Kavita/API/Services/BookService.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Web;
using API.Data.Metadata;
using API.DTOs.Reader;
using API.Entities;
using API.Entities.Enums;
using API.Parser;
using Docnet.Core;
using Docnet.Core.Converters;
using Docnet.Core.Models;
using Docnet.Core.Readers;
using ExCSS;
using HtmlAgilityPack;
using Kavita.Common;
using Microsoft.Extensions.Logging;
using Microsoft.IO;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;
using VersOne.Epub;
using VersOne.Epub.Options;
using Image = SixLabors.ImageSharp.Image;

namespace API.Services;

public interface IBookService
{
    int GetNumberOfPages(string filePath);
    string GetCoverImage(string fileFilePath, string fileName, string outputDirectory);
    Task<Dictionary<string, int>> CreateKeyToPageMappingAsync(EpubBookRef book);

    /// <summary>
    /// Scopes styles to .reading-section and replaces img src to the passed apiBase
    /// </summary>
    /// <param name="stylesheetHtml"></param>
    /// <param name="apiBase"></param>
    /// <param name="filename">If the stylesheetHtml contains Import statements, when scoping the filename, scope needs to be wrt filepath.</param>
    /// <param name="book">Book Reference, needed for if you expect Import statements</param>
    /// <returns></returns>
    Task<string> ScopeStyles(string stylesheetHtml, string apiBase, string filename, EpubBookRef book);
    ComicInfo GetComicInfo(string filePath);
    ParserInfo ParseInfo(string filePath);
    /// <summary>
    /// Extracts a PDF file's pages as images to an target directory
    /// </summary>
    /// <param name="fileFilePath"></param>
    /// <param name="targetDirectory">Where the files will be extracted to. If doesn't exist, will be created.</param>
    [Obsolete("This method of reading is no longer supported. Please use native pdf reader")]
    void ExtractPdfImages(string fileFilePath, string targetDirectory);

    Task<string> ScopePage(HtmlDocument doc, EpubBookRef book, string apiBase, HtmlNode body, Dictionary<string, int> mappings, int page);
    Task<ICollection<BookChapterItem>> GenerateTableOfContents(Chapter chapter);

    Task<string> GetBookPage(int page, int chapterId, string cachedEpubPath, string baseUrl);
}

public class BookService : IBookService
{
    private readonly ILogger<BookService> _logger;
    private readonly IDirectoryService _directoryService;
    private readonly IImageService _imageService;
    private readonly StylesheetParser _cssParser = new ();
    private static readonly RecyclableMemoryStreamManager StreamManager = new ();
    private const string CssScopeClass = ".book-content";
    private const string BookApiUrl = "book-resources?file=";
    public static readonly EpubReaderOptions BookReaderOptions = new()
    {
        PackageReaderOptions = new PackageReaderOptions()
        {
            IgnoreMissingToc = true
        }
    };

    public BookService(ILogger<BookService> logger, IDirectoryService directoryService, IImageService imageService)
    {
        _logger = logger;
        _directoryService = directoryService;
        _imageService = imageService;
    }

    private static bool HasClickableHrefPart(HtmlNode anchor)
    {
        return anchor.GetAttributeValue("href", string.Empty).Contains("#")
               && anchor.GetAttributeValue("tabindex", string.Empty) != "-1"
               && anchor.GetAttributeValue("role", string.Empty) != "presentation";
    }

    public static string GetContentType(EpubContentType type)
    {
        string contentType;
        switch (type)
        {
            case EpubContentType.IMAGE_GIF:
                contentType = "image/gif";
                break;
            case EpubContentType.IMAGE_PNG:
                contentType = "image/png";
                break;
            case EpubContentType.IMAGE_JPEG:
                contentType = "image/jpeg";
                break;
            case EpubContentType.FONT_OPENTYPE:
                contentType = "font/otf";
                break;
            case EpubContentType.FONT_TRUETYPE:
                contentType = "font/ttf";
                break;
            case EpubContentType.IMAGE_SVG:
                contentType = "image/svg+xml";
                break;
            default:
                contentType = "application/octet-stream";
                break;
        }

        return contentType;
    }

    private static void UpdateLinks(HtmlNode anchor, Dictionary<string, int> mappings, int currentPage)
    {
        if (anchor.Name != "a") return;
        var hrefParts = CleanContentKeys(anchor.GetAttributeValue("href", string.Empty))
            .Split("#");
        // Some keys get uri encoded when parsed, so replace any of those characters with original
        var mappingKey = HttpUtility.UrlDecode(hrefParts[0]);

