Bit HacksA part of being a great programmer is having your personal code library. With a personal code library I mean a repository of code that you have an intimate knowledge of and that you can reuse quickly. If you are a C programmer, you don't want to reimplement linked lists, trees, various utility functions, macros and algorithms each time you write a new program. Rather you want to take them from your repository, adjust and incorporate in your code.

A good example is the implementation of linked lists in the Linux kernel. Every kernel developer knows it and uses it if necessary. They wouldn't reimplement it. Another example is all the code written by djb. It's so good that people have taken it and turned into libdjb code library.

With this article I'd like to open a new topic in this blog where I share code from my personal code library. I'll start with a C header file that I created just recently based on my Bit Hacks You Should Know About article.

This header file is called "bithacks.h" and it contains various macros for bit manipulations. I also wrote tests for all the macros in the "bithacks-test.c" program.

The most beautiful part of "bithacks.h" is the "B8" macro that allows to write something like " x = B8(10101010) " and turns it into " x = 170 " (because 10101010 in binary is 170 in decimal). I have not yet added B16 and B32 macros but I will add them when I publish the article on advanced bithacks. The credit for the B8 idea goes to Tom Torfs who was the first to write it.

The "bithacks.h" header provides the following macros:

  • B8(x) - turns x written in binary into decimal,
  • B_EVEN(x) - tests if x is even (bithack #1),
  • B_ODD(x) - tests if x is odd (inverse of (bithack #1)),
  • B_IS_SET(x, n) - tests if n-th bit is set in x (bithack #2),
  • B_SET(x, n) - sets n-th bit in x (bithack #3),
  • B_UNSET(x, n) - unsets n-th bit in x (bithack #4),
  • B_TOGGLE(x, n) - toggles n-th bit in x (bithack #5),
  • B_TURNOFF_1(x) - turns off the right-most 1-bit in x (bithack #6),
  • B_ISOLATE_1(x) - isolates the right-most 1-bit in x (bithack #7),
  • B_PROPAGATE_1(x) - propagates the right-most 1-bit in x (bithack #8),
  • B_ISOLATE_0(x) - isolates the right-most 0-bit in x (bithack #9),
  • B_TURNON_0(x) - turn on the right-most 0-bit in x (bithack #10).

Please see "bithacks-test.c" for many examples of these macros.

For those who don't want to download bithacks.h, here is its content:

/* 
** bithacks.h - bit hacks macros. v1.0
**
** Released under the MIT license.
*/

#ifndef BITHACKS_H
#define BITHACKS_H

#define HEXIFY(X) 0x##X##LU

#define B8IFY(Y) (((Y&0x0000000FLU)?1:0)  + \
                  ((Y&0x000000F0LU)?2:0)  + \
                  ((Y&0x00000F00LU)?4:0)  + \
                  ((Y&0x0000F000LU)?8:0)  + \
                  ((Y&0x000F0000LU)?16:0) + \
                  ((Y&0x00F00000LU)?32:0) + \
                  ((Y&0x0F000000LU)?64:0) + \
                  ((Y&0xF0000000LU)?128:0))

#define B8(Z) ((unsigned char)B8IFY(HEXIFY(Z)))

/* test if x is even */
#define B_EVEN(x)        (((x)&1)==0)

/* test if x is odd */
#define B_ODD(x)         (!B_EVEN((x)))

/* test if n-th bit in x is set */
#define B_IS_SET(x, n)   (((x) & (1<<(n)))?1:0)

/* set n-th bit in x */
#define B_SET(x, n)      ((x) |= (1<<(n)))

/* unset n-th bit in x */
#define B_UNSET(x, n)    ((x) &= ~(1<<(n)))

/* toggle n-th bit in x */
#define B_TOGGLE(x, n)   ((x) ^= (1<<(n)))

/* turn off right-most 1-bit in x */
#define B_TURNOFF_1(x)   ((x) &= ((x)-1))

/* isolate right-most 1-bit in x */
#define B_ISOLATE_1(x)   ((x) &= (-(x)))

/* right-propagate right-most 1-bit in x */
#define B_PROPAGATE_1(x) ((x) |= ((x)-1))

/* isolate right-most 0-bit in x */
#define B_ISOLATE_0(x)   ((x) = ~(x) & ((x)+1))

/* turn on right-most 0-bit in x */
#define B_TURNON_0(x)    ((x) |= ((x)+1))

/*
** more bit hacks coming as soon as I post
** an article on advanced bit hacks
*/

#endif

And here are all the tests:

/* 
** bithacks-test.c - tests for bithacks.h
**
** Released under the MIT license.
*/

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

#include "bithacks.h"

int error_count;

