Notes

FA2

TZIP-21 (https://gitlab.com/tezos/tzip/-/blob/master/proposals/tzip-21/tzip-21.md)

TZIP-16 (https://gitlab.com/tezos/tzip/-/blob/master/proposals/tzip-16/tzip-16.md)

TZIP-12 (https://gitlab.com/tezos/tzip/-/blob/master/proposals/tzip-12/tzip-12.md)
- interface
  single token type & multi token type
- token transfer semantics
- metadata
  token & contract level

token_type - (token_contract_address, token_id [nat])
(if single token_type then token_id - 0n)

FA2 contract <- list of parameters (single operation query) - must not be reordered,
deduplicated, and processed in the same order

balance_of - empty - call callback with empty response

-- Interface Specification --
entrypoints - transfer, balance_of, update_operators

-- `transfer`
- transfer [ (`from_` address, `txs` [ (`to_` address, (`token_id` nat, `amount` nat)) ]) ]
1 source & multiple destinations

mint?? burn?? (specialized versions of transfer can be invoked by special administrative address only)

-- Core Transfer Behaviour
- atomic & in order
- must decrement the balance of `from_` & increment the balance of `to_` by amount of transfer
- if tranfer amount > balance of `from_` fail with `FA2_INSUFFICIENT_BALANCE`
- if token_owner does not hold any token of type `token_id` balance is 0 (no token owner can have -ve balance)
- tranfer operation must not add/remove additional transfer (transaction fees)
- 0 amount transfer is normal transfer
- same `from_` & `to_` is normal transfer
- if one of the `toke_id` is not defined fail with `FA2_TOKEN_UNDEFINED`
- transfer must apply transfer permission policy, if logic rejects whole operation must fail
x additonal contraints can be invokes if failed fail with custom mnemonic

-- Default Transfer Policy
- Token owner address must be able to perform transfer of own tokens (Tezos.sender = `from_`)
- An Operator must be permitted to manage specific owners tokens before it invokes a transfer tx (permission must be give before)
- If address that 'invokes' transfer is neither one of permitted operators  not token owner the tx must fail with `FA2_NOT_OPERATOR`
  (If atleast one of transfer is not permitted the whole batch/transaction must fail)

-- `balance_of`
- balance_of (
    `requests` [ (`owner` address, `token_id` nat) ],
    `callback` contract [ (`request` (`owner` address, `token_id` nat), `balance` nat) ]
  ) 

- there can be duplicate `balance_of_requests`
- if account does not hold any tokens , balance = 0
- if one of the `token_id` is not present then fail with `FA2_TOKEN_UNDEFINED`

Operator - Tezos address that originates transfer on behalf of owner
Owner - Tezos address which can hold tokens

-- `update_operators`
- operators are permitted per token_owner & token_id, once permitted the operator can transfer tokens of type belonging to owner
- update_operators [
  | `add_operator` (`owner` address, (`operator` address, `token_id` nat))
  | `remove_operator` (`owner` address, (`operator` address, `token_id` nat))
]

- takes a list of `update_operator` commands, if 2 different commands in list add & remove an operator for same token_owner, token_id
  the last command must take effect
- it is possible to update operator for token that does not have any balance yet
- operator relation is not transitive. c -operator-> b, b -operator-> a, then c cannot transfer a's token on behalf of b

- it is not specified who can `update_operators` but in some implementations (Tezos.sender = owner)

-- Token Metadata --
- used for user facing contexts (wallets, explorers, marketplaces)

-- Token Metadata Values --
https://gitlab.com/tezos/tzip/-/blob/master/proposals/tzip-12/tzip-12.md#token-metadata-values





NFT Tutorial (https://github.com/oxheadalpha/nft-tutorial)

NFT Boilerplate (https://github.com/klarluft/tzklar-boilerplate)

-- add tests for the NFT contract

-- add views to PE contract (token metadata) -- talk about it ask if mint entrypoint is needed?
-- add tests for views

Tests 
- Read all the contracts once
transfer:
1. balance_of empty request, callback with empty response
2. atomic transfer 
    +success - check balances after transfer, 
    +failure
3. transfer amount > balance amount (assert the error)
4. do 0 amount transfer and token_owner cannot have -ve balance
5. same to & from transfer
5. token_id not defined

(tests these in update operators tests)
6. transfer permission policy ??
    + token owner should be able to transfer
    + operator must be able to transfer on behalf of token owner
    + not owner & not operator

balance_of: 
7. duplicate balance_of requests
8. if account does not hold any tokens balance is 0
9. if token_id not present error undefined token

update_operators:
1. add operator - remove - add & do transfer
2. remove operator & do transfer
3. non transitive operator check (should fail)