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• How does heap compaction work quickly?

  - by Mason Wheeler

  They say that compacting garbage collectors are faster than traditional memory management because they only have to collect live objects, and by rearranging them in memory so everything's in one contiguous block, you end up with no heap fragmentation. But how can that be done quickly? It seems to me that that's equivalent to the bin-packing problem, which is NP-hard and can't be completed in a reasonable amount of time on a large dataset within the current limits of our knowledge about computation. What am I missing?

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• How to simulate a fake MouseOver on a Flash applet in a webpage?

  - by Mason Wheeler

  I listen to internet radio at http://player.play.it/player/player.htm and it works pretty well, except for one minor issue. The Flash applet that runs the radio player has a timer on it, where if you don't move the mouse over the player every once in a while, it decides you're idle and shuts off the stream, even if you're not actually idle, but just working on something else with the radio player running in the background. Is there any way I can send a fake MouseOver message to this applet to keep it from cutting me off in the middle of a song, maybe with a GreaseMonkey script? I'm using Firefox.

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• What does it mean that "Lisp can be written in itself?"

  - by Mason Wheeler

  Paul Graham wrote that "The unusual thing about Lisp-- in fact, the defining quality of Lisp-- is that it can be written in itself." But that doesn't seem the least bit unusual or definitive to me. ISTM that a programming language is defined by two things: Its compiler or interpreter, which defines the syntax and the semantics for the language by fiat, and its standard library, which defines to a large degree the idioms and techniques that skilled users will use when writing code in the language. With a few specific exceptions, (the non-C# members of the .NET family, for example,) most languages' standard libraries are written in that language for two very good reasons: because it will share the same set of syntactical definitions, function calling conventions, and the general "look and feel" of the language, and because the people who are likely to write a standard library for a programming language are its users, and particularly its designer(s). So there's nothing unique there; that's pretty standard. And again, there's nothing unique or unusual about a language's compiler being written in itself. C compilers are written in C. Pascal compilers are written in Pascal. Mono's C# compiler is written in C#. Heck, even some scripting languages have implementations "written in itself". So what does it mean that Lisp is unusual in being written in itself?

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• Adding an HTTP Header to the request in a servlet filter

  - by Mason

  I'm integrating with an existing servlet that pulls some properties out of the HTTP header. Basically, I'm implementing an interface that doesn't have access to the actual request, it just has access to a map of k-v for the HTTP headers. I need to pass in a request parameter. The plan is to use a servlet filter to go from parameter to header value but of course the HttpServletRequest object doesn't have an addHeader() method. Any ideas?

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• Is there any simple way to test two PNGs for equality?

  - by Mason Wheeler

  I've got a bunch of PNG images, and I'm looking for a way to identify duplicates. By duplicates I mean, specifically, two PNG files whose uncompressed image data are identical, not necessarily whose files are identical. This means I can't do something simple like compare CRC hash values. I figure this can actually be done reliably since PNGs use lossless compression, but I'm worried about speed. I know I can winnow things down a little by testing for equal dimensions first, but when it comes time to actually compare the images against each other, is there any way to do it reasonably efficiently? (ie. faster than the "double-for-loop checking pixel values against each other" brute-force method?)

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• Is there a Delphi dropdown notification component?

  - by Mason Wheeler

  You know how in Firefox, if something happens that requires your attention but isn't immediately urgent enough to require a modal dialog, it will drop down a little strip at the top of the tab with a question on it? I'd like to be able to put functionality like that in a Delphi app, but I don't know if there's a component for that. Does anyone know of one?

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• Any way to get TStringList.CommaText to not escape commas with quotes?

  - by Mason Wheeler

  I'm doing some work with code generation, and one of the things I need to do is create a function call where one of the parameters is a function call, like so: result := Func1(x, y, Func2(a, b, c)); TStringList.CommaText is very useful for generating the parameter lists, but when I traverse the tree to build the outer function call, what I end up with looks like this: result := Func1(x, y, "Func2(a, b, c)"); It's quoting the third argument because it contains commas, and that produced invalid code. But I can't do something simplistic like StringReplace all double quotes with empty strings, because it's quite possible that a function argument could be a string with double quotes inside. Is there any way to make it just not escape the lines that contain commas?