#define TEST_OK(exp, what) do { \
    if ((exp)!=(what)) { \
        error_count++; \
        printf("Test '%s' at line %d failed.\n", #exp, __LINE__); \
    } } while(0)

#define TEST_END do { \
    if (error_count) { \
        printf("Testing failed: %d failed tests.\n", error_count); \
    } else { \
        printf("All tests OK.\n"); \
    } } while (0)

void test_B8()
{
    /* test B8 */
    TEST_OK(B8(0), 0);
    TEST_OK(B8(1), 1);
    TEST_OK(B8(11), 3);
    TEST_OK(B8(111), 7);
    TEST_OK(B8(1111), 15);
    TEST_OK(B8(11111), 31);
    TEST_OK(B8(111111), 63);
    TEST_OK(B8(1111111), 127);
    TEST_OK(B8(00000000), 0);
    TEST_OK(B8(11111111), 255);
    TEST_OK(B8(1010), 10);
    TEST_OK(B8(10101010), 170);
    TEST_OK(B8(01010101), 85);
}

void test_B_EVEN()
{
    /* test B_EVEN */
    TEST_OK(B_EVEN(B8(0)), 1);
    TEST_OK(B_EVEN(B8(00000000)), 1);
    TEST_OK(B_EVEN(B8(1)), 0);
    TEST_OK(B_EVEN(B8(11111111)), 0);
    TEST_OK(B_EVEN(B8(10101010)), 1);
    TEST_OK(B_EVEN(B8(01010101)), 0);
    TEST_OK(B_EVEN(44), 1);
    TEST_OK(B_EVEN(131), 0);
}

void test_B_ODD()
{
    /* test B_ODD */
    TEST_OK(B_ODD(B8(0)), 0);
    TEST_OK(B_ODD(B8(00000000)), 0);
    TEST_OK(B_ODD(B8(1)), 1);
    TEST_OK(B_ODD(B8(11111111)), 1);
    TEST_OK(B_ODD(B8(10101010)), 0);
    TEST_OK(B_ODD(B8(01010101)), 1);
    TEST_OK(B_ODD(44), 0);
    TEST_OK(B_ODD(131), 1);
}

void test_B_IS_SET()
{
    /* test B_IS_SET */
    TEST_OK(B_IS_SET(B8(0), 0), 0);
    TEST_OK(B_IS_SET(B8(00000000), 0), 0);
    TEST_OK(B_IS_SET(B8(1), 0), 1);
    TEST_OK(B_IS_SET(B8(11111111), 0), 1);
    TEST_OK(B_IS_SET(B8(11111111), 1), 1);
    TEST_OK(B_IS_SET(B8(11111111), 2), 1);
    TEST_OK(B_IS_SET(B8(11111111), 3), 1);
    TEST_OK(B_IS_SET(B8(11111111), 4), 1);
    TEST_OK(B_IS_SET(B8(11111111), 5), 1);
    TEST_OK(B_IS_SET(B8(11111111), 6), 1);
    TEST_OK(B_IS_SET(B8(11111111), 7), 1);
    TEST_OK(B_IS_SET(B8(11110000), 0), 0);
    TEST_OK(B_IS_SET(B8(11110000), 1), 0);
    TEST_OK(B_IS_SET(B8(11110000), 2), 0);
    TEST_OK(B_IS_SET(B8(11110000), 3), 0);
    TEST_OK(B_IS_SET(B8(11110000), 4), 1);
    TEST_OK(B_IS_SET(B8(11110000), 5), 1);
    TEST_OK(B_IS_SET(B8(11110000), 6), 1);
    TEST_OK(B_IS_SET(B8(11110000), 7), 1);
    TEST_OK(B_IS_SET(B8(00001111), 0), 1);
    TEST_OK(B_IS_SET(B8(00001111), 1), 1);
    TEST_OK(B_IS_SET(B8(00001111), 2), 1);
    TEST_OK(B_IS_SET(B8(00001111), 3), 1);
    TEST_OK(B_IS_SET(B8(00001111), 4), 0);
    TEST_OK(B_IS_SET(B8(00001111), 5), 0);
    TEST_OK(B_IS_SET(B8(00001111), 6), 0);
    TEST_OK(B_IS_SET(B8(00001111), 7), 0);
    TEST_OK(B_IS_SET(B8(10101010), 0), 0);
    TEST_OK(B_IS_SET(B8(10101010), 1), 1);
    TEST_OK(B_IS_SET(B8(10101010), 2), 0);
    TEST_OK(B_IS_SET(B8(10101010), 3), 1);
    TEST_OK(B_IS_SET(B8(10101010), 4), 0);
    TEST_OK(B_IS_SET(B8(10101010), 5), 1);
    TEST_OK(B_IS_SET(B8(10101010), 6), 0);
    TEST_OK(B_IS_SET(B8(10101010), 7), 1);
    TEST_OK(B_IS_SET(B8(01010101), 0), 1);
    TEST_OK(B_IS_SET(B8(01010101), 1), 0);
    TEST_OK(B_IS_SET(B8(01010101), 2), 1);
    TEST_OK(B_IS_SET(B8(01010101), 3), 0);
    TEST_OK(B_IS_SET(B8(01010101), 4), 1);
    TEST_OK(B_IS_SET(B8(01010101), 5), 0);
    TEST_OK(B_IS_SET(B8(01010101), 6), 1);
    TEST_OK(B_IS_SET(B8(01010101), 7), 0);
}

