LibJS: Add support for floating point modulous

This change implements floating point mod based on the algorithm
used in LibM's fmod() implementation. To avoid taking a dependency
on LibM from LibJS I reimplemented the formula in LibJS.

I've incuded some of the example MDM test cases as well.
This surfaced and issue handling NaN which I've fixed as well.

1 files changed, 8 insertions, 2 deletions

diff --git a/Libraries/LibJS/Runtime/Value.cpp b/Libraries/LibJS/Runtime/Value.cpp
index 901558ecee..13687cb5b3 100644
--- a/Libraries/LibJS/Runtime/Value.cpp
+++ b/Libraries/LibJS/Runtime/Value.cpp
@@ -258,8 +258,14 @@ Value div(Value lhs, Value rhs)
 
 Value mod(Value lhs, Value rhs)
 {
-    // FIXME: It seems like JavaScript should allow modulo for doubles as well(?)
-    return Value(lhs.to_i32() % rhs.to_i32());
+    if (lhs.to_number().is_nan() || rhs.to_number().is_nan())
+        return js_nan();
+
+    double index = lhs.to_number().as_double();
+    double period = rhs.to_number().as_double();
+    double trunc = (double)(i32) (index / period);
+
+    return Value(index - trunc * period);
 }
 
 Value typed_eq(Value lhs, Value rhs)