My Stuff

Sunday, January 31, 2016

Raspberry Pi vs. Next-Big-Thing's CHIP SoC Computer

Feature	RPI 1 Model A	RPI 1 Model B	RPI 1 Model A+	RPI 1 Model B+	RPI 2ModelB	NBT CHIP
SoC	BRCM 2835	BRCM 2835	BRCM 2835	BRCM 2835		AllWinner's R8
Standard SoC Speed (MHz)	700	700	700	700	900	1000
RAM (MB)	256	512	256	512		512
Ethernet (Mbps)	N/A	100	100	100	1000	N/A
HDMI output	N/A	Yes	Yes	Yes	Yes	With extra module
Composite Video out	Yes	Yes	via 3.5 mm jack	via 3.5 mm jack	via 3.5 mm jack	Yes
Number of USB2.0	2	2	1	4	4	1
CPU Cores	1	1	1	1	4	1
Storage	SD Card	SD Card	micro SD	micro SD	MicroSD	Built-in Flash
Internal Storage Capacity	N/A	N/A	N/A	N/A	N/A	4 MB
Camera Interface (CSI)		yes	yes	yes	yes	yes
Display Interface (DSI)		yes	yes	yes	yes	yes
Video/Graphic Coprocessor	VideoCore IV	VideoCore IV	VideoCore IV	VideoCore IV	VideoCore IV	PowerVR SGX544
Architecture	ARM11v6	ARM11v6	ARM11v6	ARM11v6	ARMv7	ARMv7
GPIO Pins	26	40	40	40	40

Thursday, September 10, 2015

OpenFrameworks: Another cool Multi-platform Framework for Graphics

Steps to download and build OpenFrameWorks:

git clone https://github.com/openframeworks/openFrameworks.git
goto the OF root. This is usually the folder openFrameworks. Change directory to ./scripts/linux/ubuntu (e.g: cd openFrameworks/scripts/linux/ubuntu)

Execute the scripts in the folder as root:

sudo ./install_dependencies.sh

sudo ./install_codecs.sh

Go to ./scripts/linux and execute:

./compileOF.sh
./compilePG.sh

To generate projects (Makefile etc.) for all the examples, goto OF root, then type:

projectGenerator -v -r examples/

wxWidgets 3.0

To add the repository, first import the key:

sudo apt-key adv --fetch-keys http://repos.codelite.org/CodeLite.asc

Then add the source:


sudo tee /etc/apt/sources.list.d/wxwidgets.list
deb http://repos.codelite.org/wx3.0.2/ubuntu/  $(lsb_release -sc) universe
sudo apt-get update

Install wxWidgets modules:


sudo apt-get install libwxbase3.0-0-unofficial libwxbase3.0-dbg libwxbase3.0-dev libwxgtk3.0-0-unofficial libwxgtk3.0-dbg wx3.0-doc wx3.0-examples

Create soft-link:

sudo ln -s /usr/include/wx-3.0-unofficial/wx /usr/include/wx

To compile a single file, create a script named wxcompile and edit it with the content as below (assume with have subdirectory obj/Debug and bin/Debug:


#!/bin/bash

if [ -z "${1}" ]
then
    echo "$0 "
    exit 1
fi

FILE=`basename $1`
CPPF=$FILE
OBJF=`basename -s .cpp $CPPF`
OBJF=${OBJF}.o
g++ -g -c -Wall -DWX_PRECOMP -Winvalid-pch `wx-config --cppflags` -o obj/Debug/${OBJF} ${CPPF}

To link, create a script file name 'wxlink' and its content as below:


#!/bin/bash

if [ -z "${1}" ]
then
    echo "$0 "
    exit 1
fi

FOUT=$1
OBJF=obj/Debug/*.o

g++ -v obj/Debug/*.o `wx-config --libs` -o obj/Debug/$FOUT

For a bigger project which requires more complicated and conditional compilation, use Makefile.

Friday, July 3, 2015

Watching Nabawi-TV via VLC on Raspberry Pi

A new Indonesian da'wah TV has provided a streaming.

