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Here is some code I have that has been extracted and shrunk down from a project of mine.

#include <stdio.h>
#include <stdint.h>
#include <time.h>
#include <string.h>

#define streq(x, y) (strcmp((x), (y)) == 0)
#define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))

typedef struct
{
    const char *cmd;
    void* (*fn)(void);
} __attribute__((__packed__)) Command;

void* getTime(void)
{
    return ((void*)((uintptr_t)time(NULL)));
}

void* getDay(void)
{
    time_t t = time(NULL);
    struct tm tm = *localtime(&t);
    return ((void*)((uintptr_t)(tm.tm_wday)));
}

static Command commands[] =
{
    {"time", getTime},
    {"day", getDay},
};

int main(int argc, char *argv[])
{
    for (int i = 0; i < ARRAY_SIZE(commands); ++i)
    {
        Command *p = commands+i;
        if (streq(argv[1], p->cmd)) printf("%d\n", (int)p->fn());
    }
}

When run with some given input, it runs the method associated with that input. Since this is a scaled down version of my project, it can only run two commands.

$ ./test-command day
4

Right now, my implementation should run with \$ O \left( n\right)\$ time complexity. This would be unacceptable with the hundreds of thousands of commands my project could scale to. I am looking for reviews on scalability and optimization, and how well my Command structure and function pointer template would scale as well.

share|improve this question
    
Sort your array and bin search it, no? –  vnp Jun 12 at 23:45
    
@vnp Would a binary search be better than a hash search? –  syb0rg Jun 12 at 23:50
    
It is much simpler. Hundreds of thousands of commands translate to 30 or so binary steps. If even that is unacceptable, maybe hash is a way to go. However, the hash function must be very very good, for any conflict resolution incurs significant cost. –  vnp Jun 12 at 23:55
    
@vnp Sounds like you should be writing an answer ;) –  syb0rg Jun 12 at 23:58

2 Answers 2

up vote 3 down vote accepted
  1. Sorting the command list prior to compile time is not that much of a burden. If there a propensity to fail to do so, a simple run-time check in the DEBUG version of code would detect it.

  2. Hash is the way to go for high speed, but a simple binary search would be useful to begin with to iron out initial code development issues.

  3. To help with string compare speeds have done this ((x[0] == y[0]) && (strcmp((x), (y)) == 0)), but this may be premature optimization.

  4. Minor: Suggest replacing void* with some large unsigned integer like uintmax_t as the generic return type for the various functions void* (*fn)(void). Or better yet, a union of the various types.

    typedef union {
      time_t t;
      void *p ;
      int i;
    } Return_T;
    
  5. Minor: localtime() could return NULL. Best to test function return value before dereference.

  6. Minor: Suggest const. static const Command commands[] = ....

  7. Minor: Best to make array indexes size_t: size_t i = 0; ... Command *p = commands+i;

  8. Minor: Casting (uintptr_t)time(NULL) and then to int in printf("%d\n", (int)p->fn()); can easily lose significant data. See note #4.


bsearch() example: something like the following untested code

static int compar(const void *a, const void *b) {
  const char *key = (const char *) a;
  const Command *element = (const Command *) b;
  return strcmp(key, element->cmd);
}

void main(int argc, char *argv[]) {
  Command *match = bsearch(argv[1], commands, ARRAY_SIZE(commands),
          sizeof commands[0], compar);
  if (match != NULL) {
    printf("%d\n", (int) (match->fn()));
    }
}
share|improve this answer
    
I could not get your bsearch() example to work for me. –  syb0rg Jun 17 at 2:34
    
@syb0rg Sorry can not hep more right now, maybe tomorrow. Good luck. Try cplusplus.com/reference/cstdlib/bsearch –  chux Jun 17 at 2:42
    
An advice #3 is surely of a wrong kind. A cost of avoiding a function call is a conditional: it not taken, a function call follows; if taken, the pipeline is flushed anyway. Where's a win? Besides, strcmp is most likely inlined. –  vnp Jun 17 at 7:13
    
@vnp I doubt you would agree but most processors in 2014 are not pipelined. Consider embedded processors quantities are over 1 Billion per year. These simpler devices do benefit from code as suggested. In any case, "may be premature optimization" already addresses the concern that such a tactic may indeed be of negative value. As with many optimizations, looking at the order of complexity (hash, bsearch,, etc.) of the algorithm typically trumps linear speed improvements. –  chux Jun 17 at 14:42

As requested.

An unstructured array may only be searched in linear order. To get better asymptotics you need a more search-friendly structure. A simplest solution would be to presort the Command array, and apply a binary search to find the command. A definite advantage here is a possibility to use standard library. You may even have the array sorted during build time.

For hundreds of thousands of commands the binary search would complete in about 20 lookups. If this is unacceptable, and your commands' names let you come up with a perfect hash function, hash is a way to go. Beware that a hash function must be really perfect - that is, almost no conflicts and almost no misses. Otherwise, the asymptotic constant would defeat all the advantages of O(1).

Update: assuming a unixish environment, the best way to sort at the build time is to have a commands.c file as

static Commands commands[] = {
#include "command-list.c"
};

with a command-list.txt text file

"time", getTime
"day", getDay

and a Makefile rule saying something like (may require some backslashes)

%.c: %.txt
    sort $< | sed -e 's/.*/{&},/' > $@
share|improve this answer
    
What would be the best way to sort the array during build time? I would prefer not to do it manually. –  syb0rg Jun 13 at 0:13
2  
Terminology snipe: A perfect hash function means no conflicts at all. Technical snipe: The binary search is 20 random lookups. Even a bad hashing scheme can be made to do quite well here if you store all the command names, in hash order, in a single string. –  tmyklebu Jun 15 at 2:22
    
@tmyklebu You should form an answer out of that. It's something I would upvote. –  syb0rg Jun 17 at 1:53
    
@syb0rg: chux already nailed it. –  tmyklebu Jun 17 at 1:59

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