calchash(): work around broken RTTI.

[ginac.git] / ginac / hash_seed.h

diff --git a/ginac/hash_seed.h b/ginac/hash_seed.h
new file mode 100644 (file)
index 0000000..32b057f
--- /dev/null
+++ b/ginac/hash_seed.h
@@ -0,0 +1,45 @@
+#ifndef GINAC_HASH_SEED_H
+#define GINAC_HASH_SEED_H
+/**
+ * We need a hash function which gives different values for objects of
+ * different types. Hence we need some unique integer for each type.
+ * Fortunately, standard C++ RTTI class `type_info' stores a pointer to
+ * mangled type name. Normally this pointer is the same for all objects
+ * of the same type (although it changes from run to run), so it can be
+ * used for computing hashes. However, on some platforms (such as woe32)
+ * the pointer returned by type_info::name() might be different even for
+ * objects of the same type! Hence we need to resort to comparing string
+ * representation of the (mangled) type names. This is quite expensive,
+ * so we compare crc32 hashes of those strings. We might got more hash
+ * collisions (and slower evaluation as a result), but being a bit slower
+ * is much better than being wrong.
+ */
+#include <typeinfo>
+#include <cstring>
+#include "crc32.h"
+#include "utils.h"
+#ifdef _WIN32
+#define GINAC_HASH_USE_MANGLED_NAME 1
+#endif
+namespace GiNaC
+{
+#ifndef GINAC_HASH_USE_MANGLED_NAME
+static inline unsigned make_hash_seed(const std::type_info& tinfo)
+{
+       // this pointer is the same for all objects of the same type.
+       // Hence we can use that pointer 
+       const void* mangled_name_ptr = (const void*)tinfo.name();
+       unsigned v = golden_ratio_hash((p_int)mangled_name_ptr);
+       return v;
+}
+#else
+static unsigned make_hash_seed(const std::type_info& tinfo)
+{
+       const char* mangled_name = tinfo.name();
+       return crc32(mangled_name, std::strlen(mangled_name), 0);
+}
+#endif
+} // namespace GiNaC
+#endif /* GINAC_HASH_SEED_H */
+
+