Hyper-optimized telemetry kata in Python

Overview

This kata complements Clean Code: Advanced TDD, Ep. 20 and Clean Code: Advanced TDD, Ep. 21.

This repository contains two exercises designed to improve your skills in test-driven development.

Instructions

We will work on a telemetry system for a remote control car project. Bandwidth in the telemetry system is at a premium and you have been asked to implement a message protocol for communicating telemetry data.

Data is transmitted in a buffer (byte array). When integers are sent, the size of the buffer is reduced by employing the protocol described below.

Each value should be represented in the smallest possible C integral type (types of char and unsigned char are not included as the saving would be trivial):

From	To	Type
4_294_967_296	9_223_372_036_854_775_807	long
2_147_483_648	4_294_967_295	unsigned int
65_536	2_147_483_647	int
0	65_535	unsigned short
-32_768	-1	short
-2_147_483_648	-32_769	int
-9_223_372_036_854_775_808	-2_147_483_649	long

The value should be converted to the appropriate number of bytes for its assigned type. The complete internal 9-byte buffer comprises three parts:

• prefix byte: a byte indicating the number of the payload bytes in the buffer;
• payload bytes: the bytes holding the integer;
• trailing bytes: the zero-fill bytes to complete the buffer.

To distinguish between signed and unsigned types, the protocol introduces a little trick: for signed types, their prefix byte value is 256 minus the number of payload bytes in the buffer.

Exercise 1

Implement the static method TelemetryBuffer.to_buffer() to encode a buffer taking an integer value passed to the method.

# Type: unsigned short, bytes: 2, signed: no, prefix byte: 2
TelemetryBuffer.to_buffer(5)
# => [0x2, 0x5, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0]

# Type: int, bytes: 4, signed: yes, prefix byte: 256 - 4
TelemetryBuffer.to_buffer(2_147_483_647)
# => [0xfc, 0xff, 0xff, 0xff, 0x7f, 0x0, 0x0, 0x0, 0x0]

Hint

The BitConverter class provides a convenient way of converting integer types to and from arrays of bytes.

Exercise 2

Implement the static method TelemetryBuffer.from_buffer() to decode the buffer received, and return the value in the form of an integer.

TelemetryBuffer.from_buffer([0xfc, 0xff, 0xff, 0xff, 0x7f, 0x0, 0x0, 0x0, 0x0])
# => 2_147_483_647

If the prefix byte is of unexpected value, then return 0.

Integral numbers in C

Note

For type sizes, we assume a typical 64-bit system.

The C language provides a number of types that represent integers, each with its own range of values. The ranges are determined by the storage width of the type as allocated by the system:

Type	Width	Minimum	Maximum
char	8 bit	-128	+127
short	16 bit	-32_768	+32_767
int	32 bit	-2_147_483_648	+2_147_483_647
long	64 bit	-9_223_372_036_854_775_808	+9_223_372_036_854_775_807
unsigned char	8 bit	0	+255
unsigned short	16 bit	0	+65_535
unsigned int	32 bit	0	+4_294_967_295
unsigned long	64 bit	0	+18_446_744_073_709_551_615

Usage

You can import this project into Replit, and it will handle all dependencies automatically.

Prerequisites

• Python 3.8+
• pytest

Run main

make run

Run tests

make test

Credits and references

• https://exercism.org/tracks/csharp/exercises/hyper-optimized-telemetry