void test_B_SET()
{
    /* test B_SET */
    unsigned char x;

    x = B8(00000000);
    TEST_OK(B_SET(x, 0), B8(00000001));
    TEST_OK(B_SET(x, 1), B8(00000011));
    TEST_OK(B_SET(x, 2), B8(00000111));
    TEST_OK(B_SET(x, 3), B8(00001111));
    TEST_OK(B_SET(x, 4), B8(00011111));
    TEST_OK(B_SET(x, 5), B8(00111111));
    TEST_OK(B_SET(x, 6), B8(01111111));
    TEST_OK(B_SET(x, 7), B8(11111111));

    x = B8(11111111);
    TEST_OK(B_SET(x, 0), B8(11111111));
    TEST_OK(B_SET(x, 1), B8(11111111));
    TEST_OK(B_SET(x, 2), B8(11111111));
    TEST_OK(B_SET(x, 3), B8(11111111));
    TEST_OK(B_SET(x, 4), B8(11111111));
    TEST_OK(B_SET(x, 5), B8(11111111));
    TEST_OK(B_SET(x, 6), B8(11111111));
    TEST_OK(B_SET(x, 7), B8(11111111));
}

void test_B_UNSET()
{
    unsigned char x;
   
    x = B8(11111111);
    TEST_OK(B_UNSET(x, 0), B8(11111110));
    TEST_OK(B_UNSET(x, 1), B8(11111100));
    TEST_OK(B_UNSET(x, 2), B8(11111000));
    TEST_OK(B_UNSET(x, 3), B8(11110000));
    TEST_OK(B_UNSET(x, 4), B8(11100000));
    TEST_OK(B_UNSET(x, 5), B8(11000000));
    TEST_OK(B_UNSET(x, 6), B8(10000000));
    TEST_OK(B_UNSET(x, 7), B8(00000000));

    x = B8(00000000);
    TEST_OK(B_UNSET(x, 0), B8(00000000));
    TEST_OK(B_UNSET(x, 1), B8(00000000));
    TEST_OK(B_UNSET(x, 2), B8(00000000));
    TEST_OK(B_UNSET(x, 3), B8(00000000));
    TEST_OK(B_UNSET(x, 4), B8(00000000));
    TEST_OK(B_UNSET(x, 5), B8(00000000));
    TEST_OK(B_UNSET(x, 6), B8(00000000));
    TEST_OK(B_UNSET(x, 7), B8(00000000));
}

void test_B_TOGGLE()
{
    unsigned char x = B8(11111111);
    TEST_OK(B_TOGGLE(x, 0), B8(11111110));
    TEST_OK(B_TOGGLE(x, 0), B8(11111111));
    TEST_OK(B_TOGGLE(x, 1), B8(11111101));
    TEST_OK(B_TOGGLE(x, 1), B8(11111111));
    TEST_OK(B_TOGGLE(x, 2), B8(11111011));
    TEST_OK(B_TOGGLE(x, 2), B8(11111111));
    TEST_OK(B_TOGGLE(x, 3), B8(11110111));
    TEST_OK(B_TOGGLE(x, 3), B8(11111111));
    TEST_OK(B_TOGGLE(x, 4), B8(11101111));
    TEST_OK(B_TOGGLE(x, 4), B8(11111111));
    TEST_OK(B_TOGGLE(x, 5), B8(11011111));
    TEST_OK(B_TOGGLE(x, 5), B8(11111111));
    TEST_OK(B_TOGGLE(x, 6), B8(10111111));
    TEST_OK(B_TOGGLE(x, 6), B8(11111111));
    TEST_OK(B_TOGGLE(x, 7), B8(01111111));
    TEST_OK(B_TOGGLE(x, 7), B8(11111111));
}

void test_B_TURNOFF_1()
{
    unsigned char x;

