From fda22de83dd4633d5b938f2cf6c9e76975f9cb0d Mon Sep 17 00:00:00 2001
From: Ellpeck <me@ellpeck.de>
Date: Mon, 17 Jul 2023 15:20:36 +0200
Subject: [PATCH] Fixed TextInput not working correctly when using surrogate
 pairs

---
 CHANGELOG.md                 |  3 +++
 MLEM/Font/CodePointSource.cs | 12 ++++++++++++
 MLEM/Input/TextInput.cs      | 10 ++++++----
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 90c3afa..562a04c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,9 @@ Jump to version:
 Additions
 - Added GraphicsExtensions.WithRenderTargets, a multi-target version of WithRenderTarget
 
+Fixes
+- Fixed TextInput not working correctly when using surrogate pairs
+
 ## 6.2.0
 
 ### MLEM
diff --git a/MLEM/Font/CodePointSource.cs b/MLEM/Font/CodePointSource.cs
index fe4e3d2..cae77b2 100644
--- a/MLEM/Font/CodePointSource.cs
+++ b/MLEM/Font/CodePointSource.cs
@@ -63,6 +63,18 @@ namespace MLEM.Font {
             return (curr, 1);
         }
 
+        /// <summary>
+        /// Returns an index in this code point source that is as close to <paramref name="index"/> as possible, but not between two members of a surrogate pair. If the <paramref name="index"/> is already not between surrogate pairs, it is returned unchanged.
+        /// </summary>
+        /// <param name="index">The index to ensure is not between surrogates.</param>
+        /// <param name="increase">Whether the returned index should be increased by 1 (instead of decreased by 1) when it is between surrogates.</param>
+        /// <returns>An index close to <paramref name="index"/>, but not between surrogates.</returns>
+        public int EnsureSurrogateBoundary(int index, bool increase) {
+            if (index < this.Length && char.IsLowSurrogate(this[index]))
+                return increase && index < this.Length - 1 || index <= 0 ? index + 1 : index - 1;
+            return index;
+        }
+
         /// <summary>Returns an enumerator that iterates through the collection.</summary>
         /// <returns>A <see cref="T:System.Collections.Generic.IEnumerator`1" /> that can be used to iterate through the collection.</returns>
         /// <filterpriority>1</filterpriority>
diff --git a/MLEM/Input/TextInput.cs b/MLEM/Input/TextInput.cs
index f91fc87..2061b05 100644
--- a/MLEM/Input/TextInput.cs
+++ b/MLEM/Input/TextInput.cs
@@ -105,7 +105,8 @@ namespace MLEM.Input {
             set {
                 var val = (int) MathHelper.Clamp(value, 0F, this.text.Length);
                 if (this.caretPos != val) {
-                    this.caretPos = val;
+                    // ensure that we don't move to a location that is between high and low surrogates
+                    this.caretPos = new CodePointSource(this.text).EnsureSurrogateBoundary(val, val > this.caretPos);
                     this.caretBlinkTimer = 0;
                     this.SetTextDataDirty(false);
                 }
@@ -360,7 +361,7 @@ namespace MLEM.Input {
             if (!this.FilterText(ref strg, removeMismatching))
                 return;
             if (this.MaximumCharacters != null && strg.Length > this.MaximumCharacters)
-                strg = strg.Substring(0, this.MaximumCharacters.Value);
+                strg = strg.Substring(0, new CodePointSource(strg).EnsureSurrogateBoundary(this.MaximumCharacters.Value, false));
             this.text.Clear();
             this.text.Append(strg);
             this.CaretPos = this.text.Length;
@@ -378,7 +379,7 @@ namespace MLEM.Input {
             if (!this.FilterText(ref strg, removeMismatching))
                 return false;
             if (this.MaximumCharacters != null && this.text.Length + strg.Length > this.MaximumCharacters)
-                strg = strg.Substring(0, this.MaximumCharacters.Value - this.text.Length);
+                strg = strg.Substring(0, new CodePointSource(strg).EnsureSurrogateBoundary(this.MaximumCharacters.Value - this.text.Length, false));
             this.text.Insert(this.CaretPos, strg);
             this.CaretPos += strg.Length;
             this.SetTextDataDirty();
@@ -393,7 +394,8 @@ namespace MLEM.Input {
         public bool RemoveText(int index, int length) {
             if (index < 0 || index >= this.text.Length)
                 return false;
-            this.text.Remove(index, length);
+            var source = new CodePointSource(this.text);
+            this.text.Remove(source.EnsureSurrogateBoundary(index, false), source.EnsureSurrogateBoundary(index + length, true) - index);
             // ensure that caret pos is still in bounds
             this.CaretPos = this.CaretPos;
             this.SetTextDataDirty();