https://github.com/jarun/primepalindrome

A C program to find the 1500th 13-digit prime palindrome in < 3 secs!
https://github.com/jarun/primepalindrome

programming puzzle

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A C program to find the 1500th 13-digit prime palindrome in < 3 secs!

• Host: GitHub
• URL: https://github.com/jarun/primepalindrome
• Owner: jarun
• License: gpl-3.0
• Created: 2017-03-29T08:53:38.000Z (about 8 years ago)
• Default Branch: master
• Last Pushed: 2018-05-29T04:15:13.000Z (about 7 years ago)
• Last Synced: 2024-12-30T19:42:47.400Z (6 months ago)
• Topics: programming, puzzle
• Language: C
• Homepage:
• Size: 59.6 KB
• Stars: 28
• Watchers: 5
• Forks: 2
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
# PrimePalindrome

A fast C program to calculate the 1500th prime palindrome with an odd number (default 15) of digits.

Why _odd_? Because a palindromic number with even number of digits is always divisible by 11.

Implements space-optimized Sieve of Eratosthenes (default) with GCC-specific hardware-aided `ffsll()`as well as brute force methods.

This program is also a demonstration of using efficient algorithms and C code-optimization.

## Execution time (brute force)

/* On Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz */

**Without logic to generate next palindrome (13 digits)**

    $ ./pp

    Thu Jul  2 00:28:26 IST 2015

    Thu Jul  2 00:32:17 IST 2015

    count: 1500 val: 1015834385101

    3 minutes 51 seconds

**With logic to generate next palindrome (13 digits)**

    $ ./pp

    Tue Oct 27 23:40:41 IST 2015

    Tue Oct 27 23:40:48 IST 2015

    1500. 1015834385101

    *7 seconds!!!*

**With tweaked (but brute-force) prime detection algorithm (13 digits)**

    $ time ./pp

    Sat Mar  5 00:04:23 IST 2016

    Sat Mar  5 00:04:29 IST 2016

    1015834385101

    5.86user 0.00system 0:05.86elapsed 100%CPU (0avgtext+0avgdata 2020maxresident)k

    0inputs+0outputs (0major+420minor)pagefaults 0swaps

/* On Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz */

    // Sieve of Eratosthenes

    13 digits: 2.80user 0.00system 0:02.80elapsed 99%CPU (0avgtext+0avgdata 1428maxresident)k

    15 digits: 23.94user 0.00system 0:23.94elapsed 100%CPU (0avgtext+0avgdata 3240maxresident)k

    // Brute force

    13 digits: 3.75user 0.00system 0:03.75elapsed 100%CPU (0avgtext+0avgdata 1544maxresident)k

    15 digits: 36.94user 0.00system 0:36.94elapsed 99%CPU (0avgtext+0avgdata 1208maxresident)k

## License & Copyright

GPLv3.0

Copyright (C) 2015 [Arun Prakash Jana](mailto:[email protected])