const { createSelector } = require('reselect')

const state = {
  things: {
    byId: {
      a: { name: 'alfa', parent: null, children: ['b', 'c'] },
      b: { name: 'bravo', parent: 'a', children: [], },
      c: { name: 'charlie', parent: 'a', children: [] },
    },
    ids: ['a', 'b', 'c'],
  }
}

const thingSelector = createSelector([state => state.things.byId, state => state.things.ids],
  (byId, ids) => {
    byId = ids.reduce((_byId, id) => { _byId[id] = { ...byId[id] }; return _byId }, {})
    return ids.map(id => {
      const thing = byId[id]
      return Object.assign(thing, { parent: byId[thing.parent], children: thing.children.map(c => byId[c]) })
    })
  }
)

const things = thingSelector(state)

function printThings(things, parent, prefix = '') {
  things.filter(t => t.parent == parent).forEach(t => {
    console.log(prefix + t.name)
    printThings(t.children, t, prefix + '  ')
  })
}

printThings(things)