hellsten + algorithms   49

security - How can bcrypt have built-in salts? - Stack Overflow
Stored in the database, a bcrypt "hash" might look something like this:

$2a$10$vI8aWBnW3fID.ZQ4/zo1G.q1lRps.9cGLcZEiGDMVr5yUP1KUOYTa

This is actually three fields, delimited by "$":

2a identifies the bcrypt algorithm version that was used.
10 is the cost factor; 210 iterations of the key derivation function are used (which is not enough, by the way. I'd recommend a cost of 12 or more.)
vI8aWBnW3fID.ZQ4/zo1G.q1lRps.9cGLcZEiGDMVr5yUP1KUOYTa is the salt and the cipher...
bcrypt  authentication  gotcha  algorithms 
4 weeks ago by hellsten
Consistent Hash Rings Explained Simply
- you may want to take a URL and get back the server the website is hosted on.

- The problem of mimicking a hash table when the number of locations are constantly changing was exactly why consistent hashing was invented.

- For 2,000 keys spread across 100 locations, you now need to move only 20 keys to a new location if 1 location with only 20 keys goes down.

- This is the main benefit of consistent hashing: you now no longer need to move so many things just because one location has disappea...
algorithms  algorithm  distributed  consistent-hash  hash  cs 
5 weeks ago by hellsten
Vector Clocks Explained
Vector Clocks by Example

We’ve all had this problem:

Alice, Ben, Cathy, and Dave are planning to meet next week for
dinner. The planning starts with Alice suggesting they meet on
Wednesday. Later, Dave discuss alternatives with Cathy, and they
decide on Thursday instead. Dave also exchanges email with Ben, and
they decide on Tuesday. When Alice pings everyone again to find out
whether they still agree with her Wednesday suggestion, she gets
mixed message...
algorithms  distributed  distributed-systems  clock  vector  time 
5 weeks ago by hellsten
AdventOfCode Day 4 - High-Entropy Passphrases | Accelerated Science
Part 1: mathematical approach

1. Looking at the bottom right of the grid above we see that the maximum square root in each ring is actually giving us the number of elements in the row and column of its ring. This is easy to convert into the number of the ring, because as we have already seen we add two at each step, starting from one, so to get from the number of elements to the number of the ring we need to just subtract one and divide by two:

2. First though, to be able to do it for any numb...
advent-of-code  2017  algorithms 
10 weeks ago by hellsten
The On-Line Encyclopedia of Integer Sequences® (OEIS®)
The On-Line Encyclopedia of Integer Sequences® (OEIS®)

Most people use the OEIS to get information about a particular number sequence. If you are a new visitor, then you might ask the database if it can recognize your favorite sequence, if you have one. To do this, go to the main look-up page, enter the sequence, and click Search. You could also look for your sequence in the Index.
integers  math  mathematics  sequences  reference  cs  list  advent-of-code  algorithms 
11 weeks ago by hellsten
Let's Learn Algorithms: Implementing Binary Search - Calhoun.io
It is important to remember that a binary search can only work on data that is sorted relevant to what you are searching. For instance, if you want to find a the smallest number greater than 6 in a list, you need a sorted list, but if you want to find a commit that broke your code, you only need the commits sorted in the order ..., working, working, broken, broken, .... That is, you need all working commits to come before the breaking commit, and then all commits after the bad commit must be broken.
binary-search  binary  algorithm  algorithms 
september 2018 by hellsten
Learning… by Alexander S. Kulikov et al. [PDF/iPad/Kindle]
Learning Algorithms Through Programming and Puzzle Solving
algorithms 
august 2018 by hellsten
Constraint satisfaction - Wikipedia
[edit]
Constraint satisfaction problems on finite domains are typically solved using a form of search. The most used techniques are variants of backtracking, constraint propagation, and local search. These techniques are used on problems with nonlinear constraints.

Variable elimination and the simplex algorithm are used for solving linear and polynomial equations and inequalities, and problems containing variables with infinite domain. These are typically solved as optimization prob...
algorithms  algorithm  ai  sudoku 
may 2018 by hellsten
montanaflynn/stats
A statistics package with common functions that are missing from the Golang standard library. Completely Documented / MIT Licensed / 100% Code Coverage
golang  algorithms  statistics  stat 
october 2015 by hellsten
www.nobelprize.org/nobel_prizes/economic-sciences/laureates/2012/popular-economicsciences2012.pdf
The Gale-Shapley algorithm proved to be useful in other applications, such as high-school choice.
Up until 2003, applicants to New York City public high schools were asked to rank their five most
preferred choices, after which these preference lists were sent to the schools. The schools then decided
which students to admit, reject, or place on waiting lists. The process was repeated in two more
rounds, and students who had not been assigned to any school after the third round were allocated
through an administrative process. However, this did not provide the applicants with enough opportunities
to list their preferences, and the schools did not have enough opportunities to make offers. As
a result, about 30,000 students per year ended up at schools they had not listed. Moreover, the process
gave rise to misrepresentation of preferences. Since schools were more likely to admit students who
ranked them as their first choice, students unlikely to be admitted to their favorite school found it
in their best interest to list a more realistic option as their first choice, while applicants who simply
reported their true preferences suffered unnecessarily poor outcomes. In 2003, Roth and his colleagues
helped redesign this admissions process, based on an applicant-proposing version of the Gale-Shapley
algorithm. The new algorithm proved to be successful, with a 90 percent reduction in the number of
students assigned to schools for which they had expressed no preference. Today, a growing number of
U.S. metropolitan areas use some variant of the Gale-Shapley algorithm.
algorithm  algorithms  nobel-prize 
september 2015 by hellsten
The most important skill in software development | John D. Cook
Here’s an insightful paragraph from James Hague’s blog post Organization skills beat algorithmic wizardry:

