Unique Value Utilities

[akleeman]

A common problem is to start with some data, and wonder which unique fields the data contains. You might, for example, do

struct Foo {
  double bar;
}

std::set<double> unique_bars(const std::vector<Foo> &foos) {
  std::set<double> bars;
  for (const auto &f : foos) {
    bars.emplace(f.bar);
  }
  return bars;
}

this PR adds some helper functions which let you do things like:

auto get_bar = [](const auto &f) { return f.bar; };
const auto bars = unique_values(foos, get_bar);

If you think that all the values in a vector contain a shared field and you'd like to get the value (and assert that there is in fact only one) you can do:

auto get_bar = [](const auto &f) { return f.bar; };
const double unique_bar = unique_value(foos, get_bar);

or if there are lots of duplicate Foo types in the vector (and Foo has a < operator) you can do:

const auto unique_foos = unique_values(foos);

the function was named unique_values to avoid conflicting with std::unique which doesn't quite do the same thing. std::unique will return a new iterator which has collapsed neighboring unique values, for example:

  std::vector<int> foo = {1, 3, 2, 2, 1, 5};

  std::vector<int> copy(foo);
  const auto last = std::unique(copy.begin(), copy.end());
  copy.erase(last, copy.end());
  // copy = 1,3,2,1,5

  const auto unique_vals = unique_values(foo);
  // unique_vals = 1, 2, 3, 5

[peddie]

Good to capture this pattern.

[peddie]

Minor nit: I don't know whether we have a general policy on this, but so far I've often found asserts like assert(values.size == 1 && "expected only one unique value") to be helpful.