To watch, install vlc. Once installed, open the VLC, click "Media", and then "Open Network Stream". Enter:

rtsp://wowza60.indostreamserver.com:1935/nabawitv/live

Friday, April 3, 2015

Script to simulate multiple DHCP clients

#!/bin/sh

#simulate 255 IPhones requesting DHCP

# Apple OUI = D8:96:95

BASE="d8:96:95:08:96"

FROM=$1

shift

TO=$2

if [ -e $FROM ]; then FROM=1; fi

if [ -e $TO ]; then TO=1; fi

for i in `seq $FROM $TO`

do

    LSB=`echo "obase=16; $i" | bc`

    MAC="$BASE:$LSB"

    HNAME="`uname -n`-fakehost-$i"

    #CMD="$HOME/bin/dhtest -m $MAC -V -i eth1 -h '$HNAME'"

    CMD="$HOME/bin/dhtest -m $MAC -i eth1 -h '$HNAME'"

    echo $CMD

    $CMD

    sleep 1

done

Another way, the program dhtest (source code) can be download from https://github.com/saravana815/dhtest.

For example:

 
git clone https://github.com/saravana815/dhtest

cd dhtest

make

Daily backup of local git changes

While working on a ticket and not ready to commit, it's a good idea to backup all of our changes to an archive file.

The following script does all that and executed daily by cronjob automatically.

create a file, say "backup" in /etc/cron.daily
Type the following (change '<yourhome directory>' to your own login name and <git location> to the git directory where source codes are located):

backing up changed files to tarball

#!/bin/sh

pushd <your source-code location>

LIST=`git status --porcelain | sed -n -e '/^ [D|M]/p' | sed -e 's/^ [D|M] //'`

backupname=`git branch | sed -n -e "/^* /p" | sed  -e 's/** //'`

backupname=$backupname-`date "+%s"`.backup.tar.bz2

dest='<your home directory>'

if [ ! -d "$dest" ]

then

    mkdir -p $dest

fi

tar jcvf $dest/$backupname $LIST > /dev/null

Make the file executable:

chmod +x /etc/cron.daily/backup

Sunday, March 22, 2015

Block Access during certain period using EBTABLES

Say, we want to block any packets coming from a device with mac address 00:01:02:03:04:05 (in other words, our router/switch should just silently drop any packets coming from this MAC address) during period of time 00:00 (00:00 AM) to 6:00 AM, do:

#ebtables -A INPUT -s 00:01:02:03:04:05 --timestart 0:0 --timestop 06:00 -j DROP

If we just want to drop IPv4 packets for the above:

#ebtables -A INPUT -p IPv4 -s 00:01:02:03:04:05 --timestart 0:0 --timestop 06:00 -j DROP

So, parameters for ebtables are actually similar (yet subset) of iptables (netfilter).

Wednesday, March 11, 2015

To fix Mute button keyboard shortcut issue on LXDE

Create a script call "amixertoggle" and save it in /usr/local/bin:

#!/bin/bash

amixer $1 sset Headphone toggle
amixer $1 sset Speaker toggle
amixer $1 sset PCM toggle
amixer $1 sset Master toggle

Edit file $HOME/.config/openbox/lxde-rc.xml and replace block that has "XF86AudioMute" to call our script. For example:

...
...

<keybind key="XF86AudioMute">
<action name="Execute">
<command>amixertoggle -c 1 toggle</command>
</action>

...
...

(" -c 1" above is for machine, where the mixer control is actually on card 1. If doesn't work, we can try different number)

Reload the modified file lxde-rc.xml by doing:

openbox --reconfigure

That's it. Everytime we press "Mute" button on our PC keyboard, the mute/unmute will toggle.

Thursday, February 26, 2015

Raspberry Pi 2

Got this New Raspberry Pi 2 I ordered a few weeks ago. It's significantly faster than the first version. I only wish it had USB 3.0 ports so I can have much bigger and faster storage.

Monday, October 13, 2014

Cheap ATMEL AVR ISP ICE from QinHeng

I bought this small USB stick from eBay. It supposedly can do JTAG and many other cool stuff for ATMEL microcontrollers.