When it comes to writing code, the number one most important skill is how to keep a tangle of features from collapsing under the weight of its own complexity. I’ve worked on large telecommunications systems, console games, blogging software, a bunch of personal tools, and very rarely is there some tricky data structure or algorithm that casts a looming shadow over everything else. But there’s always lots of state to keep track of, rearranging of values, handling special cases, and carefully working out how all the pieces of a system interact. To a great extent the act of coding is one of organization. Refactoring. Simplifying. Figuring out how to remove extraneous manipulations here and there.

Algorithmic wizardry is easier to teach and easier to blog about than organizational skill, so we teach and blog about it instead. A one-hour class, or a blog post, can showcase a clever algorithm. But how do you present a clever bit of organization? If you jump to the solution, it’s unimpressive. “Here’s something simple I came up with. It may not look like much, but trust me, it was really hard to realize this was all I needed to do.” Or worse, “Here’s a moderately complicated pile of code, but you should have seen how much more complicated it was before. At least now someone stands a shot of understanding it.” Ho hum. I guess you had to be there.
software-development  engineering  algorithms  advice  best  programming 
june 2015 by hellsten
Deep Learning From The Bottom Up | Hacker News
To address some of the comments being presented here, neural nets despite being harder to train can be debugged visually.
A few tips for those of you who use neural nets:
Debug the weights with histograms. Track the gradient and make sure the magnitude is not too large and its normally distributed.
Keep track of your gradient changes when using either gradient descent or conjugate gradient.
Plot your filters, visualize what each neuron is learning.
Watch the rate of change of your cost function. If it seems like its changing too fast and stops early lower your learning rate.
Plot your activations: if they start out grey you're fine. If you start all black, you need to retune some of your parameters.
Lastly, understand the algorithm you're using. Convolutional nets are different from recursive neural tensor are different denoising autoencoders are different from RBMS/DBNs.
Pay attention to your cost function, reconstruction entropy is used differently from negative log likelihood is used differently for different objectives.
If you are trying to do feature learning, you are using RBMs, Denoising AutoEncoders and you will use reconstruction entropy. This is what you use for feature detectors. You may end up using negative log likelihood if you are dealing with continuous data.
For RBMs, pay attention to the different kinds of units[1]. Hinton recommends Gaussian visible with recitifed linear for continuous data, binary binary otherwise.
For denoising autoencoders, watch your corruption level. A higher one helps generalize better, especially with less data.
For time series or sequential data, you can use a recurrent net,moving window with DBNs, or recursive neural tensor
Other knobs:
If your deep learning framework doesn't have adagrad find one that does.
Dropout: crucial. Dropout is used in combination with mini batch learning to handle learning different "poses" of images as well as generalizing feature learning. This can be used in combination with sampling with replacement to minimize sampling error.
Regularization: L2 is typically used. Hinton once said: you want a neural net that always overfits but is regularized (youtube video...don't remember link right now).
Would love to answer questions! Source: I work on/teach this stuff.[2][3]
Lastly, tweak one knob at a time. Neural nets have a lot going on. You don't want a situation where you A/B tested 10 different parameters at once and you don't know which one worked or why.
[1]: http://www.cs.toronto.edu/~hinton/absps/guideTR.pdf
[2]: http://deeplearning4j.org/
[3]: http://zipfianacademy.com/
[4]: http://arxiv.org/abs/1206.5533 http://deeplearning4j.org/ http://deeplearning4j.org/debug.html http://yosinski.com/media/papers/Yosinski2012VisuallyDebuggi...
deep-learning  ai  algorithms 
may 2014 by hellsten
Google Code Jam
Google Code Jam is back in action challenging professional and student programmers around the globe to solve difficult algorithmic puzzles. This year's Code Jam consists of four online rounds and concludes with the Onsite World Finals under the California sun in Google's Los Angeles office. The competition will really heat up this August during the finals, where the top 25, along with last year's champion, Ivan Miatselski (mystic) of Belarus, will jam it out for the $15,000 grand prize, the coveted title of Code Jam 2014 Champion and automatic qualification in the Code Jam 2015 finals to defend the title.
algorithms  contest  challenge  programming  todo 
april 2014 by hellsten
Josh Haberman: LL and LR Parsing Demystified
Think back to our first example of 1 + 2 * 3. Here is that expression written as a tree:
parsing  parser  algorithms  python 
august 2013 by hellsten
An Exhaustive Explanation of Minimax, a Staple AI Algorithm
Remember that in ruby objects are not immutable. You can relax memory requirements by traversing game tree on demand instead of creating whole thing at the beginning.
ai  algorithms  ruby  totry  toread 
january 2012 by hellsten
Clever Algorithms
The book "Clever Algorithms: Nature-Inspired Programming Recipes" by Jason Brownlee PhD describes 45 algorithms from the field of Artificial Intelligence. All algorithm descriptions are complete and consistent to ensure that they are accessible, usable and understandable by a wide audience.

