Abacus/core/src/main/java/org/nwapw/abacus/lexing/Lexer.java

package org.nwapw.abacus.lexing;

import org.nwapw.abacus.lexing.pattern.nodes.EndNode;
import org.nwapw.abacus.lexing.pattern.Pattern;
import org.nwapw.abacus.lexing.pattern.nodes.PatternNode;

import java.util.*;

/**
 * A lexer that can generate tokens of a given type given a list of regular expressions
 * to operate on.
 *
 * @param <T> the type used to identify which match belongs to which pattern.
 */
public class Lexer<T> {

    /**
     * The registered patterns.
     */
    private Map<PatternEntry<T>, Pattern<T>> patterns;

    /**
     * Creates a new lexer with no registered patterns.
     */
    public Lexer() {
        patterns = new HashMap<>();
    }

    /**
     * Registers a single pattern.
     *
     * @param pattern the pattern regex
     * @param id      the ID by which to identify the pattern.
     */
    public void register(String pattern, T id) {
        Pattern<T> compiledPattern = new Pattern<>(pattern, id);
        if (compiledPattern.getHead() != null) patterns.put(new PatternEntry<>(pattern, id), compiledPattern);
    }

    /**
     * Unregisters a pattern.
     *
     * @param pattern the pattern to unregister
     * @param id      the ID by which to identify the pattern.
     */
    public void unregister(String pattern, T id) {
        patterns.remove(new PatternEntry<>(pattern, id));
    }

    /**
     * Reads one token from the given string.
     *
     * @param from    the string to read from
     * @param startAt the index to start at
     * @param compare the comparator used to sort tokens by their ID.
     * @return the best match.
     */
    public Match<T> lexOne(String from, int startAt, Comparator<T> compare) {
        ArrayList<Match<T>> matches = new ArrayList<>();
        HashSet<PatternNode<T>> currentSet = new HashSet<>();
        HashSet<PatternNode<T>> futureSet = new HashSet<>();
        int index = startAt;
        for (Pattern<T> pattern : patterns.values()) {
            pattern.getHead().addInto(currentSet);
        }
        while (!currentSet.isEmpty()) {
            for (PatternNode<T> node : currentSet) {
                if (index < from.length() && node.matches(from.charAt(index))) {
                    node.addOutputsInto(futureSet);
                } else if (node instanceof EndNode) {
                    matches.add(new Match<>(from.substring(startAt, index), ((EndNode<T>) node).getPatternId()));
                }
            }

            HashSet<PatternNode<T>> tmp = currentSet;
            currentSet = futureSet;
            futureSet = tmp;
            futureSet.clear();

            index++;
        }
        if (compare != null) {
            Collections.sort(matches, (a, b) -> compare.compare(a.getType(), b.getType()));
        }
        Collections.sort(matches, (o1, o2) -> o1.getContent().length() - o2.getContent().length());
        return matches.isEmpty() ? null : matches.get(matches.size() - 1);
    }

    /**
     * Reads all tokens from a string.
     *
     * @param from    the string to start from.
     * @param startAt the index to start at.
     * @param compare the comparator used to sort matches by their IDs.
     * @return the resulting list of matches, in order, or null on error.
     */
    public List<Match<T>> lexAll(String from, int startAt, Comparator<T> compare) {
        int index = startAt;
        ArrayList<Match<T>> matches = new ArrayList<>();
        Match<T> lastMatch = null;
        while (index < from.length() && (lastMatch = lexOne(from, index, compare)) != null) {
            int length = lastMatch.getContent().length();
            if (length == 0) return null;
            matches.add(lastMatch);
            index += length;
        }
        if (lastMatch == null) return null;
        return matches;
    }

    /**
     * An entry that represents a pattern that has been registered with the lexer.
     *
     * @param <T> the type used to identify the pattern.
     */
    private static class PatternEntry<T> {
        /**
         * The name of the entry.
         */
        public String name;
        /**
         * The id of the entry.
         */
        public T id;

        /**
         * Creates a new pattern entry with the given name and id.
         *
         * @param name the name of the pattern entry.
         * @param id   the id of the pattern entry.
         */
        public PatternEntry(String name, T id) {
            this.name = name;
            this.id = id;
        }

        @Override
        public int hashCode() {
            return Arrays.hashCode(new Object[] {
                    this.name,
                    this.id
            });
        }

        @Override
        public boolean equals(Object obj) {
            return obj instanceof PatternEntry &&
                    ((PatternEntry) obj).name.equals(name) &&
                    ((PatternEntry) obj).id.equals(id);
        }
    }

}