Working solution (only if EssentialCSharp.Common adds orderOnPage)

Author: Joshua-Lester3

EssentialCSharp.Web/Services/ISiteMappingService.cs

@@ -3,4 +3,5 @@
 public interface ISiteMappingService
 {
     IList<SiteMapping> SiteMappings { get; }
+    IEnumerable<SiteMappingDto> GetTocData();
 }

EssentialCSharp.Web/Services/SiteMappingDto.cs

@@ -0,0 +1,10 @@
+namespace EssentialCSharp.Web.Services;
+
+public class SiteMappingDto
+{
+    public required int Level { get; set; }
+    public required List<string> Key { get; set; }
+    public required string Href { get; set; }
+    public required string Title { get; set; }
+    public required IEnumerable<SiteMappingDto> Items { get; set; }
+}

EssentialCSharp.Web/Services/SiteMappingService.cs

@@ -12,4 +12,42 @@ public SiteMappingService(IWebHostEnvironment webHostEnvironment)
         List<SiteMapping>? siteMappings = System.Text.Json.JsonSerializer.Deserialize<List<SiteMapping>>(File.OpenRead(path)) ?? throw new InvalidOperationException("No table of contents found");
         SiteMappings = siteMappings;
     }
+
+    public IEnumerable<SiteMappingDto> GetTocData()
+    {
+        return SiteMappings.GroupBy(x => x.ChapterNumber).OrderBy(x => x.Key).Select(x =>
+        {
+            IEnumerable<SiteMapping> orderedX = x.OrderBy(i => i.PageNumber).ThenBy(i => i.OrderOnPage);
+            SiteMapping firstX = orderedX.First();
+            return new SiteMappingDto()
+            {
+                Level = 0,
+                Key = [firstX.Keys.First()],
+                Href = $"{firstX.Keys.First()}#{firstX.AnchorId}",
+                Title = $"Chapter {x.Key}: {firstX.ChapterTitle}",
+                Items = GetItems(orderedX.Skip(1), 1)
+            };
+        }
+        );
+    }
+
+    private static IEnumerable<SiteMappingDto> GetItems(IEnumerable<SiteMapping> chapterItems, int indentLevel)
+    {
+        return chapterItems
+           // Examine all items up until we move up to a level higher than where we're starting,
+           // which would indicate that we've reached the end of the entries nested under `indentationLevel`
+           .TakeWhile(i => i.IndentLevel >= indentLevel)
+            // Of all the multi-level descendants we found, take only those at the current level that we're wanting to render.
+            .Where(i => i.IndentLevel == indentLevel)
+            .Select(i => new SiteMappingDto()
+            {
+                Level = indentLevel,
+                Key = i.Keys,
+                Href = $"{i.Keys.First()}#{i.AnchorId}",
+                Title = i.RawHeading,
+                // Any children of this node will be /after/ this node,
+                // so skip any items that are /before/ the current node.
+                Items = GetItems(chapterItems.SkipWhile(q => i.Keys.First() != q.Keys.First()).Skip(1), indentLevel + 1)
+            });
+    }
 }

EssentialCSharp.Web/Views/Shared/_Layout.cshtml

@@ -266,32 +266,33 @@
     @await RenderSectionAsync("Scripts", required: false);
     <script>
         @{
-            object GetItems(IEnumerable<SiteMapping> chapterItems, int indentLevel) => chapterItems
-            // Skip the chapter entry itself
-            .Skip(1)
-            // Examine all items up until we move up to a level higher than where we're starting,
-            // which would indicate that we've reached the end of the entries nested under `indentationLevel`
-            .TakeWhile(i => i.IndentLevel >= indentLevel)
-            // Of all the multi-level descendants we found, take only those at the current level that we're wanting to render.
-            .Where(i => i.IndentLevel == indentLevel)
-            .Select(i => new
-            {
-                Level = indentLevel,
-                Key = i.Keys.First(),
-                Href = $"{i.Keys.First()}#{i.AnchorId}",
-                Title = i.RawHeading,
-                // Any children of this node will be /after/ this node,
-                // so skip any items that are /before/ the current node.
-                Items = GetItems(chapterItems.SkipWhile(q => i.Keys.First() != q.Keys.First()), indentLevel + 1)
-            });
-            var tocData = _SiteMappings.SiteMappings.GroupBy(x => x.ChapterNumber).OrderBy(x => x.Key).Select(x => new
-            {
-                Level = 0,
-                Key = x.First().Keys.First(),
-                Href = $"{x.First().Keys.First()}#{x.First().AnchorId}",
-                Title = $"Chapter {x.Key}: {x.First().ChapterTitle}",
-                Items = GetItems(x, 1)
-            });
+            // object GetItems(IEnumerable<SiteMapping> chapterItems, int indentLevel) => chapterItems
+            // // Skip the chapter entry itself
+            // .Skip(1)
+            // // Examine all items up until we move up to a level higher than where we're starting,
+            // // which would indicate that we've reached the end of the entries nested under `indentationLevel`
+            // .TakeWhile(i => i.IndentLevel >= indentLevel)
+            // // Of all the multi-level descendants we found, take only those at the current level that we're wanting to render.
+            // .Where(i => i.IndentLevel == indentLevel)
+            // .Select(i => new
+            // {
+            //     Level = indentLevel,
+            //     Key = i.Keys,
+            //     Href = $"{i.Keys.First()}#{i.AnchorId}",
+            //     Title = i.RawHeading,
+            //     // Any children of this node will be /after/ this node,
+            //     // so skip any items that are /before/ the current node.
+            //     Items = GetItems(chapterItems.SkipWhile(q => i.Keys.First() != q.Keys.First()), indentLevel + 1)
+            // });
+            // var tocData = _SiteMappings.SiteMappings.GroupBy(x => x.ChapterNumber).OrderBy(x => x.Key).Select(x => new
+            // {
+            //     Level = 0,
+            //     Key = x.First().Keys.First(),
+            //     Href = $"{x.First().Keys.First()}#{x.First().AnchorId}",
+            //     Title = $"Chapter {x.Key}: {x.First().ChapterTitle}",
+            //     Items = GetItems(x, 1)
+            // });
+            var tocData = _SiteMappings.GetTocData();
         }
 
             PREVIOUS_PAGE = @Json.Serialize(ViewBag.PreviousPage)