/* Copyright (C) 2013 TU Dortmund
 * This file is part of AutomataLib, http://www.automatalib.net/.
 * 
 * AutomataLib is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License version 3.0 as published by the Free Software Foundation.
 * 
 * AutomataLib is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public
 * License along with AutomataLib; if not, see
 * http://www.gnu.de/documents/lgpl.en.html.
 */
package net.automatalib.words;

import java.io.IOException;
import java.util.AbstractList;
import java.util.Collections;
import java.util.List;
import java.util.Objects;

import net.automatalib.commons.util.array.ArrayWritable;
import net.automatalib.commons.util.strings.AbstractPrintable;
import net.automatalib.commons.util.strings.StringUtil;

/**
 * A word is an ordered sequence of symbols. {@link Word}s are generally immutable,
 * i.e., a single {@link Word} object will never change (unless symbol objects are modified,
 * which is however highly discouraged).
 * <p>
 * This class provides the following static methods for creating words in the most common scenarios:
 * <ul>
 * <li> {@link #epsilon()} returns the empty word of length 0
 * <li> {@link #fromLetter(Object)} turns a single letter into a word of length 1
 * <li> {@link #fromSymbols(Object...)} creates a word from an array of symbols
 * <li> {@link #fromArray(Object[], int, int)} creates a word from a subrange of a symbols array
 * <li> {@link #fromList(List)} creates a word from a {@link List} of symbols
 * </ul>
 * <p>
 * Modification operations like {@link #append(Object)} or {@link #concat(Word...)} create
 * new objects, subsequently invoking these operations on the respective objects returned is
 * therefore highly inefficient. If words need to be dynamically created, a {@link WordBuilder}
 * should be used.
 * <p>
 * This is an abstract base class for word representations. Implementing classes only need to implement
 * <ul>
 * <li> {@link #getSymbol(int)}
 * <li> {@link #length()}
 * </ul>
 * <p>
 * However, for the sake of efficiency it is highly encouraged to overwrite the other methods
 * as well, providing specialized realizations.
 *
 * @param <I> symbol class
 * 
 * @author Malte Isberner <malte.isberner@gmail.com>
 */
public abstract class Word<I> extends AbstractPrintable implements ArrayWritable<I>, Iterable<I> {
        
        
        /**
         * Retrieves the empty word.
         * @return the empty word.
         * @see Collections#emptyList()
         */
        @SuppressWarnings("unchecked")
        public static <I> Word<I> epsilon() {
                return (Word<I>)(Word<?>)EmptyWord.INSTANCE;
        }
        
        /**
         * Constructs a word from a single letter.
         * @param letter the letter
         * @return a word consisting of only this letter
         */
        public static <I> Word<I> fromLetter(I letter) {
                return new LetterWord<>(letter);
        }
        
        /**
         * Creates a word from an array of symbols.
         * @param symbols the symbol array
         * @return a word containing the symbols in the specified array
         */
        @SafeVarargs
        public static <I> Word<I> fromSymbols(I ...symbols) {
                if(symbols.length == 0)
                        return epsilon();
                if(symbols.length == 1)
                        return fromLetter(symbols[0]);
                Object[] array = new Object[symbols.length];
                System.arraycopy(symbols, 0, array, 0, symbols.length);
                return new SharedWord<>(symbols);
        }
        
        /**
         * Creates a word from a subrange of an array of symbols. Note that to
         * ensure immutability, internally a copy of the array is made.
         * @param symbols the symbols array
         * @param offset the starting index in the array
         * @param length the length of the resulting word (from the starting index on)
         * @return the word consisting of the symbols in the range
         */
        public static <I> Word<I> fromArray(I[] symbols, int offset, int length) {
                if(length == 0)
                        return epsilon();
                if(length == 1)
                        return fromLetter(symbols[offset]);
                Object[] array = new Object[length];
                System.arraycopy(symbols, offset, array, 0, length);
                return new SharedWord<>(symbols);
        }
        
        /**
         * Creates a word from a list of symbols
         * @param symbolList the list of symbols
         * @return the resulting word
         */
        public static <I> Word<I> fromList(List<? extends I> symbolList) {
                int siz = symbolList.size();
                if(siz == 0)
                        return epsilon();
                if(siz == 1)
                        return Word.<I>fromLetter(symbolList.get(0));
                return new SharedWord<>(symbolList);
        }
        
        public static Word<Character> fromString(String str) {
                int len = str.length();
                Character[] chars = new Character[str.length()];
                for(int i = 0; i < len; i++)
                        chars[i] = Character.valueOf(str.charAt(i));
                return new SharedWord<>(chars);
        }
        
        /*
         * General word iterator
         */
        private class Iterator implements java.util.Iterator<I> {

                private int index = 0;
                
