Monthly Archives: August 2019

/dev/algo

Maximum Rectangular Area in Histogram

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The description of the maximum rectangular area in histogram problem is fairly simple. Given an array of the height of continuous bars with equal width, and the goal is to find the maximum rectangular area inside them. For example, the following figure shows the maximum rectangular area in pink of height array [1, 2, 3, 4, 5], which is 9.

Let's breakdown the problem. To form such a rectangular area, the following constraint must be satisfied.

All bars inside the area is at least as high as the rectangular.

Which suggests that no matter how we choose the rectangular, all involved bars must be continuous and the best height we could reach is limited to the lowest one.

As shown above, the maximum height of the rectangular area is limited to the lowest involved bar. (The height of the lowest one is mark in pink).

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/dev/null, Python

Rewrite the styled code in HTML generated by Apple to WordPress compatible HTML

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My first blog writing was in 2013, and at that time, WordPress was able to handle the styled code correctly, i.e., the code preserved the syntax highlight when I copy it from Xcode / CodeRunner and paste into the WordPress editor. The editor was capable of converting or persevering the colour info, and it did a great job of formatting the styled code into HTML.

Just like this post, https://await.moe/2013/08/assertmacros-problem/. The code shown below

typedef int (*PYStdWriter)(void *, const char *, int);
static PYStdWriter _oldStdWrite;

could be nicely formatted into the corresponding HTML code

<span style="color: #bb2ca2;">typedef</span> <span style="color: #bb2ca2;">int</span> (*PYStdWriter)(<span style="color: #bb2ca2;">void</span> *, <span style="color: #bb2ca2;">const</span> <span style="color: #bb2ca2;">char</span> *, <span style="color: #bb2ca2;">int</span>);
<span style="color: #bb2ca2;">static</span> <span style="color: #4f8187;">PYStdWriter</span> _oldStdWrite;

However, it was about the time WordPress upgraded to 3.9, the aforementioned functionality was removed. Although there are tens of syntax highlighting plugins, but I don't really like the colour schemes they offer. Besides, sometimes I may need to highlight a small portion of code. Such as this post, https://ryza.moe/2017/05/the-reason-that-codesign-remove-signature-generates-malformed-macho-still-remains-mystery/

/*
* If this has a code signature load command reuse it and just change
* the size of that data.  But do not use the old data.
*/
if(object->code_sig_cmd != NULL){
    if(object->seg_linkedit != NULL){
        object->seg_linkedit->filesize += arch_signs[i].datasize - object->code_sig_cmd->datasize; 
        if(object->seg_linkedit->filesize > object->seg_linkedit->vmsize)

As you can see, using native HTML code could enable extra control and functionality.

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/dev/null, C++

Brainfuck Interpreter in C++17——A Modern Approach to Kill Your Brain

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It has been a long time that I wrote my last C++ code. And obviously, C++ has released the new standard, C++17, for a few months. There are a lot features in which has been introduced or changed.

One out many is the pattern matching using std::variant, std::monostate and std::visit. This feature could lead the data-oriented design easier in modern C++. And it's not that hard to write a (possibly buggy) brainfuck interpreter with this feature.

Although with that said, we still need some extra 'hacking' to achieve a more elegant code style. There is a blog post by Marius Elvert explaining a two-line visitor in C++, with which you could write a few lambda expressions in one visitor and then the std::variant could be dispatched by the receiving type of those lambda. 

auto my_visitor = visitor{
    [&](int value) { /* ... */ }
    [&](std::string const & value) { /* ... */ }
};

To accomplish the goal, Marius found that lager just uses two lines of code. The details are explained in aforementioned blog post. And for the readability of the code today, I just substituted the name of the struct.

And if you're in a hurry, the full code is on my GitHub, #/brainfuck-cpp17

template<class... Ts> struct brainfuck_vm : Ts... { using Ts::operator()...; };
template<class... Ts> brainfuck_vm(Ts...) -> brainfuck_vm<Ts...>;

And the next step is to define all the 8 valid ops of brainfuck.

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Hack Space, Linux

仔细想想还是 Dockerized 吧！

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The AI Lab of my mentor was running by me for quite some months. And now it's about time to hand over the docs of the internal server to graduates. Though one of which tends to lose internet connection from time to time due to its location. However, I heart that it had been moved back to university in the middle of July.

And originally, I use Microsoft Word to keep all the records and information of almost everything, but it obviously would cause some issues.

For example, one's docs version may vary from another. Yes, I've thought about to use the cloud storage with version control even. The problem is that we cannot afford the expense of cloud drive. And we could not find someone who's willing to take the charge of reimbursement. The bills have already piled up in my mentor's desk.

Besides that, to use file as docs will inevitably introduce the ugly naming, such as docs-20190807, docs-20190607 or whatever. And it would be totally disaster if to use git for version control. Despite of the unreadable commits, the filename needs to be the same, which extremely likely to be ignored to update from the git repo for some people.

Luckily, there's one instance on AliCloud (Although personally I don't really like AliCloud, but that's another story, let's save it for next time). And lots of packages that can generate static HTML from markdown have been developed these years around.

It would be easy for everyone to access docs online and because the markdown file is pure text, we can have a very good and most important, readable track of changes with git.

The final decision is to use VuePress as the static HTML generator. And to ensure a simple installation process, dockerization is the best shot at the moment. Furthermore, basic HTTP auth is needed to keep unwanted visitors out, leaving the docs only accessible to the lab.

For your convenience, this project is located at my GitHub, #/docs. It's fully prepared and dockerized with docker-compose support.

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