        if (!mappings.ContainsKey(mappingKey))
        {
            if (HasClickableHrefPart(anchor))
            {
                var part = hrefParts.Length > 1
                    ? hrefParts[1]
                    : anchor.GetAttributeValue("href", string.Empty);
                anchor.Attributes.Add("kavita-page", $"{currentPage}");
                anchor.Attributes.Add("kavita-part", part);
                anchor.Attributes.Remove("href");
                anchor.Attributes.Add("href", "javascript:void(0)");
            }
            else
            {
                anchor.Attributes.Add("target", "_blank");
                anchor.Attributes.Add("rel", "noreferrer noopener");
            }

            return;
        }

        var mappedPage = mappings[mappingKey];
        anchor.Attributes.Add("kavita-page", $"{mappedPage}");
        if (hrefParts.Length > 1)
        {
            anchor.Attributes.Add("kavita-part",
                hrefParts[1]);
        }

        anchor.Attributes.Remove("href");
        anchor.Attributes.Add("href", "javascript:void(0)");
    }

    public async Task<string> ScopeStyles(string stylesheetHtml, string apiBase, string filename, EpubBookRef book)
    {
        // @Import statements will be handled by browser, so we must inline the css into the original file that request it, so they can be Scoped
        var prepend = filename.Length > 0 ? filename.Replace(Path.GetFileName(filename), string.Empty) : string.Empty;
        var importBuilder = new StringBuilder();
        foreach (Match match in Tasks.Scanner.Parser.Parser.CssImportUrlRegex.Matches(stylesheetHtml))
        {
            if (!match.Success) continue;

            var importFile = match.Groups["Filename"].Value;
            var key = CleanContentKeys(importFile);
            if (!key.Contains(prepend))
            {
                key = prepend + key;
            }
            if (!book.Content.AllFiles.ContainsKey(key)) continue;

            var bookFile = book.Content.AllFiles[key];
            var content = await bookFile.ReadContentAsBytesAsync();
            importBuilder.Append(Encoding.UTF8.GetString(content));
        }

        stylesheetHtml = stylesheetHtml.Insert(0, importBuilder.ToString());

        EscapeCssImportReferences(ref stylesheetHtml, apiBase, prepend);

        EscapeFontFamilyReferences(ref stylesheetHtml, apiBase, prepend);


        // Check if there are any background images and rewrite those urls
        EscapeCssImageReferences(ref stylesheetHtml, apiBase, book);

        var styleContent = RemoveWhiteSpaceFromStylesheets(stylesheetHtml);

        styleContent = styleContent.Replace("body", CssScopeClass);

        if (string.IsNullOrEmpty(styleContent)) return string.Empty;

        var stylesheet = await _cssParser.ParseAsync(styleContent);
        foreach (var styleRule in stylesheet.StyleRules)
        {
            if (styleRule.Selector.Text == CssScopeClass) continue;
            if (styleRule.Selector.Text.Contains(","))
            {
                styleRule.Text = styleRule.Text.Replace(styleRule.SelectorText,
                    string.Join(", ",
                        styleRule.Selector.Text.Split(",").Select(s => $"{CssScopeClass} " + s)));
                continue;
            }
            styleRule.Text = $"{CssScopeClass} " + styleRule.Text;
        }
        return RemoveWhiteSpaceFromStylesheets(stylesheet.ToCss());
    }

    private static void EscapeCssImportReferences(ref string stylesheetHtml, string apiBase, string prepend)
    {
        foreach (Match match in Tasks.Scanner.Parser.Parser.CssImportUrlRegex.Matches(stylesheetHtml))
        {
            if (!match.Success) continue;
            var importFile = match.Groups["Filename"].Value;
            stylesheetHtml = stylesheetHtml.Replace(importFile, apiBase + prepend + importFile);
        }
    }

    private static void EscapeFontFamilyReferences(ref string stylesheetHtml, string apiBase, string prepend)
    {
        foreach (Match match in Tasks.Scanner.Parser.Parser.FontSrcUrlRegex.Matches(stylesheetHtml))
        {
            if (!match.Success) continue;
            var importFile = match.Groups["Filename"].Value;
            stylesheetHtml = stylesheetHtml.Replace(importFile, apiBase + prepend + importFile);
        }
    }

