From aaba6508b3aef649fe8cc832e823f38893a9c514 Mon Sep 17 00:00:00 2001
From: Leonard Hecker <lhecker@microsoft.com>
Date: Fri, 30 Jun 2023 16:41:12 +0200
Subject: [PATCH] Add an ASCII fast-pass to ROW

---
 src/buffer/out/Row.cpp | 71 +++++++++++++++++++++++++++++++++++++-----
 src/buffer/out/Row.hpp | 22 ++-----------
 2 files changed, 67 insertions(+), 26 deletions(-)

diff --git a/src/buffer/out/Row.cpp b/src/buffer/out/Row.cpp
index 2d6b3e7b8d5a..295a9b287e6f 100644
--- a/src/buffer/out/Row.cpp
+++ b/src/buffer/out/Row.cpp
@@ -538,28 +538,85 @@ catch (...)
 
 [[msvc::forceinline]] void ROW::WriteHelper::ReplaceText() noexcept
 {
+    auto it = chars.begin();
+    const auto end = chars.end();
     size_t ch = chBeg;
 
-    for (const auto& s : til::utf16_iterator{ chars })
+    // First a fast-pass for ASCII. ASCII is still predominant in technical areas.
+    while (it != end)
     {
-        const auto wide = til::at(s, 0) < 0x80 ? false : IsGlyphFullWidth(s);
-        const auto colEndNew = gsl::narrow_cast<uint16_t>(colEnd + 1u + wide);
+        // MSVC is hoists the inlined call outside of the loop and  puts it at the end of ROW::ReplaceText(), past WriteHelper::Finish().
+        // Inlining the call manually technically improves performance slightly, but is less maintainable.
+        if (*it >= 0x80)
+        {
+            _replaceTextUnicode(ch, it, end);
+            return;
+        }
+
+        if (colEnd >= colLimit)
+        {
+            colEndDirty = colLimit;
+            charsConsumed = ch - chBeg;
+            return;
+        }
+
+        til::at(row._charOffsets, colEnd) = gsl::narrow_cast<uint16_t>(ch);
+        ++colEnd;
+        ++ch;
+        ++it;
+    }
+
+    colEndDirty = colEnd;
+    charsConsumed = ch - chBeg;
+}
+
+[[msvc::forceinline]] void ROW::WriteHelper::_replaceTextUnicode(size_t ch, std::wstring_view::const_iterator it, const std::wstring_view::const_iterator end)
+{
+    while (it != end)
+    {
+        unsigned int width = 1;
+        auto ptr = &*it;
+        const auto wch = *ptr;
+        size_t advance = 1;
+
+        ++it;
+
+        if (wch >= 0x80)
+        {
+            if (til::is_surrogate(wch))
+            {
+                if (it != end && til::is_leading_surrogate(wch) && til::is_trailing_surrogate(*it))
+                {
+                    advance = 2;
+                    ++it;
+                }
+                else
+                {
+                    ptr = &UNICODE_REPLACEMENT;
+                }
+            }
+
+            width = IsGlyphFullWidth({ ptr, advance }) + 1u;
+        }
+
+        const auto colEndNew = gsl::narrow_cast<uint16_t>(colEnd + width);
         if (colEndNew > colLimit)
         {
             colEndDirty = colLimit;
-            break;
+            charsConsumed = ch - chBeg;
+            return;
         }
 
         til::at(row._charOffsets, colEnd++) = gsl::narrow_cast<uint16_t>(ch);
-        if (wide)
+        if (colEnd < colEndNew)
         {
             til::at(row._charOffsets, colEnd++) = gsl::narrow_cast<uint16_t>(ch | CharOffsetsTrailer);
         }
 
-        colEndDirty = colEnd;
-        ch += s.size();
+        ch += advance;
     }
 
+    colEndDirty = colEnd;
     charsConsumed = ch - chBeg;
 }
 
diff --git a/src/buffer/out/Row.hpp b/src/buffer/out/Row.hpp
index 1ce38c0668ef..ab055611418d 100644
--- a/src/buffer/out/Row.hpp
+++ b/src/buffer/out/Row.hpp
@@ -1,22 +1,5 @@
-/*++
-Copyright (c) Microsoft Corporation
-Licensed under the MIT license.
-
-Module Name:
-- Row.hpp
-
-Abstract:
-- data structure for information associated with one row of screen buffer
-
-Author(s):
-- Michael Niksa (miniksa) 10-Apr-2014
-- Paul Campbell (paulcam) 10-Apr-2014
-
-Revision History:
-- From components of output.h/.c
-  by Therese Stowell (ThereseS) 1990-1991
-- Pulled into its own file from textBuffer.hpp/cpp (AustDi, 2017)
---*/
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
 
 #pragma once
 
@@ -170,6 +153,7 @@ class ROW final
         bool IsValid() const noexcept;
         void ReplaceCharacters(til::CoordType width) noexcept;
         void ReplaceText() noexcept;
+        void _replaceTextUnicode(size_t ch, std::wstring_view::const_iterator it, const std::wstring_view::const_iterator end);
         void CopyTextFrom(const std::span<const uint16_t>& charOffsets) noexcept;
         static void _copyOffsets(uint16_t* dst, const uint16_t* src, uint16_t size, uint16_t offset) noexcept;
         void Finish();