Commit graph

1542 commits

Author SHA1 Message Date
William Carroll
f99b61305b Commit compiled file, output.css to habit-screens
As you can see, I was previously `.gitignore`-ing this file, but because my
`default.nix` attempts to `cp output.css`, I need that file available.
2020-12-13 14:16:14 +00:00
William Carroll
6b9eae2fa3 Productionize habit-screens
At some point I should document or write a script for how I package Elm projects
with Nix to be deployed on my website. For now, I'm modeling everything after my
previous success LearnPianoChords.
2020-12-13 13:33:49 +00:00
William Carroll
e3c72f3fd6 Add project-local .gitignore
Since the `default.nix` file is specific to my tooling, I'm ignoring it.
2020-12-12 02:59:49 +00:00
William Carroll
cc4f67c388 Add usage instructions to top-level README
Also delete redundant `README` from `server` directory.
2020-12-12 02:49:49 +00:00
William Carroll
8c5e4e77ed Expose functions at API layer
Creating a simple HTTP RESTful API for exposing our `Server.semiprime`
function. It supports some help messages, primitive parsing and error handling,
and singular vs. batch processing of arguments.

For more sophisticated parsing and error-checking, I prefer to use Haskell's
Servant library.
2020-12-12 02:43:40 +00:00
William Carroll
45877a8b9c Include cache hit/miss info in return type
This can be useful downstream for diagnostics.
2020-12-12 02:43:35 +00:00
William Carroll
1a404a58de Expand 10^5 in README
I think it's more readable this way.
2020-12-12 02:41:09 +00:00
William Carroll
686766929a Simple Math tests
Calling `assert` within the `Enum.map` makes the errors more usable.
2020-12-12 01:36:22 +00:00
William Carroll
ee96a818e1 Define Server.semiprime
- Clear the boilerplate that `mix` generated
- Consume `Math.factor` to test which inputs are semiprimes
- Cache all inputs that are semiprimes as soon as we discover that they are
- semiprimes

I considered a couple things related to the Cache:
- Could save space by storing all semiprime factors in a tree. This would make
  the lookups more expensive. Also because the tree's depth would never exceed
  two (because all semiprimes only have two factors), the tree would be quite
  broad, and we may not be saving enough space for the trade to be worthwhile. I
  might be wrong about that though.
- We could consider pre-computing all semiprimes when we start the app, but
  without running some tests firsts, I'm not sure whether or not it's worth the
  trouble.
2020-12-12 01:32:31 +00:00
William Carroll
ab73220280 Define Cache.{list,clear} to help debugging
Since I'm often using `iex` for interactive development, these functions are
useful.
2020-12-12 01:31:51 +00:00
William Carroll
714ec29743 Define Cache and convert app to OTP
Define a simple in-memory key-value store for our cache.

TL;DR:
- Define `Cache` as a simple state-keeping `Agent`
- Define `Sup`, which starts our Cache process
- Define `App`, which starts our Supervisor process
- Whitelist `App` in `mix.exs`, so that it starts after calling `iex -S mix`
2020-12-12 01:04:39 +00:00
William Carroll
c0a88ba7d5 Add moduledoc to Extras
9 out of 10 doctors agree that every module needs a doc. Ask your doctor if
moduledocs are right for you!
2020-12-11 22:48:12 +00:00
William Carroll
9e2fbfde8e Move the habit-screens project into //website
I'd like to eventually deploy this to wpcarro.dev. Coming soon!
2020-12-11 22:47:04 +00:00
William Carroll
3feb8ceb9a Delete //website/habits
Accommodating space for my habit-screens project.
2020-12-11 22:46:37 +00:00
William Carroll
90035da32e Delete //website/habitgarden
This is change #2 in a series of other larger changes...
2020-12-11 22:45:39 +00:00
William Carroll
381c344563 Delete //website/days-of-week-habits
This is one small change in a series of other, larger changes.
2020-12-11 22:45:14 +00:00
William Carroll
1eab926121 Define stubbed default.nix
In case I want to package this project with Nix. For now, it's useful to store
this at the project root because it help my Emacs's `project-find-file`
function.
2020-12-11 22:43:53 +00:00
William Carroll
f1e4582392 Define Math module
Support `Math.factor` and cover it with tests.
2020-12-11 22:43:26 +00:00
William Carroll
6af5e4b82e Define Extras module
I'll use as the host for utility functions needed to extend the stdlib.
2020-12-11 22:42:55 +00:00
William Carroll
6ff814a6d3 Init Elixir project
Starting fresh with...

```shell
mix new server
```
2020-12-11 22:42:16 +00:00
William Carroll
c23a12746c Initialize the Semiprimes Service
This is an exciting take-home assignment because I get to write a service in
Elixir!
2020-12-11 18:52:35 +00:00
William Carroll
aa788b128c Update groceries/list.org
Oh the times they are a-changin'
2020-12-07 20:11:32 +00:00
William Carroll
6c3792e881 Define another function to illustrate Reservoir Sampling
Documenting a few ideas about Reservoir Sampling while it's fresh on my mind.
2020-12-07 20:10:50 +00:00
William Carroll
c6f6d5f33b Fix typo
It's Splitwise... not Transferwise!
2020-12-07 20:10:33 +00:00
William Carroll
9549dbb266 Update BFS impls
I've subtly been implementing breadth-first traversals in graphs
incorrectly. The change is subtle, but updating `seen` needs to happen
immediately after queuing an item.

