Add function decodeArrayKey

[CJ42]

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• bug report
• feature request
• question about the decisions made in the repository
• question about how to use this project

Summary

erc725.js exposes a function encodeArrayKey but does not provide the equivalent for decoding.

This function would be useful to act similar to decodeMappingKey for array data key for indexes, by returning for instance:

• the full array key name
• the index as a number

Example of how to use this function could look like this:

ERC725.decodeArrayKey(
  '0xdf30dba06db6a30e65354d9a64c6098600000000000000000000000000000005',
  'AddressPermissions[]',
);
// [
//   { 
//      keyName: 'AddressPermissions[]', 
//      index: 5
//   }
// ]

Note: this function should also easily decode large numbers as indexes. For instance:

ERC725.decodeArrayKey(
  '0xdf30dba06db6a30e65354d9a64c60986000000000000000000000000499602d2',
  'AddressPermissions[]',
);
// [
//   { 
//      keyName: 'AddressPermissions[]', 
//      index: 1234567890
//   }
// ]

Other information (e.g. detailed explanation, stack traces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)