I’ve been working on a side project with a friend for well over a year now, and in addition to the joy of working with someone you like on a problem of your choice, I’ve had quite a lot of “aha” (or “oh…”) moments.
We’re both basically happy with the state of our little project, with just a few small tweaks remaining before we can send it off into the world, except for one not-so-small thing: billing.
After about two years living in metro Vancouver, I decided to leave. I’m in a smaller city in the Fraser Valley, and it has been great!
My first time in Vancouver was for an interview at Amazon in 2019. I stayed at a hotel near the downtown office for a couple of nights, which was less time than I spent in the 6 different airplanes and a similar number of airports to get there from Brazil. Then the pandemic happened. After almost two years waiting for the visa, I finally moved to Vancouver in late 2021.
I’d like to preface this by stating that Amazon is obviously a huge company, and my opinions are just that, one person’s opinions. There will probably be some people that share my frustrations while others have had a completely different experience.
I interviewed at AWS in early 2020, pre-pandemic. The interview process is grueling and I spent considerable effort preparing for the 5-hours-long pantomime of absurd algorithms trivia and “tell me a time when you said no” behavioral questions. COVID-induced visa processing delays pushed my start date forward in time many times. The high-stress interview process and years-spanning wait built up tremendous anticipation. In hindsight I can say I probably had somewhat unrealistic expectations when finally joining the company in late 2021.
Regardless, I was quite frankly shocked after my first couple of weeks, and my first impression was that this kind of work was not for me. As a software engineer, I expected to eventually do some software engineering. I’m not sure how to describe the work that first team I joined was doing, but I can’t in good conscious call it software engineering.
Sapiens has no central point being made; rather there’s an intricate web of mostly interdependent theories and speculations. This makes for an enjoyable read, however at times it is easy to lose sight of the original premises used to build up on increasingly speculative conclusions.
Collapse is a fascinating, if somewhat exhausting, read. The central point of the book is that environmental changes, man-made or not, have been responsible for many a civilization’s collapse.
I’ve been playing with dalle-mini the last few weeks. Part of what makes it fun to play with are the bizarre and obtuse outputs. They reached that sweet spot between laughably bad and frighteningly perfect: they’re good enough to be understood and enjoyed, basically.
I think that incompleteness is part of what makes it so amusing to toy with these things, and conversely what will make future versions much less fun.
Since moving to Vancouver metro I’m continuously surprised with how common electric vehicles have become. Some may see the rise of EVs as an exciting turn towards a futuristic solarpunk utopia. I see the opposite: they are a dystopia of sorts. They are a dead end, a waste of resources in the wrong direction, a false hope.
The proliferation of personal electric vehicles is a strong marker of failure. It is a manifestation in the physical world of our inability as a society to move on from a clearly failed, car-centric way of living. Teslas kind of epitomize this – despite being just another very expensive car, despite catching fire every now and then, despite bizarre QC issues, despite its nitwit CEO, still they are seen as cool and fashionable and trendy.
Immigration sucks. In addition to the personal toll it takes on anyone, it is also mind-numbingly tedious and baroquely complex. Why aren’t things better by now?
This reminded me of the so-called “digital nomad visas”. Searching for that term will get you a thousand clickbaitey Wordpress sites with the “20 best countries with nomad visas” or whatever. But how real are they really?
Here’s something I’ve been wanting to write for a while: all the times (the ones I can remember, anyway) I bombed a software engineer job interview. There are so many “how I aced interviewing at X”/”how to pass X interview” floating around that I thought the opposite story would make for an amusing read.
My first developer job was as an intern at a big tech company in 2012. I think that was one of the worst interviews I’ve had, by the way – I could barely understand the interviewer over the cellphone, and those were the days of “how many piano players are there in New York”-kind of questions. I thought it went terrible, but I got the job somehow. On the other hand I’ve had many interviews I thought I did great but bombed anyway.
