Add text based cursor movement helpers

src/buffer/out/textBuffer.cpp

@@ -441,11 +441,20 @@ void TextBuffer::_PrepareForDoubleByteSequence(const DbcsAttribute dbcsAttribute
     }
 }
 
-void TextBuffer::ConsumeGrapheme(std::wstring_view& chars) noexcept
+// Given the character offset `position` in the `chars` string, this function returns the starting position of the next grapheme.
+// For instance, given a `chars` of L"x\uD83D\uDE42y" and a `position` of 1 it'll return 3.
+// GraphemePrev would do the exact inverse of this operation.
+// In the future, these functions are expected to also deliver information about how many columns a grapheme occupies.
+// (I know that mere UTF-16 code point iteration doesn't handle graphemes, but that's what we're working towards.)
+size_t TextBuffer::GraphemeNext(const std::wstring_view& chars, size_t position) noexcept
 {
-    // This function is supposed to mirror the behavior of ROW::Write, when it reads characters off of `chars`.
-    // (I know that a UTF-16 code point is not a grapheme, but that's what we're working towards.)
-    chars = til::utf16_pop(chars);
+    return til::utf16_iterate_next(chars, position);
+}
+
+// It's the counterpart to GraphemeNext. See GraphemeNext.
+size_t TextBuffer::GraphemePrev(const std::wstring_view& chars, size_t position) noexcept
+{
+    return til::utf16_iterate_prev(chars, position);
 }
 
 // This function is intended for writing regular "lines" of text as it'll set the wrap flag on the given row.

src/buffer/out/textBuffer.hpp

@@ -131,8 +131,10 @@ class TextBuffer final
     TextBufferTextIterator GetTextLineDataAt(const til::point at) const;
     TextBufferTextIterator GetTextDataAt(const til::point at, const Microsoft::Console::Types::Viewport limit) const;
 
+    static size_t GraphemeNext(const std::wstring_view& chars, size_t position) noexcept;
+    static size_t GraphemePrev(const std::wstring_view& chars, size_t position) noexcept;
+
     // Text insertion functions
-    static void ConsumeGrapheme(std::wstring_view& chars) noexcept;
     void Write(til::CoordType row, const TextAttribute& attributes, RowWriteState& state);
     void FillRect(const til::rect& rect, const std::wstring_view& fill, const TextAttribute& attributes);
 

src/inc/til/unicode.h

@@ -59,28 +59,30 @@ namespace til
         return { ptr, len };
     }
 
-    // Removes the first code point off of `wstr` and returns the rest.
-    constexpr std::wstring_view utf16_pop(std::wstring_view wstr) noexcept
+    // Returns the index of the next codepoint in the given wstr (i.e. after the codepoint that idx points at).
+    constexpr size_t utf16_iterate_next(const std::wstring_view& wstr, size_t idx) noexcept
     {
-        auto it = wstr.begin();
-        const auto end = wstr.end();
-
-        if (it != end)
+        if (idx < wstr.size() && is_leading_surrogate(til::at(wstr, idx++)))
         {
-            const auto wch = *it;
-            ++it;
-
-            if (is_surrogate(wch))
+            if (idx < wstr.size() && is_trailing_surrogate(til::at(wstr, idx)))
             {
-                const auto wch2 = it != end ? *it : wchar_t{};
-                if (is_leading_surrogate(wch) && is_trailing_surrogate(wch2))
-                {
-                    ++it;
-                }
+                ++idx;
             }
         }
+        return idx;
+    }
 
-        return { it, end };
+    // Returns the index of the preceding codepoint in the given wstr (i.e. in front of the codepoint that idx points at).
+    constexpr size_t utf16_iterate_prev(const std::wstring_view& wstr, size_t idx) noexcept
+    {
+        if (idx > 0 && is_trailing_surrogate(til::at(wstr, --idx)))
+        {
+            if (idx > 0 && is_leading_surrogate(til::at(wstr, idx - 1)))
+            {
+                --idx;
+            }
+        }
+        return idx;
     }
 
     // Splits a UTF16 string into codepoints, yielding `wstring_view`s of UTF16 text. Use it as:

src/terminal/adapter/adaptDispatch.cpp

@@ -176,7 +176,7 @@ void AdaptDispatch::_WriteToBuffer(const std::wstring_view string)
             //   we tried writing a wide glyph into the last column which can't work.
             if (textPositionBefore == textPositionAfter && (state.columnBegin == 0 || !wrapAtEOL))
             {
-                textBuffer.ConsumeGrapheme(state.text);
+                state.text = state.text.substr(textBuffer.GraphemeNext(state.text, 0));
             }
 
             if (wrapAtEOL)