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bug#17485: [PATCH 1/3] Let length+ return the length of dotted lists rat
From: |
Mark H Weaver |
Subject: |
bug#17485: [PATCH 1/3] Let length+ return the length of dotted lists rather than #f |
Date: |
Tue, 03 Jun 2014 23:42:26 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
Hi David,
David Kastrup <address@hidden> writes:
> * libguile/srfi-1.c (scm_srfi1_length_plus): Previously, length+
> returned #f for dotted lists. This leaves the user with no efficient
> means for determining the length of dotted lists. While the Scheme
> standard does not prescribe a behavior here, the reference
> implementation at
> <URL:http://srfi.schemers.org/srfi-1/srfi-1-reference.scm> indeed
> returns the spine length (number of successive pairs in the cdr-chain)
> of dotted lists rather than #f, providing a good endorsement of this
> behavior.
>
> As one consequence, the multi-list implementations for map, fold, and
> for-each will happen to accept dotted lists as the shortest list.
> Previously, this caused an error late during processing.
In general, rationales don't belong in the commit logs. As per the GNU
coding standards, change logs should only summarize the changes made.
>
> Signed-off-by: David Kastrup <address@hidden>
> ---
> libguile/srfi-1.c | 28 ++++++++++++++++++++++++++--
> module/srfi/srfi-1.scm | 10 +++++-----
> test-suite/tests/srfi-1.test | 28 +++++++++++++++-------------
> 3 files changed, 46 insertions(+), 20 deletions(-)
>
> diff --git a/libguile/srfi-1.c b/libguile/srfi-1.c
> index aaa3efe..0db6388 100644
> --- a/libguile/srfi-1.c
> +++ b/libguile/srfi-1.c
> @@ -614,8 +614,32 @@ SCM_DEFINE (scm_srfi1_length_plus, "length+", 1, 0, 0,
> "circular.")
> #define FUNC_NAME s_scm_srfi1_length_plus
> {
> - long len = scm_ilength (lst);
> - return (len >= 0 ? SCM_I_MAKINUM (len) : SCM_BOOL_F);
> + /* This uses the "tortoise and hare" algorithm to detect "infinitely
> + long" lists (i.e. lists with cycles in their cdrs), and returns #f
> + if it does find one.
> +
> + Dotted lists are treated just like regular lists, returning the
> + length of the spine. This is in conformance with the reference
> + implementation though not explicitly defined in the standard. */
> + long i = 0;
Please use 'size_t' instead of 'long'.
> + SCM tortoise = lst;
> + SCM hare = lst;
> +
> + do {
> + if (!scm_is_pair (hare)) return scm_from_long (i);
scm_from_size_t
> + hare = SCM_CDR(hare);
> + i++;
> + if (!scm_is_pair (hare)) return scm_from_long (i);
Ditto.
> + hare = SCM_CDR(hare);
> + i++;
> + /* For every two steps the hare takes, the tortoise takes one. */
> + tortoise = SCM_CDR(tortoise);
> + }
> + while (!scm_is_eq (hare, tortoise));
Please follow the GNU conventions for indentation.
> +
> + /* If the tortoise ever catches the hare, then the list must contain
> + a cycle. */
> + return SCM_BOOL_F;
> }
> #undef FUNC_NAME
Otherwise, this function looks good to me, but I'd prefer to give it a
new name and move it into list.c, rather than extending SRFI-1's
'length+'.
Hmm, coming up with names is hard. Maybe 'length*'?
Thanks,
Mark