I’ve been using LinkedIn basically since I started working as an intern back in 2012. My usage is mostly limited to posting my blog posts, except the couple of times I used the platform to search for a new job. So most of the time, LinkedIn has been pretty slow-paced, with maybe half a dozen random recruiters reaching out per year.
However, since the Covid-19 pandemic started, and particularly in 2021, things seem to have gone a little crazy, with a lot more recruiter activity. I was curious to see just how much things had changed, so I looked at LinkedIn’s data export.
A couple of years ago I interviewed at one of the largest Ruby shops out there. Screening went well, and some days later I was invited for an onsite.
These were the good old pre-covid days, so an onsite really meant onsite. You had to travel to the office, wherever that was.
The thing is, an onsite is actually radically different depending on where you live. It follows that onsites introduce further bias into our industry’s already problematic hiring process. I’d like to argue that although onsites have some advantages, they’re mostly a waste of time (and money).
TLDR A simple metrics-based ranking system is good enough to decide who gets how many resources.
Computational resources – CPU time, memory usage, network traffic etc – are limited. This may be more or less of a problem depending on project/company size and so on; if you’re working on a smaller product with limited traffic, it might not be meaningful at all.
Once past a certain threshold though, expenses with such resources become non-trivial and it begins to make sense to spend some time thinking about how to distribute them as efficiently as possible.
Here’s the problem that got me thinking about this: at work, we had a computational resource that needed to be consumed by a large fleet of workers (think several thousand concurrent), but each type of worker had different productivity, and that productivity changed over time. How can we decide who gets what?
TLDR I built android-analytics, a web analytics tracker running on my phone.
Say you run a blog, personal website, small-time business page or something of the sorts. Say you also want to keep an eye on how many visitors you’re getting.
The first thing that most people think at this point is “Google Analytics”. It mostly works and is free. Its also hosted by Google, which makes it very easy to start using. There aren’t many competitors that bring those points to the table, so Google Analytics usually wins by WO at this point.
I used to use Google Analytics to track this blog for those same reasons. But after finding out about Termux and writing this post about installing a web server on an Android phone, I started toying with the idea that I had this ARM-based, 2GB RAM, Linux-like device with Internet connectivity which must be more than enough for a simple webcounter-like application. After a few weeks of tinkering, here it is!
Try searching for “free dynamic dns https”, “free domain with SSL” or anything similar. There won’t be a lot of meaningful results. Sure, some of the results are pretty close, like this guide on how to get free SSL certification from Cloudflare, or this one on setting up a free dynamic hostname with SSL, but they all assume you already own a domain. If you’re looking for a completely free domain that you can use for your personal web server that also has verified SSL, there are very few results.
But why was I even looking for this?
I’m working on a side project. It has a web server that communicates with a static web page hosted on Github Pages. There are a lot of ways of setting that up; in my particular case, I have a local (as in in my house) HTTP web server accepting traffic on a non-standard port (port 80 is blocked by my ISP for commercial reasons – this detail is of paramount importance, but more on that later). It is accessible through my external IP (which is dynamic), which can be mapped to a dynamic DNS domain.
I wanted to run a simple API on the web server and access it through static pages (like this blog) hosted on Github Pages (which has a verified SSL certificate). I asked the Internet if it is possible to call from a SSL-verified page (in JavaScript) a different server that does not have a verified SSL certificate (that is, my aforementioned webapp running in my home server). It isn’t, so the conclusion was that I needed somehow to get a verified SSL certificate for my dynamic DNS domain.
Having no idea whether this was possible, I started to research.
Communicating well with your co-workers and managers is supremely important to a software developer, and even more so for the remote one. With a lot more remote workers due to the COVID-19 pandemic, this topic became a lot more relevant.
I’ve seen people hint at this more than a few times over the years, but I didn’t really “get it” until I started working as a fully remote engineer. I also find it important to understand not only what we should be doing to achieve efficient communication, but also why we should be doing those things in those ways.
