# Posts tagged "theory"

## P /= NP, Vinay Deolalikar August 9, 2010

## P and NP August 24, 2010

## David Berlinski, The Devil's Delusion: Turing machine and abacus September 18, 2010

## Some misconceptions about the CAP Theorem September 29, 2010

## Knights, knaves, and Program Construction November 11, 2010

## Measured Finger Trees December 11, 2010

## Speaking of Peano arithmetic... December 15, 2010

## Tony Morris on static types May 9, 2011

## Diagonalization and the Continuum Hypothesis May 27, 2011

## Link o' the Day: Markets are Efficient If and Only If P = NP August 24, 2011

## Link o' the day: Donald Knuth, Tony Hoare, and goto November 4, 2011

## Mad science, abstract data types, and objects December 12, 2011

## Link o' the day: Set Theory December 15, 2011

## Link o' the day: The halting poem March 14, 2012

## Link o' the day: Matt Might on parsing with derivatives March 31, 2012

## Logicomix May 28, 2012

## Quote o' the day: proofs and algorithms June 12, 2012

## On the definition of object oriented programming July 14, 2012

## The worst notational abuse October 17, 2013

## Link o' the day: static vs. dynamic typing battles January 14, 2014

## Type-safe C? August 8, 2014

## Turing machines vs. other models of computation October 30, 2014

## Quicksort February 14, 2016

## A deep dive into regular expression derivatives for lexical analysis August 15, 2016

Although I (embarrassingly) never completed the project, I have written before on derivative-based parsing,^{1} the work of Matt Might and others. The recent appearance of On the Complexity and Performance of Parsing with Derivatives and a certain immediate interest in parsing re-awakened my curiosity about the subject. (And no, it’s not because Might’s derivative-parsing tool is called Derp (but as an added bonus, the pure recognizer version is “herp” (and the accompanying regular expression package is “rerp”)).^{2}) In this particular post, I would like to demonstrate *why* I think the technique is cool, by demonstrating that derivative-based regular expressions *are* cool.

To that end, I’ll start with regular expression matching, then go on to the conversion to a deterministic finite automaton and finally discuss the minor extras needed to build a lexical analyzer, similar to lex or flex (but better).

Read more…## Applied Formal Logic: Brute Force String Search June 19, 2017

Need is there, but tools are not.

– zzz95

Let’s play with Frama-C.

Read more…## Applied Formal Logic: The bug in Quick Search June 23, 2017

In my last post, I presented a brief introduction to Frama-C and to the process of verifying properties about a very simple C function, a brute force string search. This time around, I intended to do roughly the same thing, using a slightly more complicated function, a faster string searching algorithm. Unfortunately, in doing so, I found a bug in the algorithm. Admittedly, the bug is rather minor and should not affect the actual behavior of an implementation in practice, but still, it is doing something it shouldn’t ought to be doing.

The string searching algorithm I am looking at this time is Quick Search, so named by Daniel M. Sunday in 1990 because it “is a simple, fast, practical algorithm [and] because it can be both coded and debugged quickly” (“A Very Fast Substring Search Algorithm” [PDF]. It is my personal favorite string search because it is, indeed, simple, fast, and practical. Let me quickly show why.

Read more…## Applied Formal Logic: Correctness of Quick Search July 16, 2017

When last I left, I had verified that the implementation of Quick Search was safe: the modified algorithm didn’t access memory it shouldn’t, it didn’t have numeric overflows, and it didn’t modify anything outside of its own implementation. And, using Frama-C, the process hadn’t hardly been torturous or even particularly unpleasant at all. The most important tasks were describing the requirements for calling the two functions. Those requirements are…detailed…but nothing more than what an experienced C programmer would have in mind while writing or calling the functions. A final set of shiny set of annotations ensured that the loops and the variable modifications needed by them behaved themselves.

But I hadn’t done anything about proving that the function actually did what it claimed it was supposed to do. It’s not hard to understand (one of the reasons I like Quick Search), but the ANSI/ISO C Specification Language is unfamiliar and I didn’t know how to express what went on, where. Fortunately, Yannick Moy, a software engineer at AdaCore, came to my assistance with an excellent introduction to doing the same thing with SPARK, and Loïc Correnson, one of the creators of Frama-C, popped in with some excellent suggestions (and very flattering comments).

So now, for your software verification pleasure, here is my essay at proving the correctness of Quick Search. It pretty much follows the same pattern as Yannick’s, but I did come up with a slightly different postcondition for `make_bad_shift`

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## Applied Formal Logic: Verifying Quicksort August 9, 2017

Have you ever tried to solve a problem, gotten stuck, backed off, tried again, changed approaches, failed, and then suddenly, with some approach that you’d already abandoned, solved the problem as sweetly as Penelope Doglington begging for part of your sandwich? I’ve had one of those weeks, and now I honestly don’t know what the problem was. Everything seems to be all better now.

So, here it is: my proof of Quicksort. The proof, which is mine, is mine. And therefore, my proof, which belongs to me, is as follows.

Read more…