Sunday, August 20, 2017

Are US tech companies so picky nowadays?

For last few months I have been looking for technical job in my area of expertise and experience.  I know, it is harder to find jobs in low-level or embedded system compared to application software, but I never realize even when I got some interviews, there are so picky.  A small mistake (not necessarily to judge that I will not be able to do the job), that's it.  You're done.  Next applicant, please.

I am still endeavoring to land my future job. Meanwhile, I am brushing my skills and learn some new stuff.  For example, I now know there is an opensource doing dataplane packet processing.  I also now relearning  Linux kernel and driver, Bluetooth and Wifi.

Thursday, August 17, 2017

Some videos of My Embedded projects I have done

Couple of projects done during my spare time:

ATMega32 interfacing to MAX7219 7seg via SPI 

PIC18F9520 doing ultrasonic ranging 

I wish I made video for the PIC-based IoT sensors I made (part of Computer Engineering project in my master program; It was able to push temperatur and humidty data to 'remote' server periodically, which then display it on the web)

Thursday, August 10, 2017

FizzBuzz Analysis

Recently I was asked this kind of question. As I was told not to worry too much about it and I got many other questions to answer within short time, I did not pay attention to much on it and just scribbled it on a piece of paper, no time to mentally test the algorithm.

At home I realized I did not complete the answer (blame that to the recruiter).  Anyway, the fizz buzz interview question is to ask interviewee to write a code to print number from 1 to 100, but for every multiplication of 3 to print string "FIZZ" (or something), for every multiplication of 5 to print string "BUZZ", and for every multiplication of 3 and 5 (which is another way to say multiplication of 15) to print "FIZZBUZZ".

Here's my working code in C++:

#include <iostream>

using namespace std;

const static char *s1 = "FIZZ";
const static char *s2 = "BUZZ";

int main()
{
    for(int i=1; i<=100; i++)
    {
        if (i% 3*5==0)
        {
            cout << s1 << s2 << endl;
            continue;
        }
        if (i % 3 == 0)
        {
            cout << s1 << endl;
            continue;
        }
        if (i % 5 == 0)
        {
            cout << s2 << endl;
            continue;
        }

        cout << i << endl;
    }
}

No matter whay you do, the optimal solution is always O(n) (unless you're crazy enough to just print manually without loop, such as cout << "1\n2\FIZZ\n4\BUZZ\n...|").

Some silly optimization can be performed in printing the string, though.

#include <iostream>

using namespace std;

const static string s = "FizzBuzz";

int main()
{
    for(int i=1; i<=100; i++)
    {
        if (i%15==0)
        {
            cout << s << endl;
            continue;
        }
        if (i % 3 == 0)
        {
            cout << s.substr(0,3) << endl;
            continue;
        }
        if (i % 5 == 0)
        {
            cout << s.substr(4) << endl;
            continue;
        }

        cout << i << endl;
    }
}

Another thought is to eliminate mod 15.  For example:

#include <iostream>

using namespace std;

const static string s = "FizzBuzz";

int main()
{
    int m;
    for(int i=1; i<=100; i++)
    {
        m = 0;
        if (i%3==0)
        {
            cout << s.substr(0,3);
            m++;
        }
        if (i % 5 == 0)
        {
            cout << s.substr(4);
            m++;
        }
        if (m > 0)
            cout << endl;
        else
            cout << i << endl;
    }
}

When I tested it (upper limit set to 1000 and perform "time <program>", the last algorithm shows some speedup.

The last few lines can be optimized to be:

        if (m == 0)
            cout << i;
        cout << endl;

Another tiny optimization is instead of doing post increment (i++), we do preincrement (++i).  This saves tiny bit (no copy to extra variable internally).