    private static void EscapeCssImageReferences(ref string stylesheetHtml, string apiBase, EpubBookRef book)
    {
        var matches = Tasks.Scanner.Parser.Parser.CssImageUrlRegex.Matches(stylesheetHtml);
        foreach (Match match in matches)
        {
            if (!match.Success) continue;

            var importFile = match.Groups["Filename"].Value;
            var key = CleanContentKeys(importFile);
            if (!book.Content.AllFiles.ContainsKey(key)) continue;

            stylesheetHtml = stylesheetHtml.Replace(importFile, apiBase + key);
        }
    }

    private static void ScopeImages(HtmlDocument doc, EpubBookRef book, string apiBase)
    {
        var images = doc.DocumentNode.SelectNodes("//img")
                     ?? doc.DocumentNode.SelectNodes("//image") ?? doc.DocumentNode.SelectNodes("//svg");

        if (images == null) return;


        var parent = images.First().ParentNode;

        foreach (var image in images)
        {

            string key = null;
            if (image.Attributes["src"] != null)
            {
                key = "src";
            }
            else if (image.Attributes["xlink:href"] != null)
            {
                key = "xlink:href";
            }

            if (string.IsNullOrEmpty(key)) continue;

            var imageFile = GetKeyForImage(book, image.Attributes[key].Value);
            image.Attributes.Remove(key);
            // UrlEncode here to transform ../ into an escaped version, which avoids blocking on nginx
            image.Attributes.Add(key, $"{apiBase}" + HttpUtility.UrlEncode(imageFile));

            // Add a custom class that the reader uses to ensure images stay within reader
            parent.AddClass("kavita-scale-width-container");
            image.AddClass("kavita-scale-width");
        }

    }

    /// <summary>
    /// Returns the image key associated with the file. Contains some basic fallback logic.
    /// </summary>
    /// <param name="book"></param>
    /// <param name="imageFile"></param>
    /// <returns></returns>
    private static string GetKeyForImage(EpubBookRef book, string imageFile)
    {
        if (book.Content.Images.ContainsKey(imageFile)) return imageFile;

        var correctedKey = book.Content.Images.Keys.SingleOrDefault(s => s.EndsWith(imageFile));
        if (correctedKey != null)
        {
            imageFile = correctedKey;
        }
        else if (imageFile.StartsWith(".."))
        {
            // There are cases where the key is defined static like OEBPS/Images/1-4.jpg but reference is ../Images/1-4.jpg
            correctedKey =
                book.Content.Images.Keys.SingleOrDefault(s => s.EndsWith(imageFile.Replace("..", string.Empty)));
            if (correctedKey != null)
            {
                imageFile = correctedKey;
            }
        }

        return imageFile;
    }

    private static string PrepareFinalHtml(HtmlDocument doc, HtmlNode body)
    {
        // Check if any classes on the html node (some r2l books do this) and move them to body tag for scoping
        var htmlNode = doc.DocumentNode.SelectSingleNode("//html");
        if (htmlNode == null || !htmlNode.Attributes.Contains("class")) return body.InnerHtml;

        var bodyClasses = body.Attributes.Contains("class") ? body.Attributes["class"].Value : string.Empty;
        var classes = htmlNode.Attributes["class"].Value + " " + bodyClasses;
        body.Attributes.Add("class", $"{classes}");
        // I actually need the body tag itself for the classes, so i will create a div and put the body stuff there.
        return $"<div class=\"{body.Attributes["class"].Value}\">{body.InnerHtml}</div>";
    }

    private static void RewriteAnchors(int page, HtmlDocument doc, Dictionary<string, int> mappings)
    {
        var anchors = doc.DocumentNode.SelectNodes("//a");
        if (anchors == null) return;

        foreach (var anchor in anchors)
        {
            UpdateLinks(anchor, mappings, page);
        }
    }

    private async Task InlineStyles(HtmlDocument doc, EpubBookRef book, string apiBase, HtmlNode body)
    {
        var inlineStyles = doc.DocumentNode.SelectNodes("//style");
        if (inlineStyles != null)
        {
            foreach (var inlineStyle in inlineStyles)
            {
                var styleContent = await ScopeStyles(inlineStyle.InnerHtml, apiBase, "", book);
                body.PrependChild(HtmlNode.CreateNode($"<style>{styleContent}</style>"));
            }
        }

