Source
pub fn next_f64(random: Random, comptime tables: ZigTable) f64 {
while (true) {
// We manually construct a float from parts as we can avoid an extra random lookup here by
// using the unused exponent for the lookup table entry.
const bits = random.int(u64);
const i = @as(usize, @as(u8, @truncate(bits)));
const u = blk: {
if (tables.is_symmetric) {
// Generate a value in the range [2, 4) and scale into [-1, 1)
const repr = ((0x3ff + 1) << 52) | (bits >> 12);
break :blk @as(f64, @bitCast(repr)) - 3.0;
} else {
// Generate a value in the range [1, 2) and scale into (0, 1)
const repr = (0x3ff << 52) | (bits >> 12);
break :blk @as(f64, @bitCast(repr)) - (1.0 - math.floatEps(f64) / 2.0);
}
};
const x = u * tables.x[i];
const test_x = if (tables.is_symmetric) @abs(x) else x;
// equivalent to |u| < tables.x[i+1] / tables.x[i] (or u < tables.x[i+1] / tables.x[i])
if (test_x < tables.x[i + 1]) {
return x;
}
if (i == 0) {
return tables.zero_case(random, u);
}
// equivalent to f1 + DRanU() * (f0 - f1) < 1
if (tables.f[i + 1] + (tables.f[i] - tables.f[i + 1]) * random.float(f64) < tables.pdf(x)) {
return x;
}
}
}