/// <summary>
///     Generates the sidebar navigation tree from the folder structure, front matter, and config overrides.
/// </summary>
public sealed class NavigationBuildPhase(ILogger<NavigationBuildPhase> logger) : IBuildPhase
{
    /// <inheritdoc/>
    public string Name => "NavigationBuild";
    /// <inheritdoc/>
    public int Order => 600;

    /// <inheritdoc/>
    public Task ExecuteAsync(BuildContext context, CancellationToken ct = default)
    {
        var items = new List<NavigationNode>();
        // If explicit nav config exists, use it as the skeleton
        if (context.Config.Nav.Count > 0)
        {
            items = BuildFromConfig(context.Config.Nav, context.Pages);
        }
        else
        // Auto-generate from page routes
        {
            items = BuildFromPages(context.Pages);
        }

        context.Navigation = new NavigationTree
        {
            Items = items
        };
        logger.LogInformation("Built navigation tree with {Count} top-level items", items.Count);
        return Task.CompletedTask;
    }

    private static List<NavigationNode> BuildFromConfig(List<NavItem> navItems, List<DocPage> pages)
    {
        return navItems.Select(item =>
        {
            List<NavigationNode> children = item.Children.Count > 0 ? BuildFromConfig(item.Children, pages) : BuildChildrenFromPath(item.Path ?? "", pages);
            // Check if the nav item's path points to an actual page
            DocPage? matchingPage = pages.FirstOrDefault(p => string.Equals(p.Route, item.Path, StringComparison.OrdinalIgnoreCase));
            // If no page exists at this path, resolve to first child's route
            // so clicking doesn't hit a redirect page
            string? resolvedRoute = item.Path;
            if (matchingPage is null && !string.IsNullOrEmpty(item.Path) && children.Count > 0)
            {
                resolvedRoute = children[0].Route;
            }

            return new NavigationNode
            {
                Label = item.Label,
                Route = resolvedRoute,
                Icon = item.Icon,
                Expanded = item.Expanded,
                Children = children
            };
        }).ToList();
    }

    private static List<NavigationNode> BuildChildrenFromPath(string parentPath, List<DocPage> pages)
    {
        if (string.IsNullOrEmpty(parentPath))
        {
            return[];
        }

        return pages.Where(p => p.Route.StartsWith(parentPath + "/", StringComparison.OrdinalIgnoreCase) && p.Route[parentPath.Length..].Count(c => c == '/') == 1 && p.FrontMatter.Visibility == PageVisibility.Public).OrderBy(p => p.FrontMatter.Order).ThenBy(p => p.FrontMatter.Title, StringComparer.OrdinalIgnoreCase).Select(p => new NavigationNode { Label = p.FrontMatter.Title, Route = p.Route, Icon = p.FrontMatter.Icon, Order = p.FrontMatter.Order, Expanded = p.FrontMatter.Expanded, Children = BuildChildrenFromPath(p.Route, pages) }).ToList();
    }

    private static List<NavigationNode> BuildFromPages(List<DocPage> pages)
    {
        var publicPages = pages.Where(p => p.FrontMatter.Visibility == PageVisibility.Public).OrderBy(p => p.FrontMatter.Order).ThenBy(p => p.FrontMatter.Title, StringComparer.OrdinalIgnoreCase).ToList();
        // Group by top-level path segment
        var groups = new Dictionary<string, List<DocPage>>(StringComparer.OrdinalIgnoreCase);
        foreach (DocPage page in publicPages)
        {
            string[] segments = page.Route.Trim('/').Split('/', 2);
            string topLevel = segments.Length > 0 ? segments[0] : "";
            if (!groups.TryGetValue(topLevel, out List<DocPage>? list))
            {
                list = [];
                groups[topLevel] = list;
            }

            list.Add(page);
        }

        return groups.Select(g =>
        {
            DocPage? rootPage = g.Value.FirstOrDefault(p => p.Route.Trim('/') == g.Key || p.Route.Trim('/') == g.Key + "/index");
            string label = rootPage?.FrontMatter.Title ?? FormatLabel(g.Key);
            return new NavigationNode
            {
                Label = label,
                Route = rootPage?.Route ?? "/" + g.Key,
                Order = rootPage?.FrontMatter.Order ?? 0,
                Children = g.Value.Where(p => p != rootPage).Select(p => new NavigationNode { Label = p.FrontMatter.Title, Route = p.Route, Icon = p.FrontMatter.Icon, Order = p.FrontMatter.Order }).OrderBy(n => n.Order).ThenBy(n => n.Label, StringComparer.OrdinalIgnoreCase).ToList()
            };
        }).OrderBy(n => n.Order).ThenBy(n => n.Label, StringComparer.OrdinalIgnoreCase).ToList();
    }

    private static string FormatLabel(string pathSegment)
    {
        if (string.IsNullOrEmpty(pathSegment))
        {
            return "Home";
        }

        return char.ToUpper(pathSegment[0]) + pathSegment[1..].Replace('-', ' ');
    }
}