    x = B8(11111111);
    TEST_OK(B_TURNOFF_1(x), B8(11111110));
    TEST_OK(B_TURNOFF_1(x), B8(11111100));
    TEST_OK(B_TURNOFF_1(x), B8(11111000));
    TEST_OK(B_TURNOFF_1(x), B8(11110000));
    TEST_OK(B_TURNOFF_1(x), B8(11100000));
    TEST_OK(B_TURNOFF_1(x), B8(11000000));
    TEST_OK(B_TURNOFF_1(x), B8(10000000));
    TEST_OK(B_TURNOFF_1(x), B8(00000000));
    TEST_OK(B_TURNOFF_1(x), B8(00000000));

    x = B8(10101010);
    TEST_OK(B_TURNOFF_1(x), B8(10101000));
    TEST_OK(B_TURNOFF_1(x), B8(10100000));
    TEST_OK(B_TURNOFF_1(x), B8(10000000));
    TEST_OK(B_TURNOFF_1(x), B8(00000000));
    TEST_OK(B_TURNOFF_1(x), B8(00000000));

    x = B8(01010101);
    TEST_OK(B_TURNOFF_1(x), B8(01010100));
    TEST_OK(B_TURNOFF_1(x), B8(01010000));
    TEST_OK(B_TURNOFF_1(x), B8(01000000));
    TEST_OK(B_TURNOFF_1(x), B8(00000000));
    TEST_OK(B_TURNOFF_1(x), B8(00000000));
}

void test_B_ISOLATE_1()
{
    unsigned char x;

    x = B8(11111111);
    TEST_OK(B_ISOLATE_1(x), B8(00000001));
    TEST_OK(B_ISOLATE_1(x), B8(00000001));

    x = B8(11111110);
    TEST_OK(B_ISOLATE_1(x), B8(00000010));
    TEST_OK(B_ISOLATE_1(x), B8(00000010));

    x = B8(11111100);
    TEST_OK(B_ISOLATE_1(x), B8(00000100));
    TEST_OK(B_ISOLATE_1(x), B8(00000100));

    x = B8(11111000);
    TEST_OK(B_ISOLATE_1(x), B8(00001000));
    TEST_OK(B_ISOLATE_1(x), B8(00001000));

    x = B8(11110000);
    TEST_OK(B_ISOLATE_1(x), B8(00010000));
    TEST_OK(B_ISOLATE_1(x), B8(00010000));

    x = B8(11100000);
    TEST_OK(B_ISOLATE_1(x), B8(00100000));
    TEST_OK(B_ISOLATE_1(x), B8(00100000));

    x = B8(11000000);
    TEST_OK(B_ISOLATE_1(x), B8(01000000));
    TEST_OK(B_ISOLATE_1(x), B8(01000000));

    x = B8(10000000);
    TEST_OK(B_ISOLATE_1(x), B8(10000000));
    TEST_OK(B_ISOLATE_1(x), B8(10000000));

    x = B8(00000000);
    TEST_OK(B_ISOLATE_1(x), B8(00000000));

    x = B8(10000000);
    TEST_OK(B_ISOLATE_1(x), B8(10000000));

    x = B8(10001001);
    TEST_OK(B_ISOLATE_1(x), B8(00000001));

    x = B8(10001000);
    TEST_OK(B_ISOLATE_1(x), B8(00001000));
}

void test_B_PROPAGATE_1()
{
    unsigned char x;

    x = B8(00000000);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(10000000);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11000000);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11100000);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11110000);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11111000);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11111100);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11111110);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(11111111);
    TEST_OK(B_PROPAGATE_1(x), B8(11111111));

    x = B8(00100000);
    TEST_OK(B_PROPAGATE_1(x), B8(00111111));
    TEST_OK(B_PROPAGATE_1(x), B8(00111111));

    x = B8(10101000);
    TEST_OK(B_PROPAGATE_1(x), B8(10101111));
    TEST_OK(B_PROPAGATE_1(x), B8(10101111));

    x = B8(10101010);
    TEST_OK(B_PROPAGATE_1(x), B8(10101011));
    TEST_OK(B_PROPAGATE_1(x), B8(10101011));

    x = B8(10101010);
    TEST_OK(B_PROPAGATE_1(x), B8(10101011));
    TEST_OK(B_PROPAGATE_1(x), B8(10101011));
}

void test_B_ISOLATE_0()
{
    unsigned char x;

    x = B8(00000000);
    TEST_OK(B_ISOLATE_0(x), B8(00000001));
    TEST_OK(B_ISOLATE_0(x), B8(00000010));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(00000011);
    TEST_OK(B_ISOLATE_0(x), B8(00000100));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(00000111);
    TEST_OK(B_ISOLATE_0(x), B8(00001000));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(00001111);
    TEST_OK(B_ISOLATE_0(x), B8(00010000));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(00011111);
    TEST_OK(B_ISOLATE_0(x), B8(00100000));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(00111111);
    TEST_OK(B_ISOLATE_0(x), B8(01000000));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(01111111);
    TEST_OK(B_ISOLATE_0(x), B8(10000000));
    TEST_OK(B_ISOLATE_0(x), B8(00000001));

