Capstone: Complete Counter Program

This is the capstone challenge. Build the complete counter program with:

• initialize — create the counter PDA, set to 0
• increment — add 1
• decrement — subtract 1, with underflow protection

Requirements

1. Add a decrement instruction that subtracts 1 from counter.count
2. Use require!(counter.count > 0, ErrorCode::Underflow) to prevent underflow
3. Add a Decrement accounts struct (same shape as Increment — PDA counter + user Signer)
4. Define a custom ErrorCode enum with an Underflow variant

Custom Error Enum

Anchor uses #[error_code] to define program-specific errors:

#[error_code]
pub enum ErrorCode {
    #[msg("Cannot decrement below zero")]
    Underflow,
}

The #[msg] attribute provides the human-readable error message.

This Is It

When this compiles, you've built a real Solana program from scratch using PDAs — the production-standard pattern. The next lesson covers deployment.