"Some" about the box

Thursday, August 27, 2020

High Mem vs Low Mem

https://linux-kernel-labs.github.io/refs/heads/master/labs/memory_mapping.html

kmalloc: uses low-memory

vmalloc: uses high-memory

Thursday, May 28, 2020

https://classroom.udacity.com/courses/ud001/lessons/7473321627/concepts/74276264120923

CSS Tricks: https://css-tricks.com/almanac/

CSS: how the content should look in the webpage (layouts, colors etc)
HTML: the actual contents
[Without CSS, HTML allows basic formatting such as bold, italic etc, but nothing more. To use various layouts, use CSS]
NOTE: without CSS, browser also can apply default styling settings.
Youtube without CSS is something that would look like when the internet connection is slow.

CSS has rule-sets.
And a rule-set is nothing but rules on how to render the tags
For ex: div : {color: blue} => would mean, render everything under 'division' in body in blue color
Hence it has 2 parts: tags and the declarations.
CSS rule-sets are written inside <style></style> tags inside <head> tags
Comments inside CSS is same as C or C++: /* CSS comments */
NOTE: Comments inside html is different
attributes and classes:

p { colors: red }; => will change all the paragraph colors to red.
but if we want to change a particular paragraph (id) or a group of paragraphs (classes) to a color, we need id and classes.
while declaring a tag in <body> include the id (id is similar to a variable in C) or classes (group of tags marked to apply a particular setting)
<p id="site-description">check out my site description</p>
<p class="book-summary another-class">Algorithms</p>

NOTE: an html tag can belong to multiple classes (multiple groups/selectors)

And in CSS styles,

<style>

.book-summary { => classes starts with '.'

colors:red
}

#site-description { => id (variables/attributes) starts with #

colors:pink;
}

Colors:

.sidebars {

color: rgb(0, 0, 225) => blue
color: blue => blue
color: #0000ff => blue
color: #00f => blue (short-hand representation of 0000ff)

Selectors:

Basics: Tags, classes and IDs.
More complex selectors: https://developer.mozilla.org/en-US/docs/Learn/CSS/Building_blocks/Selectors

HTML tags (trees)

Elements Link: https://developer.mozilla.org/en-US/docs/Web/HTML/Element

Each page should contain: head + body
head elements are not displayed; mainly used for title, references, meta-data, CSS-styles etc
Body elements are displayed.
Each page should have only one body
Inside body we can have: divisions, header strings, links, images, buttons, paragraphs, sub-strings, super-strings, spans etc.

divisions: <div></div>: Inside divisions, you can organize header strings, buttons etc. A body can have multiple divisions
header strings: <h1-6> </h1-6>: h1 is the highest
links: <a href="https://www.gmail.com">GMAIL</a>: a means anchor here
images: <img src="https://page-containing-image" alt="some description"> [NOTE: images are similar to links: instead of href, we have src. But images dont have end tags </img>; hence, they are one of type called "void elements"
<figure>and <figcaption>=> can be used along with image to provide figure section
For ex:

<figure>
<img src="local_file.jpg" alt="description">
<figurecaption>Caption for the image which will be displayed below the image</figurecaption>
</figure>

<em></em> => for italic
<strong></strong> => for bold
<ul><li>option1</li><li>option2</li></ul> => unordered list
<ol> with numbers by default
Comments inside html:
NOTE: Comments inside CSS is same as C: /* CSS comment */

Monday, October 7, 2019

Fetching Price of a product online

import requests
from bs4 import BeautifulSoup
import smtplib
import getpass

def get_price():
    ''' the url from which u need the product details '''
    URL = 'https://www.amazon.in/JBL-Splash-Proof-Portable-Wireless-Bluetooth/dp/B0125QA4IO?ref_=Oct_DLandingS_PC_a25847b7_13'
    ''' user-agent involved: search for 'my user agent' in your browser to get this info '''
    headers = {'User-Agent' : 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.80 Safari/537.36'}

    ''' get the whole page; if u print 'page' it will be nasty and big '''
    page = requests.get(URL, headers = headers)

    ''' parse the page (using html parser) '''
    soup = BeautifulSoup(page.content, 'html.parser')

    ''' find the id (aka variable) used in browser that holds product info '''
    product = soup.find(id = 'productTitle').get_text().strip()

    ''' similarly get the price '''
    price = soup.find(id = 'priceblock_ourprice').get_text().strip()
    print (product)
    print (price)

    ''' some manipulation needed to get the price in value (instead of string) '''
    converted_price = price.strip('₹').strip()
    converted_price = float(converted_price.replace(',', ''))
    print (converted_price)
    return converted_price

def send_mail(mailid, password, current_price):
    ''' get the SMTP object '''
    server = smtplib.SMTP('smtp.gmail.com', 587)