    x = B8(11111111);
    TEST_OK(B_ISOLATE_0(x), B8(00000000));

    x = B8(01010101);
    TEST_OK(B_ISOLATE_0(x), B8(00000010));

    x = B8(01010111);
    TEST_OK(B_ISOLATE_0(x), B8(00001000));

    x = B8(01011111);
    TEST_OK(B_ISOLATE_0(x), B8(00100000));

    x = B8(01111111);
    TEST_OK(B_ISOLATE_0(x), B8(10000000));
}

void test_B_TURNON_0()
{
    unsigned char x;

    x = B8(00000000);
    TEST_OK(B_TURNON_0(x), B8(00000001));
    TEST_OK(B_TURNON_0(x), B8(00000011));
    TEST_OK(B_TURNON_0(x), B8(00000111));
    TEST_OK(B_TURNON_0(x), B8(00001111));
    TEST_OK(B_TURNON_0(x), B8(00011111));
    TEST_OK(B_TURNON_0(x), B8(00111111));
    TEST_OK(B_TURNON_0(x), B8(01111111));
    TEST_OK(B_TURNON_0(x), B8(11111111));
    TEST_OK(B_TURNON_0(x), B8(11111111));

    x = B8(10101010);
    TEST_OK(B_TURNON_0(x), B8(10101011));
    TEST_OK(B_TURNON_0(x), B8(10101111));
    TEST_OK(B_TURNON_0(x), B8(10111111));
    TEST_OK(B_TURNON_0(x), B8(11111111));

    x = B8(10000000);
    TEST_OK(B_TURNON_0(x), B8(10000001));
    TEST_OK(B_TURNON_0(x), B8(10000011));
    TEST_OK(B_TURNON_0(x), B8(10000111));
    TEST_OK(B_TURNON_0(x), B8(10001111));
    TEST_OK(B_TURNON_0(x), B8(10011111));
    TEST_OK(B_TURNON_0(x), B8(10111111));
    TEST_OK(B_TURNON_0(x), B8(11111111));
}

int main()
{
    test_B8();
    test_B_EVEN();
    test_B_ODD();
    test_B_IS_SET();
    test_B_SET();
    test_B_UNSET();
    test_B_TOGGLE();
    test_B_TURNOFF_1();
    test_B_ISOLATE_1();
    test_B_PROPAGATE_1();
    test_B_ISOLATE_0();
    test_B_TURNON_0();

    TEST_END;

    return error_count ? EXIT_FAILURE : EXIT_SUCCESS;
}

Download "bithacks.h" header file:

Download: bithacks.h
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Download url: http://www.catonmat.net/download/bithacks.h

Download: bithacks-test.c
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Download url: http://www.catonmat.net/download/bithacks-test.c

The next post about this topic will be on advanced bithacks and extending bithacks.h with these new, advanced bithacks.

Have fun!

Google Python Search LibraryAs promised in my previous post on xgoogle library, I have added a module to get results from Google Sets.

Google Sets allows to automatically create groups of related items from a few example items. For example, you feed it "red, green, blue," and it will predict other colors such as "yellow, black, white, brown, etc."

One of the most fascinating applications that this library can be used for is predicting domain names. Most sysadmins have a coherent naming policy for their systems. For example, a sysadmin at a university might call his machines "psychology.university.edu", "art.university.edu", "geography.university.edu", etc. Now, if we feed these names "psychology, art, geography" to Google Sets, it would come up with more names such as "history, mathematics, biology, and others". Now we can do DNS scans to find if there really are such machines. This is a pretty powerful method for reconnaissance.

There are many other interesting applications. Black hat SEO's may use it to stuff their pages with related keywords and thus rank for more words on search engines. Linguists can use it for various natural language processing problems. Various word guessing games can be created.

But my personal goal in writing this library was to use it for my English language perfection and correction tool that I will release in one of the next posts about this project. I wrote more about this idea in the introductory post of xgoogle library. Please see that post for more info.

The new module is called "googlesets", and to use it, import "GoogleSets" and create an object of this type. Pass the list of items to create the prediction from to the constructor. Then use "get_results()" member function to get the list of predicted items. It returns a list of Unicode strings, so make sure to use a proper encoding when outputting them.

