stdlib: better Hash for Float64/Float32

fixes #90

dora/stdlib/primitives.dora

@@ -350,7 +350,10 @@ impl Float32 {
   @pub @internal fun isNan(): Bool;
   @pub @internal fun sqrt(): Float32;
 
-  @pub fun hash(): Int32 = self.asInt32();
+  @pub fun hash(): Int32 =
+    // discard the sign, such that 0.0 and -0.0 hash the same, at the cost of all positive numbers
+    // and their negative counterparts hashing the same, but keeping the computation branch-free.
+    self.asInt32() & 0x7FFFFFFFi32;
 
   // should be lets, not funs
   @pub @static fun bits(): Int32 = 32i32;
@@ -405,7 +408,12 @@ impl Float64 {
   @pub @internal fun isNan(): Bool;
   @pub @internal fun sqrt(): Float64;
 
-  @pub fun hash(): Int32 = self.asInt64().toInt32();
+  @pub fun hash(): Int32 = {
+    // discard the sign, such that 0.0 and -0.0 hash the same, at the cost of all positive numbers
+    // and their negative counterparts hashing the same, but keeping the computation branch-free.
+    let hash = self.asInt64() & 7FFFFFFFFFFFFFFF;
+    (hash ^ (hash.shiftRight(32))).asInt32
+  }
 
   // should be lets, not funs
   @pub @static fun bits(): Int32 = 64i32;

tests/stdlib/hashmap-contains-zero.dora

@@ -9,8 +9,8 @@ fun float64(): Unit {
   assert(map.size() == 1);
   assert(map.contains(0.0));
   assert(map.get(0.0).getOrPanic() == "a");
-  assert(map.contains(-0.0)); // FIXME: this only works by coincidence, because the hash function sucks
-  assert(map.get(-0.0).getOrPanic() == "a"); // FIXME: this only works by coincidence, because the hash function sucks
+  assert(map.contains(-0.0));
+  assert(map.get(-0.0).getOrPanic() == "a");
 }
 
 fun float32(): Unit {
@@ -19,6 +19,6 @@ fun float32(): Unit {
   assert(map.size() == 1);
   assert(map.contains(0.0f32));
   assert(map.get(0.0f32).getOrPanic() == "a");
-  assert(map.contains(-0.0f32).not()); // FIXME: should be true instead
-  // assert(map.get(-0.0f32).getOrPanic() == "a"); // FIXME: should be true instead
+  assert(map.contains(-0.0f32));
+  assert(map.get(-0.0f32).getOrPanic() == "a");
 }

tests/stdlib/hashset-contains-zero.dora

@@ -8,13 +8,13 @@ fun float64(): Unit {
 
   assert(set.size() == 1);
   assert(set.contains(0.0));
-  assert(set.contains(-0.0)); // FIXME: this only works by coincidence, because the hash function sucks
+  assert(set.contains(-0.0));
 }
 
 fun float32(): Unit {
   let set = std::HashSet[Float32]::new(0.0f32);
 
   assert(set.size() == 1);
   assert(set.contains(0.0f32));
-  assert(set.contains(-0.0f32).not()); // FIXME: should be true instead
+  assert(set.contains(-0.0f32));
 }