using System.Globalization;
using Markdig;

namespace Elternbeirat.Web.Services;

/// <summary>
/// Reads the news posts from <c>Content/posts/*.md</c>, renders them to HTML
/// once at startup and keeps them in memory, sorted newest first. Registered as
/// a singleton because the content lives in the image and does not change at
/// runtime.
/// </summary>
public sealed class PostService
{
    private readonly IReadOnlyList<Post> _posts;
    private readonly IReadOnlyDictionary<string, Post> _postsBySlug;

    /// <summary>
    /// Reads every Markdown post from the <c>Content/posts</c> directory
    /// below the application root and caches it.
    /// </summary>
    public PostService(IWebHostEnvironment environment)
    {
        var pipeline = new MarkdownPipelineBuilder()
            .UseYamlFrontMatter()
            .Build();

        var directory = Path.Combine(environment.ContentRootPath, "Content", "posts");
        var posts = new List<Post>();

        if (Directory.Exists(directory))
        {
            foreach (var path in Directory.EnumerateFiles(directory, "*.md"))
            {
                posts.Add(Read(path, pipeline));
            }
        }

        // Newest first for the list; a stable slug order breaks date ties.
        posts.Sort((a, b) =>
        {
            var byDate = b.Date.CompareTo(a.Date);
            return byDate != 0 ? byDate : string.CompareOrdinal(a.Slug, b.Slug);
        });

        _posts = posts;
        _postsBySlug = posts.ToDictionary(p => p.Slug, StringComparer.OrdinalIgnoreCase);
    }

    /// <summary>
    /// All posts, newest first. Used for the overview list.
    /// </summary>
    public IReadOnlyList<Post> All => _posts;

    /// <summary>
    /// Returns the post for the given slug, or <c>null</c> if there is none.
    /// The lookup is case-insensitive.
    /// </summary>
    public Post? Find(string slug) =>
        _postsBySlug.GetValueOrDefault(slug);

    private static Post Read(string path, MarkdownPipeline pipeline)
    {
        var slug = Path.GetFileNameWithoutExtension(path);
        var source = File.ReadAllText(path);

        var document = Markdown.Parse(source, pipeline);
        var title = FrontMatter.Read("title", document, source) ?? slug;
        var date = ParseDate(FrontMatter.Read("date", document, source));
        var html = Markdown.ToHtml(source, pipeline);

        return new Post { Slug = slug, Title = title, Date = date, ContentHtml = html };
    }

    /// <summary>
    /// Parses the <c>datum:</c> value as an ISO date (yyyy-MM-dd). Falls back to
    /// <see cref="DateOnly.MinValue"/> when the value is missing or malformed,
    /// so a single bad file does not crash startup — it just sorts last.
    /// </summary>
    private static DateOnly ParseDate(string? value)
    {
        return DateOnly.TryParse(value, CultureInfo.InvariantCulture,
            DateTimeStyles.None, out var date)
            ? date
            : DateOnly.MinValue;
    }
}