Here is an example usage of the new module. It finds items related to programming languages "python" and "perl":

from xgoogle.googlesets import GoogleSets
gs = GoogleSets(['python', 'perl'])
items = gs.get_results()
for item in items:
  print item.encode('utf8')

Output:

python
perl
php
ruby
java
javascript
c++
c
cgi
tcl
c#

The output matches that of Google Sets itself:

Google Sets Predicted Items from Perl and Python

See the readme.txt file in the xgoogle archive for more examples.

Download "xgoogle" library:

Download: xgoogle library (.zip)
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Download url: http://www.catonmat.net/download/xgoogle.zip

Have fun and let me know if you find this library useful in any way in your own projects.

This article is part of the article series "Vim Plugins You Should Know About."
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Vim Plugins, surround.vimThis is the fourth post in the article series "Vim Plugins You Should Know About". This time I am going to introduce you to a plugin called "snipmate.vim".

If you are intrigued by this topic, I suggest that you subscribe to my posts! For the introduction and first post in this article series, follow this link - Vim Plugins You Should Know About, Part I: surround.vim.

Snipmate.vim is probably the best snippets plugin for vim. A snippet is a piece of often-typed text or programming construct that you can insert into your document by using a trigger followed by a <tab>. It was written by Michael Sanders. He says he modeled this plugin after TextMate's snippets.

Here is an example usage of snipmate.vim. If you are a C programmer, then one of the most often used forms of a loop is "for (i=0; i<n; i++) { ... }". Without snippets you'd have to type this out every time. Even though it takes just another second, these seconds can add to minutes throughout the day and minutes can add to hours over longer periods of time. Why waste your time this way? With snippets you can type just "for<tab>" and snipmate will insert this whole construct in your source code automatically! If "i" or "n" weren't the variable you wanted to use, you can now use <tab> and <shift-tab> to jump to next/previous item in the loop and rename them!

Michael also created an introduction video for his plugin where he demonstrates how to use it. Check it out:

How to install snipmate.vim?

To get the latest version:

  • 1. Download snipmate.zip.
  • 2. Extract snipmate.zip to ~/.vim (on Unix/Linux) or ~\vimfiles (on Windows).
  • 3. Run :helptags ~/.vim/doc (on Unix/Linux) or :helptags ~/vimfiles/doc (on Windows) to rebuild the tags file (so that you can read :help snipmate.)
  • 4. Restart Vim.

The plugin comes with predefined snippets for more than a dozen languages (C, C++, HTML, Java, JavaScript, Objective C, Perl, PHP, Python, Ruby, Tcl, Shell, HTML, Mako templates, LaTeX, VimScript). Be sure to check out the snippet files in the "snippets" directory under your ~/.vim or ~\vimfiles directory.

If you need to define your own snippets (which you most likely will need), create a new file named "language-foo.snippets" in the "snippets" directory. For example, to define your own snippets for C language, you'd create a file called "c-foo.snippets" and place snippets in it.

To learn about snipmate snippet syntax, type ":help snipmate" and locate the syntax section in the help file.

Have Fun!

Have fun with this time saving plugin!

This article is part of the article series "Perl One-Liners Explained."
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Perl One LinersThis is the second part of a seven-part article on famous Perl one-liners. In this part I will create various one-liners for line numbering. See part one for introduction of the series.

Famous Perl one-liners is my attempt to create "perl1line.txt" that is similar to "awk1line.txt" and "sed1line.txt" that have been so popular among Awk and Sed programmers.

The article on famous Perl one-liners will consist of at least seven parts:

The one-liners will make heavy use of Perl special variables. A few years ago I compiled all the Perl special variables in a single file and called it Perl special variable cheat-sheet. Even tho it's mostly copied out of perldoc perlvar, it's still handy to have in front of you. Print it!

Awesome news: I have written an e-book based on this article series. Check it out:

And here are today's one-liners:

Line Numbering

9. Number all lines in a file.

perl -pe '$_ = "$. $_"'

As I explained in the first one-liner, "-p" causes Perl to assume a loop around the program (specified by "-e") that reads each line of input into the " $_ " variable, executes the program and then prints the " $_ " variable.

In this one-liner I simply modify " $_ " and prepend the " $. " variable to it. The special variable " $. " contains the current line number of input.

The result is that each line gets its line number prepended.

10. Number only non-empty lines in a file.

perl -pe '$_ = ++$a." $_" if /./'

Here we employ the "action if condition" statement that executes "action" only if "condition" is true. In this case the condition is a regular expression "/./", which matches any character except newline (that is, it matches a non-empty line); and the action is " $_ = ++$a." $_" ", which prepends variable " $a " incremented by one to the current line. As we didn't use strict pragma, $a was created automatically.

The result is that at each non-empty line " $a " gets incremented by one and prepended to that line. And at each empty line nothing gets modified and the empty line gets printed as is.

