NavigationGraph.cs

namespace ShellDocs.Core;

public class NavigationGraph
{
 public NavigationNode Root { get; }

 private readonly Dictionary<string, NavigationNode> _byUrl;
 private readonly List<NavigationNode> _flatPages;

 public NavigationGraph(NavigationNode root, IEnumerable<NavigationNode>? hiddenPages = null)
 {
 Root = root;
 _byUrl = new Dictionary<string, NavigationNode>(StringComparer.OrdinalIgnoreCase);
 _flatPages = new List<NavigationNode>();
 Index(root);
 // Hidden pages route (URLs resolve) but never appear in the visible
 // tree, so they're excluded from _flatPages (prev/next skips them).
 if (hiddenPages is not null)
 {
 foreach (var page in hiddenPages) IndexHidden(page);
 }
 }

 private void IndexHidden(NavigationNode node)
 {
 if (node.Kind == NodeKind.Page && !string.IsNullOrEmpty(node.Url))
 {
 _byUrl[Normalize(node.Url)] = node;
 }
 foreach (var child in node.Children) IndexHidden(child);
 }

 public NavigationNode? ResolveByUrl(string url)
 {
 var key = Normalize(url);
 return _byUrl.TryGetValue(key, out var node) ? node : null;
 }

 public (NavigationNode? Prev, NavigationNode? Next) GetPrevNext(NavigationNode node)
 {
 var i = _flatPages.IndexOf(node);
 if (i < 0) return (null, null);
 var prev = i > 0 ? _flatPages[i - 1] : null;
 var next = i < _flatPages.Count - 1 ? _flatPages[i + 1] : null;
 return (prev, next);
 }

 public IReadOnlyList<NavigationNode> GetBreadcrumb(NavigationNode node)
 {
 var chain = new List<NavigationNode>();
 var current = node;
 while (current is not null && current != Root)
 {
 chain.Add(current);
 current = current.Parent;
 }
 chain.Reverse();
 return chain;
 }

 public IEnumerable<NavigationNode> Flatten()
 {
 return FlattenFrom(Root);
 }

 private static IEnumerable<NavigationNode> FlattenFrom(NavigationNode node)
 {
 yield return node;
 foreach (var child in node.Children)
 {
 foreach (var n in FlattenFrom(child)) yield return n;
 }
 }

 private void Index(NavigationNode node)
 {
 if (node.Kind == NodeKind.Page && !string.IsNullOrEmpty(node.Url))
 {
 _byUrl[Normalize(node.Url)] = node;
 _flatPages.Add(node);
 }
 foreach (var child in node.Children) Index(child);
 }

 private static string Normalize(string url)
 {
 var s = url.Trim();
 if (!s.StartsWith('/')) s = "/" + s;
 if (s.Length > 1 && s.EndsWith('/')) s = s[..^1];
 return s;
 }
}