The results will remain the same, but the runtimes will differ dramatically. I
didn't notice this until I attempted to complete LeetCode's "count islands"
challenge, and LeetCode rejected my solution because it could not finish before
timing out. After looking at other candidates' solutions and comparing them to
mine, I couldn't see any difference... except for this subtle difference.

This SO answer provides a helpful explanation:
https://stackoverflow.com/questions/45623722/marking-node-as-visited-on-bfs-when-dequeuing

The take-away lesson here is to always call `seen.add(..)` immediately after
enqueuing.
2020-11-23 23:21:20 +00:00
William Carroll
c00eed469c Solve "cafe order checker" (again)
Perhaps my fifth iteration of solving this problem.
2020-11-21 16:31:53 +00:00
William Carroll
6ccdb06717 Solve "permutation palindrome" (again)
Python's `collections` library really shines for this problem.
2020-11-21 16:24:14 +00:00
William Carroll
60d7ea5b91 Implement a queue using two stacks
The space cost is O(n). The runtime cost of enqueue is O(1); the runtime cost of
dequeue is O(n). Using the "accounting method", the cost of an item in the
system is O(1). Here's why:

+------------+----------------------------+------+
| enqueue    | push onto lhs              | O(1) |
+------------+----------------------------+------+
| lhs -> rhs | pop off lhs; push onto rhs | O(1) |
+------------+----------------------------+------+
| dequeue    | pop off rhs                | O(1) |
+------------+----------------------------+------+
2020-11-21 16:15:43 +00:00
William Carroll
417d3b5fff Implement a bottom-up fibonacci
The bottom-up solution run in O(n) time instead of O(2^n) time, which the
recursive solution runs as:

```
def fib(n):
    return fib(n - 2) + fib(n - 1)
```

Remember that exponential algorithms are usually recursive algorithms with
multiple sibling calls to itself.
2020-11-21 14:48:12 +00:00
William Carroll
70e74a4027 Solve "linked-list-cycles"
Write a predicate for checking if a linked-list contains a cycle. For additional
practice, I also implemented a function that accepts a linked-list containing a
cycle and returns the first element of that cycle.
2020-11-21 14:47:18 +00:00
William Carroll
cbdac30643 Reimplement bst-checker
Practice makes perfect. See the previous commit for a more details about this
solution.
2020-11-21 14:21:05 +00:00
William Carroll
2b5bbb98ca Refactor existing bst-checker implementation
I believe the previous solution is invalid. This solution works and it should be
more time and space efficient.

Space-wise our stack grows proportionate to the depth of our tree, which for a
"balanced" BST should be log(n). Doing a BFT on a BST results in memory usage of
n because when we encounter the leaf nodes at the final level in the tree, they
will be 1/2 * n for a balanced BST.
2020-11-21 14:14:50 +00:00
William Carroll
1dc6695a47 Solve merge-sorted-arrays (again)
InterviewCake.com has a section on Facebook's interview, so I'm attempting to
solve all of the problems on there even if that means I'm resolving
problems. The more practice, the better. Right?

URL: interviewcake.com/facebook-interview-questions
2020-11-21 13:41:33 +00:00
William Carroll
0ccaa22032 Solve "find duplicate" using a graph
This problem is unusually difficult, but the solution is elegant.
2020-11-21 13:35:05 +00:00
William Carroll
847aad2a14 Implement the Levenstein "edit distance" algorithm
This is the mother of dynamic programming algorithms in my opinion. It computes
the minimal "edit distance" between two input strings where an edit is
considered one of:
  - inserting a character into `a`
  - deleting a character from `a`
  - substituting a character in `a` with a character from `b`

It took me awhile to grok the algorithm, but I implemented this from my
understanding of something that I read ~3 nights prior, so I must've understood
what I read. Good news!
2020-11-20 21:59:18 +00:00
William Carroll
f652ea0be6 Solve "count islands" problem
This morning, I attended the "Interview Club" and was asked this question by the
interviewer in front of ~20 FTEs. While I struggled to fully solve it during the
abridged (i.e. 20 minute) timeslot, I completed the problem afterwards.

