Threaded Equation Finder

Threaded Equation Finder

Find arithmetic equations that equates to a given 'target' value, number of terms, and operators.

View the GitHub - Download source .zip

Introduction

   You should all be familiar with how a typical computer works; you give it some variables, describe some quantities of some resources you have, choose an algorithm, let it process, and it returns to you a result or outcome. Now imagine a computer if you could work with a computer that worked the other way around. I believe it was Douglas Adams that described the notion of an all-together different type of computer; That is, you tell the computer what you want the outcome to be, and it goes off figuring out how to get there and what you need to do it. Z3, the Theorem Prover, and the constraint satisfaction problem (CSP) solver (and probably others) in Microsoft's Solver Foundation do almost exactly that.
   There is also the idea of Backcasting, which is a similar, but different idea.

   My program isn't as fancy as all that, but it does find equations that equates to a given 'target' value, albeit at random. You define constraints other than just the target value, such as what operators are allowed in the equation, the quantity of terms there, and the range or set of allowed terms.
For example, how many different ways can 9 nines equal 27, using only addition, subtraction, and multiplication, if evaluated left-to-right (ignore the order of operations)? Turns out there are only 67 ways.

(above) Application Screen Shot

How it works

   The actual approach for finding equations that equate to an arbitrary target or 'goal' value is rather primitive. By way of Brute Force, several threads are launched asynchronously to generate thousands of random equations, evaluate them and keep the results that have not been found yet.

   This is something I wrote approx. 2 years ago. I dug it up and decided to publish it, because I thought it was interesting. As this was an 'experiment', I created different ways of storing and evaluating the expressions, and have made those different 'strategies' conform to a common interface so I could easily swap them out to compare the different strategies. I have refactored the code so that each class that implements IEquation is in its own project and creates its own assembly.

   There are two fully-working strategies for representing equations: one that represented the equation as a list of 2-tuples (Term,Operator), did not perform order of operations, and was rather obtuse. The other strategy was to store the equation as a string and evaluate it using MSScriptControl.ScriptControl to Eval the string as a line of VBScript. This was unsurprisingly slower but allowed for much more robust equation evaluation. Order of operations is respected with the ScriptControl strategy, and opens the way to using using parenthesis.

   The other idea for a strategy which I have not implemented but would like to, would be a left-recursive Linq.Expression builder. Also, maybe I could somehow use Microsoft Solver Foundation for a wiser equation generation strategy than at random.

Limitations

   Today, however, there are better architectures. A concurrent system like this would benefit greatly from the Actor model. If you wanted to write complex queries against the stream of equations being generated or selected or solved, maybe reactive extensions would be a slam dunk.

   Although this project certainly is no Z3, it does provide an example of an interface... perhaps even a good one.

Running on Raspberry Pi 2

   Microsoft's Managed Extensibility Framework (MEF) might be a good thing here, but I also wrote a console client that is designed to be ran with Mono on the Raspberry Pi 2. MEF is a proprietary Microsoft .NET technology that is not supported in Mono. The extra meta data in the assembly shouldn't be a problem, but having a dependency on the MEF assembly will be. Probing of the runtime environment and dynamically loading of assemblies is required here, which I have not had time to do, so at this time, there is no MEF.

   The reason the mono client is a console application is because mono and winforms on the Raspberry Pi 2 fails for some people. The problem has something to do with a hardware floating point vs a software float, and it happens to manifest itself when using a TextBox control. The only thing that I haven't tried, and that should presumably fix it, is to re-build mono from the latest source.

Certificate Enumerator

     Recently, my windows quit updating. Just prior to that, I had been messing around with my certificate store, so I suspected that to be the cause. Running Microsoft's troubleshooter reset the download, which I had a lot of hope of fixing the issue, but the download still continued to to fail. I decided to check the Windows Event Logs, and that's where I found an error message about a certificate in the chain failing. I knew it! However, I did not know whether a trusted certificate accidentally got put in the untrusted store, or whether an untrusted certificate was accidentally put in the trusted store. I needed a way to search all of my certificates' thumbprints or serial numbers against a know repository of trusted or untrusted certificates.
    Microsoft's certificate snap-in for MMC does not allow you to view certificates in an efficient way. Opening them one at a time, manually, and then scrolling all the way down to where the thumbprint is displayed to compare it to a webpage is painful. Also, I was not satisfied with the way that Microsoft allows you to search the certificate store. The search is not very effective and you cant even search for thumbprints! Also I do not believe the search feature allows you anyway to copy any of that information to clipboard.
Most of what I needed to accomplish could simply be done if I could just export all of my computers certificates thumbprint or serial numbers to a text file, csv file, or other simple and searchable format. Then I thought to myself, I know how to do that! It was the great the lack of features of the MMC certificate snap-in, and the inability to search for certificate thumbprints that inspired me to write my own certificate utility, known simply as Certificate Enumerator.




