Add selective disclosure example to spec w/ conversion of QueryByExample to JSON pointers

[dlongley]

We will want to eventually add an algorithm that can convert a QueryByExample "example" to a list of JSON pointers that could look something like this:

import jsonpointer from 'json-pointer';

// first perform matching on `@context`, optionally performing
// JSON-LD transformations as desired, then...

// convert `vprQuery.credentialQuery.example` to JSON pointers, modulo
// `@context` field; any VCDM mandatory pointers can be automatically
// added based on VC version
// (e.g., `/issuer`, `/issuanceDate` for VC 1.x)
const object = {...example};
delete object['@context'];
const pointers = _adjustPointers(Object.keys(jsonpointer.dict(object)));

function _adjustPointers(pointers) {
  // ensure `credentialSubject` is included in any reveal because the VCDM
  // requires it; presume that if it isn't present that the entire credential
  // subject was requested and inform the user of this (as always)
  const hasCredentialSubject = pointers.some(
    pointer => pointers.includes('/credentialSubject/') ||
      pointer.endsWith('/credentialSubject'));
  if(!hasCredentialSubject) {
    pointers = pointers.slice();
    pointers.push('/credentialSubject');
  }

  pointers = _pruneShallowPointers(pointers);

  // make `type` pointers generic
  return pointers.map(pointer => {
    const index = pointer.indexOf('/type/');
    return index === -1 ? pointer : pointer.slice(0, index) + '/type';
  });
}

// gets only the deepest pointers from the given list of pointers, for example,
// `['/a/b', '/a/b/c', '/a/b/c/d']` will be pruned to: `['/a/b/c/d']`
function _pruneShallowPointers(pointers) {
  const deep = [];
  for(const pointer of pointers) {
    let isDeep = true;
    for(const p of pointers) {
      if(pointer.length < p.length && p.startsWith(pointer)) {
        isDeep = false;
        break;
      }
    }
    if(isDeep) {
      deep.push(pointer);
    }
  }
  return deep;
}

[dlongley]

Also see related: #38