                /*
                 * (non-Javadoc)
                 * @see java.util.Iterator#hasNext()
                 */
                @Override
                public boolean hasNext() {
                        return (index < Word.this.length());
                }

                /*
                 * (non-Javadoc)
                 * @see java.util.Iterator#next()
                 */
                @Override
                public I next() {
                        return Word.this.getSymbol(index++);
                }
                
                /*
                 * (non-Javadoc)
                 * @see java.util.Iterator#remove()
                 */
                @Override
                public void remove() {
                        throw new UnsupportedOperationException();
                }
        }
        
        /*
         * Representing a word as a list.
         */
        private class AsList extends AbstractList<I> {
                
                /*
                 * (non-Javadoc)
                 * @see java.util.AbstractList#get(int)
                 */
                @Override
                public I get(int index) {
                        return Word.this.getSymbol(index);
                }

                /*
                 * (non-Javadoc)
                 * @see java.util.AbstractCollection#size()
                 */
                @Override
                public int size() {
                        return Word.this.length();
                }
                
                /*
                 * (non-Javadoc)
                 * @see java.util.AbstractList#iterator()
                 */
                @Override
                public java.util.Iterator<I> iterator() {
                        return Word.this.iterator();
                }
        }

    /**
     * Return symbol that is at the specified position
     * @param i the position
     * @return symbol at position i, <tt>null</tt> if no such symbol exists
     */
    public abstract I getSymbol(int index);
    
    /**
     * Retrieves the length of this word.
     * @return the length of this word.
     */
    public abstract int length();
    
    /*
     * (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        int hash = 5;
        int len = length();
        for(int i = 0; i < len; i++) {
                hash *= 89;
                I sym = getSymbol(i);
                hash += (sym != null) ? sym.hashCode() : 0;
        }
        return hash;
    }
    
    /*
     * (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(Object other) {
        if(this == other)
                return true;
        if(other == null)
                return false;
        if(!(other instanceof Word))
                return false;
        Word<?> otherWord = (Word<?>)other;
        int len = otherWord.length();
        if(len != length())
                return false;
        for(int i = 0; i < len; i++) {
                if(!Objects.equals(getSymbol(i), otherWord.getSymbol(i)))
                        return false;
        }
        return true;
    }
    
    /*
     * (non-Javadoc)
     * @see net.automatalib.commons.util.strings.Printable#print(java.lang.Appendable)
     */
    @Override
        public void print(Appendable a) throws IOException {
                StringUtil.appendIterable(a, this, " ");
        }

        /*
     * (non-Javadoc)
     * @see net.automatalib.commons.util.array.ArrayWritable#size()
     */
    @Override
    public final int size() {
        return length();
    }
    
    
    
    /**
     * Retrieves a word representing the specified subrange of this word.
     * As words are immutable, this function usually can be realized quite efficient
     * (implementing classes should take care of this).
     * 
     * @param fromIndex the first index, inclusive.
     * @param toIndex the last index, exclusive.
     * @return the word representing the specified subrange.
     */
    public final Word<I> subWord(int fromIndex, int toIndex) {
        if(fromIndex < 0 || toIndex < fromIndex || toIndex > length())
                throw new IndexOutOfBoundsException("Invalid subword range [" + fromIndex + ", " + toIndex + ")");
        
        return _subWord(fromIndex, toIndex);
    }
   
    /**
     * Retrieves the subword of this word starting at the given index and extending
     * until the end of this word. Calling this method is equivalent to calling
     * <pre>w.subWord(fromIndex, w.length())</pre>
     * @param fromIndex the first index, inclusive
     * @return the word representing the specified subrange
     */
    public final Word<I> subWord(int fromIndex) {
        if(fromIndex <= 0) {
                if(fromIndex == 0)
                        return this;
                throw new IndexOutOfBoundsException("Invalid subword range [" + fromIndex + ",)");
        }
        return _subWord(fromIndex, length());
    }
    
    /**
     * Internal subword operation implementation. In contrast to {@link #subWord(int, int)},
     * no range checks need to be performed. As this method is flagged as <tt>protected</tt>,
     * implementations may rely on the specified indices being valid.
     * @param fromIndex the first index, inclusive (guaranteed to be valid)
     * @param toIndex the last index, exclusive (guaranteed to be valid)
     * @return the word representing the specified subrange
     */
    protected Word<I> _subWord(int fromIndex, int toIndex) {
        int len = toIndex - fromIndex;
        Object[] array = new Object[len];
        writeToArray(fromIndex, array, 0, len);
        return new SharedWord<>(array);
    }
    
    /*
     * (non-Javadoc)
     * @see java.lang.Iterable#iterator()
     */
    @Override
        public java.util.Iterator<I> iterator() {
                return new Iterator();
        }