     CertificateEnumerator can list every certificate in your various certificate stores for your local machine and currently logged in user. It can then display that information to you either in a DataGridView or TextBox (as columnarized text), and provides the ability to persist that information to file as text, comma separated values (CSV), excel format or HTML table.


     The Certificate Enumerator also has the ability to 'validate' each certificate against its CRL (certificate revocation list), if it supplied one.


     The GUI could really use some love. In case you missed it, the project is on my GitHub, so feel free to download the source and play with it. If you come up with useful, submit a pull request.

Finding a date range in SQL

    At work, we use log4net, and we have the appender (or logging output location) set up to be AdoNetAppender, so thus it logs to a SQL database. In the Log table, there is a column called [Date], and it has the sql type of datetime.


    Often, when querying the Log table, you only want to view the most recent dates. lets say within the last week. You could always ORDER BY [Date] DESC of course, but suppose we wanted more control than that, such as only last week's dates.

    The SQL keywords (and functions) that are relevant here are BETWEEN, GETDATE and DATEADD.

    Here is the SQL code:

 SELECT
     [ID],[Date],[Thread],[Level],[Logger],[Message],[Exception]
 FROM
     [DatabaseName].[dbo].[Log]
 WHERE
     [Date] BETWEEN
      DATEADD(dd, -7, GETDATE())
      AND
      DATEADD(dd,  1, GETDATE())
 ORDER BY

     [Date] DESC

    The BETWEEN keyword should be pretty self-explanatory, as should the GETDATE function. The secret sauce here lies within the DATEADD function.

    The SQL function DATEADD has this signature: DATEADD (datepart, number, date) 

    The DATEADD function adds a number to a component of DATETIME, in this case, days. This number can be negative to subtract time from a DATETIME, as is the case with our example. The datepart parameter is what determines what component of the DATETIME we are adding to. You can add as much as a year, or as little as a nanosecond (what, no picoseconds? *laugh*). Microsoft's Transact-SQL MSDN page for DATEADD supplies the following table for datepart:

DATEPART	ABBREVIATIONS
year	yy, yyyy
quarter	qq, q
month	mm, m
dayofyear	dy, y
day	dd, d
week	wk, ww
weekday	dw, w
hour	hh
minute	mi, n
second	ss, s
millisecond	ms
microsecond	mcs
nanosecond	ns

    In the example, I am subtracting 7 days from the current date. If you are making a stored procedure, this variable can be replaced with a parameter:

 CREATE PROCEDURE [dbo].[sp_GetLogEntriesRange]
     @RangeInDays int
 AS
 BEGIN
     DECLARE @DaysToAdd int
     SET @DaysToAdd = (0 - @RangeInDays)

     SELECT
      [ID],[Date],[Thread],[Level],[Logger],[Message],[Exception]
     FROM
      [DatabaseName].[dbo].[Log]
     WHERE
      [Date] BETWEEN
       DATEADD(dd, @DaysToAdd, GETDATE())
       AND
       DATEADD(dd,  1, GETDATE())
     ORDER BY
      [Date] DESC
 END

    Enjoy, I hope this helps!

Lazy IEnumerable file reader class: When to inherit from IDispose?

    Need to read just the few couple lines from a gigantic data file? Or maybe you need a forward-only, load-only-what-you-need file reader pattern? Here is a construct I have been toying with, its a class that treats a file stream like an IEnumerable.

    Note added 8/1/15: TODO: Add constructor overload that accepts the starting filepointer position, optional ending filepointer position.

    This has the benefit of not using any resources if you never use it, allows you to incrementally read gigantic files without loading it all into memory (you might need to call the StreamReader.DiscardBufferedData() method every once in a while), and because its IEnumerable, you can write queries against it that are lazy; they don't actually execute until the run-time actually NEEDS it, such as calling the IEnumerable.ToList() or 'Count()' extensions, for example. Be careful with ToList() if the file is a gigabyte or more, as calling ToList() will cause the whole thing to be read right then.

