Highlighter: Apply specific ksyntax format attributes

If a ksyntax format explicitly defines font attributes like bold,
italic, underline or strike through we now apply those attributes. If
the format defines a colors, we first check whether the color is
readable before setting it.

Fixes: QTCREATORBUG-22646
Fixes: QTCREATORBUG-22229
Fixes: QTCREATORBUG-13545
Change-Id: Id0389b3c71a078dc6d6df74ee5de7f5d479a9bcb
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/highlighter.cpp

@@ -44,6 +44,8 @@
 #include <QDir>
 #include <QMetaEnum>
 
+#include <cmath>
+
 using namespace TextEditor;
 
 static const char kDefinitionForMimeType[] = "definitionForMimeType";
@@ -322,9 +324,77 @@ void Highlighter::highlightBlock(const QString &text)
     formatSpaces(text);
 }
 
+static double luminance(const QColor &color)
+{
+    // calculate the luminance based on
+    // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+    auto val = [](const double &colorVal) {
+        return colorVal < 0.03928 ? colorVal / 12.92 : std::pow((colorVal + 0.055) / 1.055, 2.4);
+    };
+
+    static QHash<QRgb, double> cache;
+    QHash<QRgb, double>::iterator it = cache.find(color.rgb());
+    if (it == cache.end()) {
+        it = cache.insert(color.rgb(), 0.2126 * val(color.redF())
+                                           + 0.7152 * val(color.greenF())
+                                           + 0.0722 * val(color.blueF()));
+    }
+    return it.value();
+}
+
+static double contrastRatio(const QColor &color1, const QColor &color2)
+{
+    // calculate the contrast ratio based on
+    // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
+    auto contrast = (luminance(color1) + 0.05) / (luminance(color2) + 0.05);
+    if (contrast < 1)
+        return 1 / contrast;
+    return contrast;
+}
+
+static bool isReadableOn(const QColor &background, const QColor &foreground)
+{
+    // following the W3C Recommendation on contrast for large Text
+    // https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
+    return contrastRatio(background, foreground) > 3;
+}
+
 void Highlighter::applyFormat(int offset, int length, const KSyntaxHighlighting::Format &format)
 {
-    setFormat(offset, length, formatForCategory(format.textStyle()));
+    const KSyntaxHighlighting::Theme defaultTheme;
+    QTextCharFormat qformat = formatForCategory(format.textStyle());
+
+    if (format.hasTextColor(defaultTheme)) {
+        const QColor textColor = format.textColor(defaultTheme);
+        if (format.hasBackgroundColor(defaultTheme)) {
+            const QColor backgroundColor = format.hasBackgroundColor(defaultTheme);
+            if (isReadableOn(backgroundColor, textColor)) {
+                qformat.setForeground(textColor);
+                qformat.setBackground(backgroundColor);
+            } else if (isReadableOn(qformat.background().color(), textColor)) {
+                qformat.setForeground(textColor);
+            }
+        } else if (isReadableOn(qformat.background().color(), textColor)) {
+            qformat.setForeground(textColor);
+        }
+    } else if (format.hasBackgroundColor(defaultTheme)) {
+        const QColor backgroundColor = format.hasBackgroundColor(defaultTheme);
+        if (isReadableOn(backgroundColor, qformat.foreground().color()))
+            qformat.setBackground(backgroundColor);
+    }
+
+    if (format.isBold(defaultTheme))
+        qformat.setFontWeight(QFont::Bold);
+
+    if (format.isItalic(defaultTheme))
+        qformat.setFontItalic(true);
+
+    if (format.isUnderline(defaultTheme))
+        qformat.setFontUnderline(true);
+
+    if (format.isStrikeThrough(defaultTheme))
+        qformat.setFontStrikeOut(true);
+    setFormat(offset, length, qformat);
 }
 
 void Highlighter::applyFolding(int offset,