From a8b14fd29cf7cebd10d46cb44f99253085cfe43b Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 27 Jan 2023 10:08:41 +0100
Subject: [PATCH] Fix the 4rd hardest problem in computer science.

Off by one errors.
---
 .../Automation/Peers/TextBoxAutomationPeer.cs |  2 +-
 .../Automation/Provider/TextRange.cs          |  8 ++--
 .../Automation/AutomationTextRange.cs         | 38 ++++++++++++++++---
 3 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/src/Avalonia.Controls/Automation/Peers/TextBoxAutomationPeer.cs b/src/Avalonia.Controls/Automation/Peers/TextBoxAutomationPeer.cs
index 19bfcc27c6..ec70fd2831 100644
--- a/src/Avalonia.Controls/Automation/Peers/TextBoxAutomationPeer.cs
+++ b/src/Avalonia.Controls/Automation/Peers/TextBoxAutomationPeer.cs
@@ -63,7 +63,7 @@ namespace Avalonia.Automation.Peers
 
         public IReadOnlyList<TextRange> GetSelection()
         {
-            var range = TextRange.FromInclusiveStartEnd(Owner.SelectionStart, Owner.SelectionEnd);
+            var range = TextRange.FromStartEnd(Owner.SelectionStart, Owner.SelectionEnd);
             return new[] { range };
         }
 
diff --git a/src/Avalonia.Controls/Automation/Provider/TextRange.cs b/src/Avalonia.Controls/Automation/Provider/TextRange.cs
index dd732fb691..0f018ba758 100644
--- a/src/Avalonia.Controls/Automation/Provider/TextRange.cs
+++ b/src/Avalonia.Controls/Automation/Provider/TextRange.cs
@@ -45,12 +45,12 @@ namespace Avalonia.Automation.Provider
         public static TextRange Empty => new(0, 0);
 
         /// <summary>
-        /// Creates a new <see cref="TextRange"/> from an inclusive start and end index.
+        /// Creates a new <see cref="TextRange"/> from an exclusive start and end index.
         /// </summary>
-        /// <param name="start">The inclusive start index of the range.</param>
-        /// <param name="end">The inclusive end index of the range.</param>
+        /// <param name="start">The exclusive start index of the range.</param>
+        /// <param name="end">The exclusive end index of the range.</param>
         /// <returns></returns>
-        public static TextRange FromInclusiveStartEnd(int start, int end)
+        public static TextRange FromStartEnd(int start, int end)
         {
             var s = Math.Min(start, end);
             var e = Math.Max(start, end);
diff --git a/src/Windows/Avalonia.Win32/Automation/AutomationTextRange.cs b/src/Windows/Avalonia.Win32/Automation/AutomationTextRange.cs
index 66dffc3cec..5faf58fcad 100644
--- a/src/Windows/Avalonia.Win32/Automation/AutomationTextRange.cs
+++ b/src/Windows/Avalonia.Win32/Automation/AutomationTextRange.cs
@@ -12,6 +12,8 @@ namespace Avalonia.Win32.Automation
     internal class AutomationTextRange : ITextRangeProvider
     {
         private readonly AutomationNode _owner;
+        private int _start;
+        private int _end;
 
         public AutomationTextRange(AutomationNode owner, TextRange range)
             : this(owner, range.Start, range.End)
@@ -21,13 +23,37 @@ namespace Avalonia.Win32.Automation
         public AutomationTextRange(AutomationNode owner, int start, int end)
         {
             _owner = owner;
-            Start = start;
-            End = end;
+            _start = start;
+            _end = end;
+        }
+
+        public int Start
+        {
+            get => _start;
+            private set
+            {
+                if (value < 0)
+                    throw new InvalidOperationException();
+                if (value > _end)
+                    _end = value;
+                _start = value;
+            }
+        }
+
+        public int End
+        {
+            get => _end;
+            private set
+            {
+                if (value < 0)
+                    throw new InvalidOperationException();
+                if (value < _start)
+                    _start = value;
+                _end = value;
+            }
         }
 
-        public int Start { get; private set; }
-        public int End { get; private set; }
-        public TextRange Range => TextRange.FromInclusiveStartEnd(Start, End);
+        public TextRange Range => new(Start, End);
 
         private AAP.ITextProvider InnerProvider => (AAP.ITextProvider)_owner.Peer;
 
@@ -410,7 +436,7 @@ namespace Avalonia.Win32.Automation
                 case TextUnit.Word:
                     for (moved = 0; moved > count && index > 0; moved--)
                     {
-                        for (index--; index < text.Length && IsWordBreak(text[index]); index--)
+                        for (index--; index >= 0 && IsWordBreak(text[index]); index--)
                             ;
                         for (index--; !AtWordBoundary(text, index); index--)
                             ;