using Elternbeirat.Contracts;

namespace Elternbeirat.Web.Features.Layout;

/// <summary>
/// Builds the header and footer navigation from the public pages. Every page
/// carries a required <see cref="Page.Location"/> ("header" or "footer") and an
/// <see cref="Page.Order"/>; this splits them into the two menus, each ordered.
/// </summary>
public static class NavBuilder
{
    /// <summary>Header links, ordered, built from pages with location "header".</summary>
    public static IReadOnlyList<NavLinkItem> Header(IReadOnlyList<Page> pages) =>
        ToLinks(PagesAt(pages, "header"));

    /// <summary>Footer links, ordered, built from pages with location "footer".</summary>
    public static IReadOnlyList<NavLinkItem> Footer(IReadOnlyList<Page> pages) =>
        ToLinks(PagesAt(pages, "footer"));

    /// <summary>
    ///     Returns the pages of the header menu themselves, in menu order.
    /// </summary>
    /// <param name="pages">All public pages.</param>
    /// <returns>The pages with location <c>header</c>, ordered by <see cref="Page.Order"/>.</returns>
    /// <remarks>
    ///     For callers that need more than a link, such as the tiles on the home page,
    ///     which also show part of each page's body. Using the same selection as
    ///     <see cref="Header"/> keeps tiles and menu in step.
    /// </remarks>
    public static IReadOnlyList<Page> HeaderPages(IReadOnlyList<Page> pages) =>
        PagesAt(pages, "header");

    /// <summary>
    ///     Returns the URL a page is reached at.
    /// </summary>
    /// <param name="slug">The page's slug.</param>
    /// <returns><c>/</c> for the home page, otherwise <c>/{slug}</c>.</returns>
    /// <remarks>
    ///     The home page owns "/" (served by the Home component); every other slug is
    ///     reached at "/{slug}", where either a literal route or the catch-all content
    ///     page renders it.
    /// </remarks>
    public static string HrefFor(string slug) =>
        Page.IsHomeSlug(slug) ? "/" : $"/{slug}";

    private static List<Page> PagesAt(
        IReadOnlyList<Page> pages,
        string location) =>
        [.. pages
            .Where(page => string.Equals(page.Location, location, StringComparison.Ordinal))
            .OrderBy(page => page.Order)];

    private static List<NavLinkItem> ToLinks(IEnumerable<Page> pages) =>
        [.. pages.Select(page => new NavLinkItem(HrefFor(page.Slug), page.Title))];
}