To me, the single most important thing to keep in mind is that people’s mental resources: time, attention span, etc, like yours, are limited.
I’ve struggled with some kind of problem with Nvidia graphics cards on Linux since forever.
Most commonly, an external monitor wouldn’t work or the dedicated card would refuse to power off when it should.
The latter problem – a power-hogging discrete Nvidia card not turning off when it isn’t needed, specifically in Optimus-enabled laptops – has consistently haunted me throughout the years. At least in my experience, this problem is in that sweet spot of things that are definitively annoying and kind of inconvenient, but complicated enough not to be worth the several work-hours needed to definitively solve it.
Do you have an old Android phone? Sure you do! There’s a mind-blowing amount of electronic waste of all kinds, and with the average person in developed countries discarding their phones every couple of years, discarded smartphones are probably one of the most common forms of e-waste.
I had an old Motorola G5 Cedric gathering dust, so I decided to do something with it – it is now running a Puma web server with a simple Sinatra webapp.
Now, before going any further, you might be thinking: what is the real, practical use of all this? An old Android phone probably isn’t going to have a stellar performance, but neither do those t2.nanos, honestly. I’m yet to deploy any “real” code on an Android, but even the cheaper and older phones do commonly have quad-core or even octa-core CPUs, and at least 2GB RAM, so at least in theory a phone should be close – ballpark, at least – to the most modest cloud IaaS offers our there (t2.nano has 512MB for instance). Of course, a phone has an ARM processor while IaaS usually are x86; memory management is entirely different as well, but still – we’re talking ballpark estimates here.
Anyway, this is a short tutorial on how to repurpose an Android device as a web server – or any number of different things, really.
Here at Sensor Tower we handle large volumes of data, so to keep things snappy for our customers we need to think carefully about how we process and serve that data.
Understanding the data we’re handling is a fundamental part of improving the way we serve it, and by analyzing how an important backend service worked, we were able to speed it up by a factor of four.
I began blogging in the now prehistoric late 2000s.
I’ve done a few blogs about different subjects (computer science, algorithms, web development, short stories and political ramblings). I’ve had blogs on Blogspot, Wordpress and, more recently, Medium.
Those platforms were (or are, I suppose) an easy way to spew your ideas over the Internet while also being nice and comfy for other people to actually read (this last point is important for the CSS-challenged such as yours truly). In other words, those services Got Shit Done™.
Ideally, deploys should be small, concise, easily revertible, fast and with a small or nil footprint on the database. However, no matter how awesome you are, sometimes that is just unattainable and you end up needing to deploy something that is just the opposite: big, messy, hard to revert, painfully slow and rubbing the DB the wrong way. If the deploy messes with a mission-critical part of your software, all the worse for you.
But there are actually many ways you can make those situations even worse. Here are a few bullet points you can follow to guarantee a nightmarish deploy complete with nasty side-effects that will haunt you and your coworkers for days to come.
Web bloat is discussed a lot nowadays. Web pages with fairly straightforward content — such as a Google search results page — are substantially bigger today than they were a few decades ago, even though the content itself hasn’t changed that much. We, web developers, are at least partly to blame: laziness or just bad programming are definitively part of the problem (of course, laziness might stem from a tight or impossible deadline, and bad code might come from inexperienced programmers — no judgment going on here).
We’re a small team here at Guava, and we’ve always considered ourselves remote friendly. Most of us work remotely every now and then pushed by varied force majeure situations— be it the flu, the need to supervise renovation or construction work at home, flash floods near the office, receiving guests at home or any number of other situations. We’ve also had a few of us working remotely for a few days or weeks while traveling to or back from a conference, or when visiting relatives that live out of town. In other words, remote working has always been a very temporary and circumstantial thing among us.
We have a nice office (with hammocks!), excellent work equipment, great desk space, comfortable chairs, plenty of snacks and comfort food and an infinite supply of coffee. We’re also easygoing and overall pleasant people (well, most of us are) to work with several hours a day, and some of us are even mildly funny.
