using Markdig;
using Markdig.Extensions.Yaml;
using Markdig.Syntax;

namespace Elternbeirat.Web.Services;

/// <summary>
/// Reads the static content pages from <c>Content/pages/*.md</c>, renders them
/// to HTML once at startup and keeps them in memory. Registered as a singleton
/// because the content lives in the image and does not change at runtime.
/// </summary>
public sealed class PageService
{
    private readonly IReadOnlyDictionary<string, Page> _pagesBySlug;

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

        var directory = Path.Combine(environment.ContentRootPath, "Content", "pages");
        var pages = new Dictionary<string, Page>(StringComparer.OrdinalIgnoreCase);

        if (Directory.Exists(directory))
        {
            foreach (var path in Directory.EnumerateFiles(directory, "*.md"))
            {
                var page = Read(path, pipeline);
                pages[page.Slug] = page;
            }
        }

        _pagesBySlug = pages;
    }

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

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

        var document = Markdown.Parse(source, pipeline);
        var title = ReadTitle(document, source) ?? slug;
        var html = Markdown.ToHtml(source, pipeline);

        return new Page { Slug = slug, Title = title, ContentHtml = html };
    }

    /// <summary>
    /// Pulls the <c>titel:</c> value out of the YAML front matter. Kept
    /// deliberately simple (a single <c>key: value</c> line) — YamlDotNet will
    /// take over once we need structured metadata. The key stays German because
    /// the Markdown files are edited by German-speaking editors.
    /// </summary>
    private static string? ReadTitle(MarkdownDocument document, string source)
    {
        var block = document.Descendants<YamlFrontMatterBlock>().FirstOrDefault();
        if (block is null)
        {
            return null;
        }

        var frontMatter = source.Substring(block.Span.Start, block.Span.Length);
        foreach (var line in frontMatter.Split('\n'))
        {
            var trimmed = line.Trim();
            if (trimmed.StartsWith("titel:", StringComparison.OrdinalIgnoreCase))
            {
                return trimmed["titel:".Length..].Trim().Trim('"');
            }
        }

        return null;
    }
}