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• How does the destructor know when to activate itself? Can it be relied upon?

  - by Robert Mason

  Say for example i have the following code (pure example): class a { int * p; public: a() { p = new int; ~a() { delete p; } }; a * returnnew() { a retval; return(&retval); } int main() { a * foo = returnnew(); return 0; } In returnnew(), would retval be destructed after the return of the function (when retval goes out of scope)? Or would it disable automatic destruction after i returned the address and i would be able to say delete foo; at the end of main()? Or, in a similar vein (pseudocode): void foo(void* arg) { bar = (a*)arg; //do stuff exit_thread(); } int main() { while(true) { a asdf; create_thread(foo, (void*)&asdf); } return 0; } where would the destructor go? where would i have to say delete? or is this undefined behavior? Would the only possible solution be to use the STL referenced-counted pointers? how would this be implemented? Thank you- i've used C++ for a while but never quite been in this type of situation, and don't want to create memory leaks.

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• Why does my custom component raise AVs in the IDE?

  - by Mason Wheeler

  I'm trying to write a simple component that will allow you to embed one or more SDL rendering surfaces on a Delphi window, using the SDL 1.3 APIs. It will compile and install just fine, but when I try to use the component in the form designer, it raises AVs whenever I try to access its properties in the object inspector, save the form, or delete the component, and placing one on a form then trying to run gives a linker error: it apparently can't read the DFM properly for whatever reason. The DLL can be found at http://www.libsdl.org/tmp/SDL-1.3-dll.zip and the source code to my component can be downloaded here. SDL.pas is a JEDI-SDL header file; the rest is my own code. I don't see any reason for this to raise AVs in the form designer. If I dynamically create the control at runtime I don't have any stability issues. Can anyone take a look at this and maybe provide some feedback that might help me clear it up?

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• How do I get the current color of a fragment?

  - by Mason Wheeler

  I'm trying to wrap my head around shaders in GLSL, and I've found some useful resources and tutorials, but I keep running into a wall for something that ought to be fundamental and trivial: how does my fragment shader retrieve the color of the current fragment? You set the final color by saying gl_FragColor = whatever, but apparently that's an output-only value. How do you get the original color of the input so you can perform calculations on it? That's got to be in a variable somewhere, but if anyone out there knows its name, they don't seem to have recorded it in any tutorial or documentation that I've run across so far, and it's driving me up the wall.

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• How do I read an arbitrary chunk from a PNG file?

  - by Mason Wheeler

  I've got some custom metadata to put into a PNG file. It's being saved through libpng, and it's not difficult to write out a custom chunk. I just call png_write_chunk with the name, length and data. But I can't see how to get the data back out again. I'd expect there to be a png_read_chunk function that takes a chunk name and returns a pointer to the data or something like that, but there's nothing like that in png.h. Does anyone know how to accomplish this?

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• Biggest Delphi nitpicks

  - by Mason Wheeler

  What sort of minor annoyances do you run into using Delphi? I'm not looking for major issues such as "I want a 64-bit compiler." Just little things that can be easily worked around but still should have been implemented better so you don't have to work around them? Marking this CW. I'm more interested in the answers than the points.

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• How to know when my control changes size?

  - by Mason Wheeler

  I'm building a custom control, and I need it to be able to respond when it gets resized. I need the old dimensions and the new dimensions available in order to do some calculations. Unfortunately, the SetWidth and SetHeight methods are private to TControl, not protected, and so I can't override them. Is there any other way to know that my control's about to be resized, and to have the old size and the new size both available?

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• How do I make lines scale when using GLOrtho?

