// Author: Gockner, Simon
// Created: 2020-02-12
// Copyright(c) 2020 SimonG. All Rights Reserved.

using System;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Xml.Linq;
using GBase.DataHandling.Exceptions;
using GBase.Interfaces.DataHandling;
using GBase.Interfaces.DataHandling.Xml;

namespace GBase.DataHandling
{
    /// <summary>
    /// A <see cref="IDataWriter"/> that writes to a xml file
    /// </summary>
    public class XmlDataWriter : IXmlDataWriter
    {
        /// <summary>
        /// Initialize the <see cref="XmlDataWriter"/>
        /// </summary>
        /// <param name="file"></param>
        /// <param name="rootElementName"></param>
        /// <param name="cancellationToken">A <see cref="CancellationToken"/> to cancel the async operation</param>
        /// <returns>Returns true if successful, false if not</returns>
        /// <exception cref="Exception">No root element found</exception>
        public async Task<bool> InitFile(FileStream file, string rootElementName, CancellationToken cancellationToken)
        {
            //if the xml file isn't empty, return
            if (file.Length > 3) //> 3 because of BOM
                return true; //TODO: Don't return bool?
            
            XDocument xmlDocument = new XDocument();
            xmlDocument.Add(new XElement(rootElementName));
            file.Seek(0, SeekOrigin.Begin); //reset stream to it's beginning to be able to save it
            await xmlDocument.SaveAsync(file, SaveOptions.OmitDuplicateNamespaces, cancellationToken);

            return true;
        }

        /// <summary>
        /// Write the data of a property
        /// </summary>
        /// <typeparam name="T">The <see cref="Type"/></typeparam>
        /// <typeparam name="TProperty">The <see cref="Type"/> of the property</typeparam>
        /// <param name="file"></param>
        /// <param name="propertyName">The name of the property</param>
        /// <param name="value">The value of the property</param>
        /// <param name="overwrite">If true an existing value is overwritten, if false an additional value is added</param>
        /// <param name="cancellationToken"></param>
        /// <returns>A <see cref="Task"/> to await</returns>
        public async Task Write<T, TProperty>(FileStream file, string propertyName, string value, bool overwrite, CancellationToken cancellationToken) => 
            await Write<T>(file, propertyName, value, typeof(TProperty), overwrite, cancellationToken);

        /// <summary>
        /// Write the data of a property
        /// </summary>
        /// <typeparam name="T">The <see cref="Type"/> that implements the property</typeparam>
        /// <param name="file"></param>
        /// <param name="propertyName">The name of the property</param>
        /// <param name="value">The value of the property</param>
        /// <param name="propertyType"></param>
        /// <param name="overwrite">If true an existing value is overwritten, if false an additional value is added</param>
        /// <param name="cancellationToken"></param>
        /// <returns>A <see cref="Task"/> to await</returns>
        /// <exception cref="ArgumentNullException"><paramref name="propertyName"/></exception>
        /// <exception cref="InvalidXmlFileException">No root element is set</exception>
        public async Task Write<T>(FileStream file, string propertyName, string value, Type propertyType, bool overwrite, CancellationToken cancellationToken)
        {
            string typeName = typeof(T).FullName;
            if (typeName == null)
                throw new ArgumentNullException(nameof(typeName));
            
            file.Seek(0, SeekOrigin.Begin); //reset stream to it's beginning to be able to save it
            XDocument xmlDocument = await XDocument.LoadAsync(file, LoadOptions.None, cancellationToken);
            file.Seek(0, SeekOrigin.Begin); //reset stream to it's beginning to be able to save it

            XElement rootElement = xmlDocument.Root;
            if (rootElement == null)
                throw new InvalidXmlFileException("No root element is set.");
            
            XElement propertyElement = rootElement.Element(propertyName);
            if (propertyElement != null) //property element already exists
            {
                XElement valueElement = propertyElement.Element(XmlDataHandler.VALUE_ELEMENT_NAME);
                if (valueElement != null && overwrite) //value element exists and overwrite is true
                {
                    valueElement.Value = value; //overwrite existing value
                }
                else if (!overwrite && propertyElement.Elements(XmlDataHandler.VALUE_ELEMENT_NAME).Any(v => v.Value.Equals(value))) //no overwrite and same value exists already
                {
                    XElement sameValueElement = propertyElement.Elements(XmlDataHandler.VALUE_ELEMENT_NAME).First(v => v.Value.Equals(value));
                    sameValueElement.Remove(); //remove the already existing value from its current position
                    propertyElement.AddFirst(sameValueElement); //add it as the first element again
                }
                else //no value element exists or overwrite is false
                    propertyElement.AddFirst(new XElement(XmlDataHandler.VALUE_ELEMENT_NAME) { Value = value }); //add a new value element
            }
            else //property element doesn't exist
            {
                propertyElement = new XElement(propertyName, new XElement(XmlDataHandler.VALUE_ELEMENT_NAME) { Value = value }); //create the new property element with the value element
                propertyElement.SetAttributeValue(XmlDataHandler.VALUE_TYPE_ATTRIBUTE_NAME, propertyType.FullName); //add the property type attribute

                rootElement.Add(propertyElement); //create new property element with the value element
            }

            //TODO: check if whole file is overwritten (probably) -> performance issues for large files?
            await xmlDocument.SaveAsync(file, SaveOptions.OmitDuplicateNamespaces, cancellationToken); //save the document with the added elements
        }

        /// <summary>
        /// Remove the value for the given property
        /// </summary>
        /// <typeparam name="T">The <see cref="Type"/> of the property</typeparam>
        /// <typeparam name="TProperty">The <see cref="Type"/> of the property</typeparam>
        /// <param name="file"></param>
        /// <param name="propertyName">The name of the property</param>
        /// <param name="value">The value to set</param>
        /// <param name="cancellationToken"></param>
        /// <returns>A <see cref="Task"/> to await</returns>
        /// <exception cref="ArgumentNullException"><paramref name="propertyName"/></exception>
        /// <exception cref="InvalidXmlFileException">No root element is set</exception>
        public async Task Remove<T, TProperty>(FileStream file, string propertyName, string value, CancellationToken cancellationToken)
        {
            string typeName = typeof(T).FullName;
            if (typeName == null)
                throw new ArgumentNullException(nameof(typeName));

            XDocument xmlDocument = await XDocument.LoadAsync(file, LoadOptions.None, cancellationToken);
            file.Seek(0, SeekOrigin.Begin); //reset stream to it's beginning to be able to save it

            XElement rootElement = xmlDocument.Root;
            if (rootElement == null)
                throw new InvalidXmlFileException("No root element is set.");
            
            XElement typeElement = rootElement.Element(typeName);
            XElement propertyElement = typeElement?.Element(propertyName);
            XElement valueElement = propertyElement?.Elements(XmlDataHandler.VALUE_ELEMENT_NAME).FirstOrDefault(e => e.Value.Equals(value));
            if (valueElement == null)
                return;

            valueElement.Remove();

            //TODO: check if whole file is overwritten (probably) -> performance issues for large files?
            await xmlDocument.SaveAsync(file, SaveOptions.OmitDuplicateNamespaces, cancellationToken); //save the document with the added elements
        }
    }
}