Yes, this article will use the 5 stages of grief as an analogy for something software development-related. There are at least a few thousand other articles with a similar motif (424,000 results for “grief stages software” according to Google). But bear with me for the next 5 minutes and I promise you’ll get something out of this — if nothing else, at least the smirk of those who read their past follies put on text by someone else.
I have been working on a rather big Rails project for the past year and half. The project is nearly 7 years old, and has an all-too-common successful-startup-bought-by-industry-giant background story. In a project with this kind of background, some things are bound to happen: many developers of many skill ranges have come and gone, many software fads (cough, Meteor, cough), and above all else a lot of legacy code that is, well, let’s put it nicely, not so great. None of this should be taken personally in any way — it is just natural for such things to occur in such projects.
We got a 6-year-old Rails app with ~370k LOC and a ~6k-test suite which took 24 minutes to complete. Not good! We took a few days off of the main project to see if we could make things better.
More often than not, test suites are the nasty underbelly of a Rails app. Size and age just aggravate the problem. Tests are seldom a high priority in any project, and speed might not be an issue at all in smaller apps where the whole test suite might take just a few seconds to complete. As the project grows and the CI takes increasingly longer to complete, spec speed suddenly becomes more of an issue.
“Small” and “new” are not exactly the case for a certain Rails project we’re working on here at Guava. We’re talking about a 6-year-old e-commerce portal with ~370k LOC, a couple million customers and a ~6k-test, 300-spec suite which took, on average, a whopping 24 minutes to complete in our CI. Not good! So we took a couple of days off the main project to see if we could make things better — or less worse.
Providing feedback is one of the most important parts of any software. Unfortunately, more often than not we tend to downplay or ignore the very simple yet crucial task of letting the user know what is going on. In this article I’ll use a short cautionary tale of how the lack of proper user feedback (and some laziness, I admit) almost cost me an entire HDD with years of personal data.
What happens when your project’s RDBMS is just not enough to deal with unexpectedly huge amounts of data?
You could try to de-normalize some tables here and there to avoid unnecessary JOINs, create a few indexes, implement some kind of pagination or even pre-process the data into a more palatable format. However, if you did all that and it still was not enough, the “natural impulse” is to give up on the RDBMS altogether and just use Elasticsearch. Sounds like a no-brainer, right?
Being clever is a good thing for a developer. Ingenuity allows us to write software that solves complex real-world problems. However, “clever” code is not always a good thing. In many cases — I dare say in most cases — it is a very bad thing. I consciously try to avoid writing code that might be seen as “clever”. The smart thing to do is trying hard not to be smart (yes, very 1984).
Developers tend to see themselves (quite indulgently) as smart people. Not many people understand what we do, and society sees a developer as a kind of modern wizard, writing unreadable magic spells in a small metal box. In reality, though, we are not half as smart as we think: for instance, if you are a developer, you are certainly familiar with the frustration of trying to understand some cryptic piece of code that seemed perfectly reasonable and straightforward when you wrote it a couple of months earlier.
Figure 1
How do old-time languages such as C, Fortran and others survive in a world with Python, Ruby and so on?
There is plenty legacy code still around which need maintaining, of course. And there are (will always be?) a few specific applications where low level is needed. But one of the great things with software is building upon old stuff using new tools, which brings us to our topic today: building a shared library containing some of our C stuff and using it in nice and comfy Python. Figure 1 shows an example of what we can achieve by using graphical tools available in Python to improve our existing code’s text-based output. More on that later on.
For our purposes, we consider shared libraries as a collection of compiled objects condensed into a single file, which may then be called by other software. This is, of course, a simplification. A longer discussion about shared and static libraries can be found in [1].
In our previous installments we implemented two of the most well-known self-balancing binary search trees: AVL and Red-black trees.