Here is my solution.
2020-11-20 21:32:22 +00:00
William Carroll
fa717e8a6f Re-implement suffix_tree function
Create a suffix tree from an input string. This implementation uses a stack to
control the flow of the program.

I expected this attempt to be easier than my first attempt, but surprisingly, it
was similarly difficult. It took me ~30-45 minutes to successfully implement
this function, and I'm still not pleased with the final result.
2020-11-19 21:12:36 +00:00
William Carroll
1088e4143d Implement a suffix tree
While it took me awhile to implement, this exercise was definitely worth
doing. I think there should be a more elegant way to construct the tree using
maybe a stack, but I couldn't find it.

All of this was part of a larger effort to search a string for a variety of
patterns. The solution is to compile the string into a suffix tree and then
search the suffix tree for each of the patterns.

I'm glad I didn't gloss over this exercise.
2020-11-19 00:35:23 +00:00
William Carroll
c0268ed31a Refactor random-choice
Prefer initializing `result` to an empty array of size `m`, which makes the
algorithm a bit more elegant.
2020-11-17 23:54:54 +00:00
William Carroll
751b5327a9 Solve algorithms dealing with randomness
Tonight I learned that random sample where each element in the sampling corpus
has an equal likelihood of being chosen is a brand of algorithms known as
"reservoir sampling".

- Implement random.shuffle(..)
- Implement random.choice(..)

Surprisingly, candidates are expected to encounter problems like this during
interviews.
2020-11-17 22:28:24 +00:00
William Carroll
572fb0fe5f Solve "nearby words" function
Given an input like "gello" suggest an correction like "hello".

This is a proof-of-concept problem for writing a simplistic auto-correction
algorithm for a mobile device.
2020-11-16 17:17:28 +00:00
William Carroll
6989c3a91a Implement the Rabin Karp string matching algorithm
This algorithm is pretty interesting because it runs in linear time with respect
to the length of the `corpus` string. It does this by using a sliding window
hash. This hash -- because it's a sliding window -- runs in constant time for
each iteration; we're only adding and subtracting one character each time and
not re-hashing the whole "window".

When our hashes match, only then do we compare the "window" to the
`pattern`. String comparisons are linear because they compare each character to
each character one at a time. But because we only compare strings when are
hashes match (a check which runs in constant time), this spares us the
performance hit.
2020-11-16 17:14:08 +00:00
William Carroll
a2fa88f561 Prefer mutative variant of delete for HashTable
Instead of calling `filter(..)`.
2020-11-16 17:13:39 +00:00
William Carroll
a457a81bbb Add another solution to the "move zeroes to end" problem
Support the optimally performance solution of which I'm aware.
2020-11-16 17:13:03 +00:00
William Carroll
ff08b723db Solve "find pairs for sum"
I have encountered this problem 3x in the wild thus far:
  1. www.InterviewCake.com
  2. Cracking the Coding Interview
  3. www.Pramp.com
2020-11-16 17:12:05 +00:00
William Carroll
92ab94943e Start working on the "Hard" problems
Firstly, implement a function that adds two arguments together... without using
the `+` operator. I need to drill this problem. Thankfully I took a Coursera
course that taught me how to make a half-adder and a full-adder, but the
recommended solution for this is a bit more difficult.
2020-11-16 17:10:57 +00:00
William Carroll
30f4d6f4a4 Implement a simple hash function and hash table
I was always curious how hashing functions were implemented, so I read about the
"polynomial rolling hash function", and I decided implementing it would be a
good exercise. After writing that, writing a hash table was simple.
2020-11-16 00:35:01 +00:00
William Carroll
363519273a Find the intersection (if any) between two linked lists
As with most linked list questions, this one involves an arcane trick from the
neck-bearded playbook.
2020-11-15 17:42:44 +00:00
William Carroll
c8330adfcb Solve "Move Zeroes to End"
Write a function to modify an array of integers in-place such that all of the
zeroes in the array are at the end, and the order of the other integers is not
changed.
2020-11-15 13:51:46 +00:00
William Carroll
09cd819a70 Include re-roll strategy for rand7
After seeing the solution that my book advocated, I implemented it using
recursion.
2020-11-14 17:36:04 +00:00