Programs Are Stateless

use anchor_lang::prelude::*;

declare_id!("Counter11111111111111111111111111111111111");

#[program]
pub mod counter {
    use super::*;

    pub fn increment(ctx: Context<Increment>) -> Result<()> {
        let counter = &mut ctx.accounts.counter;
        counter.value = counter.value.checked_add(1).unwrap();
        Ok(())
    }
}

// The program's code lives in the program account.
// The COUNTER VALUE lives in a separate account (PDA or user-owned).
#[derive(Accounts)]
pub struct Increment<'info> {
    #[account(mut)]
    pub counter: Account<'info, Counter>,
}

#[account]
pub struct Counter {
    pub value: u64,
}

// To call increment, the user submits a tx with:
//   - program_id: <Counter program pubkey>
//   - accounts: [counter_account_pubkey]
//   - data: <serialized "increment" instruction>
// The program code reads/writes counter_account's data field.
// The program itself stores nothing across invocations.