        var styleNodes = doc.DocumentNode.SelectNodes("/html/head/link");
        if (styleNodes != null)
        {
            foreach (var styleLinks in styleNodes)
            {
                var key = CleanContentKeys(styleLinks.Attributes["href"].Value);
                // Some epubs are malformed the key in content.opf might be: content/resources/filelist_0_0.xml but the actual html links to resources/filelist_0_0.xml
                // In this case, we will do a search for the key that ends with
                if (!book.Content.Css.ContainsKey(key))
                {
                    var correctedKey = book.Content.Css.Keys.SingleOrDefault(s => s.EndsWith(key));
                    if (correctedKey == null)
                    {
                        _logger.LogError("Epub is Malformed, key: {Key} is not matching OPF file", key);
                        continue;
                    }

                    key = correctedKey;
                }

                try
                {
                    var cssFile = book.Content.Css[key];

                    var styleContent = await ScopeStyles(await cssFile.ReadContentAsync(), apiBase,
                        cssFile.FileName, book);
                    if (styleContent != null)
                    {
                        body.PrependChild(HtmlNode.CreateNode($"<style>{styleContent}</style>"));
                    }
                }
                catch (Exception ex)
                {
                    _logger.LogError(ex, "There was an error reading css file for inlining likely due to a key mismatch in metadata");
                }
            }
        }
    }

    public ComicInfo GetComicInfo(string filePath)
    {
        if (!IsValidFile(filePath) || Tasks.Scanner.Parser.Parser.IsPdf(filePath)) return null;

        try
        {
            using var epubBook = EpubReader.OpenBook(filePath, BookReaderOptions);
            var publicationDate =
                epubBook.Schema.Package.Metadata.Dates.FirstOrDefault(date => date.Event == "publication")?.Date;

            if (string.IsNullOrEmpty(publicationDate))
            {
                publicationDate = epubBook.Schema.Package.Metadata.Dates.FirstOrDefault()?.Date;
            }
            var dateParsed = DateTime.TryParse(publicationDate, out var date);
            var year = 0;
            var month = 0;
            var day = 0;
            switch (dateParsed)
            {
                case true:
                    year = date.Year;
                    month = date.Month;
                    day = date.Day;
                    break;
                case false when !string.IsNullOrEmpty(publicationDate) && publicationDate.Length == 4:
                    int.TryParse(publicationDate, out year);
                    break;
            }

            var info =  new ComicInfo()
            {
                Summary = epubBook.Schema.Package.Metadata.Description,
                Writer = string.Join(",", epubBook.Schema.Package.Metadata.Creators.Select(c => Tasks.Scanner.Parser.Parser.CleanAuthor(c.Creator))),
                Publisher = string.Join(",", epubBook.Schema.Package.Metadata.Publishers),
                Month = month,
                Day = day,
                Year = year,
                Title = epubBook.Title,
                Genre = string.Join(",", epubBook.Schema.Package.Metadata.Subjects.Select(s => s.ToLower().Trim())),
                LanguageISO = epubBook.Schema.Package.Metadata.Languages.FirstOrDefault() ?? string.Empty
            };
            ComicInfo.CleanComicInfo(info);

            // Parse tags not exposed via Library
            foreach (var metadataItem in epubBook.Schema.Package.Metadata.MetaItems)
            {
                switch (metadataItem.Name)
                {
                    case "calibre:rating":
                        info.UserRating = float.Parse(metadataItem.Content);
                        break;
                    case "calibre:title_sort":
                        info.TitleSort = metadataItem.Content;
                        break;
                    case "calibre:series":
                        info.Series = metadataItem.Content;
                        info.SeriesSort = metadataItem.Content;
                        break;
                }
            }

            return info;
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "[GetComicInfo] There was an exception getting metadata");
        }

        return null;
    }

    private bool IsValidFile(string filePath)
    {
        if (!File.Exists(filePath))
        {
            _logger.LogWarning("[BookService] Book {EpubFile} could not be found", filePath);
            return false;
        }

        if (Tasks.Scanner.Parser.Parser.IsBook(filePath)) return true;