    /*
     * (non-Javadoc)
     * @see net.automatalib.commons.util.array.ArrayWritable#writeToArray(int, java.lang.Object[], int, int)
     */
    @Override
        public void writeToArray(int offset, Object[] array, int tgtOffset, int length) {
        int idx = offset, arrayIdx = tgtOffset;
        
        while(length-- > 0)
                array[arrayIdx++] = getSymbol(idx++);
    }
    
    
    /**
     * Retrieves a {@link List} view on the contents of this word.
     * @return an unmodifiable list of the contained symbols.
     */
    public List<I> asList() {
        return new AsList();
    }
    
    /**
         * Retrieves a prefix of the given length. If <code>length</code>
         * is negative, then a prefix consisting of all but the last
         * <code>-length</code> symbols is returned.
         * 
         * @param prefixLen the length of the prefix (may be negative, see above).
         * @return the prefix of the given length.
         */
        public final Word<I> prefix(int prefixLen) {
                if(prefixLen < 0)
                        prefixLen = length() + prefixLen;
                
                return subWord(0, prefixLen);
        }
        
        /**
         * Retrieves a suffix of the given length. If <code>length</code> is
         * negative, then a suffix consisting of all but the first
         * <code>-length</code> symbols is returned.
         * 
         * @param suffixLen the length of the suffix (may be negative, see above).
         * @return the suffix of the given length.
         */
        public final Word<I> suffix(int suffixLen) {
                int wordLen = length();
                int startIdx = (suffixLen < 0) ? -suffixLen : (wordLen - suffixLen);
                
                return subWord(startIdx, wordLen);
        }
        
        /**
         * Retrieves the list of all prefixes of this word. In the default implementation,
         * the prefixes are lazily instantiated upon the respective calls of {@link List#get(int)}
         * or {@link Iterator#next()}.
         * @param longestFirst whether to start with the longest prefix (otherwise, the first prefix
         * in the list will be the shortest).
         * @return a (non-materialized) list containing all prefixes 
         */
        public List<Word<I>> prefixes(boolean longestFirst) {
                return new SubwordList<>(this, true, longestFirst);
        }
        
        /**
         * Retrieves the list of all suffixes of this word. In the default implementation,
         * the suffixes are lazily instantiated upon the respective calls of {@link List#get(int)}
         * or {@link Iterator#next()}.
         * @param longestFirst whether to start with the longest suffix (otherwise, the first suffix
         * in the list will be the shortest).
         * @return a (non-materialized) list containing all suffix 
         */
        public List<Word<I>> suffixes(boolean longestFirst) {
                return new SubwordList<>(this, false, longestFirst);
        }
        
        
        /**
         * Retrieves the next word after this in canonical order. Figuratively
         * speaking, if there are <tt>k</tt> alphabet symbols, one can think of
         * a word of length <tt>n</tt> as an <tt>n</tt>-digit radix-<tt>k</tt>
         * representation of the number. The next word in canonical order
         * is the representation for the number represented by this word plus one.
         * @param sigma the alphabet
         * @return the next word in canonical order
         */
        public Word<I> canonicalNext(Alphabet<I> sigma) {
                int len = length();
                Object[] symbols = new Object[len];
                writeToArray(0, symbols, 0, len);
                
                int alphabetSize = sigma.size();
                
                int i = 0;
                boolean overflow = true;
                for(I sym : this) {
                        int nextIdx = (sigma.getSymbolIndex(sym) + 1) % alphabetSize; 
                        symbols[i++] = sigma.getSymbol(nextIdx);
                        if(nextIdx != 0) {
                                overflow = false;
                                break;
                        }
                }
                
                while(i < len) {
                        symbols[i] = getSymbol(i);
                        i++;
                }
                
                if(overflow) {
                        Object[] newSymbols = new Object[len+1];
                        newSymbols[0] = sigma.getSymbol(0);
                        System.arraycopy(symbols, 0, newSymbols, 1, len);
                        symbols = newSymbols;
                }
                
                return new SharedWord<>(symbols);
        }
        
        /**
         * Retrieves the last symbol of this word.
         * @return the last symbol of this word.
         */
        public I lastSymbol() {
                return getSymbol(length() - 1);
        }
        
        /**
         * Retrieves the first symbol of this word.
         * @return the first symbol of this word
         */
        public I firstSymbol() {
                return getSymbol(0);
        }
        
        /**
         * Appends a symbol to this word and returns the result as a new word.
         * @param symbol the symbol to append
         * @return the word plus the given symbol
         */
        public Word<I> append(I symbol) {
                int len = length();
                Object[] array = new Object[len + 1];
                writeToArray(0, array, 0, len);
                array[len] = symbol;
                return new SharedWord<>(array);
        }
        
