strlentest.cpp

#include <chrono>
#include <algorithm>
#include <functional>
#include <iostream>
#include <random>
#include <string>

// Return time in seconds as a double.
static std::chrono::high_resolution_clock::time_point GetTime(void)
{
	//double seconds;
	std::chrono::high_resolution_clock::time_point h_start;
	h_start = std::chrono::high_resolution_clock::now();
	return h_start;
}

/* Return the length of the null-terminated string STR.  Scan for
the null terminator quickly by testing four bytes at a time.  */
size_t strlenopt(const char *str)
{
	const char *char_ptr;
	const unsigned long int *longword_ptr;
	unsigned long int longword, himagic, lomagic;

	/* Handle the first few characters by reading one character at a time.
	Do this until CHAR_PTR is aligned on a longword boundary.  */
	for (char_ptr = str; ((unsigned long int) char_ptr
		& (sizeof(longword) - 1)) != 0;
		++char_ptr)
		if (*char_ptr == '\0')
			return char_ptr - str;

	/* All these elucidatory comments refer to 4-byte longwords,
	but the theory applies equally well to 8-byte longwords.  */

	longword_ptr = (unsigned long int *) char_ptr;

	/* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
	the "holes."  Note that there is a hole just to the left of
	each byte, with an extra at the end:
	bits:  01111110 11111110 11111110 11111111
	bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
	The 1-bits make sure that carries propagate to the next 0-bit.
	The 0-bits provide holes for carries to fall into.  */
	himagic = 0x80808080L;
	lomagic = 0x01010101L;
	if (sizeof(longword) > 4)
	{
		/* 64-bit version of the magic.  */
		/* Do the shift in two steps to avoid a warning if long has 32 bits.  */
		himagic = ((himagic << 16) << 16) | himagic;
		lomagic = ((lomagic << 16) << 16) | lomagic;
	}
	if (sizeof(longword) > 8)
		abort();

	/* Instead of the traditional loop which tests each character,
	we will test a longword at a time.  The tricky part is testing
	if *any of the four* bytes in the longword in question are zero.  */
	for (;;)
	{
		longword = *longword_ptr++;
		if (((longword - lomagic) & ~longword & himagic) != 0)
		{
			/* Which of the bytes was the zero?  If none of them were, it was
			a misfire; continue the search.  */
			const char *cp = (const char *)(longword_ptr - 1);

			if (cp[0] == 0)
				return cp - str;
			if (cp[1] == 0)
				return cp - str + 1;
			if (cp[2] == 0)
				return cp - str + 2;
			if (cp[3] == 0)
				return cp - str + 3;
			if (sizeof(longword) > 4)
			{
				if (cp[4] == 0)
					return cp - str + 4;
				if (cp[5] == 0)
					return cp - str + 5;
				if (cp[6] == 0)
					return cp - str + 6;
				if (cp[7] == 0)
					return cp - str + 7;
			}
		}
	}
}

size_t strlenHw(const char *str)
{
	const char *p;
	if (str == NULL)
		return 0;
	p = str;
	while (*p != '\0')
		p++;
	return p - str;
}

std::string randString(int len)
{
	std::mt19937_64 gen{ std::random_device{}() };
	std::uniform_int_distribution<short> dist{ 'a', 'z' };

	const unsigned length = len + dist(gen);
	//std::cout << "Generated string of len " << length << std::endl;
	std::string str(length, '\0');
	for (auto& c : str)
		c = dist(gen);
	return str;
}

int main()
{
	std::string t;
	std::chrono::duration<double> tot_cpp_time;
	auto start_cpp = GetTime();
	for (int i = 0; i < 10; i++)
	{
		t = randString(0xfffffff);
		strlenopt(t.c_str());
	}
	auto stop_cpp = GetTime();
	tot_cpp_time = std::chrono::duration_cast<std::chrono::duration<double>>(stop_cpp - start_cpp);
	std::cout << "GLib Optimized:\t\t\t\t\t\t" << tot_cpp_time.count() << " Seconds\n";

	start_cpp = GetTime();
	for (int i = 0; i < 10; i++)
	{
		t = randString(0xfffffff);
		strlenHw(t.c_str());
	}
	stop_cpp = GetTime();
	tot_cpp_time = std::chrono::duration_cast<std::chrono::duration<double>>(stop_cpp - start_cpp);
	std::cout << "Simple Implementation Hand Written:\t\t\t" << tot_cpp_time.count() << " Seconds\n";

	start_cpp = GetTime();
	for (int i = 0; i < 10; i++)
	{
		t = randString(0xfffffff);
		strlen(t.c_str());
	}
	stop_cpp = GetTime();
	tot_cpp_time = std::chrono::duration_cast<std::chrono::duration<double>>(stop_cpp - start_cpp);
	std::cout << "Built-In strlen:\t\t\t\t\t" << tot_cpp_time.count() << " Seconds\n";

	start_cpp = GetTime();
	for (int i = 0; i < 1000; i++)
	{
		t = randString(0xff);
		strlenopt(t.c_str());
	}

	stop_cpp = GetTime();
	tot_cpp_time = std::chrono::duration_cast<std::chrono::duration<double>>(stop_cpp - start_cpp);
	std::cout << "Short String GLib Optimized:\t\t\t\t" << tot_cpp_time.count() << " Seconds\n";

	start_cpp = GetTime();
	for (int i = 0; i < 1000; i++)
	{
		t = randString(0xff);
		strlenHw(t.c_str());
	}
	stop_cpp = GetTime();
	tot_cpp_time = std::chrono::duration_cast<std::chrono::duration<double>>(stop_cpp - start_cpp);
	std::cout << "Short String Simple Implementation Hand Written:\t" << tot_cpp_time.count() << " Seconds\n";

	start_cpp = GetTime();
	for (int i = 0; i < 1000; i++)
	{
		t = randString(0xff);
		strlen(t.c_str());
	}
	stop_cpp = GetTime();
	tot_cpp_time = std::chrono::duration_cast<std::chrono::duration<double>>(stop_cpp - start_cpp);
	std::cout << "Short String Built-In strlen:\t\t\t\t" << tot_cpp_time.count() << " Seconds\n";

	return 0;
}