Re: bottleneck: string2hex

[Sean Conner <sean@...>]

It was thus said that the Great Valerio Schiavoni once stated:
> Hello,
> profiling my application, the big bottleneck of it comes from this function:
> 
> function string2hex(str)
> assert(str,"String2hex got null argument")
>  local h,s =  string.gsub(str, "(.)", function(c) return
> string.format("%02X", string.byte(c)) end)
> return h
> end
> 
> 
> The size of input strings can be from 1kb to 1Mb.
> How would you optimize it ?

  I'd try some Lua variants and if it still proved too slow, I would
probably test a C implementation.  Which I did.  Test harness:

start = os.clock()
h = string2hex(data)
stop = os.clock()
print("string2hex()",stop - start)

start = os.clock()
h = strhex(data) -- C version
stop = os.clock()
print("strhex()",stop - start)

[spc]lucy:/tmp>lua tlua.lua
string2hex1()   3.52
strhex()        0.65
[spc]lucy:/tmp>

C code is as follows:

/*
 * Sample code---feel free to use.
 *
 * compiled and tested under Linux
 * gcc -Wall -Wextra -pedantic -ansi -g -shared -fpic -o strhex.so strhex.c
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <lua.h>
#include <lauxlib.h>
#include <lualib.h> 

static int strhex(lua_State *L)
{
  const char *s;
  size_t      size;
  size_t      max; 
  char       *ns;  
  char       *p;  
  
  s  = luaL_checklstring(L,1,&size);
  ns = malloc((size * 2) + 1);

  for ( p = ns , max = size ; max > 0 ; max--)
  {
    sprintf(p,"%02X",*s++);
    p += 2;
  }
   
  lua_pushlstring(L,ns,size * 2);
  free(ns);
  return 1;
}
 
int luaopen_strhex(lua_State *L)
{
  lua_register(L,"strhex",strhex);
  return 0;
}

  Then again, I've been programming in C for years now, so this type of
stuff is second nature to me.

  -spc