        /**
         * Prepends a symbol to this word and returns the result as a new word.
         * @param symbol the symbol to prepend
         * @return the given symbol plus to word.
         */
        public Word<I> prepend(I symbol) {
                int len = length();
                Object[] array = new Object[len+1];
                array[0] = symbol;
                writeToArray(0, array, 1, len);
                
                return new SharedWord<>(array);
        }
        
        /**
         * Concatenates this word with several other words and returns the result
         * as a new word.
         * 
         * Note that this method cannot be overridden. Implementing classes need to
         * override the {@link #_concat(Word...)} method instead.
         *  
         * @param words the words to concatenate with this word
         * @return the result of the concatenation
         * @see #_concat(Word...)
         */
        @SafeVarargs
        public final Word<I> concat(Word<I> ...words) {
                return _concat(words);
        }
        
        /**
         * Realizes the concatenation of this word with several other words.
         * @param words the words to concatenate
         * @return the results of the concatenation
         */
        @SuppressWarnings("unchecked")
        protected Word<I> _concat(Word<I> ...words) {
                if(words.length == 0)
                        return this;
                
                int len = length();
                
                int totalSize = len;
                for(int i = 0; i < words.length; i++)
                        totalSize += words[i].length();
                
                Object[] array = new Object[totalSize];
                writeToArray(0, array, 0, len);
                int currOfs = len;
                for(int i = 0; i < words.length; i++) {
                        Word<I> w = words[i];
                        int wLen = w.length();
                        w.writeToArray(0, array, currOfs, wLen);
                        currOfs += wLen;
                }

                return new SharedWord<>(array);
        }
        
        /**
         * Checks if this word is a prefix of another word.
         * @param other the other word
         * @return <tt>true</tt> if this word is a prefix of the other word, <tt>false</tt>
         * otherwise.
         */
        public boolean isPrefixOf(Word<I> other) {
                int len = length(), otherLen = other.length();
                if(otherLen < len)
                        return false;
                
                for(int i = 0; i < len; i++) {
                        I sym1 = getSymbol(i), sym2 = other.getSymbol(i);
                        
                        if(!Objects.equals(sym1,  sym2))
                                return false;
                }
                return true;
        }
        
        /**
         * Determines the longest common prefix of this word and another
         * word.
         * @param other the other word
         * @return the longest common prefix of this word and the other word
         */
        public Word<I> longestCommonPrefix(Word<I> other) {
                int len = length(), otherLen = other.length();
                int maxIdx = (len < otherLen) ? len : otherLen;
                
                int i = 0;
                while(i < maxIdx) {
                        I sym1 = getSymbol(i), sym2 = getSymbol(i);
                        
                        if(!Objects.equals(sym1, sym2))
                                break;
                        i++;
                }
                
                return prefix(i);
        }
        
        /**
         * Checks if this word is a suffix of another word
         * @param other the other word
         * @return <tt>true</tt> if this word is a suffix of the other word, <tt>false</tt>
         * otherwise. 
         */
        public boolean isSuffixOf(Word<I> other) {
                int len = length(), otherLen = other.length();
                if(otherLen < len)
                        return false;
                
                int ofs = otherLen - len;
                for(int i = 0; i < len; i++) {
                        I sym1 = getSymbol(i), sym2 = other.getSymbol(ofs + i);
                        if(!Objects.equals(sym1, sym2))
                                return false;
                }
                return true;
        }
        
        /**
         * Determines the longest common suffix of this word and another word.
         * @param other the other word
         * @return the longest common suffix
         */
        public Word<I> longestCommonSuffix(Word<I> other) {
                int len = length(), otherLen = other.length();
                int minLen = (len < otherLen) ? len : otherLen;
                
                int idx1 = len, idx2 = otherLen;
                int i = 0;
                while(i < minLen) {
                        I sym1 = getSymbol(--idx1), sym2 = other.getSymbol(--idx2);
                        
                        if(!Objects.equals(sym1, sym2))
                                break;
                        
                        i++;
                }
                
                return suffix(i);
        }
        
        /**
         * Retrieves a "flattened" version of this word, i.e., without any hierarchical structure
         * attached.
         * This can be helpful if {@link Word} is subclassed to allow representing, e.g., a concatenation
         * dynamically, but due to performance concerns not too many levels of indirection
         * should be introduced.
         * @return a flattened version of this word.
         */
        public Word<I> flatten() {
                int len = length();
                Object[] array = new Object[len];
                writeToArray(0, array, 0, len);
                return new SharedWord<>(array);
        }
        
        public Word<I> trimmed() {
                int len = length();
                Object[] array = new Object[len];
                writeToArray(0, array, 0, len);
                return new SharedWord<>(array);
        }

        /**
         * Checks if this word is empty, i.e., contains no symbols.
         * @return <tt>true</tt> if this word is empty, <tt>false</tt> otherwise.
         */
        public boolean isEmpty() {
                return (length() == 0);
        }
}