c++ - Is there a reverse function for strstr

Question

I am trying to find a similar function to strstr that searches a substring starting from the end towards the beginning of the string.

Answer 1

The standard C library does not have a "reverse strstr" function, so you have to find or write your own.

I came up with a couple of solutions of my own, and added some testing and benchmarking code together with the other functions in this thread. For those curious, running on my laptop (Ubuntu karmic, amd64 architecture) the output looks like this:

$ gcc -O2 --std=c99 strrstr.c && ./a.out
#1 0.123 us last_strstr
#2 0.440 us theo
#3 0.460 us cordelia
#4 1.690 us digitalross
#5 7.700 us backwards_memcmp
#6 8.600 us sinan

Your results may be different and, depending on your compiler and library, the ordering of the results may also be different.

To get the offset (index) of the match from the beginning of the string, use pointer arithmetic:

char *match = last_strstr(haystack, needle);
ptrdiff_t index;
if (match != NULL)
    index = match - haystack;
else
    index = -1;

And now, the larch (note that this is purely in C, I do not know C++ well enough to give an answer for it):

#include <string.h>
#include <stdlib.h>

/* By liw. */
static char *last_strstr(const char *haystack, const char *needle)
{
    if (*needle == '')
        return (char *) haystack;

    char *result = NULL;
    for (;;) {
        char *p = strstr(haystack, needle);
        if (p == NULL)
            break;
        result = p;
        haystack = p + 1;
    }

    return result;
}


/* By liw. */
static char *backwards_memcmp(const char *haystack, const char *needle)
{
    size_t haylen = strlen(haystack);

    if (*needle == '')
        return (char *) haystack;

    size_t needlelen = strlen(needle);
    if (needlelen > haylen)
        return NULL;

    const char *p = haystack + haylen - needlelen;
    for (;;) {
        if (memcmp(p, needle, needlelen) == 0)
            return (char *) p;
        if (p == haystack)
            return NULL;
        --p;
    }
}


/* From http://stuff.mit.edu/afs/sipb/user/cordelia/Diplomacy/mapit/strrstr.c
 */
static char *cordelia(const char *s1, const char *s2)
{
 const char *sc1, *sc2, *psc1, *ps1;

 if (*s2 == '')
  return((char *)s1);

 ps1 = s1 + strlen(s1);

 while(ps1 != s1) {
  --ps1;
  for (psc1 = ps1, sc2 = s2; ; )
   if (*(psc1++) != *(sc2++))
    break;
   else if (*sc2 == '')
    return ((char *)ps1);
 }
 return ((char *)NULL);
}


/* From http://stackoverflow.com/questions/1634359/
   is-there-a-reverse-fn-for-strstr/1634398#1634398 (DigitalRoss). */
static char *reverse(const char *s)
{
  if (s == NULL)
    return NULL;
  size_t i, len = strlen(s);
  char *r = malloc(len + 1);

  for(i = 0; i < len; ++i)
    r[i] = s[len - i - 1];
  r[len] = 0;
  return r;
}
char *digitalross(const char *s1, const char *s2)
{
  size_t  s1len = strlen(s1);
  size_t  s2len = strlen(s2);
  const char *s;

  if (s2len == 0)
    return (char *) s1;
  if (s2len > s1len)
    return NULL;
  for (s = s1 + s1len - s2len; s >= s1; --s)
    if (strncmp(s, s2, s2len) == 0)
      return (char *) s;
  return NULL;
}


/* From http://stackoverflow.com/questions/1634359/
  is-there-a-reverse-fn-for-strstr/1634487#1634487 (Sinan ünür). */

char *sinan(const char *source, const char *target)
{
    const char *current;
    const char *found = NULL;

    if (*target == '')
        return (char *) source;

    size_t target_length = strlen(target);
    current = source + strlen(source) - target_length;

    while ( current >= source ) {
        if ( (found = strstr(current, target)) ) {
            break;
        }
        current -= 1;
    }

    return (char *) found;
}


/* From http://stackoverflow.com/questions/1634359/
  is-there-a-reverse-fn-for-strstr/1634441#1634441 (Theo Spears). */
char *theo(const char* haystack, const char* needle)
{
  size_t needle_length = strlen(needle);
  const char* haystack_end = haystack + strlen(haystack) - needle_length;
  const char* p;
  size_t i;

  if (*needle == '')
    return (char *) haystack;
  for(p = haystack_end; p >= haystack; --p)
  {
    for(i = 0; i < needle_length; ++i) {
      if(p[i] != needle[i])
        goto next;
    }
    return (char *) p;

    next:;
  }
  return 0;
}


/*
 * The rest of this code is a test and timing harness for the various
 * implementations above.
 */


