vlib/math: Add a pure V backend for vlib/math (#11267)

vlib/math/modf.v

@@ -0,0 +1,29 @@
+module math
+
+const (
+	modf_maxpowtwo = 4.503599627370496000e+15
+)
+
+// modf returns integer and fractional floating-point numbers
+// that sum to f. Both values have the same sign as f.
+//
+// special cases are:
+// modf(±inf) = ±inf, nan
+// modf(nan) = nan, nan
+pub fn modf(f f64) (f64, f64) {
+	abs_f := abs(f)
+	mut i := 0.0
+	if abs_f >= math.modf_maxpowtwo {
+		i = f // it must be an integer
+	} else {
+		i = abs_f + math.modf_maxpowtwo // shift fraction off right
+		i -= math.modf_maxpowtwo // shift back without fraction
+		for i > abs_f { // above arithmetic might round
+			i -= 1.0 // test again just to be sure
+		}
+		if f < 0.0 {
+			i = -i
+		}
+	}
+	return i, f - i // signed fractional part
+}