[Icons] Add SVG currentColor sentinel resolver

Qt's SVG renderer has no CSS cascade, so a `currentColor` fill has to be
substituted before the bytes reach QSvgRenderer.

- Match and replace on bytes: both the sentinel and the replacement are
  ASCII, so the document never goes through a UTF-16 round trip, and files
  that are not valid UTF-8 survive unchanged.
- Document that the match is a plain case-insensitive substring, so an
  identifier that merely starts with the sentinel gets rewritten too.

libcockatrice_utility/CMakeLists.txt

@@ -8,6 +8,7 @@ set(UTILITY_SOURCES
     libcockatrice/utility/passwordhasher.cpp
     libcockatrice/utility/report_categories.cpp
     libcockatrice/utility/server_rate_limiter.cpp
+    libcockatrice/utility/svg_recolor.cpp
     libcockatrice/utility/translation_loader.cpp
     libcockatrice/utility/user_data_backup.cpp
     libcockatrice/utility/warning_categories.cpp
@@ -34,6 +35,7 @@ set(UTILITY_HEADERS
     libcockatrice/utility/zone_names.h
     libcockatrice/utility/days_years_between.h
     libcockatrice/utility/server_rate_limiter.h
+    libcockatrice/utility/svg_recolor.h
     libcockatrice/utility/translation_loader.h
     libcockatrice/utility/user_data_backup.h
     libcockatrice/utility/warning_categories.h

libcockatrice_utility/libcockatrice/utility/svg_recolor.cpp

@@ -0,0 +1,44 @@
+#include "svg_recolor.h"
+
+#include <QByteArrayView>
+
+namespace
+{
+constexpr QByteArrayView SENTINEL = "currentColor";
+
+/// Case-insensitive byte search for the sentinel, starting at @p from.
+/// QByteArray::indexOf() only matches case-sensitively for QByteArrayView, and
+/// its char overload cannot match a word.
+qsizetype findSentinel(const QByteArray &bytes, qsizetype from)
+{
+    // QByteArrayView::sliced() is a view, so the scan does not allocate a copy
+    // at every byte position (QByteArray::sliced() would).
+    const QByteArrayView view(bytes);
+    for (qsizetype i = from; i + SENTINEL.size() <= view.size(); ++i) {
+        if (view.sliced(i, SENTINEL.size()).compare(SENTINEL, Qt::CaseInsensitive) == 0) {
+            return i;
+        }
+    }
+    return -1;
+}
+} // namespace
+
+QByteArray resolveSvgColorSentinel(const QByteArray &svg, const QByteArray &color)
+{
+    // The sentinel and the replacement are both ASCII, so stay in bytes: a
+    // QString round trip would decode the whole document (and mangle bytes that
+    // are not valid UTF-8) even on the no-op path.
+    qsizetype pos = findSentinel(svg, 0);
+    if (pos < 0) {
+        return svg;
+    }
+
+    QByteArray resolved = svg;
+    while (pos >= 0) {
+        resolved.replace(pos, SENTINEL.size(), color);
+        // Skip past the inserted color: a caller's color never contains the
+        // sentinel, and this keeps the scan linear.
+        pos = findSentinel(resolved, pos + color.size());
+    }
+    return resolved;
+}

libcockatrice_utility/libcockatrice/utility/svg_recolor.h

@@ -0,0 +1,31 @@
+/**
+ * @file svg_recolor.h
+ * @ingroup Core
+ */
+
+#ifndef SVG_RECOLOR_H
+#define SVG_RECOLOR_H
+
+#include <QByteArray>
+
+/**
+ * @brief Resolves the `currentColor` sentinel in authored SVG data.
+ *
+ * Qt's SVG renderer has no CSS cascade or ambient color context, so the
+ * sentinel is not resolved by Qt. Callers pass the concrete color they want
+ * (for example a QPalette role serialized to #RRGGBB) and receive SVG bytes
+ * that are safe to hand to QSvgRenderer.
+ *
+ * Matching is a plain, case-insensitive substring search, not a CSS parse: an
+ * identifier that merely starts with the sentinel (for example
+ * `id="currentColorStop"`) is rewritten too. That is acceptable for the assets
+ * this project authors, and keeps the resolver dependency-free.
+ *
+ * @param svg SVG source bytes, expected to be UTF-8. Invalid UTF-8 is preserved
+ *            byte-for-byte, since the resolver never decodes the document.
+ * @param color Concrete SVG color (for example "#ff00ff"), as ASCII bytes.
+ * @return The recolored SVG source; the input is left untouched.
+ */
+QByteArray resolveSvgColorSentinel(const QByteArray &svg, const QByteArray &color);
+
+#endif // SVG_RECOLOR_H

