59 lines
2.2 KiB
Zig
59 lines
2.2 KiB
Zig
const Int = @import("std").meta.Int;
|
|
const math = @import("std").math;
|
|
|
|
pub fn intToFloat(comptime T: type, x: anytype) T {
|
|
if (x == 0) return 0;
|
|
|
|
// Various constants whose values follow from the type parameters.
|
|
// Any reasonable optimizer will fold and propagate all of these.
|
|
const Z = Int(.unsigned, @bitSizeOf(@TypeOf(x)));
|
|
const uT = Int(.unsigned, @bitSizeOf(T));
|
|
const inf = math.inf(T);
|
|
const float_bits = @bitSizeOf(T);
|
|
const int_bits = @bitSizeOf(@TypeOf(x));
|
|
const exp_bits = math.floatExponentBits(T);
|
|
const fractional_bits = math.floatFractionalBits(T);
|
|
const exp_bias = math.maxInt(Int(.unsigned, exp_bits - 1));
|
|
const implicit_bit = if (T != f80) @as(uT, 1) << fractional_bits else 0;
|
|
const max_exp = exp_bias;
|
|
|
|
// Sign
|
|
var abs_val = math.absCast(x);
|
|
const sign_bit = if (x < 0) @as(uT, 1) << (float_bits - 1) else 0;
|
|
var result: uT = sign_bit;
|
|
|
|
// Compute significand
|
|
var exp = int_bits - @clz(abs_val) - 1;
|
|
if (int_bits <= fractional_bits or exp <= fractional_bits) {
|
|
const shift_amt = fractional_bits - @intCast(math.Log2Int(uT), exp);
|
|
|
|
// Shift up result to line up with the significand - no rounding required
|
|
result = (@intCast(uT, abs_val) << shift_amt);
|
|
result ^= implicit_bit; // Remove implicit integer bit
|
|
} else {
|
|
var shift_amt = @intCast(math.Log2Int(Z), exp - fractional_bits);
|
|
const exact_tie: bool = @ctz(abs_val) == shift_amt - 1;
|
|
|
|
// Shift down result and remove implicit integer bit
|
|
result = @intCast(uT, (abs_val >> (shift_amt - 1))) ^ (implicit_bit << 1);
|
|
|
|
// Round result, including round-to-even for exact ties
|
|
result = ((result + 1) >> 1) & ~@as(uT, @boolToInt(exact_tie));
|
|
}
|
|
|
|
// Compute exponent
|
|
if ((int_bits > max_exp) and (exp > max_exp)) // If exponent too large, overflow to infinity
|
|
return @bitCast(T, sign_bit | @bitCast(uT, inf));
|
|
|
|
result += (@as(uT, exp) + exp_bias) << math.floatMantissaBits(T);
|
|
|
|
// If the result included a carry, we need to restore the explicit integer bit
|
|
if (T == f80) result |= 1 << fractional_bits;
|
|
|
|
return @bitCast(T, sign_bit | result);
|
|
}
|
|
|
|
test {
|
|
_ = @import("int_to_float_test.zig");
|
|
}
|