using System;
using System.IO;
using System.IO.Compression;
using System.Threading.Tasks;
using System.Xml.Linq;

namespace VersOne.Epub.Internal
{
    internal static class RootFilePathReader
    {
        public static async Task<string> GetRootFilePathAsync(ZipArchive epubArchive)
        {
            const string EPUB_CONTAINER_FILE_PATH = "META-INF/container.xml";
            ZipArchiveEntry containerFileEntry = epubArchive.GetEntry(EPUB_CONTAINER_FILE_PATH);
            if (containerFileEntry == null)
            {
                throw new Exception(String.Format("EPUB parsing error: {0} file not found in archive.", EPUB_CONTAINER_FILE_PATH));
            }
            XDocument containerDocument;
            using (Stream containerStream = containerFileEntry.Open())
            {
                containerDocument = await XmlUtils.LoadDocumentAsync(containerStream).ConfigureAwait(false);
            }
            XNamespace cnsNamespace = "urn:oasis:names:tc:opendocument:xmlns:container";
            XAttribute fullPathAttribute = containerDocument.Element(cnsNamespace + "container")?.Element(cnsNamespace + "rootfiles")?.Element(cnsNamespace + "rootfile")?.Attribute("full-path");
            if (fullPathAttribute == null)
            {
                throw new Exception("EPUB parsing error: root file path not found in the EPUB container.");
            }
            return fullPathAttribute.Value;
        }
    }
}