_section: Events

Explain how topics and such work

_subsection: Solidity Topics

How to compute the topic...

_subsection: Logs and Filtering

Example hog logs are used.

Link to provider.getLogs and contract.on

_heading: Filters

Filter are used as a way to query ... efficient, explain bloom filters lightly

A filter may have up to 4 topic-sets, where each topic-set refers
to a condition that must match the log topic in that position (i.e. each
condition is ``AND``-ed together).

If a topic-set is ``null``, a log topic in that position is not filtered
at all and **any value** matches.

If a topic-set is a single topic, a log topic in that position must match
**that topic**.

If a topic-set is an array of topics, a log topic in that position must
match any **one** of the topics (i.e. the topic in this position are ``OR``-ed).


_table: Example Log Matching @style<full>

$TopicABaCD: **[** (topic[0] = A) **OR** (topic[0] = B) **]** **AND**
             **[** (topic[1] = C) **OR** (topic[1] = D) **]**

|  **Topic-Sets**           |  **Matching Logs**                    <<|
|   [ A ]                   | topic[0] = A                          <<|
|   [ A, null ]             |   ^                                     |
|   [ null, B ]             | topic[1] = B                          <<|
|   [ null, [ B ] ]         |   ^                                     |
|   [ null, [ B ], null ]   |   ^                                     |
|   [ A, B ]                | (topic[0] = A) **AND** (topic[1] = B) <<|
|   [ A, [ B ] ]            |   ^                                     |
|   [ A, [ B ], null ]      |   ^                                     |
|   [ [ A, B ] ]            | (topic[0] = A) **OR** (topic[0] = B)  <<|
|   [ [ A, B ], null ]      |   ^                                     |
|   [ [ A, B ], [ C, D ] ]  | $TopicABaCD                       <<|


_code: ERC-20 Transfer Filter Examples @lang<javascript>

// <hide>
const tokenAddress = ethers.constants.AddressZero;
const myAddress = ethers.constants.AddressZero;
const myOtherAddress = ethers.constants.AddressZero;
const id = ethers.utils.id;
const hexZeroPad = ethers.utils.hexZeroPad;
// </hide>

// Short example of manually creating filters for an ERC-20
// Transfer event.
//
// Most users should generally use the Contract API to
// compute filters, as it is much simpler, but this is
// provided as an illustration for those curious. See
// below for examples of the equivalent Contract API.

// ERC-20:
//   Transfer(address indexed src, address indexed dst, uint val)
//
// -------------------^
// ----------------------------------------^
//
// Notice that only *src* and *dst* are *indexed*, so ONLY they
// qualify for filtering.
//
// Also, note that in Solidity an Event uses the first topic to
// identify the Event name; for Transfer this will be:
//   id("Transfer(address,address,uint256)")
//
// Other Notes:
//  - A topic must be 32 bytes; so shorter types must be padded

// List all token transfers  *from*  myAddress
filter = {
    address: tokenAddress,
    topics: [
        id("Transfer(address,address,uint256)"),
        hexZeroPad(myAddress, 32)
    ]
}

// List all token transfers  *to*  myAddress:
filter = {
    address: tokenAddress,
    topics: [
        id("Transfer(address,address,uint256)"),
        null,
        hexZeroPad(myAddress, 32)
    ]
}

// List all token transfers  *to*  myAddress or myOtherAddress:
filter = {
    address: tokenAddress,
    topics: [
        id("Transfer(address,address,uint256)"),
        null,
        [
            hexZeroPad(myAddress, 32),
            hexZeroPad(myOtherAddress, 32),
        ]
    ]
}

_null:

To simplify life, ..., explain here, the contract API


_code: ERC-20 Contract Filter Examples @lang<javascript>

// <hide>
const tokenAddress = "0x6B175474E89094C44Da98b954EedeAC495271d0F"; // DAI
const myAddress = "0x8ba1f109551bD432803012645Ac136ddd64DBA72";
const otherAddress = "0xEA517D5a070e6705Cc5467858681Ed953d285Eb9";
const provider = ethers.getDefaultProvider();
const Contract = ethers.Contract;
// </hide>

const abi = [
  "event Transfer(address indexed src, address indexed dst, uint val)"
];

const contract = new Contract(tokenAddress, abi, provider);

// List all token transfers *from* myAddress
contract.filters.Transfer(myAddress)
//!

// List all token transfers *to* myAddress:
contract.filters.Transfer(null, myAddress)
//!

// List all token transfers *from* myAddress *to* otherAddress:
contract.filters.Transfer(myAddress, otherAddress)
//!

// List all token transfers *to* myAddress OR otherAddress:
contract.filters.Transfer(null, [ myAddress, otherAddress ])
//!

_heading: Other Things? TODO

Explain what happens to strings and bytes, how to filter and retain the value