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const builtin = @import("builtin");
const is_test = builtin.is_test;
const std = @import("std");
const maxInt = std.math.maxInt;
const DBL_MANT_DIG = 53;
pub extern fn __floatuntidf(arg: u128) f64 {
@setRuntimeSafety(is_test);
if (arg == 0)
return 0.0;
var a = arg;
const N: u32 = @sizeOf(u128) * 8;
const sd = @bitCast(i32, N - @clz(a)); // number of significant digits
var e: i32 = sd - 1; // exponent
if (sd > DBL_MANT_DIG) {
// start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
// finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
// 12345678901234567890123456
// 1 = msb 1 bit
// P = bit DBL_MANT_DIG-1 bits to the right of 1
// Q = bit DBL_MANT_DIG bits to the right of 1
// R = "or" of all bits to the right of Q
switch (sd) {
DBL_MANT_DIG + 1 => {
a <<= 1;
},
DBL_MANT_DIG + 2 => {},
else => {
const shift_amt = @bitCast(i32, N + (DBL_MANT_DIG + 2)) - sd;
const shift_amt_u7 = @intCast(u7, shift_amt);
a = (a >> @intCast(u7, sd - (DBL_MANT_DIG + 2))) |
@boolToInt((a & (u128(maxInt(u128)) >> shift_amt_u7)) != 0);
},
}
// finish
a |= @boolToInt((a & 4) != 0); // Or P into R
a += 1; // round - this step may add a significant bit
a >>= 2; // dump Q and R
// a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
if ((a & (u128(1) << DBL_MANT_DIG)) != 0) {
a >>= 1;
e += 1;
}
// a is now rounded to DBL_MANT_DIG bits
} else {
a <<= @intCast(u7, DBL_MANT_DIG - sd);
// a is now rounded to DBL_MANT_DIG bits
}
const high: u64 = @bitCast(u32, (e + 1023) << 20) | // exponent
(@truncate(u32, a >> 32) & 0x000FFFFF); // mantissa-high
const low = @truncate(u32, a); // mantissa-low
return @bitCast(f64, low | (high << 32));
}
test "import floatuntidf" {
_ = @import("floatuntidf_test.zig");
}
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