Add same-letter cycling for file-jumping

This will allow files selected by their
first letter to be cycled when the same
letter is pressed.

e.g. having files "a.txt" and "ab.txt"
Pressing "a" the first time will select
"a.txt", pressing "a" a second time will
selected "ab.txt" instead of reselecting
"a.txt"

Author: jeffsieu

Files UWP/Filesystem/ItemViewModel.cs

@@ -182,15 +182,36 @@ public string JumpString
             }
             set
             {
+                // If current string is "a", and the next character typed is "a",
+                // search for next file that starts with "a" (a.k.a. _jumpString = "a")
+                if (_jumpString.Length == 1 && value == _jumpString + _jumpString)
+                {
+                    value = _jumpString;
+                }
                 if (value != "")
                 {
                     ListedItem jumpedToItem = null;
-                    try
+                    ListedItem previouslySelectedItem = null;
+                    var candidateItems = _filesAndFolders.Where(f => f.FileName.Length >= value.Length && f.FileName.Substring(0, value.Length).ToLower() == value);
+
+                    if (App.selectedTabInstance.accessibleContentFrame.SourcePageType == typeof(GenericFileBrowser))
+                    {
+                        previouslySelectedItem = (App.selectedTabInstance.accessibleContentFrame.Content as GenericFileBrowser).AllView.SelectedItem as ListedItem;
+                    }
+                    else if (App.selectedTabInstance.accessibleContentFrame.SourcePageType == typeof(PhotoAlbum))
+                    {
+                        previouslySelectedItem = (App.selectedTabInstance.accessibleContentFrame.Content as PhotoAlbum).gv.SelectedItem as ListedItem;
+                    }
+
+                    // If the user is trying to cycle through items
+                    // starting with the same letter
+                    if (value.Length == 1 && previouslySelectedItem != null)
                     {
-                        jumpedToItem = _filesAndFolders.Where(f => f.FileName.Substring(0, value.Length).ToLower() == value).First();
+                        // Try to select item lexicographically bigger than the previous item
+                        jumpedToItem = candidateItems.FirstOrDefault(f => f.FileName.CompareTo(previouslySelectedItem.FileName) > 0);
                     }
-                    catch (ArgumentOutOfRangeException) { }
-                    catch (InvalidOperationException) { }
+                    if (jumpedToItem == null)
+                        jumpedToItem = candidateItems.FirstOrDefault();
 
                     if (jumpedToItem != null)
                     {