BugFix: try and prevent crashing during highlighting

It seems that one of the tests in `(void) TTextEdit::highlightSelection()`
was being done too late - with the result that out of range indexing was
being attempted on the `(std::deque<std::deque<TChar>) TBuffer::buffer`
member. Specifically the test for `currentY >= totaly` should have been
done before `currentY` was used to select one from the outer `std::deque`
container so as to establish the size of it (to be stored as `maxX`).

The revised code replaces both the highlight and un-highlight code nested
loops with components that are more conventionally used, i.e. with the
test being based on a less than condition only with the limit being
determined within the top parts of the for loops and no additional tests
to break from the loop within the body thereof...

Signed-off-by: Stephen Lyons <slysven@virginmedia.com>

src/TTextEdit.cpp

@@ -645,28 +645,27 @@ void TTextEdit::highlightSelection()
 
     mSelectedRegion = mSelectedRegion.subtracted(newRegion);
 
-    int startY = mPA.y();
-    auto totalY = static_cast<int>(mpBuffer->buffer.size());
-    for (int currentY = startY, total = mPB.y(); currentY <= total; ++currentY) {
-        int currentX = 0;
-        int maxX = static_cast<int>(mpBuffer->buffer.at(currentY).size());
-        if (currentY == startY) {
-            currentX = mPA.x();
-        }
-
-        if (currentY >= totalY) {
-            break;
-        }
-
-        for (; currentX < maxX; ++currentX) {
-            if ((currentY == mPB.y()) && (currentX > mPB.x())) {
-                break;
-            }
-            if (!(mpBuffer->buffer.at(currentY).at(currentX).isSelected())) {
-                mpBuffer->buffer.at(currentY).at(currentX).select();
-            }
+    // mpA represents the first (zero-based) line/row (y) and position/column
+    // (x) that IS to be selected, mpB represents the last (zero-based) line and
+    // column that IS to be selected - which means that in the traditional 'for'
+    // loop construct where the test is a '<' based one, the test limit is
+    // mpB.y() + 1 for the row and mpB.x() + 1 for the column:
+    // clang-format off
+    for (int y = std::max(0, mPA.y()), endY = std::min((mPB.y() + 1), static_cast<int>(mpBuffer->buffer.size()));
+         y < endY;
+         ++y) {
+
+        for (int x = (y == mPA.y()) ? std::max(0, mPA.x()) : 0,
+                 endX = (y == (mPB.y()))
+                        ? std::min((mPB.x() + 1), static_cast<int>(mpBuffer->buffer.at(y).size()))
+                        : static_cast<int>(mpBuffer->buffer.at(y).size());
+             x < endX;
+             ++x) {
+
+            mpBuffer->buffer.at(y).at(x).select();
         }
     }
+    // clang-format on
 
     update(mSelectedRegion.boundingRect());
     update(newRegion);
@@ -676,23 +675,24 @@ void TTextEdit::highlightSelection()
 void TTextEdit::unHighlight()
 {
     normaliseSelection();
-    int y1 = mPA.y();
 
-    if (y1 < 0) {
-        return;
-    }
-    for (int y = y1, total = mPB.y(); y <= total; ++y) {
-        int x = 0;
-
-        if (y >= static_cast<int>(mpBuffer->buffer.size())) {
-            break;
+    // clang-format off
+    for (int y = std::max(0, mPA.y()), endY = std::min((mPB.y() + 1), static_cast<int>(mpBuffer->buffer.size()));
+         y < endY;
+         ++y) {
+
+        for (int x = (y == mPA.y()) ? std::max(0, mPA.x()) : 0,
+                 endX = (y == (mPB.y()))
+                        ? std::min((mPB.x() + 1), static_cast<int>(mpBuffer->buffer.at(y).size()))
+                        : static_cast<int>(mpBuffer->buffer.at(y).size());
+             x < endX;
+             ++x) {
+
+            mpBuffer->buffer.at(y).at(x).deselect();
         }
-
-        for (; x < static_cast<int>(mpBuffer->buffer.at(y).size()); ++x)
-            if (mpBuffer->buffer.at(y).at(x).isSelected()) {
-                mpBuffer->buffer.at(y).at(x).deselect();
-            }
     }
+    // clang-format on
+
     forceUpdate();
 }