Re^3: Faster Luhn Check Digit Calculation?

[AnomalousMonk]

Here's another twist that might squeeze out a few more computrons: use array literals (if that's the correct terminology; introduced with C99 IIRC; I assume Inline::C supports C99) to compile the arrays rather than building local arrays on each subroutine call (if you don't want to just declare the arrays static| static const, which also works | should work). I haven't looked at the assembler output, but the idea (the hope?) is that the compiler would make these statics. There are two versions of the function below; both work. One is more heavily macro-ized because I wanted to see how far I could push this approach; for reasons of maintainability, I'm not sure I'd want it in production code. Only minimal testing has been done; I leave that to you :).

t_array_literal_1.c:

// t_array_literal_1.c                                      01dec18waw

// compile:
//  gcc -std=c99 t_array_literal_1.c

#include "stdio.h"
#include "string.h"
#include "stdlib.h"

#define elementsof(ra) (sizeof(ra) / sizeof(ra[0]))

int cd4_a (char *);
int cd4_b (char *);

void main (void) {

    char * suffices[] = { "000", "999", "100", "020", "003", "123", };

    for (size_t i = 0;  i < elementsof(suffices);  ++i) {

        char ccn[20] = "401135000000";
        strcat(ccn, suffices[i]);

        int cda = cd4_a(ccn);
        int cdb = cd4_b(ccn);  // cdb = 8;  // for testing

        printf("ccn: '%s'  a: %d  b: %d", ccn, cda, cdb);

        if (cda != cdb) {
            printf("  (digits differ: stopping) \n", cda, cdb);
            exit(1);
            }

        printf("\n");

        }

    }

int cd4_a( char *ccn ) {  // from pm#1226599
    int total = 0;
    int i;
    int co9[]={0,2,4,6,8,1,3,5,7,9};  // or maybe just make these arra
+ys static?
    int straight[]={1,3,5,7,9,11,13};
    int adjusted[]={0,2,4,6,8,10,12,14};
    for (i=0;i<7;i++) {
        total += ccn[straight[i]]-48;
    }
    for (i=0;i<8;i++) {
        total += co9[ccn[adjusted[i]]-48];
    }
    total *=9;
    return total %10;
}

#define X2_CO9  0, 2, 4, 6, 8, 1, 3, 5, 7, 9
#define STRAIGHT_I  1, 3, 5, 7, 9, 11, 13
#define ADJUSTED_I  0, 2, 4, 6, 8, 10, 12, 14

int cd4_b (char * ccn) {

    int total = 0;

    for (int i = 0;  i < 7;  i++) {
        total += ccn[ ((int []){ STRAIGHT_I })[i] ] - (int)'0';
        }

    for (int i = 0;  i < 8;  i++) {
        total += ((int []){ X2_CO9 })[ ccn[ ((int []){ ADJUSTED_I })[i
+] ] - (int)'0'];
        }

    total *= 9;
    return total % 10;

    }

// #define X2_CO9(n)  (((int []){ 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 })[n])
// #define STRAIGHT_I(i)  (((int []){ 1, 3, 5, 7, 9, 11, 13 })[i])
// #define ADJUSTED_I(i)  (((int []){ 0, 2, 4, 6, 8, 10, 12, 14 })[i])
//
// int cd4_b (char * ccn) {
//
//     int total = 0;
//
//     for (int i = 0;  i < 7;  i++) {
//         total += ccn[ STRAIGHT_I(i) ] - (int)'0';
//         }
//
//     for (int i = 0;  i < 8;  i++) {
//         total += X2_CO9( ccn[ ADJUSTED_I(i) ] - (int)'0' );
//         }
//
//     total *= 9;
//     return total % 10;
//
//     }