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

entrypoint!(process_instruction);

pub fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    let accounts_iter = &mut accounts.iter();
    let account = next_account_info(accounts_iter)?;

    if account.owner != program_id {
        msg!("Account does not have the correct program id");
        return Err(ProgramError::IncorrectProgramId);
    }

    let mut data = account.try_borrow_mut_data()?;
    let amount = instruction_data
        .get(..8)
        .and_then(|slice| slice.try_into().ok())
        .map(u64::from_le_bytes)
        .ok_or(ProgramError::InvalidInstructionData)?;

    if instruction_data[0] == 0 {
        // Deposit
        *data.get_mut(0..8).unwrap() += amount.to_le_bytes();
        msg!("Deposited {} lamports", amount);
    } else if instruction_data[0] == 1 && account.is_signer {
        // Withdraw (only by owner)
        let balance = u64::from_le_bytes(data.get(0..8).unwrap().try_into().unwrap());
        if balance < amount {
            return Err(ProgramError::InsufficientFunds);
        }
        *data.get_mut(0..8).unwrap() -= amount.to_le_bytes();
        msg!("Withdrew {} lamports", amount);
    }

    Ok(())
}