Peter Limkilde Svendsen
91 lines
3.4 KiB
Rust
91 lines
3.4 KiB
Rust
//! Helper functions for pseudorandom number generation using LCG, see https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/functions/drand48.html
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use crate::platform::types::*;
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/* The default element buffer for the linear congruential generator's
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* sequence. Implemented using a c_ushort array for consistency between
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* the drand48()/lrand48()/mrand48() and erand48()/nrand48()/jrand48()
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* functions, and with SEED48_XSUBI (see below). */
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pub static mut DEFAULT_XSUBI: [c_ushort; 3] = [0; 3];
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// Used by seed48() (returns a pointer to this array).
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pub static mut SEED48_XSUBI: [c_ushort; 3] = [0; 3];
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/* Multiplier and addend, which may be set through lcong48(). Default
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* values as specified in POSIX. */
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const A_DEFAULT_VALUE: u64 = 0x5deece66d;
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const C_DEFAULT_VALUE: u16 = 0xb;
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pub static mut A: u64 = A_DEFAULT_VALUE;
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pub static mut C: u16 = C_DEFAULT_VALUE;
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/// Used by `srand48()` and `seed48()`.
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pub unsafe fn reset_a_and_c() {
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A = A_DEFAULT_VALUE;
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C = C_DEFAULT_VALUE;
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}
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/// Build a 48-bit integer from a size-3 array of unsigned short.
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///
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/// Pointers to c_ushort can be converted to &[c_ushort; 3] by taking
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/// &*(YOUR_C_USHORT_POINTER as *const [c_ushort; 3])
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///
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/// See also this cbindgen issue for why the stdlib functions can't just
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/// have an xsubi: *mut [c_ushort; 3] parameter:
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/// https://github.com/eqrion/cbindgen/issues/171
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pub fn u48_from_ushort_arr3(arr: &[c_ushort; 3]) -> u64 {
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/* Cast via u16 to ensure we get only the lower 16 bits of each
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* element, as specified by POSIX. */
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u64::from(arr[0] as u16) | (u64::from(arr[1] as u16) << 16) | (u64::from(arr[2] as u16) << 32)
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}
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/// Make a size-3 array of unsigned short from a 48-bit integer.
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pub fn ushort_arr3_from_u48(value: u64) -> [c_ushort; 3] {
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[
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c_ushort::from(value as u16),
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c_ushort::from((value >> 16) as u16),
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c_ushort::from((value >> 32) as u16),
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]
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}
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/// Advances the buffer from the input argument to the next element in
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/// the linear congruential generator's sequence.
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///
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/// Modifies the passed argument in-place and returns the new value as a
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/// u64.
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pub unsafe fn generator_step(xsubi: &mut [c_ushort; 3]) -> u64 {
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let old_xsubi_value: u64 = u48_from_ushort_arr3(xsubi);
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/* The recurrence relation of the linear congruential generator,
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* X_(n+1) = (a * X_n + c) % m,
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* with m = 2**48. The multiplication and addition can overflow a
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* u64, but we just let it wrap since we take mod 2**48 anyway. */
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let new_xsubi_value: u64 =
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A.wrapping_mul(old_xsubi_value).wrapping_add(u64::from(C)) & 0xffff_ffff_ffff;
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*xsubi = ushort_arr3_from_u48(new_xsubi_value);
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new_xsubi_value
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}
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/// Get a C `double` from a 48-bit integer (for `drand48()` and
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/// `erand48()`).
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pub fn f64_from_x(x: u64) -> c_double {
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/* We set the exponent to 0, and the 48-bit integer is copied into the high
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* 48 of the 52 significand bits. The value then lies in the range
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* [1.0, 2.0), from which we simply subtract 1.0. */
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f64::from_bits(0x3ff0_0000_0000_0000_u64 | (x << 4)) - 1.0
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}
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/// Get the high 31 bits of a 48-bit integer (for `lrand48()` and
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/// `nrand48()`).
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pub fn u31_from_x(x: u64) -> c_long {
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(x >> 17) as c_long
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}
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/// Get the high 32 bits, signed, of a 48-bit integer (for `mrand48()`
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/// and `jrand48()`).
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pub fn i32_from_x(x: u64) -> c_long {
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// Cast via i32 to ensure we get the sign correct
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c_long::from((x >> 16) as i32)
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}
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