    If instead you just need to iterate through each line only until you find what you are looking for, or use Linq and a predicate to search for a particular line that satisfies a condition, then this pattern will save your application from having the load the whole thing in memory:

public class EnumerableFileReader
{
    public FileInfo File { get { return _file; } }
    public bool FileExists { get { return _file.Exists; } }

    private FileInfo _file;

    public EnumerableFileReader(string fileLocation)
        : this(new FileInfo(fileLocation))
    {
    }

    public EnumerableFileReader(FileInfo file)
    {
        if (!file.Exists)
        {
            throw new FileNotFoundException();
        }

        _file = file;
    }

    public IEnumerable FileLines()
    {
        if (!FileExists) yield break;

        string line;
        //long internalBufferSize = 0;

        using (StreamReader reader = _file.OpenText())
        {
            while ((line = reader.ReadLine()) != null)
            {
                //if (internalBufferSize++ > 90000) {   reader.DiscardBufferedData(); internalBufferSize = 0; }
                yield return line;
            }
        }

        yield break;
    }
}

    It struck me that it might be a good idea to make the class inherit from IDisposable, so the StreamReader doesn't get left around in memory, holding a file open. Indeed; all those yield keywords make it look like the stream object will just hang around there if FileLines is never called again to finish the enumeration. However, it turns out this is probably not necessary but the answer is, as you might expect: IT DEPENDS. It depends... on how you are going to use the class. Looking into the subject, I discovered that when you use the yield keyword, the compiler generates a nested class which implements the IEnumerable, IEnumerator and IDisposable interfaces and stores all context data for you under the hood. I'm not going to drop the IL (or CIL) here, but if you are curious, just open up your IEnumerable class in ILSpy. Just make sure you change the language in the drop-down box at the top from C# to IL, otherwise it will be hidden.

    So just when is our class disposed of? Well anytime you explicitly call Dispose on the Enumerator or the Stream, which one might expect. However, this will dispose of a lot more than just the stream or the enumerator alone, but all of the associated constructs that are generated around the enumerator/yield pattern. to be disposed of. Dispose will also be called at the end of enumeration. This includes when you run out of things to enumerate, any time you use the yield break or return keyword, or the program flow leaves the using statement surrounding the Stream. Here is something I didn't know: Dispose is also called when you call the IEnumerable.First() or FirstOrDefault() extension. Also, any time you use a foreach loop around the IEnumerator, the object will get disposed after you are done looping or after you break or leave the loop.

  

So, in short: As long as you're using LINQ extensions or foreach loops, the disposal is taken care of for you automatically. However, if you are manually calling the Enumerator().MoveNext() on the method, then you need to call dispose yourself, or rather, implement the IDisposable pattern.

    Being able to use EnumerableFileReader in a using statement/disposable pattern would likely be expected of a file reader class. You could have your dispose method set a boolean flag and then call FileLines(), and add an if statement in the while look of your FileLines() method that will yield break when the dispose flag is set to true, but cleaning up properly can be tricky if your IEnumerator has more than one or two return yield statements. I would instead suggest that that we use one of the tricks we just leaned above and just have our Dispose() function call .FirstOrDefault() on the FileLines() method:

public class EnumerableFileReader : IDisposable
{
[...]

    public void Dispose()
    {
        FileLines().FirstOrDefault();
    }

[...]
}

Humorous Software Licence Agreement #2

See also: Humorous Software Licence Agreement #1

Please note: I do not know what kind of legal protection against liability this licence will actually afford you. Please confer with an actual lawyer before attempting to use this in any legal capacity.

Software Usage License Agreement
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All rights reserved.

 * WITH NO GUARANTEES OF ANY KIND
THIS SOFTWARE IS PROVIDED:
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 * The above copyright notice and this permission notice shall be included in all copies or substantial portions of the software.

UNDER NO CIRCUMSTANCES AND IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT, COURSE OF BUSINESS, LATE-NIGHT DRUNKEN PRANKS, OR ACT OF,
OR OTHERWISE, ARISING FROM, OUT OF, OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE

Want to read more? I wrote a Humorous Software Licence Agreement #1 post with a similar-themed software licence.