using Elternbeirat.Contracts;

namespace Elternbeirat.Web.Features.Posts;

/// <summary>
///     The posts next to a given post in time: the one published before it and the
///     one published after it.
/// </summary>
/// <param name="Previous">
///     The older post, or <see langword="null"/> if the post is the oldest.
/// </param>
/// <param name="Next">
///     The newer post, or <see langword="null"/> if the post is the newest.
/// </param>
/// <remarks>
///     "Previous" and "next" follow time, as a reader moving through the news would:
///     "Vorheriger Beitrag" is the older one, "Nächster Beitrag" the newer one. The
///     list order of the posts page (newest first) is the other way round, which is
///     why the direction is pinned down here instead of being left to the markup.
/// </remarks>
/// <seealso cref="PostDetail"/>
public sealed record PostNeighbors(Post? Previous, Post? Next)
{
    /// <summary>
    ///     Finds the neighbours of the post with the given slug.
    /// </summary>
    /// <param name="posts">
    ///     All public posts, newest first, as <c>PocketBaseClient.GetPostsAsync</c>
    ///     returns them.
    /// </param>
    /// <param name="slug">The slug of the post being shown.</param>
    /// <returns>
    ///     The older and the newer post. Both are <see langword="null"/> if the slug is
    ///     not in the list, or if it is the only post.
    /// </returns>
    /// <remarks>
    ///     The slug is compared case-insensitively, like the route lookup in
    ///     <see cref="PostDetail"/>, so a page reached as <c>/posts/New-Board</c> still
    ///     gets its neighbours.
    /// </remarks>
    /// <example>
    ///     <code>
    ///         // posts = [March, February, January] (newest first)
    ///         var neighbors = PostNeighbors.Of(posts, "february");
    ///         // neighbors.Previous is January, neighbors.Next is March
    ///     </code>
    /// </example>
    public static PostNeighbors Of(IReadOnlyList<Post> posts, string slug) =>
        IndexOf(posts, slug) is var index and >= 0
            ? new PostNeighbors(
                Previous: index + 1 < posts.Count ? posts[index + 1] : null,
                Next: index > 0 ? posts[index - 1] : null)
            : new PostNeighbors(null, null);

    /// <summary>
    ///     The position of the post with <paramref name="slug"/>, or <c>-1</c>.
    /// </summary>
    private static int IndexOf(IReadOnlyList<Post> posts, string slug) =>
        posts
            .Select((post, index) => (post, index))
            .FirstOrDefault(
                entry => string.Equals(entry.post.Slug, slug, StringComparison.OrdinalIgnoreCase),
                (null!, -1))
            .index;
}