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Go small at first.
Follow the code paths from UI to the finish line for that particular path. I cannot stress this technique enough. Pick a path that is relevant to your current task or project.
Don't try to learn the entire system at one time.
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If a debugger is available, use it to see the call stack, for the functionality you're currently working on. This will give you an idea of the control flow for that functionality.
Once the first functionality is reasonably well understood, go to the next functionality, and so on.
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Try reaching out to the dev community, and see if they can suggest easy items for a newcomer to the codebase. I know Mozilla has a list of them, but doubt they're the only project to do so.
Did you ever see history portrayed as an old man with a wise brow and pulseless heart, waging all things in the balance of reason?
Is not rather the genius of history like an eternal, imploring maiden, full of fire, with a burning heart and flaming soul, humanly warm and humanly beautiful?
--Zachris Topelius
Training a telescope on one’s own belly button will only reveal lint. You like that? You go right on staring at it. I prefer looking at galaxies.
-- Sarah Hoyt
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TheOnlyRealTodd wrote: when I open the project file for these, it can be tough trying to figure out where to even start.
Getting them to compile is the hardest step! 
TheOnlyRealTodd wrote: and also looking at the unit tests to get an idea of whats happening
That's a good approach, especially single stepping through the tests.
And write down everything you learn, and your questions for what you don't understand yet.
Personally, I think most of these open source sites could benefit greatly from writing some "programmer documentation", as you're not the only one with "how the elephant does this stuff work?" question. And that might be the most useful contribution at first!
Marc
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Play with it without looking at the code, first -- get the grand view, and get to know the whys and wherefores.
I wanna be a eunuchs developer! Pass me a bread knife!
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To analyse a large code base you could try running a dependency tool such as DeepEnds
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The essence of your learning question is advice as to how you can most efficiently organize and sequence your code-reading-and-understanding learning tasks so as to attain a reasonable level of proficiency in the time you have allotted and at the same time reduce the time you will spend on the study of code without merit, which frankly is not possible until you have already learned it.
My suggestion is to, at the outset, refuse to learn and study code written in throw-away computer languages.
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As always, working backwards is a good approach.
You can find previously FIXED Items, and review the posted code changes that fixed the item.
I would recommend being able to build/test the previous version, and verify the bug.
Apply the fix.
Verify the bug is gone.
If you get decent at that. Then get realistic. It takes approximately 5,000hrs to master a new skill.
Assuming you have mastered programming in general, lets assume a large code base will take you about 1,000hrs for a solid basic understanding. (Half a work year).
Yeah, it is easy to jump in and hack away. But actually mastering a code base.
This gets to the REASON others suggest you support a code base that you already use, like, and would like to extend.
BTW, as you setup your environment to test/validate prior updates. Considering reviewing and enhancing the documentation that helps others get to where you got to.
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If you can find an issue that does not have a UnitTest, contribute a UnitTest that reproduces the issue.
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I found the doxygen tool to be very helpful. I can show some call traces and is very convenient for navigating objects.
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In GREP we trust. Use GREP, find in files, or find usages to see all the references to a particular class and it's public methods.
Start the new dev with a small task - a bug fix or minor enhancement. Then have the new dev document every class and method that contributes in some way to the scenario. Have the new dev document every other class and method that depends on the code that is changed, all the way up to the UI or interface.
Also, I absolutely concur with the idea of debugging and examining the stack. If unit tests and integration tests are available then run these in the debugger. If a developer went to the trouble to write unit tests, then it must be important.
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Could I also echo the responses from some of the other repliers that that is a chronic shortage of documenters for most open source products?
They are like rocking horse poop.
If you want to hone your coding skills and can find a project that deeply interests you, then hack away.
If you are just looking to contribute to a project that matters to you, then documenters are always welcomed with open arms.
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I think your first impulse (find and solve an issue) was the right one.
At least you have a "goal" in mind; the rest ("reading code") gets old pretty fast.
Ultimately, you will find out your value is in seeing the big picture quickly, and prioritizing what needs to be done.
A lot of code never gets executed or deals with fringe cases; better to focus on the stuff that actually gets run; i.e. the "buggy" parts.
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Since you're a novice first focus on what you are comfy with.
Pick that layer.
Pick up a tool like Ndepend or Nitriq and see how the layers interact.
Then and only then start playing on the keyboard.
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Here are a few tips.
If nothing else fix layout issues - indentation, spacing etc - and add (sensible) comments where it makes sense. The act of tidying up layout and having to think about what a small section of code is doing, in its own right, will help build your understanding of the bigger picture. From there is probably won't take long for you to start spotting refactoring opportunities.
If you do decide to make changes, start with the small trivial things since these will often be overlooked or tolerated for the sake of the big things. Build up a testing regime for your changes BEFORE you make the changes.
Don't focus on code structure or control too much. That will become obvious. The key to any code base is how it organizes its data and moves it around. This you can analyse and diagram.
Remember, fundamentally all any software is really about is moving data from A to B.
With this in mind, pay special attention to the interfaces between modules, components and systems. This is likely where the most problems are. Especially when either side of the interface has been independently developed.
Also look for places where data is transformed from one form to another e.g. conversions, lookups. Another source for faults.
If you are able to run the software, another way of gaining understanding is to include detailed logging/tracing of the software's operation as it is running. In this context, look to trace the initial state of variables and when variables change, the function calls (including explicit variable values) and function returns and error events in the code. Those three categories of logging should be enough for you to hone in on most problems with the code when it is running. This is ofc verbose and has performance implications so make sure it can be turned off or removed from the release product entirely.
Finally, study design patterns and identify where they have been used in the code. Either intentionally or unwittingly. You may be lucky and the patterns may be explicitly named e.g. WidgetFactory or WangleAdaptor. Design patterns are not the be all and end all, but they are a useful shorthand for common development problems.
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Look Big, but start small.
First get an overview of the what it's fundamentally trying to achieve.
But then,look at a function/class at the bottom that needs a little maintenance and update it.
Maybe only a little refactoring to make it clearer or updating the inline documentation.
Then follow up into the calling classes and see what's happening there.
Like pulling a piece of knitted wool, each tug takes you further into unraveling it until you've touched every part of the main codebase.
And then review/throw away most of your changes. You had no clue what you were doing at the start.
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The two main tips I have are:
1) Use doxygen to generate a hyperlinked, annotated version of the source code you can navigate in your browser.
2) Do code reviews: this gives you an opportunity to ask questions about code that developers who are currently hot in. Only works if you have a functioning code review system in place, of course.
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Garfield[^]
Skipper: We'll fix it.
Alex: Fix it? How you gonna fix this?
Skipper: Grit, spit and a whole lotta duct tape.
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I've got a drawer like that. I put a pair of socks into it, but only ever get one of them out. I wonder where they all go.
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For me it's a coat pocket!
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You keep your socks in your coat pockets?
Are you residentially challenged? 
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
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OriginalGriff wrote: Are you residentially challenged?
No! He is homeless...
Skipper: We'll fix it.
Alex: Fix it? How you gonna fix this?
Skipper: Grit, spit and a whole lotta duct tape.
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That would explain why he's always "Working" or "InThePub" 
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
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Well the flat I live in at the moment had two optics in the kitchen when I moved in...
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