        _logger.LogWarning("[BookService] Book {EpubFile} is not a valid EPUB/PDF", filePath);
        return false;
    }

    public int GetNumberOfPages(string filePath)
    {
        if (!IsValidFile(filePath)) return 0;

        try
        {
            if (Tasks.Scanner.Parser.Parser.IsPdf(filePath))
            {
                using var docReader = DocLib.Instance.GetDocReader(filePath, new PageDimensions(1080, 1920));
                return docReader.GetPageCount();
            }

            using var epubBook = EpubReader.OpenBook(filePath, BookReaderOptions);
            return epubBook.Content.Html.Count;
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "[BookService] There was an exception getting number of pages, defaulting to 0");
        }

        return 0;
    }

    public static string EscapeTags(string content)
    {
        content = Regex.Replace(content, @"<script(.*)(/>)", "<script$1></script>");
        content = Regex.Replace(content, @"<title(.*)(/>)", "<title$1></title>");
        return content;
    }

    /// <summary>
    /// Removes all leading ../
    /// </summary>
    /// <param name="key"></param>
    /// <returns></returns>
    public static string CleanContentKeys(string key)
    {
        return key.Replace("../", string.Empty);
    }

    public async Task<Dictionary<string, int>> CreateKeyToPageMappingAsync(EpubBookRef book)
    {
        var dict = new Dictionary<string, int>();
        var pageCount = 0;
        foreach (var contentFileRef in await book.GetReadingOrderAsync())
        {
            if (contentFileRef.ContentType != EpubContentType.XHTML_1_1) continue;
            dict.Add(contentFileRef.FileName, pageCount);
            pageCount += 1;
        }

        return dict;
    }

    /// <summary>
    /// Parses out Title from book. Chapters and Volumes will always be "0". If there is any exception reading book (malformed books)
    /// then null is returned. This expects only an epub file
    /// </summary>
    /// <param name="filePath"></param>
    /// <returns></returns>
    public ParserInfo ParseInfo(string filePath)
    {
        if (!Tasks.Scanner.Parser.Parser.IsEpub(filePath)) return null;

        try
        {
            using var epubBook = EpubReader.OpenBook(filePath, BookReaderOptions);

            // <meta content="The Dark Tower" name="calibre:series"/>
            // <meta content="Wolves of the Calla" name="calibre:title_sort"/>
            // If all three are present, we can take that over dc:title and format as:
            // Series = The Dark Tower, Volume = 5, Filename as "Wolves of the Calla"
            // In addition, the following can exist and should parse as a series (EPUB 3.2 spec)
            // <meta property="belongs-to-collection" id="c01">
            //   The Lord of the Rings
            // </meta>
            // <meta refines="#c01" property="collection-type">set</meta>
            // <meta refines="#c01" property="group-position">2</meta>
            try
            {
                var seriesIndex = string.Empty;
                var series = string.Empty;
                var specialName = string.Empty;


                foreach (var metadataItem in epubBook.Schema.Package.Metadata.MetaItems)
                {
                    // EPUB 2 and 3
                    switch (metadataItem.Name)
                    {
                        case "calibre:series_index":
                            seriesIndex = metadataItem.Content;
                            break;
                        case "calibre:series":
                            series = metadataItem.Content;
                            break;
                        case "calibre:title_sort":
                            specialName = metadataItem.Content;
                            break;
                    }

                    // EPUB 3.2+ only
                    switch (metadataItem.Property)
                    {
                        case "group-position":
                            seriesIndex = metadataItem.Content;
                            break;
                        case "belongs-to-collection":
                            series = metadataItem.Content;
                            break;
                        case "collection-type":
                            // These look to be genres from https://manual.calibre-ebook.com/sub_groups.html
                            break;
                    }
                }

                if (!string.IsNullOrEmpty(series) && !string.IsNullOrEmpty(seriesIndex))
                {
                    if (string.IsNullOrEmpty(specialName))
                    {
                        specialName = epubBook.Title;
                    }
                    var info = new ParserInfo()
                    {
                        Chapters = Tasks.Scanner.Parser.Parser.DefaultChapter,
                        Edition = string.Empty,
                        Format = MangaFormat.Epub,
                        Filename = Path.GetFileName(filePath),
                        Title = specialName?.Trim(),
                        FullFilePath = filePath,
                        IsSpecial = false,
                        Series = series.Trim(),
                        SeriesSort = series.Trim(),
                        Volumes = seriesIndex
                    };

                    return info;
                }
            }
            catch (Exception)
            {
                // Swallow exception
            }

