Fix math.fround

math.lua

@@ -152,6 +152,9 @@ function tostring(number, base, digit_function, precision)
 end
 
 -- See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/fround#polyfill
+-- Rounds a 64-bit float to a 32-bit float;
+-- if the closest 32-bit float is out of bounds,
+-- the appropriate infinity is returned.
 function fround(number)
 	if number == 0 or number ~= number then
 		return number
@@ -161,14 +164,18 @@ function fround(number)
 		sign = -1
 		number = -number
 	end
-	local exp = math_floor(math.log(number, 2))
+	local _, exp = math.frexp(number)
+	exp = exp - 1 -- we want 2^exponent >= number > 2^(exponent-1)
 	local powexp = 2 ^ math.max(-126, math.min(exp, 127))
-	local leading = exp < -127 and 0 or 1
-	local mantissa = math_floor((leading - number / powexp) * 0x800000 + 0.5)
-	if mantissa <= -0x800000 then
+	local leading = exp <= -127 and 0 or 1 -- subnormal number?
+	local mantissa = math.floor((number / powexp - leading) * 0x800000 + 0.5)
+	if
+		mantissa > 0x800000 -- doesn't fit in mantissa
+		or (exp >= 127 and mantissa == 0x800000) -- fits if the exponent can be increased
+	then
 		return sign * inf
 	end
-	return sign * powexp * (leading - mantissa / 0x800000)
+	return sign * powexp * (leading + mantissa / 0x800000)
 end
 
 -- Export environment