  - by Mason Wheeler

  I'm using GLOrtho to set up a 2D view that I can render textures onto. It works really well, up until I try to zoom in on the image. If I pass half the width and half the height of the viewport to GLOrtho, I end up with all my textures displayed twice as big as normal, which is exactly what I expect. But then I try to draw a box around part of the image and it all falls apart. I call glBegin(GL_LINE_LOOP), place the four vertices, and call glEnd, and I expect to see the same thing I would see if I drew it at normal zoom level, doubled. Instead, I get lines that are all the right length, but they all come out one pixel wide, instead of two, and it looks really bad. What am I missing?

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• Anyone know how to use TValue.AsType<TNotifyEvent> properly?

  - by Mason Wheeler

  I'm trying to use RTTI to add an event handler to a control, that may already have an event handler set. The code looks something like this: var prop: TRttiProperty; val: TValue; begin prop := FContext.GetType(MyControl.ClassInfo).GetProperty('OnChange'); val := prop.GetValue(MyControl); FOldOnChange := val.AsType<TNotifyEvent>; prop.SetValue(MyControl, TValue.From<TNotifyEvent>(self.MyOnChange)); end; I want this so I can do this in MyOnChange: begin if assigned(FOldOnChange) then FOldOnChange(Sender); //additional code here end; Unfortunately, the compiler doesn't seem to like the line FOldOnChange := val.AsType<TNotifyEvent>;. It says E2010 Incompatible types: 'procedure, untyped pointer or untyped parameter' and 'TNotifyEvent' Anyone know why that is or how to fix it? It looks right to me...

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• How to publish a list of integers?

  - by Mason Wheeler

  I want to make a component that includes a list of integers as one of its serialized properties. I know I can't declare a TList<integer> as a published property, because it doesn't descend from TPersistent. I've read that you can define "fake" published properties if you override DefineProperties, but I'm not quite sure how that works, especially when it comes to creating a fake property that's a list, not a single value. Can someone point me in the right direction?

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• Can I get a PTypeInfo from a string?

  - by Mason Wheeler

  This is probably going to be a "no", but is there any way I can use Delphi's RTTI, either old-school or the 2010 extended RTTI, to pass in a string containing the name of a type, specifically the name of an enumerated type, and have it give me the PTypeInfo for that type? I've looked through RTTI.pas and TypInfo.pas and I don't see any function that would do that, but I might have missed something. What I'm looking for: var info: PTypeInfo; begin info := GetTypeInfoFromName('TComponentStyle'); end; Or something like that. Thing is, the name of the enumerated type would be passed in; it wouldn't be known at compile time.

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• How do I retrieve an automated report and save it to a database?

  - by Mason Wheeler

  I've got a web server that will take scripts in Python, PHP or Perl. I don't know much about any of those languages, but of the three, Python seems the least scary. It has a MySql database set up, and I know enough SQL to manage it and write queries for it. I also have a program that I want to add automated error reporting to. Something goes wrong, it sends a bug report to my server. What I don't know how to do is write a Python script that will sit on the web server and, when my program sends in a bug report, do the following: Receive the bug report. Parse it out into sections. Insert it into the database. Have the server send me an email. From what little I understand, this seems like it shouldn't be too difficult if I only knew what I was doing. Could someone point me to a site that explains the basic principles I'd need to create a script like this?

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• Is there any way to run "dir" directly?

  - by Mason Wheeler

  In my answer to this question, where the asker needed a fast way to get a directory listing of a folder on a network drive, I suggested using the DOS "dir" command. Unfortunately, it's a command, not a program, so you can't execute it with CreateProcess and so I had to put it in a batch file. I don't really like that solution. It feels like a hack to me. Does anyone know a way to run dir from Delphi instead of from an external batch file?

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• Avoiding "variable might not have been initialized"

  - by Mason Wheeler

  I recently ran across a routine that looks something like this: procedure TMyForm.DoSomething(list: TList<TMyObject>; const flag: boolean); var local: integer; begin if flag then //do something else local := ExpensiveFunctionCallThatCalculatesSomething; //do something else for i := 0 to list.Count do if flag then //do something else if list[i].IntValue > local then //WARNING HERE //do something else end; This gives Variable 'local' might not have been initialized even though you can tell by reading the code that you won't hit that line unless the code branch that initializes it has run. Now, I could get rid of this warning by adding a useless local := 0; at the top of the procedure, but I wonder if there might not be a better way to structure this to avoid the issue. Anyone have any ideas?

