# ERC-20 / BEP-20 token

**Problem:** Token balances, decimals, transfer and total supply.

**Model:** Every public transaction changes a deterministic state machine. Distinguish token balances from native gas coin, transaction submission from finality, and contract permissions from off-chain operator policy.

**BSC vs Ethereum:** Both execute EVM-compatible contracts, but chain ID, native gas asset, RPC and network finality assumptions differ. ERC-20/BEP-20 interface compatibility does not move balances across networks.

**Safety:** Training implementations are intentionally minimal. For deployed tokens/NFTs prefer audited OpenZeppelin contracts, access control, testing and independent review.

# ERC-20 / BEP-20 token — executable lab

## Bài toán → Vấn đề → Giải pháp
Token balances, decimals, transfer and total supply. A centralized implementation cannot assume on-chain transfers and application accounting are automatically synchronized. Implement the contract with explicit validation, accounting and revert behavior.

## Setup
1. Install Foundry (`forge --version`) and Solidity compiler 0.8.24 (Foundry can download it when online).
2. `cd solution && forge build && forge test -vvv`.
3. Read `src/Token.sol` and `test/Lab.t.sol`, then implement your own version in `starter/`.

## Test plan
- Successful state transition, correct getter values and events where relevant.
- Revert on invalid amount, unauthorized call or duplicate identifier as applicable.
- Add a fuzz test and one failure injection case.

## Expected output
`forge test` exits 0 with `[PASS]` for each included test. This is an expected result, not a verified runtime log.

## Exercises
1. Add tests for two distinct actors and zero addresses.
2. Explain the trust boundary and identify missing production controls.
3. Extend the contract using OpenZeppelin in a separate production-style branch.

## Interview
Q: Why is an event not enough to prove final payment? A: Logs can be removed by chain reorg; verify receipt, confirmations/finality, chain ID, token address, and log identity.
Q: Is this contract production ready? A: No; the examples omit standards compliance, audits, upgrade and operational controls.

# Interview — ERC-20 / BEP-20 token

1. **What does on-chain state guarantee?** Deterministic execution under consensus, not truthful off-chain inputs.
2. **Can a transaction be considered final immediately?** No; account for inclusion, reorg and chain-specific finality.
3. **Why avoid homegrown token contracts in production?** Missing standard methods, approvals, security review and integration edge cases.