5 Reasons To Read:

1. 45 algorithms described.
2. Designed specifically for Programmers, Research Scientists and Interested Amateurs.
3. Complete code examples in the Ruby programming language.
4. Standardized algorithm descriptions.
5. Algorithms drawn from the popular fields of Computational Intelligence, Metaheuristics, and Biologically Inspired Computation.
ai  algorithms  programming  book  ruby  free  tosite1 
february 2011 by hellsten
Using the Python NLTK Bayesian Classifier for word sense disambiguation - 92% accuracy - Jim Plush's Programming Paradise
Today's article will be going over some basic word sense disambiguation using the NLTK toolkit in Python and Wikipedia. Word sense disambiguation is the process of trying to determine if you mention the world "apple" are you talking about Apple the company or apple the fruit? I've read a few white papers on the subject and decided to try out some of my own tests to compare results.
nlp  nltk  bayesian  algorithms  python  analysis  text-classification 
december 2010 by hellsten
Self-Improving Bayesian Sentiment Analysis for Twitter
The simplest way to do this was to remove all ‘noise’ words from the tweets and classification process – those words that do not imply positivity or negativity, but that may falsely skew the results.

There are plenty of noise word lists around, so I took one of those and removed any words that are relevant to sentiment analysis (e.g. ‘unfortunately’, that appears in the MySQL list, may be useful for identifying negative tweets).

Each word (‘feature’) has a strength of positive/negative sentiment, based on the number of positive/negative tweets it is previously featured in. For example, in my dataset, the word ‘new’ is fairly positive, but the word ‘kudos’ is extremely positive. By calculating the number of strong words and variation in positive/negative words, a confidence can be calculated (e.g. a tweet that includes 5 negative words, 1 positive word, 2 extremely negative words and no extremely positive words can be confidently assumed to be negative).
ai  text-classification  bayesian  twitter  algorithms  bayes  classification 
august 2010 by hellsten
Introduction to Information Retrieval
The book aims to provide a modern approach to information retrieval from a computer science perspective. It is based on a course we have been teaching in various forms at Stanford University and at the University of Stuttgart.
information-retrieval  rocchio  bayesian  svm  lsi  text-classification  classification  td-idf  best  development  computer-science  book  free  algorithms  datamining  ir 
july 2010 by hellsten
You Don’t Need Math Skills To Be A Good Developer But You Do Need Them To Be A Great One
Learning a little bit about search exposed me to all sorts of interesting software-y and computer science-y related things/problems (machine learning, natural language processing, algorithm analysis etc.) and now everywhere I turn I see math and so feel my lack of skills all the more keenly. I've come to the realization that you need a decent level of math skill if you want to do cool and interesting things with computers.

Normally people choose a framework or two and a programming language and go with that, which is fine and worthwhile. But consider the fact that frameworks and to a lesser extent languages have a limited shelf life. If you're building a career on being a Hibernate, Rails or Struts expert (the struts guys should really be getting worried now :)), you will have to rinse and repeat all over again in a few years when new frameworks come along to supersede the current flavour of the month.
skills  career  programming  math  development  best  tosite1  algorithms 
june 2010 by hellsten
Stevey's Blog Rants: Math For Programmers
The right way to learn math is to ignore the actual algorithms and proofs, for the most part, and to start by learning a little bit about all the techniques: their names, what they're useful for, approximately how they're computed, how long they've been around, (sometimes) who invented them, what their limitations are, and what they're related to. Think of it as a Liberal Arts degree in mathematics.
math  algorithms  programming  career  education  mathematics  learning  best  development  tosite1 
march 2010 by hellsten
A Thousand Foot View of Machine Learning « Awwthor Blog
For further reading, I suggest taking a look at Andrew Moore’s tutorials, which I have found to be very helpful. Andrew Moore is a well-known AI researcher from CMU. Mainly, I suggest taking a look at his tutorials on Decision Trees, Gaussian Mixture Models, K-Means Clustering and Support Vector Machines. For a broad look at the field, his Intro to AI tutorial might be helpful.
ai  machine-learning  algorithms  programming  svm 
january 2010 by hellsten
Data Structures and Algorithms with Object-Oriented Design Patterns in Ruby
Data Structures and Algorithms
with Object-Oriented Design Patterns in Ruby
programming  algorithms  book  ruby 
april 2007 by hellsten

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