core: prevent allocations when looking up an existing Core::Id

Change-Id: I7bd2f83ffc6a57135aab9f76ff929eca48d16885
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/coreplugin/id.cpp

@@ -49,20 +49,66 @@ namespace Core {
 
 */
 
+class StringHolder
+{
+public:
+    explicit StringHolder(const char *s)
+        : str(s)
+    {
+        n = strlen(s);
+        int m = n;
+        h = 0;
+        while (m--) {
+            h = (h << 4) + *s++;
+            h ^= (h & 0xf0000000) >> 23;
+            h &= 0x0fffffff;
+        }
+    }
+    int n;
+    const char *str;
+    uint h;
+};
+
+static bool operator==(const StringHolder &sh1, const StringHolder &sh2)
+{
+    // sh.n is unlikely to discriminate better than the hash.
+    return sh1.h == sh2.h && strcmp(sh1.str, sh1.str) == 0;
+}
+
+
+static uint qHash(const StringHolder &sh)
+{
+    return sh.h;
+}
+
+struct IdCache : public QHash<StringHolder, int>
+{
+#ifndef QTC_ALLOW_STATIC_LEAKS
+    ~IdCache()
+    {
+        for (IdCache::iterator it = begin(); it != end(); ++it)
+            free(const_cast<char *>(it.key().str));
+    }
+#endif
+};
+
+
 static int lastUid = 0;
 static QVector<QByteArray> stringFromId;
-static QHash<QByteArray, int> idFromString;
+static IdCache idFromString;
 
-static int theId(const QByteArray &ba)
+static int theId(const char *str)
 {
-    QTC_CHECK(!ba.isEmpty());
-    int res = idFromString.value(ba);
+    QTC_ASSERT(str && *str, return 0);
+    StringHolder sh(str);
+    int res = idFromString.value(sh, 0);
     if (res == 0) {
         if (lastUid == 0)
             stringFromId.append(QByteArray());
         res = ++lastUid;
-        idFromString[ba] = res;
-        stringFromId.append(ba);
+        sh.str = strdup(sh.str);
+        idFromString[sh] = res;
+        stringFromId.append(QByteArray::fromRawData(sh.str, sh.n));
     }
     return res;
 }