Hashtbl.hash is not compatible with (=)

[vicuna]

Original bug ID: 5222
Reporter: Pascal Cuoq
Assigned to: @xavierleroy
Status: closed (set by @xavierleroy on 2012-09-25T18:06:17Z)
Resolution: fixed
Priority: normal
Severity: major
Version: 3.12.0
Category: ~DO NOT USE (was: OCaml general)
Related to: #5225
Monitored by: bobot till "Julien Signoles" @dbuenzli

Bug description

According to http://caml.inria.fr/pub/docs/manual-ocaml/libref/Hashtbl.html :

Hashtbl.hash x associates a positive integer to any value of any type. It is guaranteed that if x = y or Pervasives.compare x y = 0, then hash x = hash y.

But:

    Objective Caml version 3.12.0

Hashtbl.hash (+0.) ;;

• : int = 0

Hashtbl.hash (-0.) ;;

• : int = 128

(+0.) = (-0.) ;;

• : bool = true

[vicuna]

Comment author: @xavierleroy

Well spotted, thanks. Some normalization of the float is in order before hashing its bits.

[vicuna]

Comment author: @xavierleroy

As part of a major overhaul of the generic hash function, fixed the +0.0/-0.0 issue, and also made sure that all NaNs hash to the same value. The code is in SVN trunk and should go into release 3.13.