            return new ParserInfo()
            {
                Chapters = Tasks.Scanner.Parser.Parser.DefaultChapter,
                Edition = string.Empty,
                Format = MangaFormat.Epub,
                Filename = Path.GetFileName(filePath),
                Title = epubBook.Title.Trim(),
                FullFilePath = filePath,
                IsSpecial = false,
                Series = epubBook.Title.Trim(),
                Volumes = Tasks.Scanner.Parser.Parser.DefaultVolume,
            };
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "[BookService] There was an exception when opening epub book: {FileName}", filePath);
        }

        return null;
    }

    /// <summary>
    /// Extracts a pdf into images to a target directory. Uses multi-threaded implementation since docnet is slow normally.
    /// </summary>
    /// <param name="fileFilePath"></param>
    /// <param name="targetDirectory"></param>
    public void ExtractPdfImages(string fileFilePath, string targetDirectory)
    {
        _directoryService.ExistOrCreate(targetDirectory);

        using var docReader = DocLib.Instance.GetDocReader(fileFilePath, new PageDimensions(1080, 1920));
        var pages = docReader.GetPageCount();
        Parallel.For(0, pages, pageNumber =>
        {
            using var stream = StreamManager.GetStream("BookService.GetPdfPage");
            GetPdfPage(docReader, pageNumber, stream);
            using var fileStream = File.Create(Path.Combine(targetDirectory, "Page-" + pageNumber + ".png"));
            stream.Seek(0, SeekOrigin.Begin);
            stream.CopyTo(fileStream);
        });
    }

    /// <summary>
    /// Responsible to scope all the css, links, tags, etc to prepare a self contained html file for the page
    /// </summary>
    /// <param name="doc">Html Doc that will be appended to</param>
    /// <param name="book">Underlying epub</param>
    /// <param name="apiBase">API Url for file loading to pass through</param>
    /// <param name="body">Body element from the epub</param>
    /// <param name="mappings">Epub mappings</param>
    /// <param name="page">Page number we are loading</param>
    /// <returns></returns>
    public async Task<string> ScopePage(HtmlDocument doc, EpubBookRef book, string apiBase, HtmlNode body, Dictionary<string, int> mappings, int page)
    {
        await InlineStyles(doc, book, apiBase, body);

        RewriteAnchors(page, doc, mappings);

        ScopeImages(doc, book, apiBase);

        return PrepareFinalHtml(doc, body);
    }

    /// <summary>
    /// This will return a list of mappings from ID -> page num. ID will be the xhtml key and page num will be the reading order
    /// this is used to rewrite anchors in the book text so that we always load properly in our reader.
    /// </summary>
    /// <param name="chapter">Chapter with at least one file</param>
    /// <returns></returns>
    public async Task<ICollection<BookChapterItem>> GenerateTableOfContents(Chapter chapter)
    {
        using var book = await EpubReader.OpenBookAsync(chapter.Files.ElementAt(0).FilePath, BookReaderOptions);
        var mappings = await CreateKeyToPageMappingAsync(book);

        var navItems = await book.GetNavigationAsync();
        var chaptersList = new List<BookChapterItem>();

        foreach (var navigationItem in navItems)
        {
            if (navigationItem.NestedItems.Count == 0)
            {
                CreateToCChapter(navigationItem, Array.Empty<BookChapterItem>(), chaptersList, mappings);
                continue;
            }

            var nestedChapters = new List<BookChapterItem>();

            foreach (var nestedChapter in navigationItem.NestedItems.Where(n => n.Link != null))
            {
                var key = BookService.CleanContentKeys(nestedChapter.Link.ContentFileName);
                if (mappings.ContainsKey(key))
                {
                    nestedChapters.Add(new BookChapterItem()
                    {
                        Title = nestedChapter.Title,
                        Page = mappings[key],
                        Part = nestedChapter.Link.Anchor ?? string.Empty,
                        Children = new List<BookChapterItem>()
                    });
                }
            }

            CreateToCChapter(navigationItem, nestedChapters, chaptersList, mappings);
        }

        if (chaptersList.Count != 0) return chaptersList;
        // Generate from TOC
        var tocPage = book.Content.Html.Keys.FirstOrDefault(k => k.ToUpper().Contains("TOC"));
        if (tocPage == null) return chaptersList;