    ''' extended hello '''
    server.ehlo()

    ''' put the smtp connection in tls mode; this to get encrypted connection '''
    server.starttls()
    server.ehlo()

    ''' login using your credentials '''
    server.login(mailid, password)

    #subject = 'Hey, the price fell down to ₹ {}/-'.format(current_price).encode('utf-8')
    #subject = 'Hey, the price fell down to ₹ %s' % (str(current_price)).encode('utf-8')
    subject = 'Hey, the price fell down'
    body = 'Check the amazon link for more details: https://www.amazon.in/JBL-Splash-Proof-Portable-Wireless-Bluetooth/dp/B0125QA4IO?ref_=Oct_DLandingS_PC_a25847b7_13'    msg = f"Subject: {subject}\n\n{body}"    server.sendmail('hkumar.sekar@gmail.com', 'harish.sekar@broadcom.com', msg)

    ''' don't forget to delete the smtp object aka. close connection '''
    server.quit()

    print ("Hey, the price dropped. Mail is sent")

def main():
    mailid = input('Enter your email ID: ')
    p = getpass.getpass()
    current_price = get_price()

    if current_price < 6000:
        send_mail(mailid, p, current_price)

if __name__ == '__main__':
    main()

PROGRAM OUTPUT:
C:\Users\hs411053\PycharmProjects\scraper\venv\Scripts\python.exe C:/Users/hs411053/PycharmProjects/scraper/scraper
Enter your email ID: hkumar.sekar@gmail.com
Password:
JBL Flip 3 Portable Wireless Speaker with Powerful Sound & Mic (Blue)
₹ 5,599.00
5599.0
Hey, the price dropped. Mail is sent

Process finished with exit code 0








And the Mail would have been sent.

Sunday, October 6, 2019

How to Print in Makefile

Source: https://stackoverflow.com/questions/11775733/how-can-i-print-message-in-makefile/11776179

15 # for TRACEGEN
16 include $(MAKEDIR)/Tools.make
17
18 ifeq ($(CONFIG_SOMETHING),y)
19 EXTRA_CFLAGS += -fsanitize=kernel-address
20 $(info ************ TEST VERSION ************)
21 else
22 $(info ************ ELSE VERSION ************)
23 endif

$(info your_text) : Information. This doesn't stop the execution.

$(warning your_text) : Warning. This shows the text as a warning.

$(error your_text) : Fatal Error. This will stop the execution.

To print a variable in Makefile:

18 $(info $$CONFIG_VAR1 is [${CONFIG_VAR1}])

19 $(info $$CONFIG_VAR2 is [${CONFIG_VAR2}])

Tuesday, October 1, 2019

What is the smallest integer greater than 95555 in which there will be 4 identical numbers?

Ans: 96666? nope, its 95999 :-)

Saturday, August 31, 2019

Direct Memory Access (DMA)

Reference:

If IO device (such as disk controller) needs data to be written or read from the memory, it used to to through the CPU like below.

In the above image, if IO can talk to/access memory directly, the number of cycles will reduce drastically. To achieve this direct memory access by an IO device, a special called DMA controller is needed.

Now, there are 4 components involved in DMA theory. Picture below.

YOUTUBE LINK: https://www.youtube.com/watch?v=Xkpu8BXi3aI

Here is how it works:
1.

Monday, May 13, 2019

Bubble Sort

Pseudocode:

for j = 0 to n -2 // no need till n-2 as it will validate already properly aligned values(see code)
if a[j] > a[j+1]
swap (a[j], a[j+1])
After the above pass, the largest element will come at the end.

Perform the above pass for each of the elements in the array.

Code
#include <stdio.h>

#define NELEMS(x) (sizeof (x) / sizeof (x[0]))

void display(int arr[], int n)
{
int i;
for (i = 0; i < n; ++i)
printf("%d ", arr[i]);

printf("\n");
}

void swap(int *x, int *y)
{
int t = *x;
*x = *y;
*y = t;
}

void bubble(int arr[], int n)
{
int i, j;
int swapped = 0;
for (i = 0; i < n - 1; ++i) { // no need of <n as bigger will find its way anyway
swapped = 0;
for (j = 0; j < n - i - 1; ++j) { // j < n-2 also works, but this is not needed
if (arr[j] > arr[j + 1]) {
swap(arr + j, arr + j + 1);
swapped = 1;
display(arr, 8);
}
}
if (swapped == 0) break; // if no swap happened, no need to continue any further
}
}

int main()
{
int arr[] = { 17, 21, 19, 15, 5, 99, 1, 0 };
/* int arr[] = { 17, 21, 19, 98, 99, 100, 101, 102 }; // almost sorted (usage of swapped flag helps here) */

printf("Unsorted: ");
display(arr, NELEMS(arr));

bubble(arr, NELEMS(arr));

printf("Sorted: ");
display(arr, NELEMS(arr));
}

Output:

