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Build Your Own Blockchain (BYOB) - PART 1

You first need to implement a simple blockchain data structure in blockchain.js. It should support adding a string of text as the data in a block, and printing the contents of the whole blockchain. Make sure the tests pass.

You are provided mychain.js, which is a simple REPL (read-evaluate-print-loop) style program. Edit this program to include your blockchain, and support commands that add text messages to your blockchain and dump its contents.

This program should also load a blockchain from a .json file via the --load=.. command-line parameter, but only if all blocks loaded are valid. Finally, this program should save the blockchain to a .json file via the .save command.

Details

The provided REPL program mychain.js accepts an optional command-line parameter: --load={FILE}, which specifies a JSON file {FILE} to load a blockchain from.

The program expects input from the keyboard in the format: .command other data. The leading . identifies a command. Supported commands:

  • .help: Print the help

  • .q: Quit the program (also: Ctrl+C twice)

  • .add {text}: Add text to the blockchain

  • .print: Print the blocks of the blockchain

  • .save {FILE}: Save the blockchain to {FILE} as JSON.

Tasks

Implement a blockchain data structure in blockchain.js. Required operations:

  • addBlock(data): specifiy a string of text to add as a new block to the blockchain

  • print(): loop through all the blocks and print them as JSON

  • isValid(): validate all the blocks in the chain (including the genesis block)

The blockchain should automatically add a genesis block with "000000" as its special hash.

Each block should include the following fields:

  • hash: the computed hash for the block (see below)

  • prevHash: the hash of the previous block (undefined for the genesis block)

  • index: the numeric index of the block (0-based starting with the genesis block)

  • data: a string of text

  • timestamp: The timestamp the block was created (Date.now())

Block hashes should be computed as the SHA256 hash of a string containing the block's prevHash, index, data, and timestamp fields, each separated by ;.

Verify that the tests all pass.

Edit repl.js to add handlers for the .print and .save commands the same as the .add command is handled.

Put the loaded blocks (when using --load=.. parameter) into the blockchain. Check for validity with isValid(); if invalid, quit with an error message.

Add listeners (via listener.on(..)) for the add/print/save commands, with appropriate calls to the blockchain's methods.

Saving a blockchain should JSON.stringify(..) the blockchain's blocks as an array.