Anyway, here is the detail how to use it:

The other end of the stick has dual-line 10-pin connector. The pins are:

Pin Purpose
------------------------------
1    TCK
2    GND
3    TDO
4    VTref
5    TMS
6    nSRT
7    VSupply
8    (nSRT)
9    TDI
10   GND

These pins are actually JTAG pins. The connections to the Microcontroller is as follow (the pullup resistors are all 4.7k):

The USB id:

$ lsusb
...
Bus 003 Device 006: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
...

Upon inserting the stick to my PC's USB (running Ubuntu Linux):

[118845.955500] ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0
[118845.955541] ch341 3-2:1.0: device disconnected
[118851.692044] usb 3-2: new full-speed USB device number 6 using ohci-pci
[118851.901261] usb 3-2: New USB device found, idVendor=1a86, idProduct=7523
[118851.901271] usb 3-2: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[118851.901276] usb 3-2: Product: USB2.0-Serial
[118851.904683] ch341 3-2:1.0: ch341-uart converter detected
[118851.940428] usb 3-2: ch341-uart converter now attached to ttyUSB0

To use it as programmer, we can use avrdude:

$ sudo avrdude -c avrisp -P /dev/ttyUSB0 -pm328p <other options>

To debug, we can use another opensource, AvaRICE (from ubuntu, just do "apt-get install avarice").

For example (this is just to show how to use it, as my ICE stick is not connected to any target device yet so it reports it "No configuration available for device ID: ffff"):

$ sudo avarice --jtag /dev/ttyUSB0 -1
AVaRICE version 2.11, Jan 17 2014 02:53:19

Defaulting JTAG bitrate to 250 kHz.

JTAG config starting.
Hardware Version: 0xc3
Software Version: 0x80
Reported JTAG device ID: 0xFFFF
No configuration available for device ID: ffff

The avarice can be set as a gdb server mode, so we can debug the target removely. For example, to make it as gdb-server listening on port 4242.

$ sudo avarice --jtag /dev/ttyUSB0 :4242

From another PC (or the same PC, if host and server is on the same machine), we open gdb, and connect:

$ gdb
GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) target remote 127.0.0.1:4242

Using PicKit2 Programmer to program Atmel's microcontrollers

Many hobbists are familiar with Arduino kit. This beast uses various Microcontrollers from Atmel. For example, Arduino UNO uses AtMega 328p.

Normally, we use the pcb kit to program the chip, as the Arduino (or the clones) use USB to serial chip to translate USB to serial needed to program Arduino.

For people who has done hacking on PIC microcontrollers from MicroChip and wants to use bare-metal chip, they can program the Atmel chips using the existing cheap PicKit2 programmer. What they need just the Pickit2 programmer and a software tool called avrdude.

The pin connections:

AVR   - PICKit2 (pin):

-----------------------

RST   - VPP/MCLR (1)
VDD   - VDD Target (2) -- optional if AVR self powered
GND   - GND (3)
MISO  - PGD (4)
SCLK  - PDC (5)
MOSI  - AUX (6)

Some of command examples are shown below:

To write our firmware to the flash in AtMega 328p:

$ avrdude -c pickit2 -p m328p -v -V -U flash:w:"myfirmware.hex":a

To upload bootloader (e.g. to upload ARDUINO bootloader):

$ avrdude -c pickit2 -p m328p -v -V -U boot:w:"boot.hex":a

Left shift by negative number used to test unique characters in a string

What is the outcome of this?

val=-32;
mask = 1<<val;

I tried to execute it on Python, it failed with error message.

But if I do it in C or C++, I get something else. Turn out, this depends o the compiler and CPU we use. As I use Pentium4 and GCC to compile this, the following snippet of the generated assembly language is interesting:

.L4:
movsbl (%edx), %ecx
movl %esi, %edi
addl $1, %edx
subl $97, %ecx
sall %cl, %edi
testl %eax, %edi
jg .L5
orl %edi, %eax

.L2:

What is "sall" above? It is Intel CPU's mnemonic for "Shift Arithmetic Left for Long Integer".

In the machine code, -32 is transformed to Second-complement of 32:

32 = 0b00100000

~32 = 0b11011111

~32+1 = 0b11100000 = -32

According Wikipedia, arithmetic shift left maintains the MSBit, so in this case MSB is always 1. Also, Intel CPUs only shifts the first 5-bits of the shifting bits (in this case, 5-bit LSB of 0b11100000 which is 0), so 1<<-32 is actuall 1<<0 = 1.