11. Number and print only non-empty lines in a file (drop empty lines).

perl -ne 'print ++$a." $_" if /./'

This one-liner uses the "-n" program argument that places the line in " $_ " variable and then executes the program specified by "-e". Unlike "-p", it does not print the line after executing code in "-e", so we have to call "print" explicitly to get it printed.

The one-liner calls "print" only on lines that have at least one character in them. And exactly like in the previous one-liner, it increments the line number in variable " $a " by one for each non-empty line.

The empty lines simply get ignored and never get printed.

12. Number all lines but print line numbers only non-empty lines.

perl -pe '$_ = "$. $_" if /./'

This one-liner is similar to one-liner #10. Here I modify the " $_ " variable that holds the entire line only if the line has at least one character. All other lines (empty ones) get printed without line numbers.

13. Number only lines that match a pattern, print others unmodified.

perl -pe '$_ = ++$a." $_" if /regex/'

Here we again use the "action if condition" statement but the condition in this case is a pattern (regular expression) "/regex/". The action is the same as in one-liner #10. I don't want to repeat, see #10 for explanation.

14. Number and print only lines that match a pattern.

perl -ne 'print ++$a." $_" if /regex/'

This one-liner is almost exactly like #11. The only difference is that it prints numbered lines that match only "/regex/".

15. Number all lines, but print line numbers only for lines that match a pattern.

perl -pe '$_ = "$. $_" if /regex/'

This one-liner is similar to the previous one-liner and to one-liner #12. Here the line gets its line number prepended if it matches a /regex/, otherwise it just gets printed without a line number.

16. Number all lines in a file using a custom format (emulate cat -n).

perl -ne 'printf "%-5d %s", $., $_'

This one-liner uses the formatted print "printf" function to print the line number together with line. In this particular example the line numbers are left aligned on 5 char boundary.

Some other nice format strings are "%5d" that right-aligns line numbers on 5 char boundary and "%05d" that zero-fills and right-justifies the line numbers.

Here my Perl printf cheat sheet might come handy that lists all the possible format specifiers.

17. Print the total number of lines in a file (emulate wc -l).

perl -lne 'END { print $. }'

This one-liner uses the "END" block that Perl probably took as a feature from Awk language. The END block gets executed after the program has executed. In this case the program is the hidden loop over the input that was created by the "-n" argument. After it has looped over the input, the special variable " $. " contains the number of lines there was in the input. The END block prints this variable. The " -l " parameter sets the output record separator for "print" to a newline (so that we didn't have to print "$.\n").

Another way to do the same is:

perl -le 'print $n=()=<>'

This is a tricky one, but easy to understand if you know about Perl contexts. In this one-liner the " ()=<> " part causes the <> operator (the diamond operator) to evaluate in list context, that causes the diamond operator to read the whole file in a list. Next, " $n " gets evaluated in scalar context. Evaluating a list in a scalar context returns the number of elements in the list. Thus the " $n=()=<> " construction is equal to the number of lines in the input, that is number of lines in the file. The print statement prints this number out. The " -l " argument makes sure a newline gets added after printing out this number.

This is the same as writing the following, except longer:

perl -le 'print scalar(()=<>)'

And completely obvious version:

perl -le 'print scalar(@foo=<>)'

Yet another way to do it:

perl -ne '}{print $.'

This one-liner uses the eskimo operator "}{" in conjunction with "-n" command line argument. As I explained in one-liner #11, the "-n" argument forces Perl to assume a " while(<>) { } " loop around the program. The eskimo operator forces Perl to escape the loop, and the program turns out to be:

while (<>) {
}{                    # eskimo operator here
    print $.;
}

It's easy to see that this program just loops over all the input and after it's done doing so, it prints the " $. ", which is the number of lines in the input.

18. Print the number of non-empty lines in a file.

perl -le 'print scalar(grep{/./}<>)'

This one-liner uses the "grep" function that is similar to the grep Unix command. Given a list of values, " grep {condition} " returns only those values that match condition. In this case the condition is a regular expression that matches at least one character, so the input gets filtered and the "grep{/./}" returns all lines that were non empty. To get the number of characters we evaluate the list in scalar context and print the result. (As I mentioned in the previous one-liner list in scalar context evaluates to number of elements in the list).

A golfer's version of this one-liner would be to replace "scalar()" with " ~~ " (double bitwise negate), thus it can be shortened:

perl -le 'print ~~grep{/./}<>'

This can be made even shorter:

perl -le 'print~~grep/./,<>'

19. Print the number of empty lines in a file.

perl -lne '$a++ if /^$/; END {print $a+0}'

Here I use variable $a to count how many empty lines have I encountered. Once I have finished looping over all the lines, I print the value of $a in the END block. I use " $a+0 " construction to make sure " 0 " gets output if no lines were empty.

