This challenge allows you to practice the concepts and techniques learned over the past week and apply them in a concrete project. This Sprint, we learned how hash tables combine two data structures to get the best of both worlds and were introduced into the fascinating world of blockchains. In your challenge this week, you will demonstrate proficiency by solving algorithms in Python using hash tables and add another key feature to your blockchain.

Read these instructions carefully. Understand exactly what is expected before starting this Sprint Challenge.

This is an individual assessment. All work must be your own. Your challenge score is a measure of your ability to work independently using the material covered through this sprint. You need to demonstrate proficiency in the concepts and objectives introduced and practiced in preceding days.

You are not allowed to collaborate during the Sprint Challenge. However, you are encouraged to follow the twenty-minute rule and seek support from your PM and Instructor in your cohort help channel on Slack. Your work reflects your proficiency in Python and your command of the concepts and techniques in related to hash tables and blockchains.

You have three hours to complete this challenge. Plan your time accordingly.

Commit your code regularly and meaningfully. This helps both you (in case you ever need to return to old code for any number of reasons and your project manager.

This sprint challenge is divided up into three parts: Hash tables coding, blockchain coding, and a short interview covering parts of hash tables and blockchain.

Explain in detail the workings of a dynamic array: Dyanmic arrays are arrays with automatic resizing, and adjacent relationship in memory

• What is the runtime complexity to access an array, add or remove from the front, and add or remove from the back?

O(n) add/remove from front, O(1) add/remove from back, add/remove in middle O(n), O(1) access via given index, O(n) to find item

• What is the worse case scenario if you try to extend the storage size of a dynamic array?

O(n)

Explain how a blockchain is structured. What are the blocks, what is the chain? How is the data organized?

Blockchain is a secured chain of blocks that contain specific information grouped together in a network. It is a decentralized combination or network of clients vs a centralized server. Data is separated into these block "containers", that are validated through proof of work and consensus model

Explain how proof of work functions. How does it operate. How does this protect the chain from attack. What kind of attack is possible?

PoW functions are algorithms that confirm transactions and produce new blocks in the chain by introducing an intentionally difficult mathematical problem that can only be solved with brute force (highly dependent on computer power). It is very secure due to the inherent security properties of the block through hashing and the chain's relationship thanks to the consensus model. The chain could be attacked via a 51% attack where superior computing power is involved to fork the chain and invalidate transactions.

For the hash tables portion of the sprint challenge, you'll be working through two algorithm problems that are amenable to being solved efficiently using a hash table. You know the drill at this point. Navigate into each exercise's directory, read the instructions for the exercise laid out in the README, implement your solution in the .py skeleton file, then make sure your code passes the tests by running the test script with make tests.

A hash table implementation has been included for you already. Your task is to get the tests passing (using a hash table to do it). You can remind yourself of what hash table functions are available by looking at the hashtable.py file that is included in each exercise directory (note that the hash table implementations for both exercises differ slightly).

You may not use any advanced, built-in Python functions to solve these problems.

For the blockchain portion of the challenge, you will be writing code for a new miner that will solve a different Proof of Work algorithm than the one we have been working with.

Your goal is to mine at least 16 coins. Keep in mind that with many people competing over the same coins, this may take a long time. By our math, we expect that an average solution should be the first to find a solution at least 20 times in an hour or two of mining.

The grade for this sprint challenge is the average of the number of points received (points/15)