What about 1<<-33?

33 = 0b00100001

~33 = 0b11011110

~32+1 = 0b11100001 = -33

The first five bits is 1, so 1<<-33 = 1<<1 = 2

What about 1<<-31?

31 = 0b00011111

~31 = 0b11100000

~31+1 = 0b11100001 = -31

The first five bits is 1, so 1<<-31 = 1<<1 = 0x00000010

What about 1<<-7?

7 = 0b00000111

~7 = 0b11111000

~7+1 = 0b11111001 = -7

The first five bits is 1, so 1<<-7 = 1<<25 = 0x02000000

Now, how do we use this to check a uniqueness of characters in a string?

Alphabet 'A' to 'Z' has 26 characters. Alphabet 'a' to 'z' has also 26 characters.

The difference between the character i in a string is: str[i] - 'a'. Now this is the interesting part: if we declare a 32-bit variable (which is enough to cover 'A' to 'Z') where each bit corresponds to the occurence of each character in alphabet, we can determine if the character has been used before and tell the string has unique characters or not (e.g, bit 1 to tell if character 'A' has been used or not, etc.)

'A' - 'a' = -32

...

'Z' - 'a' = -7

...

'a' - 'a' = 0

...

'Z' - 'a' = 25

if we shift 1 to -32, it is the same as 1<<0. If we shift 1 to -33 it is the same as 1<<1, so on.

1<<-32 = 0x01

1<<-31 = 0x02

...

1<<-7 = 1<<25 = 0x2000000

1<<0 = 0x01

1<<1 = 0x02

...

1<<25 = 0x2000000

so:

1<<('A'-'a') == 1<<('a'-'a') = 0x01

1<<('B'-'a') == 1<<('b'-'a') = 0x02

...

1<<('Z'-'a') == 1<<('z'-'a') = 0x2000000

So the code is:

for (i=0; i<strlen(s); i++)

{

val = s[i] - 'a';

if (flags & (1 << val))

return 0; /* the string is not unique */

flags |= 1 << val;

}
return 1;

Notice this algorithm might not work on different machine with different compiler (as shifting by negative number is undefined in standard C).

The Use of c++decl

cdecl and c++decl are two nice tools to check syntax or to generate declaration of C and C++ respectively.

for example, using c++decl (the text in bold is the one we type):

$ cdecl

Type `help' or `?' for help
c++decl>

c++decl> declare foo as pointer to member of class X function (arg1, arg2) returning reference to class Y

class Y &(X::*foo)(arg1, arg2)

c++decl>

c++decl> declare foo as reference to class X

class X &foo

c++decl>

c++decl> cast foo into reference to class X

(class X &)foo

c++decl>

We can also use them to check a declaration syntax, for example:

c++decl> explain int (*a)[5]

declare a as pointer to array 5 of int

c++decl>

c++decl> explain long **foo[10]

declare foo as array 10 of pointer to pointer to long

c++decl>

c++decl> explain int*&(foo);

declare foo as reference to pointer to int

c++decl>

The function can also be used to translate an english declaration to C (or C++) declaration. For example:

$ c++decl declare "i as array 5 of pointer to function(int,int) returning reference to class X"

class X &(*i[5])(int , int )

$

$ c++decl declare "i as array 5 of pointer to function(reference to class X,constant reference to int) returning reference to class X"

class X &(*i[5])(class X &, int & const )

$

$ c++decl declare "const_ptr as const pointer to const int"

const int * const const_ptr

$ c++decl declare "const_ptr as const pointer to const reference to class X"

class X & const * const const_ptr

c++decl> declare myfunc as const pointer to function (const pointer to int, reference to class X) returning void

void (* const myfunc)(int * const , class X &)

c++decl>

c++decl> declare myfunctpr as array of pointer to function (void) returning int

int (*myfunctpr[])(void )

c++decl>

declare myptr as pointer to array 10 of struct MyStruct

struct MyStruct (*myptr)[10]

c++decl>

Saturday, October 4, 2014

Odd Numbers with Parity

#!/usr/bin/python

table=[]
for n in range(0,256):
    p=False
    while n:
        p = not p
        n &= n-1
    table.append(p)

parity_odd = []

print "Odd numbers with Parity:"



for i in range(0,256):
    if (i%2) and (table[i]):
        parity_odd.append(i)
        print i,