We had a few classes on AVL trees in our basic data structures & algorithms class back in college, which made its implementation far less of a challenge than the Red-black tree. So besides the fundamental guidance of CLRS I had to do quite some googling to get it working. While googling I noticed there were quite a lot of questions about which (AVL or RB) tree was “better” in some sense, be it insertion, search time, deletion time, etc. Most textbooks and articles dismiss this question just by stating the factor differences in either trees’ worst case heights, as we briefly mentioned in the past installment. If you’re anything like me, however, you’ll want to see some comparisons where the trees are actually tested. So I decided to do some simple benchmarking to test those theoretical worst-cases. Here’s what I found out.
In our last installment on trees, we studied and implemented the AVL tree. The AVL tree is one of many self-balancing binary search trees, a special kind of BST that enforces sub-linear operation costs by maintaining tree height close to the theoretical minimum of $latex log_{2}(n)$. This is usually done by what is called tree rotation, which is basically moving around tree nodes (and updating some special node properties).
As you can see in the Wikipedia page¹, AVL trees guarantee that the tree height is strictly less than $latex \approx 1.44~log_{2}(n)$, while Red-black trees have a slightly worse threshold of $latex \approx 2~log_{2}(n)$; thus, AVL trees will provide significantly better search times than Red-black trees. However, while AVL trees may need to do $latex O(log(n))$ rotations after each insertion, Red-black trees must do at most 2 rotations per insertion. So either one may be your tree of choice depending on the application: if search time is critical but data doesn’t get updated too often, an AVL tree will perform better; whereas a Red-black tree will perform better in scenarios where data is constantly being changed.
Self-balancing BSTs add some kind of property to tree nodes that make way for tree balancing: with AVL trees, it was the “balance factor”. With Red-black trees, a “color” property is added to each node. This leads us to the Red-black tree properties:
In the previous installment we built a simple text generator using some Ruby meta-programming tricks. It was still far from being our desired context-free grammar (CFG) generator, though, since it lacked many CFG prerequisites. Most flagrantly, we had no rule recursion and only one production (rule definition) per rule. Here’s the what a script that would use both features:
dictionary
noun 'dog', 'bus'
verb 'barked', 'parked'
preposition 'at'
rule 'phrase'
opt 'The', noun, verb, preposition, 'a', noun
opt 'Here goes some', phrase, 'recursion.'
opt 'Meet me', preposition, 'the station.'
grammar phrase: 10The dictionary section is just as we left it. Let’s see what changed in the rule section.
I’ve been working with Ruby for nearly a year now, which means I’m starting to feel the urge to tell people how awesome the language is. One of the most interesting aspects of Ruby to me is metaprogramming, which it seems to have quite a vocation for.
Since college I have a fondness for automata and formal languages theory. One of the topics I particularly like is text generation (if you haven’t already, check out the excellent SCIgen and the Dada engine), so I thought that building a Context-free grammar (CFG)-like text generator in Ruby would be a nice little exercise and an opportunity to use some of the language’s coolest features. Also I’ve implemented one of those using Java several years ago, and it was a mess, so I was curious as to how much of an improvement would Ruby offer.
Suppose the following script:
dictionary 'noun', 'dog', 'bus'
dictionary 'verb', 'barked', 'parked'
dictionary 'preposition', 'at'
rule 'phrase', 'noun', 'verb', 'preposition', 'noun'
codex 'phrase'We’d like dictionary to store some words according to their classes, and rule to define a specific ordering of words. For now let’s not worry about codex (it’s just a collection of rules).
At this point the seasoned programmer is mentally sketching some kind of text parser. It’s an okay solution, but isn’t there something nicer we can do? Well, there is: DSLs! In fact, Ruby is quite an excellent tool to build a DSL, and many famed Ruby-powered applications such as Rspec (and many others) define some kind of DSL.
Masters classes started a few weeks ago, taking their toll on my productivity here. Sorry about that!
