /**
         * Create a grammar object with the given rules. Duplicated rules are merged
         * into one rule. The <em>start</em> symbol of the first rule is chosen as
         * the start symbol of the created CFG
         *
         * @param rules the rules the grammar consists of
         * @throws IllegalArgumentException if the list of rules is empty
         * @throws NullPointerException if the list of rules is {@code null}
         */
        public static <T> Cfg<T> of(final List<Rule<T>> rules) {
                if (rules.isEmpty()) {
                        throw new IllegalArgumentException(
                                "The list of rules must not be empty."
                        );
                }

                final List<Rule<T>> normalizedRules = normalize(rules);

                final List<Symbol<T>> symbols = normalizedRules.stream()
                        .flatMap(Cfg::ruleSymbols)
                        .distinct()
                        .toList();

                final List<NonTerminal<T>> nonTerminals = symbols.stream()
                        .filter(NonTerminal.class::isInstance)
                        .map(nt -> (NonTerminal<T>)nt)
                        .toList();

                final List<Terminal<T>> terminals = symbols.stream()
                        .filter(Terminal.class::isInstance)
                        .map(nt -> (Terminal<T>)nt)
                        .toList();

                return new Cfg<>(
                        nonTerminals,
                        terminals,
                        normalizedRules.stream()
                                .map(r -> rebuild(r, symbols))
                                .toList(),
                        (NonTerminal<T>)select(normalizedRules.get(0).start(), symbols)
                );
        }

        /**
         * Create a grammar object with the given rules. Duplicated rules are merged
         * into one rule. The <em>start</em> symbol of the first rule is chosen as
         * the start symbol of the created CFG
         *
         * @param rules the rules the grammar consists of
         * @throws IllegalArgumentException if the list of rules is empty
         * @throws NullPointerException if the list of rules is {@code null}
         */
        @SafeVarargs
        public static <T> Cfg<T> of(final Rule<T>... rules) {
                return Cfg.of(List.of(rules));
        }

        private static <T> List<Rule<T>> normalize(final List<Rule<T>> rules) {
                final Map<NonTerminal<T>, List<Rule<T>>> grouped = rules.stream()
                        .collect(groupingBy(
                                Rule::start,
                                LinkedHashMap::new,
                                toCollection(ArrayList::new)));

                return grouped.entrySet().stream()
                        .map(entry -> merge(entry.getKey(), entry.getValue()))
                        .toList();
        }

        private static <T> Rule<T> merge(final NonTerminal<T> start, final List<Rule<T>> rules) {
                return new Rule<>(
                        start,
                        rules.stream()
                                .flatMap(rule -> rule.alternatives().stream())
                                .toList()
                );
        }

        private static <T> Stream<Symbol<T>> ruleSymbols(final Rule<T> rule) {
                return Stream.concat(
                        Stream.of(rule.start),
                        rule.alternatives.stream()
                                .flatMap(expr -> expr.symbols().stream())
                );
        }

        private static <T> Rule<T> rebuild(final Rule<T> rule, final List<Symbol<T>> symbols) {
                return new Rule<>(
                        (NonTerminal<T>)select(rule.start, symbols),
                        rule.alternatives.stream()
                                .map(e -> rebuild(e, symbols))
                                .toList()
                );
        }

        private static <T> Expression<T>
        rebuild(final Expression<T> expression, final List<Symbol<T>> symbols) {
                return new Expression<>(
                        expression.symbols.stream()
                                .map(s -> select(s, symbols))
                                .toList()
                );
        }

        private static <T> Symbol<T> select(
                final Symbol<T> symbol,
                final List<Symbol<T>> symbols
        ) {
                for (var s : symbols) {
                        if (s.name().equals(symbol.name())) {
                                return s;
                        }
                }
                throw new AssertionError("Symbol not found: " + symbol);
        }

        @SuppressWarnings("unchecked")
        static <A, B extends A> Cfg<A> upcast(final Cfg<B> seq) {
                return (Cfg<A>)seq;
        }


        /* *************************************************************************
         * Static factory methods for rule creation.
         * ************************************************************************/

        /**
         * Factory method for creating a terminal symbol with the given
         * {@code name} and {@code value}.
         *
         * @param name the name of the terminal symbol
         * @param value the value of the terminal symbol
         * @param <T> the terminal symbol value type
         * @return a new terminal symbol
         */
        public static <T> Terminal<T> T(final String name, final T value) {
                return new Terminal<>(name, value);
        }

        /**
         * Factory method for creating a terminal symbol with the given
         * {@code name}.
         *
         * @param name the name of the terminal symbol
         * @return a new terminal symbol
         */
        public static Terminal<String> T(final String name) {
                return new Terminal<>(name, name);
        }

        /**
         * Factory method for creating non-terminal symbols.
         *
         * @param name the name of the symbol.
         * @param <T> the terminal symbol value type
         * @return a new non-terminal symbol
         */
        public static <T> NonTerminal<T> N(final String name) {
                return new NonTerminal<>(name);
        }

        /**
         * Factory method for creating an expression with the given
         * {@code symbols}.
         *
         * @param symbols the list of symbols of the expression
         * @throws IllegalArgumentException if the list of {@code symbols} is
         *         empty
         * @param <T> the terminal symbol value type
         * @return a new expression
         */
        @SafeVarargs
        public static <T> Expression<T> E(final Symbol<T>... symbols) {
                return new Expression<>(List.of(symbols));
        }

        /**
         * Factory method for creating a new rule.
         *
         * @param name the name of start symbol of the rule
         * @param alternatives the list af alternative rule expressions
         * @throws IllegalArgumentException if the given list of
         *         {@code alternatives} is empty
         * @throws NullPointerException if one of the arguments is {@code null}
         * @param <T> the terminal symbol value type
         * @return a new rule
         */
        @SafeVarargs
        public static <T> Rule<T> R(
                final String name,
                final Expression<T>... alternatives
        ) {
                return new Rule<>(new NonTerminal<>(name), List.of(alternatives));
        }

}