        // Find all anchor tags, for each anchor we get inner text, to lower then title case on UI. Get href and generate page content
        var doc = new HtmlDocument();
        var content = await book.Content.Html[tocPage].ReadContentAsync();
        doc.LoadHtml(content);
        var anchors = doc.DocumentNode.SelectNodes("//a");
        if (anchors == null) return chaptersList;

        foreach (var anchor in anchors)
        {
            if (!anchor.Attributes.Contains("href")) continue;

            var key = BookService.CleanContentKeys(anchor.Attributes["href"].Value).Split("#")[0];
            if (!mappings.ContainsKey(key))
            {
                // Fallback to searching for key (bad epub metadata)
                var correctedKey = book.Content.Html.Keys.SingleOrDefault(s => s.EndsWith(key));
                if (!string.IsNullOrEmpty(correctedKey))
                {
                    key = correctedKey;
                }
            }

            if (string.IsNullOrEmpty(key) || !mappings.ContainsKey(key)) continue;
            var part = string.Empty;
            if (anchor.Attributes["href"].Value.Contains('#'))
            {
                part = anchor.Attributes["href"].Value.Split("#")[1];
            }
            chaptersList.Add(new BookChapterItem()
            {
                Title = anchor.InnerText,
                Page = mappings[key],
                Part = part,
                Children = new List<BookChapterItem>()
            });
        }

        return chaptersList;
    }

    /// <summary>
    /// This returns a single page within the epub book. All html will be rewritten to be scoped within our reader,
    /// all css is scoped, etc.
    /// </summary>
    /// <param name="page">The requested page</param>
    /// <param name="chapterId">The chapterId</param>
    /// <param name="cachedEpubPath">The path to the cached epub file</param>
    /// <param name="baseUrl">The API base for Kavita, to rewrite urls to so we load though our endpoint</param>
    /// <returns>Full epub HTML Page, scoped to Kavita's reader</returns>
    /// <exception cref="KavitaException">All exceptions throw this</exception>
    public async Task<string> GetBookPage(int page, int chapterId, string cachedEpubPath, string baseUrl)
    {
        using var book = await EpubReader.OpenBookAsync(cachedEpubPath, BookReaderOptions);
        var mappings = await CreateKeyToPageMappingAsync(book);
        var apiBase = baseUrl + "book/" + chapterId + "/" + BookApiUrl;

        var counter = 0;
        var doc = new HtmlDocument {OptionFixNestedTags = true};


        var bookPages = await book.GetReadingOrderAsync();
        foreach (var contentFileRef in bookPages)
        {
            if (page != counter)
            {
                counter++;
                continue;
            }

            var content = await contentFileRef.ReadContentAsync();
            if (contentFileRef.ContentType != EpubContentType.XHTML_1_1) return content;

            // In more cases than not, due to this being XML not HTML, we need to escape the script tags.
            content = BookService.EscapeTags(content);

            doc.LoadHtml(content);
            var body = doc.DocumentNode.SelectSingleNode("//body");

            if (body == null)
            {
                if (doc.ParseErrors.Any())
                {
                    LogBookErrors(book, contentFileRef, doc);
                    throw new KavitaException("The file is malformed! Cannot read.");
                }
                _logger.LogError("{FilePath} has no body tag! Generating one for support. Book may be skewed", book.FilePath);
                doc.DocumentNode.SelectSingleNode("/html").AppendChild(HtmlNode.CreateNode("<body></body>"));
                body = doc.DocumentNode.SelectSingleNode("/html/body");
            }

            return await ScopePage(doc, book, apiBase, body, mappings, page);
        }

        throw new KavitaException("Could not find the appropriate html for that page");
    }

    private static void CreateToCChapter(EpubNavigationItemRef navigationItem, IList<BookChapterItem> nestedChapters, IList<BookChapterItem> chaptersList,
        IReadOnlyDictionary<string, int> mappings)
    {
        if (navigationItem.Link == null)
        {
            var item = new BookChapterItem()
            {
                Title = navigationItem.Title,
                Children = nestedChapters
            };
            if (nestedChapters.Count > 0)
            {
                item.Page = nestedChapters[0].Page;
            }

            chaptersList.Add(item);
        }
        else
        {
            var groupKey = CleanContentKeys(navigationItem.Link.ContentFileName);
            if (mappings.ContainsKey(groupKey))
            {
                chaptersList.Add(new BookChapterItem()
                {
                    Title = navigationItem.Title,
                    Page = mappings[groupKey],
                    Children = nestedChapters
                });
            }
        }
    }