So we (pardon the nosism, but I think it sounds less egocentric than writing “I” all the time) hinted at AVL trees back on our Trees, Part I post. Specifically, we learned that:
a binary search tree (BST), provides O(h) time search, insert and delete operations (h is the tree height.
Linear time (O(h)) doesn’t sound very good - if h is close to n, we’ll have the same performance as a linked list. What if there were a way to bound the tree height to some sub-linear factor? As it turns out, there are several ways to do so, and the general idea of somehow keeping the tree height limited to a certain factor of the number of elements it holds is called height balancing. Ergo we’ll want to look into (height) balanced/self-balancing binary search trees **(BBST). **
Burger
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D . I Y
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FAVL tree
Since binary search trees have at most two children, the best tree height (i.e. smallest) we can achieve is log2 n (n being the number of elements in the tree). There are several self-balancing BSTs developed over the years. It seems that up there in the US college professors tend to prefer the red-black tree when studying BBSTs, whilst over here AVL is preferred. In any case, AVL tree was the first BBST ever devised, so we’ll adopt it as our BBST model.
AVL trees (named after its two Soviet inventors Adelson-Velsky and Landis) use a series of rotations to keep the tree balanced. To keep track of when a certain subtree rooted at some node needs to be rotated, we maintain (or calculate) a balance factor variable for each node, which is the difference between the node’s left and right children’s heights, i.e.:
balance_factor(n) = n.left_child.height - n.right_child.height
Now that we have a way to represent graphs, we can discuss one of the most important problems in graph theory: the shortest path problem (SPP). More or less formally, we’ll define SPP as:
Given a weighted graph G(V,E), find the sequence P = {v0, v1, v2, …, v(n-1)}, vi ∈ V, from vertex V0 to vertex V(n-1), such that the list of edges EP = {(v0,v1), (v1,v2), … (v(n-2), v(n-1))} exists and the summation of costs of all elements e ∈ EP is the smallest possible.
In other words, find the less expensive (ergo “shortest”) path between two vertices.
The trivial solution is using BFS starting at vertex A and stopping when it reaches vertex B. However, BFS doesn’t look at the edge costs: it calculates the path with least edges, not the path with least total cost.
Although not necessarily the fastest, Dijkstra’s algorithm is probably the most popular way to solve the shortest path problem due to its simplicity and elegance. The algorithm relies heavily on priority queues, so make sure to take a look at that if you haven’t already.
Pseudocode
dist[from] = 0
for v : G
if v != source
dist[v] = infinity
prev[v] = -1
PQ.add(v, dist[v])
while PQ.hasNext()
u = PQ.pop()
for each neighbor v of u
alt = dist[u] + length(u, v)
if alt < dist[v]
dist[v] = alt
prev[v] = u
PQ.decrease_key(v,alt)
return prevrender(‘/image.*’, caption: ‘Bright green tree - Waikat’, src: ‘//upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Bright_green_tree_-Waikato.jpg/512px-Bright_green_tree-Waikato.jpg)](http://commons.wikimedia.org/wiki/File%3ABright_green_tree-_Waikato.jpg)
We used trees to build the heap data structure before, but we didn’t bother with the theory behind trees, which are abstract and concrete data structures themselves. There’s a huge range of material to cover so I’ll split this in several posts.
In this first post we’ll cover the basic theory and implement a binary search tree (BST), which provides O(h) time search, insert and delete operations (h is the tree height). First, the basics:
Trees are graphs with a few extra properties and interpretations/conventions. * Trees have height (longest branch length) and depth (distance to root). * The uppermost level consists of at most one node (the tree root). * All nodes may have children. * There are no edges other than parent-child edges.
Trees are classified according to some of those properties above and some others we’ll mention later. Most commonly, there is a constraint to the maximum number of children per node -e.g. the binary tree limits children to 2 per node.