Result:

Odd numbers with Parity:
1 7 11 13 19 21 25 31 35 37 41 47 49 55 59 61 67 69 73 79 81 87 91 93 97 103 107 109 115 117 121 127 131 133 137 143 145 151 155 157 161 167 171 173 179 181 185 191 193 199 203 205 211 213 217 223 227 229 233 239 241 247 251 253

Thursday, October 2, 2014

Fastest Fibonacci Sequencer

This algorithm is based on Fast-doubling as explained in Fast Fibonacci algorithms.

Given F(k) and F(k+1), we can calculate these equations:

F(2k) = F(k)[2F(k+1)−F(k)]
F(2k+1) = F(k+1)2+F(k)2.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>

#ifdef DEBUG
#define DBGMSG(x)       printf x 
#else
#define DBGMSG(x)
#endif

typedef unsigned long long num_type;

void _fib(int n, num_type *fn, num_type *fn_1);

num_type fibonacci(uint n, num_type *s)
{
 int i;
    num_type fn=0, fn_1=1;

 if (!s)
  return 0;

 if (n < 2)
    {
  return 1;
    }
    for(i=1; i<n; i++)
    {
        _fib(i, &fn, &fn_1);
        DBGMSG(("i=%u, n=%u, fn=%llu, fn+1=%llu\n", i, n, fn, fn_1));
        s[i] = fn;
    }
    return s[n];
}

/*
 * This is base on matrix exponentiation algorithm:
 * where:
 *  f(2k) = f(k)*{ 2*f(k+1) - f(k)}         ---> c
 *  f(2k+1) = f(k)^2 + f(k+1)^2             ---> s
 *
 *  if we plugin k = n/2, we get f(n) and f(n+1)
 */
void _fib(int n, num_type *fn, num_type *fn_1)
{
    num_type c,d;
    num_type a, b;

    DBGMSG( ("%s: Initially n=%u, fn=%llu, fn_1=%llu\n", __FUNCTION__, n, *fn, *fn_1) );
    if (n==0)
    {
        *fn = 0;        // fn
        *fn_1 = 1;      // fn+1
        return; 
    }
    else {
        a = *fn;
        b = *fn_1;
    }
    _fib(n>>1, &a, &b);
    DBGMSG (("%s: local fn=%llu, fn_1=%llu\n", __FUNCTION__, a, b));
    c = a * (2*b - a);
    d = b*b + a*a;
    DBGMSG( ("%s: c=%llu, d=%llu\n", __FUNCTION__, c, d) );
    if (n % 2 == 0)
    {
        // n is even
        *fn = c;
        *fn_1 = d;
    }
    else {
        *fn = d;
        *fn_1 = c+d;
    }
}

int main(int argc, char *argv[])
{
 uint n;
 uint f;
 int i;
 num_type *result;
    char *sbuf;
    int status = 0;

    if (argc > 1)
        n = atoi(argv[1]);
    else {
     printf("Enter a number: ");
        sbuf = (char *)malloc(50);
     status = (getline(&sbuf, &n, stdin) > 0);
        if (status == 0) {
      n = atoi(sbuf);
            free(sbuf);
        }
       else {
            printf("Error in input\n");
            return (1);
        }
    }
    if (n <= 0)
    {
        printf("What have yo done???\n");
        return(0);
    }
    if (status == 0)
 {
     result = (num_type *)malloc(n*sizeof(unsigned long long));
     f = fibonacci(n, result);
  printf("Fibonacci sequence: ");
  for(i=0; i<n; i++)
   printf("%llu ", result[i]);
  printf("\n");
        if (result)
      free(result);
    }
}

Friday, September 26, 2014

Fastest bit counting

The best bit counting algorithm as far as I know is the one invented by folks at Stanford University, which is always O(1).

int bitcount(int n)
{
int cnt = 0;

n = n - ((n >> 1) & 0x55555555);
n = (n & 0x33333333) + ((n >> 2) & 0x33333333);

return (((n + (n >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
}

For example, after I compile it to x86 Assembly:

.cfi_startproc

movl 4(%esp), %eax

movl %eax, %edx

sarl %edx

andl $1431655765, %edx

subl %edx, %eax

movl %eax, %edx

sarl $2, %eax

andl $858993459, %edx

andl $858993459, %eax

addl %edx, %eax

movl %eax, %edx

sarl $4, %edx

addl %edx, %eax

andl $252645135, %eax

imull $16843009, %eax, %eax

sarl $24, %eax

ret

.cfi_endproc

But, how does actually the algorithm work?

