From: Christian Bauer <Christian.Bauer@uni-mainz.de>
Date: Thu, 11 Dec 2003 18:57:50 +0000 (+0000)
Subject: hash keys of indices must not depend on the index dimension; doing so breaks
X-Git-Tag: release_1-2-0~48
X-Git-Url: https://www.ginac.de/ginac.git//ginac.git?p=ginac.git;a=commitdiff_plain;h=79e49016f3a732fe1758bc3efd10f0727862017c

hash keys of indices must not depend on the index dimension; doing so breaks
the ordering of dummy indices that simplify_index() depends on
---

diff --git a/ginac/idx.cpp b/ginac/idx.cpp
index 67603cf2..5739f80e 100644
--- a/ginac/idx.cpp
+++ b/ginac/idx.cpp
@@ -325,11 +325,17 @@ bool spinidx::match_same_type(const basic & other) const
 
 unsigned idx::calchash() const
 {
+	// NOTE: The code in simplify_indexed() assumes that canonically
+	// ordered sequences of indices have the two members of dummy index
+	// pairs lying next to each other. The hash values for indices must
+	// be devised accordingly. The easiest (only?) way to guarantee the
+	// desired ordering is to make indices with the same value have equal
+	// hash keys. That is, the hash values must not depend on the index
+	// dimensions or other attributes (variance etc.).
+	// The compare_same_type() methods will take care of the rest.
 	unsigned v = golden_ratio_hash(tinfo());
 	v = rotate_left(v);
 	v ^= value.gethash();
-	v = rotate_left(v);
-	v ^= dim.gethash();
 
 	// Store calculated hash value only if object is already evaluated
 	if (flags & status_flags::evaluated) {