Mathematically, a graph is a set of vertices and edges, thus a graph G is usually written as G(V,E). Besides linking vertices in the graph, edges can also carry a specific value which may be interpreted as cost, weight, distance etc.
In computer science, we’re interested in the (abstract) data structure used to implement the graph mathematical concept. Let’s first discuss the basic elements in a graph - vertices and edges:
typedef struct vertex
{
unsigned long id;
int status;
double x,y;
void* data;
} vertex;Vertices should be able to hold any kind of data, so we’ll just throw in a void pointer for that. Other than that we have an id, status (marked or unmarked - more on that later) and 2D coordinates so we can draw the vertices somewhere.
typedef struct edge
{
vertex* from, *to;
int cost;
} edge;Edges consist of just pointers to the vertices they link and an optional value used as weight, distance, cost etc. Strictly speaking we could use a void pointer for that value as well, as long as we also defined a comparison function. But let’s save the hassle and just use an integer instead - most algorithms will be fine with that.
Priority queues (PQs) are abstract data types that work just like regular stacks, but the popping order depends on each element’s priority instead of the sequence they were pushed onto the queue (FIFO or LIFO).
The naïve way of implementing a PQ consists of using an unsorted list or array and searching for the highest-priority element at each pop, which takes O(n) time. There are several more efficient implementations, of which the most usual is the heap.
Heaps are complete (i.e. all levels except possibly the last are filled) binary trees that work as PQs by maintaining the following property: children nodes always have a smaller priority than their parent, i.e. for any node A with children B and C, priority(B) < priority(A) && priority(C) < priority(A). Note that there is no assumed relation between siblings or cousins.
Each element of a heap has two pieces of information: a key and a value, hence we call them key-value (KV) pair. The key identifies the specific element, and the value determines the element’s priority within the heap. Heaps can be min-heaps (low value = high priority) or max-heaps (high value = high priority).
Several times I find myself wanting to visualize something in 2D, but can’t bother to fire up OpenGL or other cumbersome API.
So I wrote a simple program which I called BurgerGFX, with 2 core functionalities: draw point and draw line. I find this to be quite enough for simple applications such as viewing a graph.
Setting up the drawing canvas, which I call burger, is simple: call create(width, height), which returns a pointer to the burger. Then simply call the draws, prints and cleans as needed.
Using our implementation of a doubly linked (DL) list, we can very simply build the most basic LIFO (last in, first out) data structure: the stack.
Stacks have two basic operations: push and pop. Push pushes data onto the stack (i.e., end of the DL list) and pop pops data off the list’s tail, which is only possible because we can set the new tail as tail->prev, since we’re using a DL list, with previous pointers. Another useful function is peek, which returns a pointer to the stack’s top.
A doubly linked list is like our previously implemented Linked List, but instead of only having pointers to the next element, it also has pointers to the _previous _element:
This property makes the doubly linked list very useful as a base for other data structures such as the stack: having a previous pointer means we can quickly (O(1)) remove objects from the list’s tail, which would be impossible with a linked list.
We won’t discuss implementation since it so similar to a linked list. If anything implementation is even simpler than a linked list because of the previous pointer access.
Very simple Vector implementation with add, add_all, get and delete operations using arrays of void pointers.
The downside to this as compared to simply using an array is that here we have an array of pointers, which means the data will most likely be scattered over the memory, not coalesced.
Mergesort is an important sorting algorithm when you don’t have efficient random memory access, since it doesn’t rely on that and has good time complexity - O(n logn) specifically.
As a typical divide-and-conquer algorithm, Mergesort has two steps: first it recursively splits the lists in two until each half is unitary, then it recursively mends back the lists until it reaches the original size.
But before we dive into the actual algorithm, we need to make some changes to the linked list algorithm we’ll be using.
Here’s a very simple implementation of the linked list data structure.
A pointer to the head element is enough to define a linked list. Each element consists of one pointer to the subsequent element in the list and one pointer to the element’s data:
