secret_santa.js

function secretSanta(values, mutuality = 0.5) {
    let offset = 0;
    return values
        .map((val, index) => ({index: index, order: Math.random()}))
        .sort((a, b) => a.order - b.order)
        .map((item, next, items) => {
            [next, offset] = (++next - offset > 1 && items.length - next > 1 && Math.random() < mutuality)
                ? [offset, next]
                : [next < items.length ? next : offset, offset];
            return [values[item.index], values[items[next].index]];
        });
}


// Example output
for (let mutuality=0.0; mutuality <= 1.0; mutuality += 0.5) {
    console.log(`Mutuality ${mutuality}: `, secretSanta(['Alice', 'Bob', 'Craig', 'Dave', 'Eve', 'Fiona', 'Gloria'], mutuality));
}
/* [ [from who, to who], ... ]
Mutuality 0:  [
  [ 'Craig', 'Alice' ],
  [ 'Alice', 'Fiona' ],
  [ 'Fiona', 'Dave' ],
  [ 'Dave', 'Gloria' ],
  [ 'Gloria', 'Bob' ],
  [ 'Bob', 'Eve' ],
  [ 'Eve', 'Craig' ]
]
Mutuality 0.5:  [
  [ 'Gloria', 'Dave' ],
  [ 'Dave', 'Craig' ],
  [ 'Craig', 'Bob' ],
  [ 'Bob', 'Eve' ],
  [ 'Eve', 'Gloria' ],
  [ 'Alice', 'Fiona' ],
  [ 'Fiona', 'Alice' ]
]
Mutuality 1:  [
  [ 'Alice', 'Bob' ],
  [ 'Bob', 'Alice' ],
  [ 'Dave', 'Gloria' ],
  [ 'Gloria', 'Dave' ],
  [ 'Eve', 'Fiona' ],
  [ 'Fiona', 'Craig' ],
  [ 'Craig', 'Eve' ]
]
*/