Unsorted: 17 21 19 15 5 99 1 0

17 19 21 15 5 99 1 0

17 19 15 21 5 99 1 0

17 19 15 5 21 99 1 0

17 19 15 5 21 1 99 0

17 19 15 5 21 1 0 99

17 15 19 5 21 1 0 99

17 15 5 19 21 1 0 99

17 15 5 19 1 21 0 99

17 15 5 19 1 0 21 99

15 17 5 19 1 0 21 99

15 5 17 19 1 0 21 99

15 5 17 1 19 0 21 99

15 5 17 1 0 19 21 99

5 15 17 1 0 19 21 99

5 15 1 17 0 19 21 99

5 15 1 0 17 19 21 99

5 1 15 0 17 19 21 99

5 1 0 15 17 19 21 99

1 5 0 15 17 19 21 99

1 0 5 15 17 19 21 99

0 1 5 15 17 19 21 99

Sorted: 0 1 5 15 17 19 21 99

Time Complexity: O(n^n)

Selection Sort

Pseudocode:

Take element in 0th position
From 1st to (n-1)th position, find the smallest element and its index.
If we found a smallest element, swap it with 0th position
Repeat the above for all the positions
NOTE: Selection sort basically means, fill the position of each index (starting from 0) by it's appropriate value (by choosing the minimum amount in the remaining indexes in each loop)

Psuedocode flow:

SelectionSort (A, n)

{

for i = 0 to n -1 // we can do it till n-2, as n-1 will automatically have biggest element in its position

{

iMin = i
for j = i+1 to n-1

{

if (A[j] < A[iMin])

iMin = j
}

swap (A[iMin], A[i])

}

C Code:

#include <stdio.h>

#define NELEMS(x) (sizeof (x)/sizeof(x[0]))

void swap(int *x, int *y)
{
int t = *x;
*x = *y;
*y = t;
}

void display(int arr[], int size)
{
int i;
for (i = 0; i < size; ++i) {
printf("%d ", arr[i]);
}
printf("\n");
}

void selection(int arr[], int n)
{
int i, j, iMin;

for (i = 0; i < n - 1; ++i) { // no need for last iteration, as the biggest number will be there anyways (due to previous operation)
iMin = i; // find the right element for this position
for (j = i + 1; j < n; ++j) {
if (arr[j] < arr[iMin]) {
iMin = j;
}
}
swap(arr + iMin, arr + i);
display(arr, 7); // NELEMS wont' work as it will be converted as pointer _here_
}
}

int main()
{
int arr[] = { 21, 5, 22, 74, 0, 99, 18 };

printf("UnSorted: ");
display(arr, NELEMS(arr));

selection(arr, NELEMS(arr));

printf("Sorted: ");
display(arr, NELEMS(arr));
return (0);

}

Output:

UnSorted: 21 5 22 74 0 99 18

0 5 22 74 21 99 18

0 5 18 74 21 99 22

0 5 18 21 74 99 22

0 5 18 21 22 99 74

0 5 18 21 22 74 99

Sorted: 0 5 18 21 22 74 99

[NOTE that, each index starting from 0 (referred by iMin), will pull the appropriate value, by going through the loop]

Sorting Basics

Sorting

http://www.youtube.com/playlist?list=PL2_aWCzGMAwKedT2KfDMB9YA5DgASZb3U

Sorting is basically essential to do the searching.

Linear Search:
If we are to search a data in an unsorted pool of data, we call the search as Linear Search.
If the number of data in the pool is 'n', and we are to search for a data 'd', then we need to compare 'd' against (n-1) elements.
If n is 2^64, and if each comparison takes 1 ms, then we will need 2^64 ms (worst-case) to find the data.

size = n
n = 2^64 => 2^64 comparisons

Sorted:
If we the pool is sorted, then we can use Binary search.
size = n
And if n = 2^64 => 64 ms

Number of Sorting algorithms:

Selection Sort
Bubble Sort
Insertion Sort
Merge Sort
Quick Sort
Heap Sort
Counting Sort
Radix Sort
[And many more]

Classification of these algorithms is based on:

Time Complexity (Big O Notation)
Space Complexity or Memory Usage [Merge sort, uses temporary storage]
Stability (already sorted items in the input, will be preserved in the sorted list). For ex, cards of 6heart and 6clubs along with other cards, will remain in same order even in sorted list
Internal (RAM) or External Sort (data in Disc or tapes)
Recursive (Quick and Merge) and Non-Recursive

"Some" about the box - Computer