Ok, let try a simple one. Assume n is a 4-bit number, and the bits can be represented as a set such that n= {a,b,c,d}., where a,b,c.. can only have either 0 or 1. The decimal value of the n is: 8*a + 4*b + 2*c + d. Total number of bit one is: a + b + c +d.

For example, for n=0b1101, a=1, b=1, c=0, d=1 (which in decimal is 8*1 + 4*1 + 2*0 + 1 = 13), and total number of bit one is a+b+c+d = 1 + 1 + 0 + 1 = 3. So far so good?

Now, we know that to count the 1-bits is as simple as: a+b+c+d. But, wait.... n itself is not a+b+c+d, but 8*a + 4*b + 2*c + d. Ok, let's conquer this step-by-step.

If we shift n one bit to the right the LSbit is gone and other numbers just divided by two, so n becomes 4*a + 2*b + c, right? Now substract this to the original n.

n = 8*a + 4*b + 2*c + d

n>>1 = 0 + 4*a + 2*b + c

----------------------------- -

= 0 + 4*a + 2*b + c + d

That's a good direction! Now if (n>>1) is written in the 4-bit nibble it is actually 0 + 4a + 2b + c. If I just want to subtract 4a and c, we have to mask out 2*b. so we mask it with binary 0101 (0x5), so we get only 4a + c:

n = 8*a + 4*b + 2*c + d

(n>>1)&0x5 = 4*a + 0 + c

---------------------------------- -

n -(n>>1)&0x05 = 4*a + 4*b + c + d

Now store this result back to n, so from now on n is 4*a + 4*b + c + d

To get c + d only, we mask n with 0b11 or n & 0x03

if we shift n above once, we get 0 + 2a + 2b + 0, but if shift it again it becomes a + b!

To make sure we only get the lowest two bits (a + b), we mask it to 0x03 or:

n & 0x03 = c + d

(n >> 2)&0x03) = a + b

------------------------ +

Nice! now we have this expression: a + b + c +d .

so, for 4-bit bit counting, we can use the expression:

n = n - (n>>1) & 0x05

bits = (n & 0x3) + (n>>2) & 0x3

Proof: as example above, n=13 (0b1101). so a=1,b=1, c=0, d=1

n = 0b1101 - (0b0110) & 0b0101 = 0b1101 - 0b100 = 13 - 4 = 9 = 0b1001

then the next step:

bits =(0b1001 & 0b0011) + (0b1001>>2) & 0b0011, or bits = 0b0001 + (0b0010) & 0b0011

or bits = 1 + 2 = 3 !!

For 32-bit, it is based on the same idea, except we have to do more.

Say n has set of coefficients {a[0], a[1], ...., a[31]} to represent the number, so n = a[0]*2^31 + a[1]*2^30 + .... + a[15]*2^0

The mask should be 32-bit, so instead of 0x5, we use 0x55555555 = 0b0101010101...0101

n = 2^31*a[0] + ... + 2^1*a[30] + 2^0*a[31]

(n>>1)&0b0101..0101 = 2^30*a[0] + ... + 2^2*a[28] + 2^0*a[30]

-------------------------------------------------------------- -

n -(n>>1)&0x055555555 = 2^31*a[0] - 2^28*a[0] + (2^30*a[1] - 2^26*a[1]) + ... + 4*a[29] - 2*a[30] + a[31]

Let's review binary arithmetics.