tests/CMakeLists.txt

@@ -177,6 +177,7 @@ cockatrice_add_test(NAME test_age_formatting SOURCES test_age_formatting.cpp LIB
 cockatrice_add_test(NAME user_data_backup_test SOURCES user_data_backup_test.cpp LIBS libcockatrice_utility)
 cockatrice_add_test(NAME password_hash_test SOURCES password_hash_test.cpp LIBS libcockatrice_utility)
 cockatrice_add_test(NAME report_categories_test SOURCES report_categories_test.cpp LIBS libcockatrice_utility)
+cockatrice_add_test(NAME svg_recolor_test SOURCES svg_recolor_test.cpp LIBS libcockatrice_utility)
 cockatrice_add_test(NAME warning_categories_test SOURCES warning_categories_test.cpp LIBS libcockatrice_utility)
 cockatrice_add_test(
   NAME

tests/svg_recolor_test.cpp

@@ -0,0 +1,66 @@
+#include "libcockatrice/utility/svg_recolor.h"
+
+#include <QByteArray>
+#include <gtest/gtest.h>
+
+namespace
+{
+
+TEST(SvgRecolor, ReplacesEverySentinel)
+{
+    const QByteArray svg = R"(<svg><path fill="currentColor"/><path stroke="currentColor"/></svg>)";
+    const QByteArray result = resolveSvgColorSentinel(svg, "#ff8800");
+
+    EXPECT_EQ(result.count("#ff8800"), 2);
+    EXPECT_FALSE(result.contains("currentColor"));
+}
+
+TEST(SvgRecolor, ReplacesSentinelRegardlessOfCase)
+{
+    const QByteArray svg = R"(<svg><path fill="CurrentColor"/><path stroke="CURRENTCOLOR"/></svg>)";
+
+    EXPECT_EQ(resolveSvgColorSentinel(svg, "#ff8800").count("#ff8800"), 2);
+}
+
+TEST(SvgRecolor, LeavesTheRestOfTheDocumentIntact)
+{
+    const QByteArray svg =
+        R"(<svg viewBox="0 0 24 24"><!-- an ordinary comment --><path d="M0 0h24v24" fill="currentColor"/></svg>)";
+    const QByteArray result = resolveSvgColorSentinel(svg, "#123456");
+
+    EXPECT_TRUE(result.contains(R"(viewBox="0 0 24 24")"));
+    EXPECT_TRUE(result.contains(R"(d="M0 0h24v24")"));
+    EXPECT_TRUE(result.contains("an ordinary comment"));
+}
+
+TEST(SvgRecolor, DoesNotModifyTheInput)
+{
+    const QByteArray svg = R"(<svg><path fill="currentColor"/></svg>)";
+    const QByteArray original = svg;
+
+    resolveSvgColorSentinel(svg, "#000000");
+
+    EXPECT_EQ(svg, original);
+}
+
+TEST(SvgRecolor, PreservesBytesThatAreNotValidUtf8)
+{
+    QByteArray svg = "<svg>";
+    svg.append("\xff\xfe");
+    svg.append(R"(<path fill="currentColor"/></svg>)");
+
+    const QByteArray result = resolveSvgColorSentinel(svg, "#ff8800");
+
+    EXPECT_TRUE(result.contains("\xff\xfe"));
+    EXPECT_FALSE(result.contains("currentColor"));
+    EXPECT_EQ(result.count("#ff8800"), 1);
+}
+
+TEST(SvgRecolor, NoSentinelIsANoOp)
+{
+    const QByteArray svg = R"(<svg><path fill="#abcdef"/></svg>)";
+
+    EXPECT_EQ(resolveSvgColorSentinel(svg, "#ffffff"), svg);
+}
+
+} // namespace