#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>


/* Check that the given function works. */
static bool works(const char *name, char *(*func)(const char *, const char *))
{
    struct {
        const char *haystack;
        const char *needle;
        int offset;
    } tests[] = {
        { "", "", 0 },
        { "", "x", -1 },
        { "x", "", 0 },
        { "x", "x", 0 },
        { "xy", "x", 0 },
        { "xy", "y", 1 },
        { "xyx", "x", 2 },
        { "xyx", "y", 1 },
        { "xyx", "z", -1 },
        { "xyx", "", 0 },
    };
    const int num_tests = sizeof(tests) / sizeof(tests[0]);
    bool ok = true;

    for (int i = 0; i < num_tests; ++i) {
        int offset;
        char *p = func(tests[i].haystack, tests[i].needle);
        if (p == NULL)
            offset = -1;
        else
            offset = p - tests[i].haystack;
        if (offset != tests[i].offset) {
            fprintf(stderr, "FAIL %s, test %d: returned %d, haystack = '%s', "
                            "needle = '%s', correct return %d
",
                            name, i, offset, tests[i].haystack, tests[i].needle,
                            tests[i].offset);
            ok = false;
        }
    }
    return ok;
}


/* Dummy function for calibrating the measurement loop. */
static char *dummy(const char *haystack, const char *needle)
{
    return NULL;
}


/* Measure how long it will take to call the given function with the
   given arguments the given number of times. Return clock ticks. */
static clock_t repeat(char *(*func)(const char *, const char *),
                       const char *haystack, const char *needle,
                       long num_times)
{
    clock_t start, end;

    start = clock();
    for (long i = 0; i < num_times; ++i) {
        func(haystack, needle);
    }
    end = clock();
    return end - start;
}


static clock_t min(clock_t a, clock_t b)
{
    if (a < b)
        return a;
    else
        return b;
}


/* Measure the time to execute one call of a function, and return the
   number of CPU clock ticks (see clock(3)). */
static double timeit(char *(*func)(const char *, const char *))
{
    /* The arguments for the functions to be measured. We deliberately
       choose a case where the haystack is large and the needle is in
       the middle, rather than at either end. Obviously, any test data
       will favor some implementations over others. This is the weakest
       part of the benchmark. */

    const char haystack[] = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "b"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    const char needle[] = "b";

    /* First we find out how many repeats we need to do to get a sufficiently
       long measurement time. These functions are so fast that measuring
       only a small number of repeats will give wrong results. However,
       we don't want to do a ridiculously long measurement, either, so 
       start with one repeat and multiply it by 10 until the total time is
       about 0.2 seconds. 

       Finally, we measure the dummy function the same number of times
       to get rid of the call overhead.

       */

    clock_t mintime = 0.2 * CLOCKS_PER_SEC;
    clock_t clocks;
    long repeats = 1;
    for (;;) {
        clocks = repeat(func, haystack, needle, repeats);
        if (clocks >= mintime)
            break;
        repeats *= 10;
    }

    clocks = min(clocks, repeat(func, haystack, needle, repeats));
    clocks = min(clocks, repeat(func, haystack, needle, repeats));

    clock_t dummy_clocks;

    dummy_clocks = repeat(dummy, haystack, needle, repeats);
    dummy_clocks = min(dummy_clocks, repeat(dummy, haystack, needle, repeats));
    dummy_clocks = min(dummy_clocks, repeat(dummy, haystack, needle, repeats));

    return (double) (clocks - dummy_clocks) / repeats / CLOCKS_PER_SEC;
}


/* Array of all functions. */
struct func {
    const char *name;
    char *(*func)(const char *, const char *);
    double secs;
} funcs[] = {
#define X(func) { #func, func, 0 }
    X(last_strstr),
    X(backwards_memcmp),
    X(cordelia),
    X(digitalross),
    X(sinan),
    X(theo),
#undef X
};
const int num_funcs = sizeof(funcs) / sizeof(funcs[0]);


/* Comparison function for qsort, comparing timings. */
int funcmp(const void *a, const void *b)
{
    const struct func *aa = a;
    const struct func *bb = b;

    if (aa->secs < bb->secs)
        return -1;
    else if (aa->secs > bb->secs)
        return 1;
    else
        return 0;
}


int main(void)
{

    bool ok = true;
    for (int i = 0; i < num_funcs; ++i) {
        if (!works(funcs[i].name, funcs[i].func)) {
            fprintf(stderr, "%s does not work
", funcs[i].name);            
            ok = false;
        }
    }
    if (!ok)
        return EXIT_FAILURE;

    for (int i = 0; i < num_funcs; ++i)
        funcs[i].secs = timeit(funcs[i].func);
    qsort(funcs, num_funcs, sizeof(funcs[0]), funcmp);
    for (int i = 0; i < num_funcs; ++i)
        printf("#%d %.3f us %s
", i+1, funcs[i].secs * 1e6, funcs[i].name);

    return 0;
}