//
// ========================================================================
// Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under
// the terms of the Eclipse Public License 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0
//
// This Source Code may also be made available under the following
// Secondary Licenses when the conditions for such availability set
// forth in the Eclipse Public License, v. 2.0 are satisfied:
// the Apache License v2.0 which is available at
// https://www.apache.org/licenses/LICENSE-2.0
//
// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
// ========================================================================
//

package org.eclipse.jetty.util;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

A Trie String lookup data structure using a fixed size array.
This implementation is always case insensitive and is optimal for
a small number of fixed strings with few special characters.  The
Trie is stored in an array of lookup tables, each indexed by the
next character of the key.   Frequently used characters are directly
indexed in each lookup table, whilst infrequently used characters
must use a big character table.

This Trie is very space efficient if the key characters are
from ' ', '+', '-', ':', ';', '.', 'A' to 'Z' or 'a' to 'z'.
Other ISO-8859-1 characters can be used by the key, but less space
efficiently.

This Trie is not Threadsafe and contains no mutual exclusion
or deliberate memory barriers.  It is intended for an ArrayTrie to be
built by a single thread and then used concurrently by multiple threads
and not mutated during that access.  If concurrent mutations of the
Trie is required external locks need to be applied.

Type parameters:
<V> – the entry type/**
 * <p>A Trie String lookup data structure using a fixed size array.</p>
 * <p>This implementation is always case insensitive and is optimal for
 * a small number of fixed strings with few special characters.  The
 * Trie is stored in an array of lookup tables, each indexed by the
 * next character of the key.   Frequently used characters are directly
 * indexed in each lookup table, whilst infrequently used characters
 * must use a big character table.
 * </p>
 * <p>This Trie is very space efficient if the key characters are
 * from ' ', '+', '-', ':', ';', '.', 'A' to 'Z' or 'a' to 'z'.
 * Other ISO-8859-1 characters can be used by the key, but less space
 * efficiently.
 * </p>
 * <p>This Trie is not Threadsafe and contains no mutual exclusion
 * or deliberate memory barriers.  It is intended for an ArrayTrie to be
 * built by a single thread and then used concurrently by multiple threads
 * and not mutated during that access.  If concurrent mutations of the
 * Trie is required external locks need to be applied.
 * </p>
 *
 * @param <V> the entry type
 */
class ArrayTrie<V> extends AbstractTrie<V>
{
    
The Size of a Trie row is how many characters can be looked
up directly without going to a big index.  This is set at
32 to cover case insensitive alphabet and a few other common
characters.
/**
     * The Size of a Trie row is how many characters can be looked
     * up directly without going to a big index.  This is set at
     * 32 to cover case insensitive alphabet and a few other common
     * characters.
     */
    private static final int ROW_SIZE = 32;

    
The index lookup table, this maps a character as a byte
(ISO-8859-1 or UTF8) to an index within a Trie row
/**
     * The index lookup table, this maps a character as a byte
     * (ISO-8859-1 or UTF8) to an index within a Trie row
     */
    private static final int[] LOOKUP =
        {
            // 0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
            /*0*/-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            /*1*/-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            /*2*/31, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 26, -1, 27, 30, -1,
            /*3*/-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 28, 29, -1, -1, -1, -1,
            /*4*/-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
            /*5*/15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
            /*6*/-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
            /*7*/15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1
        };

    
The Trie rows in a single array which allows a lookup of row,character
to the next row in the Trie.  This is actually a 2 dimensional
array that has been flattened to achieve locality of reference.
The first ROW_SIZE entries are for row 0, then next ROW_SIZE
entries are for row 1 etc.   So in general instead of using
_rows[row][index], we use _rows[row*ROW_SIZE+index] to look up
the next row for a given character.
The array is of characters rather than integers to save space.
/**
     * The Trie rows in a single array which allows a lookup of row,character
     * to the next row in the Trie.  This is actually a 2 dimensional
     * array that has been flattened to achieve locality of reference.
     * The first ROW_SIZE entries are for row 0, then next ROW_SIZE
     * entries are for row 1 etc.   So in general instead of using
     * _rows[row][index], we use _rows[row*ROW_SIZE+index] to look up
     * the next row for a given character.
     *
     * The array is of characters rather than integers to save space.
     */
    private final char[] _rowIndex;

    
The key (if any) for a Trie row.
A row may be a leaf, a node or both in the Trie tree.
/**
     * The key (if any) for a Trie row.
     * A row may be a leaf, a node or both in the Trie tree.
     */
    private final String[] _key;

    
The value (if any) for a Trie row.
A row may be a leaf, a node or both in the Trie tree.
/**
     * The value (if any) for a Trie row.
     * A row may be a leaf, a node or both in the Trie tree.
     */
    private final V[] _value;

