using System;
using System.Collections.Generic;
using System.IO;
using System.Xml;
using Google.ProtocolBuffers.Descriptors;

namespace Google.ProtocolBuffers.Serialization
{
    /// <summary>
    /// Parses a proto buffer from an XML document or fragment.  .NET 3.5 users may also
    /// use this class to process Json by setting the options to support Json and providing
    /// an XmlReader obtained from <see cref="System.Runtime.Serialization.Json.JsonReaderWriterFactory"/>.
    /// </summary>
    public class XmlFormatReader : AbstractTextReader
    {
        public const string DefaultRootElementName = XmlFormatWriter.DefaultRootElementName;
        private readonly XmlReader _input;
        private string _rootElementName;

        static XmlReaderSettings DefaultSettings
        {
            get { return new XmlReaderSettings() { CheckCharacters=false, IgnoreComments=true, IgnoreProcessingInstructions = true }; }
        }

        /// <summary>
        /// Constructs the XmlFormatReader using the stream provided as the xml
        /// </summary>
        public XmlFormatReader(Stream input) : this(XmlReader.Create(input, DefaultSettings)) { }
        /// <summary>
        /// Constructs the XmlFormatReader using the string provided as the xml to be read
        /// </summary>
        public XmlFormatReader(String input) : this(XmlReader.Create(new StringReader(input))) { }
        /// <summary>
        /// Constructs the XmlFormatReader using the xml in the TextReader
        /// </summary>
        public XmlFormatReader(TextReader input) : this(XmlReader.Create(input)) { }
        /// <summary>
        /// Constructs the XmlFormatReader with the XmlReader
        /// </summary>
        public XmlFormatReader(XmlReader input) : this(input, XmlReaderOptions.None) { }
        /// <summary>
        /// Constructs the XmlFormatReader with the XmlReader and options
        /// </summary>
        public XmlFormatReader(XmlReader input, XmlReaderOptions options)
        {
            _input = input;
            _rootElementName = DefaultRootElementName;
            Options = options;
        }
        
        /// <summary>
        /// Gets or sets the options to use when reading the xml
        /// </summary>
        public XmlReaderOptions Options { get; set; }

        /// <summary>
        /// Gets or sets the default element name to use when using the Merge&lt;TBuilder>()
        /// </summary>
        public string RootElementName 
        {
            get { return _rootElementName; } 
            set { ThrowHelper.ThrowIfNull(value, "RootElementName"); _rootElementName = value; } 
        }

        private XmlFormatReader CloneWith(XmlReader rdr)
        {
            return new XmlFormatReader(rdr, Options);
        }
        private void NextElement()
        {
            while (!_input.IsStartElement() && _input.Read())
                continue;
        }
        private static void Assert(bool cond)
        {
            if (!cond) throw new FormatException();
        }

        /// <summary>
        /// Merge the provided builder as an element named <see cref="RootElementName"/> in the current context
        /// </summary>
        public override TBuilder Merge<TBuilder>(TBuilder builder, ExtensionRegistry registry)
        { return Merge(_rootElementName, builder, registry); }

        /// <summary>
        /// Merge the provided builder as an element of the current context
        /// </summary>
        public TBuilder Merge<TBuilder>(string element, TBuilder builder) where TBuilder : IBuilderLite
        { return Merge(element, builder, ExtensionRegistry.Empty); }

        /// <summary>
        /// Merge the provided builder as an element of the current context
        /// </summary>
        public TBuilder Merge<TBuilder>(string element, TBuilder builder, ExtensionRegistry registry) where TBuilder : IBuilderLite
        {
            string field;
            Assert(PeekNext(out field) && field == element);
            ReadMessage(builder, registry);
            return builder;
        }

        /// <summary>
        /// Peeks at the next field in the input stream and returns what information is available.
        /// </summary>
        /// <remarks>
        /// This may be called multiple times without actually reading the field.  Only after the field
        /// is either read, or skipped, should PeekNext return a different value.
        /// </remarks>
        protected override bool PeekNext(out string field)
        {
            NextElement();
            if(_input.IsStartElement())
            {
                field = _input.LocalName;
                return true;
            }
            field = null;
            return false;
        }

        /// <summary>
        /// Causes the reader to skip past this field
        /// </summary>
        protected override void Skip()
        {
            if (_input.IsStartElement())
            {
                if (!_input.IsEmptyElement)
                {
                    int depth = _input.Depth;
                    while (_input.Depth >= depth && _input.NodeType != XmlNodeType.EndElement)
                        Assert(_input.Read());
                }
                _input.Read();
            }
        }

        /// <summary>
        /// returns true if it was able to read a single value into the value reference.  The value
        /// stored may be of type System.String, System.Int32, or an IEnumLite from the IEnumLiteMap.
        /// </summary>
        protected override bool ReadEnum(ref object value)
        {
            int number;
            string temp;
            if (null != (temp = _input.GetAttribute("value")) && int.TryParse(temp, out number))
            {
                Skip();
                value = number;
                return true;
            }
            return base.ReadEnum(ref value);
        }

        /// <summary>
        /// Returns true if it was able to read a String from the input
        /// </summary>
        protected override bool ReadAsText(ref string value, Type type)
        {
            Assert(_input.NodeType == XmlNodeType.Element);
            value = _input.ReadElementContentAsString();
            
            return true;
        }

        /// <summary>
        /// Merges the input stream into the provided IBuilderLite 
        /// </summary>
        protected override bool ReadMessage(IBuilderLite builder, ExtensionRegistry registry)
        {
            Assert(_input.IsStartElement());

            if (!_input.IsEmptyElement)
            {
                int depth = _input.Depth;
                XmlReader child = _input.ReadSubtree();
                while (!child.IsStartElement() && child.Read())
                    continue;
                child.Read();
                builder.WeakMergeFrom(CloneWith(child), registry);
                Assert(depth == _input.Depth && _input.NodeType == XmlNodeType.EndElement);
            }
            _input.Read();
            return true;
        }

        private IEnumerable<string> NonNestedArrayItems(string field)
        {
            return base.ForeachArrayItem(field);
        }

        /// <summary>
        /// Cursors through the array elements and stops at the end of the array
        /// </summary>
        protected override IEnumerable<string> ForeachArrayItem(string field)
        {
            bool isNested = (Options & XmlReaderOptions.ReadNestedArrays) != 0;

            if (!isNested)
            {
                foreach (string item in NonNestedArrayItems(field))
                    yield return item;
                yield break;
            }
            if (!_input.IsEmptyElement)
            {
                int depth = _input.Depth;
                XmlReader child = _input.ReadSubtree();

                while (!child.IsStartElement() && child.Read())
                    continue;
                child.Read();

                foreach (string item in CloneWith(child).NonNestedArrayItems("item"))
                    yield return item;
                Assert(depth == _input.Depth && _input.NodeType == XmlNodeType.EndElement);
            }
            _input.Read();
            yield break;
        }
    }
}