Let’s write a native solana contract that has a

1. Double
2. Half
3. Add
4. Subtract

function that can be applied to a data account on the blockchain.

Assume that the data account needs to be initialised on the client before calling the client with it.

• Go to solana playground - https://beta.solpg.io/
• Create a new native rust contract
• Add imports

use borsh::{BorshDeserialize, BorshSerialize};
use solana_program::{
    entrypoint, 
    account_info::{
        next_account_info,
        AccountInfo
    }, 
    entrypoint::ProgramResult, 
    pubkey::Pubkey,
    program_error::ProgramError,
};

• Define the on chain state data structure

#[derive(BorshSerialize, BorshDeserialize)]
struct CounterState {
    count: u32
}

• Define the instruction

#[derive(BorshSerialize, BorshDeserialize)]
enum Instruction {
    Double,
    Half,
    Add { amount: u32 },
    Subtract { amount: u32 },
}

• Define the main funciton

entrypoint!(process_instruction);
fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8]
) -> ProgramResult {

}

• Get the data account from the accounts array

let mut iter = accounts.iter();
let data_account = next_account_info(&mut iter)?;

if !data_account.is_signer {
    return Err(ProgramError::MissingRequiredSignature);
}

• Parse the instruction