Add hashing support compatible with std::hash (#11)

Author: brodeuralexis

c_include/fine.hpp

@@ -65,13 +65,16 @@ inline ERL_NIF_TERM make_atom(ErlNifEnv *env, const char *msg) {
 // A representation of an atom term.
 class Atom {
 public:
-  Atom(std::string name) : name(name), term(std::nullopt) {
+  Atom(std::string name) : name(std::move(name)), term(std::nullopt) {
     if (!Atom::initialized) {
       Atom::atoms.push_back(this);
     }
   }
 
-  std::string to_string() const { return this->name; }
+public:
+  const std::string &to_string() const & noexcept { return this->name; }
+
+  std::string to_string() && noexcept { return this->name; }
 
   bool operator==(const Atom &other) const { return this->name == other.name; }
 
@@ -90,6 +93,8 @@ class Atom {
   }
 
   friend struct Encoder<Atom>;
+  friend struct Decoder<Atom>;
+  friend struct ::std::hash<Atom>;
 
   friend int __private__::load(ErlNifEnv *env, void **priv_data,
                                ERL_NIF_TERM load_info);
@@ -1182,4 +1187,18 @@ inline int load(ErlNifEnv *env, void **, ERL_NIF_TERM) {
 
 } // namespace fine
 
+namespace std {
+template <> struct hash<::fine::Term> {
+  size_t operator()(const ::fine::Term &term) noexcept {
+    return enif_hash(ERL_NIF_INTERNAL_HASH, term, 0);
+  }
+};
+
+template <> struct hash<::fine::Atom> {
+  size_t operator()(const ::fine::Atom &atom) noexcept {
+    return std::hash<std::string_view>{}(atom.to_string());
+  }
+};
+} // namespace std
+
 #endif

test/c_src/finest.cpp

@@ -1,5 +1,6 @@
 #include <cstring>
 #include <exception>
+#include <functional>
 #include <memory_resource>
 #include <optional>
 #include <stdexcept>
@@ -388,6 +389,17 @@ bool compare_ge(ErlNifEnv *, fine::Term lhs, fine::Term rhs) noexcept {
   return lhs >= rhs;
 }
 FINE_NIF(compare_ge, 0);
+
+std::uint64_t hash_term(ErlNifEnv *, fine::Term term) noexcept {
+  return std::invoke(std::hash<fine::Term>{}, term);
+}
+FINE_NIF(hash_term, 0);
+
+std::uint64_t hash_atom(ErlNifEnv *, fine::Atom atom) noexcept {
+  return std::invoke(std::hash<fine::Atom>{}, atom);
+}
+FINE_NIF(hash_atom, 0);
+
 } // namespace finest
 
 FINE_INIT("Elixir.Finest.NIF");

test/lib/finest/nif.ex

@@ -71,5 +71,8 @@ defmodule Finest.NIF do
   def compare_gt(_lhs, _rhs), do: err!()
   def compare_ge(_lhs, _rhs), do: err!()
 
+  def hash_term(_term), do: err!()
+  def hash_atom(_term), do: err!()
+
   defp err!(), do: :erlang.nif_error(:not_loaded)
 end

test/test/finest_test.exs

@@ -359,4 +359,18 @@ defmodule FinestTest do
       assert NIF.compare_ge("fine", "fine")
     end
   end
+
+  describe "hash" do
+    test "term" do
+      for value <- [42, "fine", ["it", %{"should" => {"just", "work"}}], :atom] do
+        assert NIF.hash_term(value) == NIF.hash_term(value)
+      end
+    end
+
+    test "atom" do
+      for value <- [:ok, :error, :"with spaces", Enum, nil, true, false] do
+        assert NIF.hash_atom(value) == NIF.hash_atom(value)
+      end
+    end
+  end
 end