    /// <summary>
    /// Extracts the cover image to covers directory and returns file path back
    /// </summary>
    /// <param name="fileFilePath"></param>
    /// <param name="fileName">Name of the new file.</param>
    /// <param name="outputDirectory">Where to output the file, defaults to covers directory</param>
    /// <returns></returns>
    public string GetCoverImage(string fileFilePath, string fileName, string outputDirectory)
    {
        if (!IsValidFile(fileFilePath)) return string.Empty;

        if (Tasks.Scanner.Parser.Parser.IsPdf(fileFilePath))
        {
            return GetPdfCoverImage(fileFilePath, fileName, outputDirectory);
        }

        using var epubBook = EpubReader.OpenBook(fileFilePath, BookReaderOptions);

        try
        {
            // Try to get the cover image from OPF file, if not set, try to parse it from all the files, then result to the first one.
            var coverImageContent = epubBook.Content.Cover
                                    ?? epubBook.Content.Images.Values.FirstOrDefault(file => Tasks.Scanner.Parser.Parser.IsCoverImage(file.FileName))
                                    ?? epubBook.Content.Images.Values.FirstOrDefault();

            if (coverImageContent == null) return string.Empty;
            using var stream = coverImageContent.GetContentStream();

            return _imageService.WriteCoverThumbnail(stream, fileName, outputDirectory);
        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex, "[BookService] There was a critical error and prevented thumbnail generation on {BookFile}. Defaulting to no cover image", fileFilePath);
        }

        return string.Empty;
    }


    private string GetPdfCoverImage(string fileFilePath, string fileName, string outputDirectory)
    {
        try
        {
            using var docReader = DocLib.Instance.GetDocReader(fileFilePath, new PageDimensions(1080, 1920));
            if (docReader.GetPageCount() == 0) return string.Empty;

            using var stream = StreamManager.GetStream("BookService.GetPdfPage");
            GetPdfPage(docReader, 0, stream);

            return _imageService.WriteCoverThumbnail(stream, fileName, outputDirectory);

        }
        catch (Exception ex)
        {
            _logger.LogWarning(ex,
                "[BookService] There was a critical error and prevented thumbnail generation on {BookFile}. Defaulting to no cover image",
                fileFilePath);
        }

        return string.Empty;
    }

    /// <summary>
    /// Returns an image raster of a page within a PDF
    /// </summary>
    /// <param name="docReader"></param>
    /// <param name="pageNumber"></param>
    /// <param name="stream"></param>
    private static void GetPdfPage(IDocReader docReader, int pageNumber, Stream stream)
    {
        using var pageReader = docReader.GetPageReader(pageNumber);
        var rawBytes = pageReader.GetImage(new NaiveTransparencyRemover());
        var width = pageReader.GetPageWidth();
        var height = pageReader.GetPageHeight();
        var image = Image.LoadPixelData<Bgra32>(rawBytes, width, height);

        stream.Seek(0, SeekOrigin.Begin);
        image.SaveAsPng(stream);
        stream.Seek(0, SeekOrigin.Begin);
    }

    private static string RemoveWhiteSpaceFromStylesheets(string body)
    {
        if (string.IsNullOrEmpty(body))
        {
            return string.Empty;
        }

        // Remove comments from CSS
        body = Regex.Replace(body, @"/\*[\d\D]*?\*/", string.Empty);

        body = Regex.Replace(body, @"[a-zA-Z]+#", "#");
        body = Regex.Replace(body, @"[\n\r]+\s*", string.Empty);
        body = Regex.Replace(body, @"\s+", " ");
        body = Regex.Replace(body, @"\s?([:,;{}])\s?", "$1");
        try
        {
            body = body.Replace(";}", "}");
        }
        catch (Exception)
        {
            /* Swallow exception. Some css don't have style rules ending in ; */
        }

        body = Regex.Replace(body, @"([\s:]0)(px|pt|%|em)", "$1");


        return body;
    }

    private void LogBookErrors(EpubBookRef book, EpubContentFileRef contentFileRef, HtmlDocument doc)
    {
        _logger.LogError("{FilePath} has an invalid html file (Page {PageName})", book.FilePath, contentFileRef.FileName);
        foreach (var error in doc.ParseErrors)
        {
            _logger.LogError("Line {LineNumber}, Reason: {Reason}", error.Line, error.Reason);
        }
    }
}