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[bug#29132] [PATCH] system: vm: Use 2^32 - 1 as hash size.
From: |
Ludovic Courtès |
Subject: |
[bug#29132] [PATCH] system: vm: Use 2^32 - 1 as hash size. |
Date: |
Sun, 05 Nov 2017 21:48:52 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Hello!
Mathieu Othacehe <address@hidden> skribis:
> * gnu/system/vm.scm (operating-system-uuid): Use 2^32 - 1 instead of
> 2^32 as hash size.
>
> On some 32 bit system (ARM for example), 2^32 exceeds hash max
> size (ULONG_MAX = 2^32 - 1).
> ---
> gnu/system/vm.scm | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/gnu/system/vm.scm b/gnu/system/vm.scm
> index 3127b30..4424608 100644
> --- a/gnu/system/vm.scm
> +++ b/gnu/system/vm.scm
> @@ -372,13 +372,13 @@ TYPE (one of 'iso9660 or 'dce). Return a UUID object."
> (bytevector->uuid
> (uint-list->bytevector
> (list (hash file-system-type
> - (expt 2 32))
> + (- (expt 2 32) 1))
‘hash’ is documented like this:
-- Scheme Procedure: hash key size
-- Scheme Procedure: hashq key size
-- Scheme Procedure: hashv key size
-- C Function: scm_hash (key, size)
-- C Function: scm_hashq (key, size)
-- C Function: scm_hashv (key, size)
Return a hash value for KEY. This is a number in the range 0 to
SIZE-1, which is suitable for use in a hash table of the given
SIZE.
So I take it that you always get something in the range 0–2³²-1, no?
Ludo’.