2³ - 2² = 8 - 4 = 4

2¹⁶ - 2¹⁵ = 65536 - 32768 = 32768
or: 2^(a+1) - 2^a = 2^a

2^(a+2) - 2^a = 2^a * 2² - 2^a = 2^a (4 - 1) = 3*2^a

So the result is:

a[0] * (2^31 - 2^28) + a[1] * (2^30 - 2^26) + ..... + a[30] (4 -2) + a[31]
= 3*2^28*a[0] + .3*2^26*a[1].. + 2*a[30] + a[31]

n - n(>>1) & 0x055555555 = 3*2^28*a[0] + .3*2^26*a[1].. + 2*a[30] + a[31]

stored this as a new n.

n>>2 = 2^24*a[0] + 2^22*a[1] + ...2*a[28] + a[29]

The rest is actually manipulation to count this a[0] + a[1] + ... + a[31]

A variant, but this one is invented by folks at MIT:

int bitcount(unsigned int n)
{
int cnt = 0;
register unsigned int tmp;

tmp = n - ((n >> 1) & 033333333333) - ((n >> 2) & 011111111111);

return ((tmp + (tmp >>3)) & 0030707070707) % 63;
}

It uses the similar method. but with different approach (notice the number 01..., 0333... and 00307... are in octal. We could use Hex version but then it is harder to remember)

Monday, June 2, 2014

Audio on Laptop HP dv7-6c80us not working

Problem:

HP-PAVILION-DV7:~$ aplay -l
aplay: device_list:268: no soundcards found...
HP-PAVILION-DV7:~$

HP-PAVILION-DV7:~$ lsmod | grep snd

snd_hda_codec_hdmi 41154 0

snd_hda_codec_idt 50341 1

snd_hda_intel 52267 0

snd_hda_codec 188738 3 snd_hda_codec_hdmi,snd_hda_codec_idt,snd_hda_intel

snd_hwdep 13602 1 snd_hda_codec

snd_pcm 102033 3 snd_hda_codec_hdmi,snd_hda_codec,snd_hda_intel

snd_page_alloc 18710 2 snd_pcm,snd_hda_intel

snd_seq_midi 13324 0

snd_seq_midi_event 14899 1 snd_seq_midi

snd_rawmidi 30095 1 snd_seq_midi

snd_seq 61560 2 snd_seq_midi_event,snd_seq_midi

snd_seq_device 14497 3 snd_seq,snd_rawmidi,snd_seq_midi

snd_timer 29433 2 snd_pcm,snd_seq

snd 69141 11 snd_hwdep,snd_timer,snd_hda_codec_hdmi,snd_hda_codec_idt,snd_pcm,snd_seq,snd_rawmidi,snd_hda_codec,snd_hda_intel,snd_seq_device,snd_seq_midi

soundcore 12680 1 snd

HP-PAVILION-DV7:~$ cat /proc/asound/pcm
00-00: 92HD81B1X5 Analog : 92HD81B1X5 Analog : playback 1 : capture 1

HP-PAVILION-DV7:~$ lspci -v
...
...
00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High Definition Audio Controller (rev 05)
Subsystem: Hewlett-Packard Company Device 165b
Flags: bus master, fast devsel, latency 0, IRQ 54
Memory at c6500000 (64-bit, non-prefetchable) [size=16K]
Capabilities: [50] Power Management version 2
Capabilities: [60] MSI: Enable+ Count=1/1 Maskable- 64bit+
Capabilities: [70] Express Root Complex Integrated Endpoint, MSI 00
Capabilities: [100] Virtual Channel
Capabilities: [130] Root Complex Link
Kernel driver in use: snd_hda_intel

Thursday, February 6, 2014

The Magic of 37

If x is a multiplication of 3 (3,6,9,12,15,....)

37 * x = y₁y₂y₃ = z

y_{1 +}y_{2 +}y₃ = x
y₁ = z div 100
y₂ = (z mod 100) div 10
y₃ = (z mod 10) div 1

Examples:
37*3 = 111
1+1+1 = 3

37*6 = 222
2+2+2 = 6

Interesting, isn't??

Not really so. x is multiplication of 3, so x = 3*b (where b=1,2,3...)
37*x = 37 * (3*b) = 111*b = bbb

so bbb = y₁y₂y₃ = yyy

or y = b

When we do y₁ + y₂ + y₃, it is actually b + b + b. For example, pick b=2 (so x=6), 111*b = 222, 2+2+2=6=x.