# Human Hour: Volume 3 - Introduction to Smart Contracts and Solidity

## Reading smart contracts using Etherscan

Verified vs Unverified contracts:

Verified: Humans Of NFT
https://etherscan.io/address/0x8575B2Dbbd7608A1629aDAA952abA74Bcc53d22A#code

Unverified: Opensea shared Contract
https://etherscan.io/address/0x495f947276749ce646f68ac8c248420045cb7b5e#code

Decompiler: https://ethervm.io/decompile

## The Ethereum Virtual Machine

Runtime env for smart contracts (sandboxed / isolated).

no access to network / filesystem etc.

EVM - 2 kinds of accounts, external accounts (public-private key) and contract accounts.

When a contract is deployed, it gets an account.

## The Ethereum Network

A fun visualizer: https://txstreet.com/v/eth-btc


## Understanding your seedphrase

https://metamask.zendesk.com/hc/en-us/articles/4404722782107-User-guide-Secret-Recovery-Phrase-password-and-private-keys

## What is an RPC node?

An RPC (Remote Procedure Call) Node is a type of server, which you communicate with through an API (application programming interface). When your dapp (or a wallet like metamask) wants to execute a transaction, you don't speak directly to the blockchain. Instead, you're communicating with an RPC node that's responsible for reading blockchain data or sending transactions to the network.

## What is Solidity?

It's a language (which gets compiled down to bytecode). Inspired by c++ and python etc. Similar to OOP (Object Oriented Programming);

## What is a smart contract?

Are digital contracts (programs), stored on the the blockchain.

## What is pragma?

The `pragma` keyword tells the compiler what version of solidity you're using. As bugs are fixed, and new features are introduced, new versions of solidity are released. `pragma` allows you to tell the compiler exactly what version you want to use.

eg. `pragma solidity 0.8.0`

The `^` defines the minimum version the compiler accepts.

## Comments

Comments allow you to make notes (comments) inside the code that aren't processed by the compiler (ie the program does not try and execute those lines).

Eg.

// this is a single line comment

/*
    This is a multi-line comment.
    It can be as long as you want.
*/

## Variables

`uint256` - a whole number (256 bits)

`string` - a string of characters (ie text)

`mapping` - a data-structure that stores key-value pairs

`array` - a "list" of variables

`bool` - variable that can only be `true` or `false`

`address` - an account/contract address

## function visibility

`public` - available inside, outside, and in contracts that inherit from the contract its declared in.

`external` - available outside the contract, ie it cannot be called inside the contract.

`internal` - only available inside the contract and contracts that inherit from it.

`private` - only available in the current contract, not to contracts that inherit from it.

## Using Remix

Remix is an opensource IDE (integrated development environment) for developing and deploying smart contracts to EVM compatible (Ethereum Virtual Machine) Compatible blockchains (such as Ethereum).

https://remix.ethereum.org/#optimize=false&runs=200&evmVersion=null&version=soljson-v0.8.7+commit.e28d00a7.js

## Gas

`1 Ether = 1,000,000,000,000,000,000 Wei`

`1 Ether = 1,000,000,000 Gwei`

`1 Gwei = 1 Giga Gwei`

Calculate gas price from Etherscan TX:

`Gas Limit x Gas Fee` = Gas Usage (GWEI)

Convert GWEI => Ether

Unit Converter: https://eth-converter.com/

Op Codes cost gas, you can pull up a table of op code costs: https://github.com/crytic/evm-opcodes

Contracts can run out of gas to prevent them from getting stuck in an infinite loop.

## Failed Transactions

Examine the state to see the difference (before vs after) of what was actually lost in the transaction.


## Gas Estimator

https://www.blocknative.com/gas-estimator

## OpenZeppelin, ERC721, ERC1155

Openzeppelin contracts: https://docs.openzeppelin.com/contracts/4.x/

OpenZeppelin Wizard: https://wizard.openzeppelin.com/