    
A big index for each row.
If a character outside of the lookup map is needed,
then a big index will be created for the row, with
256 entries, one for each possible byte.
/**
     * A big index for each row.
     * If a character outside of the lookup map is needed,
     * then a big index will be created for the row, with
     * 256 entries, one for each possible byte.
     */
    private char[][] _bigIndex;

    
The number of rows allocated
/**
     * The number of rows allocated
     */
    private char _rows;

    
Params:
capacity – The capacity of the trie, which at the worst case
is the total number of characters of all keys stored in the Trie.
The capacity needed is dependent of the shared prefixes of the keys.
For example, a capacity of 6 nodes is required to store keys "foo"
and "bar", but a capacity of only 4 is required to
store "bar" and "bat"./**
     * @param capacity The capacity of the trie, which at the worst case
     * is the total number of characters of all keys stored in the Trie.
     * The capacity needed is dependent of the shared prefixes of the keys.
     * For example, a capacity of 6 nodes is required to store keys "foo"
     * and "bar", but a capacity of only 4 is required to
     * store "bar" and "bat".
     */
    @SuppressWarnings("unchecked")
    ArrayTrie(int capacity)
    {
        super(true);
        capacity++;
        _value = (V[])new Object[capacity];
        _rowIndex = new char[capacity * ROW_SIZE];
        _key = new String[capacity];
    }

    @SuppressWarnings("unchecked")
    ArrayTrie(Map<String, V> initialValues)
    {
        super(true);
        // The calculated requiredCapacity does not take into account the
        // extra reserved slot for the empty string key, so we have to add 1.
        int capacity = requiredCapacity(initialValues.keySet(), false) + 1;
        _value = (V[])new Object[capacity];
        _rowIndex = new char[capacity * ROW_SIZE];
        _key = new String[capacity];
        for (Map.Entry<String, V> entry : initialValues.entrySet())
        {
            if (!put(entry.getKey(), entry.getValue()))
                throw new AssertionError("Invalid capacity calculated (" + capacity + ") at '" + entry + "' for " + initialValues);
        }
    }

    @Override
    public void clear()
    {
        _rows = 0;
        Arrays.fill(_value, null);
        Arrays.fill(_rowIndex, (char)0);
        Arrays.fill(_key, null);
    }

    @Override
    public boolean put(String s, V v)
    {
        int t = 0;
        int k;
        int limit = s.length();
        for (k = 0; k < limit; k++)
        {
            char c = s.charAt(k);

            int index = LOOKUP[c & 0x7f];
            if (index >= 0)
            {
                int idx = t * ROW_SIZE + index;
                t = _rowIndex[idx];
                if (t == 0)
                {
                    if (++_rows >= _value.length)
                        return false;
                    t = _rowIndex[idx] = _rows;
                }
            }
            else if (c > 127)
                throw new IllegalArgumentException("non ascii character");
            else
            {
                if (_bigIndex == null)
                    _bigIndex = new char[_value.length][];
                if (t >= _bigIndex.length)
                    return false;
                char[] big = _bigIndex[t];
                if (big == null)
                    big = _bigIndex[t] = new char[128];
                t = big[c];
                if (t == 0)
                {
                    if (_rows == _value.length)
                        return false;
                    t = big[c] = ++_rows;
                }
            }
        }

        if (t >= _key.length)
        {
            _rows = (char)_key.length;
            return false;
        }

        _key[t] = v == null ? null : s;
        _value[t] = v;
        return true;
    }

    @Override
    public V get(String s, int offset, int len)
    {
        int t = 0;
        for (int i = 0; i < len; i++)
        {
            char c = s.charAt(offset + i);
            int index = LOOKUP[c & 0x7f];
            if (index >= 0)
            {
                int idx = t * ROW_SIZE + index;
                t = _rowIndex[idx];
                if (t == 0)
                    return null;
            }
            else
            {
                char[] big = _bigIndex == null ? null : _bigIndex[t];
                if (big == null)
                    return null;
                t = big[c];
                if (t == 0)
                    return null;
            }
        }
        return _value[t];
    }

    @Override
    public V get(ByteBuffer b, int offset, int len)
    {
        int t = 0;
        for (int i = 0; i < len; i++)
        {
            byte c = b.get(offset + i);
            int index = LOOKUP[c & 0x7f];
            if (index >= 0)
            {
                int idx = t * ROW_SIZE + index;
                t = _rowIndex[idx];
                if (t == 0)
                    return null;
            }
            else
            {
                char[] big = _bigIndex == null ? null : _bigIndex[t];
                if (big == null)
                    return null;
                t = big[c];
                if (t == 0)
                    return null;
            }
        }
        return (V)_value[t];
    }

    @Override
    public V getBest(byte[] b, int offset, int len)
    {
        return getBest(0, b, offset, len);
    }

