solana-program-examples/basics/counter/native/program/src/lib.rs

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

mod state;
use state::*;

declare_id!("Fg6PaFpoGXkYsidMpWTK6W2BeZ7FEfcYkg476zPFsLnS");

#[cfg(not(feature = "no-entrypoint"))]
use solana_program::entrypoint;

#[cfg(not(feature = "no-entrypoint"))]
entrypoint!(process_instruction);

pub fn process_instruction(
    _program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    let (instruction_discriminant, instruction_data_inner) = instruction_data.split_at(1);
    match instruction_discriminant[0] {
        0 => {
            msg!("Instruction: Increment");
            process_increment_counter(accounts, instruction_data_inner)?;
        }
        _ => {
            msg!("Error: unknown instruction")
        }
    }
    Ok(())
}

pub fn process_increment_counter(
    accounts: &[AccountInfo],
    _instruction_data: &[u8],
) -> Result<(), ProgramError> {
    let account_info_iter = &mut accounts.iter();

    let counter_account = next_account_info(account_info_iter)?;
    assert!(
        counter_account.is_writable,
        "Counter account must be writable"
    );

    let mut counter = Counter::try_from_slice(&counter_account.try_borrow_mut_data()?)?;
    counter.count += 1;
    counter.serialize(&mut *counter_account.data.borrow_mut())?;

    msg!("Counter state incremented to {:?}", counter.count);
    Ok(())
}