I could have also modified the previous one-liner:

perl -le 'print scalar(grep{/^$/}<>)'

Or written it with " ~~ ":

perl -le 'print ~~grep{/^$/}<>'

These last two versions are not as effective, as they would read the whole file in memory. Where as the first one would do it line by line.

20. Print the number of lines in a file that match a pattern (emulate grep -c).

perl -lne '$a++ if /regex/; END {print $a+0}'

This one-liner is basically the same as the previous one, except it increments the line counter $a by one in case a line matches a regular expression /regex/.

Perl one-liners explained e-book

I've now written the "Perl One-Liners Explained" e-book based on this article series. I went through all the one-liners, improved explanations, fixed mistakes and typos, added a bunch of new one-liners, added an introduction to Perl one-liners and a new chapter on Perl's special variables. Please take a look:

Have Fun!

Have fun with these one-liners. These were really easy this time. The next part is going to be about various calculations.

Can you think of other numbering operations that I did not include here?

A Year of BloggingHoly smokes! It has now been two years since I started this blog. It seems almost like yesterday when I posted the "A Year of Blogging" article. And now it's two! With this post I'd like to celebrate the 2nd birthday and share various interesting statistics that I managed to gather.

During this year (July 20, 2008 - July 26, 2009) I wrote 55 posts, which received around 1000 comments. According to StatCounter and Google Analytics my blog was visited by 1,050,000 unique people who viewed 1,700,000 pages. Wow, 1 million visitors! That's very impressive!

Here is a Google Analytics graph of monthly page views for the last year (click for a larger version):

Catonmat.Net Page Views Per Month (Second Year of Blogging)

In the last three months I did not manage to write much and you can see how that reflected on the page views. A good lesson to be learned is to be persistent and keep writing articles consistently.

Here is the same graph with two years of data, showing a complete picture of my blog's growth:

Catonmat.Net Page Views Per Month (Two Years of Blogging)

I like this seemingly linear growth. I hope it continues the same way the next year!

Here are the top 5 referring sites that my visitors came from:

And here are the top 5 referring blogs:

I found that just a handful of blogs had linked to me during this year. The main reason, I suspect, is that I do not link out much myself... It's something to improve upon.

If you remember, I ended the last year's post with the following words (I had only 1000 subscribers at that time):

I am setting myself a goal of reaching 5000 subscribers by the end of the next year of blogging (July 2009)! I know that this is very ambitious goal but I am ready to take the challenge!

I can proudly say that I reached my ambitious goal! My blog now has almost 7000 subscribers! If you have not yet subscribed, click here to do it!

Here is the RSS subscriber graph for the whole two years:

RSS Subscriber Count, Two Years of Blogging

Several months ago I approximated the subscriber data with an exponent function and it produced a good fit. Probably if I had continued writing articles at the same pace I did three months ago, I'd have over 10,000 subscribers now.

Anyway, let's now turn to the top 10 most viewed posts:

The article that I liked the most myself but which didn't make it to top ten was the "Set Operations in Unix Shell". I just love this Unix stuff I did there.

I am also very proud for the following three article series that I wrote:

  • 1. Review of MIT's Introduction to Algorithms course (14 parts).
  • 2. Famous Awk One-Liners Explained (4 parts: 1, 2, 3, 4).
  • 3. Famous Sed One-Liners Explained (3 parts: 1, 2, 3)

Finally, here is a list of ideas that I have thought for the third year of blogging:

  • Publish three e-books on Awk One-Liners, Sed One-Liners and Perl One-Liners.
  • Launch mathematics, physics and general science blog.
  • Write about mathematical foundations of cryptography and try to implement various cryptosystems and cryptography protocols.
  • Publish my review of MIT's Linear Algebra course (in math blog, so the main topic of catonmat stays computing).
  • Publish my review of MIT's Physics courses on Mechanics, Electromagnetism, and Waves (in physics blog).
  • Publish my notes on how I learned the C++ language.
  • Write more about computer security and ethical hacking.
  • Write several book reviews.
  • Create a bunch of various fun utilities and programs.
  • Create at least one useful web project.
  • Add a knowledge database to catonmat, create software to allow easy publishing to it.
  • If time allows, publish reviews of important computer science publications.

I'll document everything here as I go, so if you are interested in these topics stay with me by subscribing to my rss feed!

And to make things more challenging again, I am setting a new goal for the next year of blogging. The goal is to reach 20,000 subscribers by July 2010!

Hope to see you all on my blog again! Now it's time for this delicious cake:

Second Birthday Portal Game Cake