decode-django

Decode in Django

Components:

• Decoding logic
• API wrapping decoding logic
• End-user web page

The components fit together as follows:

[web page -> api -> decoding logic]

Trying it out

Requires Python 3 with virtualenv.

$ cd decode-django
$ virtualenv-3.7 venv37
$ . ./venv37
$ pip install -r ./requirements.txt
$ make serve

Then open a browser and navigate to http://localhost:8000/app/web/.

Encoding/Decoding

First lets dig into the encoding/decoding algorithm as it is lies at the core of the app and is probably the most challenging. Consider that a simple ASCII-like character encoding exists with the following mapping:

• 'A' = 1
• 'B' = 2
• ..
• 'Z' = 26

Let's quickly look at some sample encodings:

• "BBF" -> 226
• "AB" -> 12
• "L" -> 12

You probably already see an issue lurking here. Both "AB" and "L" are encoded in the same way. This makes decoding the result quite the challenge.

The decoding of encoded strings assumes we're only dealing with numerical digits (as per the encoding). The decoding algorithm takes an encoded string and returns all the different ways the input could possibly be decoded. For example, the input string, "226" can be decoded in 3 different ways.

• (2, 2, 6) or "BBF"
• (2, 26) or "BZ"
• (22, 6) or "VF"

To get the combinations I've used a tree-based approach. Perhaps there are other ways and I'd be interested in hearing about them. Here goes!

We'll use "1234" as our input string. Let's see if we can break the problem down a bit, ah la "divide and conquer". We know the encoding only goes up to 26 and so we only need to consider single and double-digit combinations. So 12 is valid but 123 is definitely falls outside of the encoding. So looking at the first two digits (12) we can say it is encoded as either 1 (followed by 2 or 23) or 12 (followed by 3 or 34). We can apply the same logic to 2... it is followed by either 3 or 34.

{{ Tree Diagram here }}

Recursion

Each node has an optional left and right edge/node.

You'll notice that we always have a possible decoding by taking each digit individually. This also gives an indication of the depth of the tree. So the depth is equal to the length of the input string.

Each unique path from root to leaf is a possible decoding. Traversing the tree could be done after it has been built but can also be done while building the tree.

API

The decoding logic is exposed via a simple HTTP API. There is a single endpoint which accepts the input string and returns all possible decodings.

Request:

GET /api/decode/?input=226

Response:

200 OK

{
  "result": [
    "BBF",
    "BZ",
    "VF"
  ]
}

If the input string contains invalid input a 400 status code is returned.

End-user Web Page

An end-user may navigate their browser to a web page and provide an input string for decoding. On submitting the input an API call is made to retrieve all decodings which are then itemized on the page.

Bugs and Limitations

For now the input string is capped to 30 characters. Still need to re-evaluate the maximum length of the input string. There are a number of factors to consider here:

• The decoding algorithm, i.e. run time, recursion limit (call stack)
• Requests timing out
• The maximum size of an HTTP request
• The actual end-user use case

ToDo

• Get clarification if nodes > 27 (outside of encoding range) may be eliminated. Done => Eliminate them.
• API error handling, i.e. timeouts
• Add property based testing for codec