Nexus.Reader/src/NexusReader.Infrastructure/Services/EpubService.cs

using System.Text;
using System.Text.RegularExpressions;
using FluentResults;
using NexusReader.Application.Abstractions.Services;
using NexusReader.Application.Queries.Reader;
using VersOne.Epub;

namespace NexusReader.Infrastructure.Services;

public class EpubService : IEpubService
{
    private const string EpubPath = "wwwroot/assets/book.epub";
    private const int WordThreshold = 1000;

    public async Task<Result<ReaderPageViewModel>> GetEpubContentAsync(int chapterIndex)
    {
        try
        {
            // Path handling: Recursive search upwards to find the asset in development or production
            var relativePath = Path.Combine("wwwroot", "assets", "book.epub");
            string? fullPath = null;
            var searchPaths = new List<string>();

            var currentDir = new DirectoryInfo(AppDomain.CurrentDomain.BaseDirectory);
            while (currentDir != null)
            {
                var checkPath1 = Path.Combine(currentDir.FullName, relativePath);
                var checkPath2 = Path.Combine(currentDir.FullName, "src", "NexusReader.Web.New", relativePath);
                
                searchPaths.Add(checkPath1);
                if (File.Exists(checkPath1)) { fullPath = checkPath1; break; }
                
                searchPaths.Add(checkPath2);
                if (File.Exists(checkPath2)) { fullPath = checkPath2; break; }
                
                currentDir = currentDir.Parent;
            }
            
            if (fullPath == null)
            {
                return Result.Fail($"EPUB file not found. Checked {searchPaths.Count} locations, including: {string.Join(", ", searchPaths.Take(3))}");
            }

            if (!File.Exists(fullPath))
            {
                return Result.Fail($"EPUB file at '{fullPath}' is not accessible or does not exist.");
            }

            using var bookRef = await EpubReader.OpenBookAsync(fullPath);
            var readingOrder = bookRef.GetReadingOrder();

            if (readingOrder == null || !readingOrder.Any())
            {
                return Result.Fail("The EPUB has no readable content files in ReadingOrder.");
            }

            // Ensure index is within bounds
            if (chapterIndex < 0 || chapterIndex >= readingOrder.Count)
            {
                chapterIndex = 0; // Default to first chapter
            }

            var chapterRef = readingOrder[chapterIndex];
            
            // Try to find a better title from navigation (TOC)
            var navigation = bookRef.GetNavigation();
            var chapterTitle = FindTitleInNavigation(navigation, chapterRef.FilePath) 
                               ?? Path.GetFileNameWithoutExtension(chapterRef.FilePath) 
                               ?? $"Chapter {chapterIndex + 1}";

            var chapterContent = await chapterRef.ReadContentAsTextAsync();

            var blocks = new List<ContentBlock>();
            int totalWordCount = 0;
            int blockCounter = 0;

            var paragraphs = ExtractParagraphs(chapterContent);
            foreach (var p in paragraphs)
            {
                var sanitizedContent = SanitizeParagraph(p);
                if (string.IsNullOrWhiteSpace(sanitizedContent)) continue;

                // Requirement: Each paragraph mapped to its own TextSegmentBlock
                blocks.Add(new TextSegmentBlock($"seg-{blockCounter++}", sanitizedContent));

                int wordsInP = CountWords(sanitizedContent);
                totalWordCount += wordsInP;

                // Requirement: Smart Injection after 1000 words
                if (totalWordCount >= WordThreshold)
                {
                    blocks.Add(CreateAiTrigger($"trigger-{blockCounter++}"));
                    totalWordCount = 0;
                }
            }

            // End of chapter section trigger
            if (blocks.Any() && blocks.Last() is not AiActionTriggerBlock)
            {
                blocks.Add(CreateAiTrigger($"trigger-{blockCounter++}"));
            }

            return Result.Ok(new ReaderPageViewModel(blocks, chapterIndex, readingOrder.Count, chapterTitle));
        }
        catch (Exception ex)
        {
            return Result.Fail(new Error($"Failed to process EPUB: {ex.Message}").CausedBy(ex));
        }
    }

    private List<string> ExtractParagraphs(string html)
    {
        var bodyMatch = Regex.Match(html, @"<body\b[^>]*>(.*?)</body>", RegexOptions.IgnoreCase | RegexOptions.Singleline);
        var content = bodyMatch.Success ? bodyMatch.Groups[1].Value : html;

        var paragraphs = new List<string>();
        // Match block-level elements: h1-h6, p, ul, ol, blockquote, pre
        // We match the whole tag to preserve it for sanitization
        var matches = Regex.Matches(content, @"<(p|h[1-6]|ul|ol|blockquote|pre)\b[^>]*>.*?</\1>|<hr\b[^>]*>", RegexOptions.IgnoreCase | RegexOptions.Singleline);
        
        foreach (Match match in matches)
        {
            paragraphs.Add(match.Value);
        }

        // Fallback: split by double newlines if no block tags found
        if (paragraphs.Count == 0)
        {
            paragraphs = content.Split(new[] { "<br />", "<br>", "\n\n", "\r\n\r\n" }, StringSplitOptions.RemoveEmptyEntries).ToList();
        }

        return paragraphs;
    }

    private string SanitizeParagraph(string html)
    {
        // 1. Remove <style> and <script> blocks
        var clean = Regex.Replace(html, @"<(style|script)\b[^>]*>.*?</\1>", "", RegexOptions.IgnoreCase | RegexOptions.Singleline);
        
        // 2. Remove all tags except allowed structural and formatting tags
        clean = Regex.Replace(clean, @"<(?!/?(b|i|strong|em|h[1-6]|p|ul|ol|li|blockquote|pre|code|br|hr)\b)[^>]+>", "", RegexOptions.IgnoreCase);
        
        // 3. Requirement: Aggressively strip attributes (class, style, id) from allowed tags
        clean = Regex.Replace(clean, @"<(b|i|strong|em|h[1-6]|p|ul|ol|li|blockquote|pre|code|br|hr)\b[^>]*>", "<$1>", RegexOptions.IgnoreCase);
        
        // 4. Decode HTML entities
        clean = System.Net.WebUtility.HtmlDecode(clean);

        return clean.Trim();
    }

    private int CountWords(string text)
    {
        if (string.IsNullOrWhiteSpace(text)) return 0;
        return text.Split(new[] { ' ', '\r', '\n', '\t' }, StringSplitOptions.RemoveEmptyEntries).Length;
    }

    private AiActionTriggerBlock CreateAiTrigger(string id)
    {
        return new AiActionTriggerBlock(
            id,
            "Wykryto ciekawy fragment! Czy chcesz, abym wygenerował podsumowanie lub quiz z tego rozdziału?",
            new List<string> { "Podsumuj", "Generuj Quiz", "Pomiń" }
        );
    }

    private string? FindTitleInNavigation(IEnumerable<EpubNavigationItemRef> navigation, string? filePath)
    {
        if (string.IsNullOrEmpty(filePath)) return null;
        
        var fileName = Path.GetFileName(filePath);

        foreach (var item in navigation)
        {
            // Match by full path or just filename as fallback
            if (item.Link?.ContentFilePath == filePath || item.Link?.ContentFilePath == fileName)
                return item.Title;
                
            if (item.NestedItems != null && item.NestedItems.Any())
            {
                var childTitle = FindTitleInNavigation(item.NestedItems, filePath);
                if (childTitle != null) return childTitle;
            }
        }
        return null;
    }
}