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• Is there a way to programatically check dependencies of an EXE?

  - by Mason Wheeler

  I've got a certain project that I build and distribute to users. I have two build configurations, Debug and Release. Debug, obviously, is for my use in debugging, but there's an additional wrinkle: the Debug configuration uses a special debugging memory manager, with a dependency on an external DLL. There's been a few times when I've accidentally built and distributed an installer package with the Debug configuration, and it's then failed to run once installed because the users don't have the special DLL. I'd like to be able to keep that from happening in the future. I know I can get the dependencies in a program by running Dependency Walker, but I'm looking for a way to do it programatically. Specifically, I have a way to run scripts while creating the installer, and I want something I can put in the installer script to check the program and see if it has a dependency on this DLL, and if so, cause the installer-creation process to fail with an error. I know how to create a simple CLI program that would take two filenames as parameters, and could run a DependsOn function and create output based on the result of it, but I don't know what to put in the DependsOn function. Does anyone know how I'd go about writing it?

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• Making a table in a scrolling div resizable?

  - by Mason Jones

  I've got a table in a div, with a vertical scrollbar on the div to allow the table to be longer than the div can hold. Works fine. But I'd like to allow the user to resize the div vertically if they want to be able to view more of the table. I've been playing with the jQueryUI resizable interaction, but it doesn't seem to quite do what I want; at least, not so far. I've tried making the wrapper div resizable, but the behavior's erratic. If I have the style "height:20em; overflow:auto;" on it, then I can resize the table horizontally, but not vertically. If I remove the overflow, then the table flows outside the div of course. If I remove the height, then the table is actually resizable, but it is initially drawn at full height. Anyone know of a way to specify an initial height, but allow it to be resized larger than that? If I make the table resizable rather than the div, then I can resize the table horizontally within the div but I can't increase the height of the displayed table. Which makes sense, of course, but I thought I'd mention it. Also, is there a way to make the resize "handle" the corner between the horizontal and vertical scrollbars? Right now it's a sort of invisible handle in the bottom-right of the table. Thanks for any thoughts.

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• How do I make VC++'s debugger break on exceptions?

  - by Mason Wheeler

  I'm trying to debug a problem in a DLL written in C that keeps causing access violations. I'm using Visual C++ 2008, but the code is straight C. I'm used to Delphi, where if an exception occurs while running under the debugger, the program will immediately break to the debugger and it will give you a chance to examine the program state. In Visual C++, though, all I get is a message in the Output tab: First-chance exception at blah blah blah: Access violation reading location 0x04410000. No breaks, nothing. It just goes and unwinds the stack until it's back in my Delphi EXE, which recognizes something's wrong and alerts me there, but by that point I've lost several layers of call stack and I don't know what's going on. I've tried other debugging techniques, but whatever it's doing is taking place deep within a nested loop inside a C macro that's getting called more than 500 times, and that's just a bit beyond my skill (or my patience) to trace through. I figure there has to be some way to get the "first-chance" exception to actually give me a "chance" to handle it. There's probably some "break immediately on first-chance exceptions" configuration setting I don't know about, but it doesn't seem to be all that discoverable. Does anyone know where it is and how to enable it?

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• Does anyone know how to register VC++ Express 2008?

  - by Mason Wheeler

  I recently reinstalled Visual C++ 2008 Express after a system crash, and now it wants me to register. But when I go to the registration page, all I get is information about registering VS Express 2010, which is not what I'm trying to do. Does anyone know if it's still possible to register VS Express 2008?

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• Draggable tab control for Delphi?

  - by Mason Wheeler

  Are there any tab controls available for Delphi where the tabs can be clicked and dragged to reorder them, like in a web browser?

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