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Hashtbl.hash is not compatible with (=) #5222

vicuna opened this issue Feb 9, 2011 · 2 comments


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commented Feb 9, 2011

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 :

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


    Objective Caml version 3.12.0

Hashtbl.hash (+0.) ;;

  • : int = 0

Hashtbl.hash (-0.) ;;

  • : int = 128

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

  • : bool = true


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commented Mar 4, 2011

Comment author: @xavierleroy

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


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commented May 29, 2011

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.

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