    @Override
    public V getBest(ByteBuffer b, int offset, int len)
    {
        if (b.hasArray())
            return getBest(0, b.array(), b.arrayOffset() + b.position() + offset, len);
        return getBest(0, b, offset, len);
    }

    @Override
    public V getBest(String s, int offset, int len)
    {
        return getBest(0, s, offset, len);
    }

    private V getBest(int t, String s, int offset, int len)
    {
        int pos = offset;
        for (int i = 0; i < len; i++)
        {
            char c = s.charAt(pos++);
            int index = LOOKUP[c & 0x7f];
            if (index >= 0)
            {
                int idx = t * ROW_SIZE + index;
                int nt = _rowIndex[idx];
                if (nt == 0)
                    break;
                t = nt;
            }
            else
            {
                char[] big = _bigIndex == null ? null : _bigIndex[t];
                if (big == null)
                    return null;
                int nt = big[c];
                if (nt == 0)
                    break;
                t = nt;
            }

            // Is the next Trie is a match
            if (_key[t] != null)
            {
                // Recurse so we can remember this possibility
                V best = getBest(t, s, offset + i + 1, len - i - 1);
                if (best != null)
                    return best;
                return (V)_value[t];
            }
        }
        return (V)_value[t];
    }

    private V getBest(int t, byte[] b, int offset, int len)
    {
        for (int i = 0; i < len; i++)
        {
            byte c = b[offset + i];
            int index = LOOKUP[c & 0x7f];
            if (index >= 0)
            {
                int idx = t * ROW_SIZE + index;
                int nt = _rowIndex[idx];
                if (nt == 0)
                    break;
                t = nt;
            }
            else
            {
                char[] big = _bigIndex == null ? null : _bigIndex[t];
                if (big == null)
                    return null;
                int nt = big[c];
                if (nt == 0)
                    break;
                t = nt;
            }

            // Is the next Trie is a match
            if (_key[t] != null)
            {
                // Recurse so we can remember this possibility
                V best = getBest(t, b, offset + i + 1, len - i - 1);
                if (best != null)
                    return best;
                break;
            }
        }
        return (V)_value[t];
    }

    private V getBest(int t, ByteBuffer b, int offset, int len)
    {
        int pos = b.position() + offset;
        for (int i = 0; i < len; i++)
        {
            byte c = b.get(pos++);
            int index = LOOKUP[c & 0x7f];
            if (index >= 0)
            {
                int idx = t * ROW_SIZE + index;
                int nt = _rowIndex[idx];
                if (nt == 0)
                    break;
                t = nt;
            }
            else
            {
                char[] big = _bigIndex == null ? null : _bigIndex[t];
                if (big == null)
                    return null;
                int nt = big[c];
                if (nt == 0)
                    break;
                t = nt;
            }

            // Is the next Trie is a match
            if (_key[t] != null)
            {
                // Recurse so we can remember this possibility
                V best = getBest(t, b, offset + i + 1, len - i - 1);
                if (best != null)
                    return best;
                break;
            }
        }
        return (V)_value[t];
    }

    @Override
    public String toString()
    {
        StringBuilder buf = new StringBuilder();
        toString(buf, 0);

        if (buf.length() == 0)
            return "{}";

        buf.setCharAt(0, '{');
        buf.append('}');
        return buf.toString();
    }

    private void toString(Appendable out, int t)
    {
        if (_value[t] != null)
        {
            try
            {
                out.append(',');
                out.append(_key[t]);
                out.append('=');
                out.append(_value[t].toString());
            }
            catch (IOException e)
            {
                throw new RuntimeException(e);
            }
        }

        for (int i = 0; i < ROW_SIZE; i++)
        {
            int idx = t * ROW_SIZE + i;
            if (_rowIndex[idx] != 0)
                toString(out, _rowIndex[idx]);
        }

        char[] big = _bigIndex == null ? null : _bigIndex[t];
        if (big != null)
        {
            for (int i : big)
            {
                if (i != 0)
                    toString(out, i);
            }
        }
    }

    @Override
    public Set<String> keySet()
    {
        Set<String> keys = new HashSet<>();
        keySet(keys, 0);
        return keys;
    }

    @Override
    public int size()
    {
        return keySet().size();
    }

    @Override
    public boolean isEmpty()
    {
        return keySet().isEmpty();
    }

    private void keySet(Set<String> set, int t)
    {
        if (t < _value.length && _value[t] != null)
            set.add(_key[t]);

        for (int i = 0; i < ROW_SIZE; i++)
        {
            int idx = t * ROW_SIZE + i;
            if (idx < _rowIndex.length && _rowIndex[idx] != 0)
                keySet(set, _rowIndex[idx]);
        }

        char[] big = _bigIndex == null || t >= _bigIndex.length ? null : _bigIndex[t];
        if (big != null)
        {
            for (int i : big)
            {
                if (i != 0)
                    keySet(set, i);
            }
        }
    }
}