Convert __DATE__-style string to sortable string

Question

Goal is to write the shortest possible C89 and C99-compliant single-module C program which will compute and print out a single-line string whose sort order will correspond with the date given by the predefined __DATE__ macro (in other words, later dates will yield later-sorting strings). The programmer is free to arbitrarily select the mapping between dates and strings, but every entry should specify the mapping and it should be obvious that it will sort correctly (e.g. a programmer could decide to compute (day + month*73 + year*4129) and output that as a number, though it's likely that particular encoding would probably require a longer program than some others).

The program should yield identical results on any standards-compliant compiler on which 'int' is 32 bits or larger and both source and target character sets are 7-bit ASCII, and should not rely upon any implementation-defined or undefined behavior nor print any characters outside the 32-126 range except for a single newline at the end. The program should contain the following aspects indicated below (replacing «CODE» with anything desired):

♯include <stdio.h>
«CODE»int main(void){«CODE»}

All output shall be produced by the printf at the end (i.e. the correct value will be in an int called z). The required elements will be included in the character total for each entry.

Code should operate correctly for all future dates through Dec 31 9999. Libraries which are standard in both C89 and C99 may be used, provided that appropriate headers are included. Note that standard date libraries may not be assumed to operate beyond the Unix limits.

Note: Code is permitted to perform Undefined Behavior if and only if the __DATE__ macro expands to a macro other than a date between Feb 11 2012 and Dec 31 9999 (expressed in that format, using C-standard abbreviated English month names Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec)

Note 2: For compliance with C99 standards, 'main' should return an arbitrary, but defined value, and the newline is required. The first requirement, btw, adds 7 characters to my best effort; the single new-line adds 5.

Answer 1

C, 137 184 184 140 120 106 103 characters

Replaced the month name lookup with a magic formula.
The formula (m[1]*4388^m[2]*7)%252 is ascending for month names.
Changed it to nicely return 0, at no cost.
It no longer prints a number. Instead it prints a string, which should sort right.
Implemented supercat's %*s idea, which inserts more spaces for earlier months, along with a function that's descending for month names - (m[1]*29^m[2]+405)%49.

#include<stdio.h>
int main(void){
    char*m=__DATE__"%*.6s\n"+1;
    return!printf(m+6,(*m*29^m[1]+405)%49,m);
}

I thought single digit days are represented as Jan_1_2012 (_ being a space), when in fact it's Jan__1_2012 (extra space). This complicated things, so my previous versions were more complicated:

#include<stdio.h>
int main(void){
    char*m=__DATE__+1,*t=m+m[4]/16;
    return!printf("%s%3d%s\n",t+3,(*m*4388^m[1]*7)%252,t);
}

Answer 2

C, 194 characters

#include <stdio.h>
#include <string.h>
d,y,n[3];
int main(void)
{
sscanf(__DATE__,"%s %d %d",n,&d,&y);
return printf("%d%02d%02d",y,13-strlen(strstr("JanFebMarAprMayJunJulAugSepOctNovDec",n))/3,d);
}

I think most of the newlines are unnecessary, but I've left them in for readability.

Not sure what your feelings are about compiler warnings - this throws a few but runs fine. Also not sure whether the declarations with no type are valid C89 or not.

Answer 3

Here are the solutions that I had come up with (for brevity, I'm just writing the #include line once--copy and paste as needed to assemble a testable program).

(my entries)
#include <stdio.h>
int main(void){char*d=__DATE__"%s%sDFCwu-vBxE-t%c"+1;return printf(d+6,d+(*d+d[1])%17+9,d,10);}
int main(void){char*d="h-elbj-ikcfdga-"__DATE__+16;return printf("%s%s\n",d+6,d-(*d+d[1])%17);}
int main(void){char*d=__DATE__+1;return printf("%s%x%s\n",d+6,3**d+"![ WT -#[ 8"[d[1]%12],d);}
int main(void){char*d=__DATE__"%x%.5s\n"+1;return printf(d+6,3**d+"![ WT -#[ 8"[d[1]%12],d);}
int main(void){char*d=__DATE__"%x%.5s\n";return printf(d+7,d[1]+"1C0EB042E0:"[d[2]%12],d);}
(ugoren's entry, golfed, for comparison, and adding .5 to the %s format specifier)
int main(void){char*m=__DATE__"%3d%.5s\n"+1;return!printf(m+6,(*m*803^m[1]*95)%94,m);}

My initial approaches made use of a month-to-number approach which can turn each month into an arbitrary character. If in a production embedded-systems environment one had to turn an alphabetic month into a number 1-12, the approach might actually be an efficient algorithm to do so (perhaps using repeated subtract-17 instead of mod, if no other divisions are required).

Allowing the computation to return values which were sortable but not consecutive made it possible to use a smaller table indexed using the third character of the month, and add to that character the second. A further savings was achieved by realizing that the multiplication by three I'd used to facilitate "tiebreaking" was in fact not needed, since the months which would otherwise yield matching hash values were correctly sortable by their first character.

I tried quite a few variations, but nothing came close to the operator combination that ugoren found, which don't require any table. I was also impressed by his entry which would have accommodated single-digit dates without padding, which would have been a challenge in its own right, and one which I doubt that I could have handled as nicely.

Answer 4

C, 163 characters

A different approach from my other solution.
I can save 13 characters by making t an int array, and relying on the internal layout of struct tm. But I guess it violates the rules.

#define _XOPEN_SOURCE
#include<time.h>
#include<stdio.h>
int main(void){
    struct tm t;
    strptime(__DATE__,"%b%d%Y",&t);
    return printf("%d\n",t.tm_year*366+t.tm_yday);
}