modules:

module Capability {

    // Capability is responsible for declaring an account's permissions.
    // We define the notion of an owner, minter and blacklisted.
    // Only the owner can assign blacklisted and minter capabilities.

    // The owner is defined by hardcoding its account before publishing the module.

    // -----------------------------------------------------------------

    // Declare owner as a resource. It's only meant to be published once
    resource Owner { }

    // Declare a resource that declares an address's capabilities.
    // Every address using EToken will need to publish a resource themselves.
    // However, only the owner can change its content.
    resource T {
        minter: bool,
        blacklisted: bool,
    }

    // Every account should execute this once before using Capability and EToken module.
    // If the sender is the hardcoded owner, then owner capability is published.
    // Reverts if already published
    public publish() {
        let sender: address;
        sender = get_txn_sender();

        // Publish owner capability if sender is the privileged account
        // Uncomment the following line in production and use a real owner account: if (move(sender) == 0x0) {
        if (true) {
            // Always branch to here when testing, otherwise the test can't complete as the sender address is randomly chosen.
            Self.grant_owner_capability();
        }

        // Publish a new capability with no permissions.
        move_to_sender<T>(T{ minter: false, blacklisted: false });

        return;    
    }

    // Internal function that grants owner capability
    grant_owner_capability() {
        move_to_sender<Owner>(Owner {});
        return;
    }

    // Grants minter capability to receiver, but can only succeed if sender owns the owner capability.
    public grant_minter_capability(receiver: address, owner_capability: &R#Self.Owner) {
        let capability_ref: &mut R#Self.T;

        release(move(owner_capability));

        // Pull a mutable reference to the receiver's capability, and change its permission.
        capability_ref = borrow_global<T>(move(receiver));
        *(&mut move(capability_ref).minter) = true;

        return;
    }

    // Grants blacklist capability to receiver, but can only succeed if sender owns the owner capability.
    public grant_blacklisted_capability(receiver: address, owner_capability: &R#Self.Owner) {
        let capability_ref: &mut R#Self.T;

        release(move(owner_capability));

        // Pull a mutable reference to the receiver's capability, and change its permission.
        capability_ref = borrow_global<T>(move(receiver));
        *(&mut move(capability_ref).blacklisted) = true;

        return;
    }

    // This returns an immutable reference to the owner capability if it exists.
    // Is used by the owner to show ownership to privileged functions.
    // Reverts if owner capability does not exist.
    public borrow_owner_capability(): &R#Self.Owner {
        let sender: address;
        let owner_capability_ref: &mut R#Self.Owner;
        let owner_capability_immut_ref: &R#Self.Owner;

        sender = get_txn_sender();
        owner_capability_ref = borrow_global<Owner>(move(sender));
        owner_capability_immut_ref = freeze(move(owner_capability_ref));

        return move(owner_capability_immut_ref);
    }

    // This returns an immutable reference to the general capability if it exists.
    // Should be used by every account to prove capabilities.
    // Reverts if capability does not exist.
    public borrow_capability(): &R#Self.T {
        let sender: address;
        let capability_ref: &mut R#Self.T;
        let capability_immut_ref: &R#Self.T;

        sender = get_txn_sender();
        capability_ref = borrow_global<T>(move(sender));
        capability_immut_ref = freeze(move(capability_ref));

        return move(capability_immut_ref);
    }

    // Return whether the capability allows minting.
    public is_minter(capability: &R#Self.T): bool {
        let is_minter: bool;
        is_minter = *(&move(capability).minter);
        return move(is_minter);
    }

    // Return true the capability is not blacklisted.
    public is_not_blacklisted(capability: &R#Self.T): bool {
        let is_blacklisted: bool;
        is_blacklisted = *(&move(capability).blacklisted);
        return !move(is_blacklisted);
    }

    // Reverts if capability does not allow minting
    public require_minter(capability: &R#Self.T) {
        let is_minter: bool;
        is_minter = Self.is_minter(move(capability));
        assert(move(is_minter), 0);
        return;
    }

    // Reverts if capability is blacklisted
    public require_not_blacklisted(capability: &R#Self.T) {
        let is_not_blacklisted: bool;
        is_not_blacklisted = Self.is_not_blacklisted(move(capability));
        assert(move(is_not_blacklisted), 0);
        return;
    }
}

module EToken {

    // This module is responsible for an actual eToken.
    // For it to be useful a capability has to be published by using the Capability module above.
    
