/*
 * Copyright (c) OSGi Alliance (2012, 2014). All Rights Reserved.
 * 
 * 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.
 */

package org.osgi.dto;

import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.IdentityHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * Super type for Data Transfer Objects.
 * 
 * <p>
 * A Data Transfer Object (DTO) is easily serializable having only public fields
 * of primitive types and their wrapper classes, Strings, and DTOs. List, Set,
 * Map and array aggregates may also be used. The aggregates must only hold
 * objects of the listed types or aggregates.
 * 
 * <p>
 * The object graph from a Data Transfer Object must be a tree to simplify
 * serialization and deserialization.
 * 
 * @author $Id$
 * @NotThreadSafe
 */
public abstract class DTO {

    /**
     * Return a string representation of this DTO suitable for use when
     * debugging.
     * 
     * <p>
     * The format of the string representation is not specified and subject to
     * change.
     * 
     * @return A string representation of this DTO suitable for use when
     *         debugging.
     */
    @Override
    public String toString() {
        return appendValue(new StringBuilder(), new IdentityHashMap<Object, String>(), "#", this).toString();
    }

    /**
     * Append the specified DTO's string representation to the specified
     * StringBuilder.
     * 
     * @param result StringBuilder to which the string representation is
     *        appended.
     * @param objectRefs References to "seen" objects.
     * @param refpath The reference path of the specified DTO.
     * @param dto The DTO whose string representation is to be appended.
     * @return The specified StringBuilder.
     */
    private static StringBuilder appendDTO(final StringBuilder result, final Map<Object, String> objectRefs, final String refpath, final DTO dto) {
        result.append("{");
        String delim = "";
        for (Field field : dto.getClass().getFields()) {
            if (Modifier.isStatic(field.getModifiers())) {
                continue;
            }
            result.append(delim);
            final String name = field.getName();
            appendString(result, name);
            result.append(":");
            Object value = null;
            try {
                value = field.get(dto);
            } catch (IllegalAccessException e) {
                // use null value;
            }
            appendValue(result, objectRefs, refpath + "/" + name, value);
            delim = ", ";
        }
        result.append("}");
        return result;
    }

    /**
     * Append the specified value's string representation to the specified
     * StringBuilder.
     * 
     * <p>
     * This method handles cycles in the object graph, using path-based
     * references, even though the specification requires the object graph from
     * a DTO to be a tree.
     * 
     * @param result StringBuilder to which the string representation is
     *        appended.
     * @param objectRefs References to "seen" objects.
     * @param refpath The reference path of the specified value.
     * @param value The object whose string representation is to be appended.
     * @return The specified StringBuilder.
     */
    private static StringBuilder appendValue(final StringBuilder result, final Map<Object, String> objectRefs, final String refpath, final Object value) {
        if (value == null) {
            return result.append("null");
        }
        // Simple Java types
        if (value instanceof String || value instanceof Character) {
            return appendString(result, compress(value.toString()));
        }
        if (value instanceof Number || value instanceof Boolean) {
            return result.append(value.toString());
        }

        // Complex types
        final String path = objectRefs.get(value);
        if (path != null) {
            result.append("{\"$ref\":");
            appendString(result, path);
            result.append("}");
            return result;
        }
        objectRefs.put(value, refpath);

        if (value instanceof DTO) {
            return appendDTO(result, objectRefs, refpath, (DTO) value);
        }
        if (value instanceof Map) {
            return appendMap(result, objectRefs, refpath, (Map<?, ?>) value);
        }
        if (value instanceof List || value instanceof Set) {
            return appendIterable(result, objectRefs, refpath, (Iterable<?>) value);
        }
        if (value.getClass().isArray()) {
            return appendArray(result, objectRefs, refpath, value);
        }
        return appendString(result, compress(value.toString()));
    }

    /**
     * Append the specified array's string representation to the specified
     * StringBuilder.
     * 
     * @param result StringBuilder to which the string representation is
     *        appended.
     * @param objectRefs References to "seen" objects.
     * @param refpath The reference path of the specified array.
     * @param array The array whose string representation is to be appended.
     * @return The specified StringBuilder.
     */
    private static StringBuilder appendArray(final StringBuilder result, final Map<Object, String> objectRefs, final String refpath, final Object array) {
        result.append("[");
        final int length = Array.getLength(array);
        for (int i = 0; i < length; i++) {
            if (i > 0) {
                result.append(",");
            }
            appendValue(result, objectRefs, refpath + "/" + i, Array.get(array, i));
        }
        result.append("]");
        return result;
    }

    /**
     * Append the specified iterable's string representation to the specified
     * StringBuilder.
     * 
     * @param result StringBuilder to which the string representation is
     *        appended.
     * @param objectRefs References to "seen" objects.
     * @param refpath The reference path of the specified list.
     * @param iterable The iterable whose string representation is to be
     *        appended.
     * @return The specified StringBuilder.
     */
    private static StringBuilder appendIterable(final StringBuilder result, final Map<Object, String> objectRefs, final String refpath, final Iterable<?> iterable) {
        result.append("[");
        int i = 0;
        for (Object item : iterable) {
            if (i > 0) {
                result.append(",");
            }
            appendValue(result, objectRefs, refpath + "/" + i, item);
            i++;
        }
        result.append("]");
        return result;
    }

    /**
     * Append the specified map's string representation to the specified
     * StringBuilder.
     * 
     * @param result StringBuilder to which the string representation is
     *        appended.
     * @param objectRefs References to "seen" objects.
     * @param refpath The reference path of the specified map.
     * @param map The map whose string representation is to be appended.
     * @return The specified StringBuilder.
     */
    private static StringBuilder appendMap(final StringBuilder result, final Map<Object, String> objectRefs, final String refpath, final Map<?, ?> map) {
        result.append("{");
        String delim = "";
        for (Map.Entry<?, ?> entry : map.entrySet()) {
            result.append(delim);
            final String name = String.valueOf(entry.getKey());
            appendString(result, name);
            result.append(":");
            final Object value = entry.getValue();
            appendValue(result, objectRefs, refpath + "/" + name, value);
            delim = ", ";
        }
        result.append("}");
        return result;
    }

    /**
     * Append the specified string to the specified StringBuilder.
     * 
     * @param result StringBuilder to which the string is appended.
     * @param string The string to be appended.
     * @return The specified StringBuilder.
     */
    private static StringBuilder appendString(final StringBuilder result, final CharSequence string) {
        result.append("\"");
        int i = result.length();
        result.append(string);
        while (i < result.length()) { // escape if necessary
            char c = result.charAt(i);
            if ((c == '"') || (c == '\\')) {
                result.insert(i, '\\');
                i = i + 2;
                continue;
            }
            if (c < 0x20) {
                result.insert(i + 1, Integer.toHexString(c | 0x10000));
                result.replace(i, i + 2, "\\u");
                i = i + 6;
                continue;
            }
            i++;
        }
        result.append("\"");
        return result;
    }

    /**
     * Compress, in length, the specified string.
     * 
     * @param in The string to potentially compress.
     * @return The string compressed, if necessary.
     */
    private static CharSequence compress(final CharSequence in) {
        final int length = in.length();
        if (length <= 21) {
            return in;
        }
        StringBuilder result = new StringBuilder(21);
        result.append(in, 0, 9);
        result.append("...");
        result.append(in, length - 9, length);
        return result;
    }
}