Last, we can write it down in Assembly if we want.  The following is suitable for embedded device:

main:
leal 4(%esp), %ecx
andl    $-16, %esp
pushl -4(%ecx)
pushl %ebp
movl %esp, %ebp
pushl %edi
pushl %esi
pushl %ebx
pushl %ecx

subl $8, %esp
movl $1, %ebx         ; EBX=1
movl $3, %esi         ; ESI=3
movl $5, %edi         ; EDI=5
jmp CHECK_FIZZ

FOR_I_NEXT:
movl %ebx, %eax       ; eax stores current value of i
cltd
idivl %edi             ; i / 5, result in eax and mod in edx
testl %edx, %edx       ; i % 5
jne PRINT_I          ; i % 5 != 0 -> jump to PRINT_I

CHECK_FUZZ:
subl $8, %esp
pushl stdout
pushl 'f'
call _IO_putc
popl %edx
popl %ecx
pushl stdout
pushl 'i'
call _IO_putc
popl %eax
popl %edx
pushl stdout
pushl 'z'
call _IO_putc
popl %ecx
popl %eax
pushl stdout
pushl 'z'
call _IO_putc
addl $16, %esp

PRINT_CR:
subl   $8, %esp
pushl stdout
pushl '\n'
call _IO_putc
incl %ebx
addl $16, %esp
cmpl $101, %ebx           ; i == 101 ?
je MAIN_EXIT            ; if (i ==101) goto MAIN_EXIT

CHECK_FIZZ:
movl %ebx, %eax
cltd
idivl %esi
testl %edx, %edx           ; i % 3 == 0
jne FOR_I_NEXT
subl $8, %esp             ; if (i%3==0):
pushl stdout
pushl 'b'
call _IO_putc
popl %eax
popl %edx
pushl stdout
pushl 'u'
call _IO_putc
popl %ecx
popl %eax
pushl stdout
pushl 'z'
call _IO_putc
popl %eax
popl %edx
pushl stdout
pushl 'z'
call _IO_putc
movl %ebx, %eax
cltd
idivl %edi
addl $16, %esp
testl %edx, %edx
jne PRINT_CR
jmp CHECK_FUZZ

PRINT_I:
pushl %eax
pushl %ebx
pushl $.LC0
pushl $1
call __printf_chk
addl $16, %esp
jmp PRINT_CR
MAIN_EXIT:
xorl   %eax, %eax
leal -16(%ebp), %esp
popl %ecx
popl %ebx
popl %esi
popl %edi
popl %ebp
leal -4(%ecx), %esp
ret



Thursday, June 22, 2017

Onion Omega2+ Terminal on browser

After I installed the web-based Terminal on my Omega2, I was able to login to the terminal via web-browser. Cool!


Tuesday, June 20, 2017

My Omeaga2+ OpenWRT Card

Finally, after waiting for almost 10 months, my order of Omega2+ from Onion arrived yesterday.
Will post later about the getting started experience.  For now, only pictures.



Tuesday, June 13, 2017

Interesting opinions About Job Interviews

I one day stumbled upon this site (Things I Learned From Failing Technical Interviews), a section called "Do not be Yourself" intrigued me.

Contrary to the popular suggestions from many career advisers to be yourself during job interview, the person suggests the interviewee not to be him/herself, but to be a robot.  I totally agree with his opinion.  If you've done multiple job interviews for software engineer job position, you might have experience unease when the interviewer expect you to be perfect in coding in front of him/her. No syntax errors, have to memorize the APIs, etc.  The blogger continues to say that hiring a candidate is just like another upgrade of server or adding another PC in the server cluster, which is to offload work from the team to the new hired.

It is almost a guarantee that you will be asked something you don't know and even an hour of thinking about it and problem solving around it you still won't know.  Sometimes you get asked something you've known it long time ago, but can no longer able to grasp it and explain it to them.  You feel like you're an idiot with glamorous title and said experiences (and preconceive that the interviewer must says in his mind, "why the heck this guy is applying for this position"?)

Another jobseeking-related blog: I will get That Job at Google