    // -----------------------------------------------------------------

    import Transaction.Capability;

    // Declare the eToken resource, storing an account's total balance.
    resource T {
        value: u64,
    }

    // Publishes an initial zero eToken to the sender.
    // Should be called once before using this module.
    public publish() {
        move_to_sender<T>(T{ value: 0 });
        return;
    }

    // Mint new eTokens.
    // Reverts if capability does not allow it.
    public mint(value: u64, capability: &R#Capability.T): R#Self.T {
        Capability.require_minter(move(capability));
        return T{value: move(value)};
    }

    // Returns an account's eToken balance.
    // Reverts if an initial eToken hasn't been published.
    public balance(): u64 {
        let sender: address;
        let token_ref: &mut R#Self.T;
        let token_value: u64;

        sender = get_txn_sender();
        token_ref = borrow_global<T>(move(sender));
        token_value = *(&move(token_ref).value);

        return move(token_value);
    }

    // Deposit owned tokens to an payee's address, and destroy the tokens to deposit,
    // Reverts if user is blacklisted.
    public deposit(payee: address, to_deposit: R#Self.T, capability: &R#Capability.T) {
        let payee_token_ref: &mut R#Self.T;
        let payee_token_value: u64;
        let to_deposit_value: u64;

        Capability.require_not_blacklisted(move(capability));

        payee_token_ref = borrow_global<T>(move(payee));
        payee_token_value = *(©(payee_token_ref).value);

        // Unpack and destroy to_deposit tokens
        T{ value: to_deposit_value } = move(to_deposit);

        // Increase the payees balance with the destroyed token amount
        *(&mut move(payee_token_ref).value) = move(payee_token_value) + move(to_deposit_value);

        return;
    }

    // Withdraw an amount of tokens of the sender and return it.
    // This works by splitting the token published and returning the specified amount as tokens. 
    public withdraw(amount: u64, capability: &R#Capability.T): R#Self.T {
        let sender: address;
        let sender_token_ref: &mut R#Self.T;
        let value: u64;

        Capability.require_not_blacklisted(move(capability));

        sender = get_txn_sender();
        sender_token_ref = borrow_global<T>(move(sender));
        value = *(©(sender_token_ref).value);

        // Make sure that sender has enough tokens
        assert(copy(value) >= copy(amount), 1);

        // Split the senders token and return the amount specified
        *(&mut move(sender_token_ref).value) = move(value) - copy(amount);
        return T{ value: move(amount) };
    }
}

script:

// Performs simple testing to crudely verify the published modules above.

import Transaction.Capability;
import Transaction.EToken;

main() {
    let sender: address;
    let owner_capability: &R#Capability.Owner;
    let capability: &R#Capability.T;
    let minted_tokens: R#EToken.T;
    let balance: u64;

    sender = get_txn_sender();

    // Publish initial capability
    Capability.publish();

    // Borrow owner_capability for minter delegation
    owner_capability = Capability.borrow_owner_capability();

    // Delegate itself as a minter
    Capability.grant_minter_capability(copy(sender), move(owner_capability));

    // Borrow general capability for proof of minting capability
    capability = Capability.borrow_capability();

    // Publish an eToken account
    EToken.publish();

    // Mint 100 eTokens and prove minter capability
    minted_tokens = EToken.mint(100, copy(capability));

    // Deposit the freshly minted tokens to itself
    EToken.deposit(move(sender), move(minted_tokens), move(capability));

    // Test that the balance corresponds with the intended behaviour
    balance = EToken.balance();
    assert(move(balance) == 100, 3);

    return;
}

cargo test -p functional_tests eToken