[llvm] [FileCheck] Add split view diff option for FileCheck (PR #189269)

[Henrik G. Olsson via llvm-commits]

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@@ -2762,11 +2762,45 @@ static DiffContext getDiffContext(SourceMgr &SM, unsigned LineNo,
           getLineText(LineNo + 1)};
 }
 
+// Extracts a fixed-width "window" from string S, centered around DiffPos.
+static std::string getCenteredView(StringRef S, size_t DiffPos, size_t Width) {
+  if (S.size() <= Width)
+    return S.str() + std::string(Width - S.size(), ' ');
+
+  size_t HalfWidth = Width / 2;
+  size_t Start = (DiffPos > HalfWidth) ? DiffPos - HalfWidth : 0;
+
+  if (Start + Width > S.size())
+    Start = (S.size() > Width) ? S.size() - Width : 0;
+
+  std::string View = S.substr(Start, Width).str();
+
+  if (Start > 0 && Width > 3)
+    View.replace(0, 3, "...");
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hnrklssn wrote:

I think this could technically still overlap with `DiffPos` if e.g. `Width` is 4.

https://github.com/llvm/llvm-project/pull/189269