pattern

/*
 * both functions do the same thing, find repeating pattern in a string
 * in assembly, uses 7 different instructions, occupies 31 lines
 * in C, uses (a lot) of stack instructions, also occupies 31 lines
 */

/*
 * speed comparison, two different tests were done (different strings)
 * results (in cycles):
 * 1) c version - 782 | asm version - 357
 * 2) c version - 1127 | asm version - 425
 */

/***********************************************************************************/

.globl _asm_find_pattern
_asm_find_pattern:

    movq    $0, %rcx
fp_out_loop:
    addq    $1, %rcx
    movq    %rsi, %rdx
fp_inf_loop:
    movq    %rcx, %rbx
    movq    %rsi, %r8
    movq    %rdx, %r9
fp_cmp_loop:
    addq    $1, %r8
    addq    $1, %r9
    movb    -1(%r8), %al
    cmpb    %al, -1(%r9)
    jne fp_out_loop
    subq    $1, %rbx
    jne fp_cmp_loop
    addq    %rcx, %rdx
    cmpb    $0, (%rdx)
    jne fp_inf_loop
    subq    %rcx, %rdx
fp_cpy_loop:
    movb    (%rdx), %al
    movb    %al, (%rdi)
    addq    $1, %rdx
    addq    $1, %rdi
    testb   %al, %al
    jne fp_cpy_loop
    movb    $0, (%rdi)
    ret

/***********************************************************************************/

void new_find_pattern(char *output, char *input)
{
    int i = 0, j;
    char *s1, *s2;
    char *c1, *c2;

    while (1) {
        i++;
        c1 = input;
        while (1) {
            j = i;
            s1 = c1;
            s2 = input;
            while (j) {
                if (*s1++ != *s2++)
                    break;
                j--;
            }
            if (j) {
                break;
            }
            c1 += i;
            if (!*c1) {
                c1 -= i;
                for (c2 = output; *c1; c2++)
                    *c2 = *c1++;
                *c2 = '\0';
                return;
            }
        }
    }
}

/***********************************************************************************/