Implement random() and friends

src/header/stdlib/random.rs

@@ -0,0 +1,89 @@
+//! Helper functions for random() and friends, see https://pubs.opengroup.org/onlinepubs/7908799/xsh/initstate.html
+/* Ported from musl's implementation (src/prng/random.c). Does not
+ * currently implement locking, though. */
+
+use crate::platform::types::*;
+use core::{convert::TryFrom, ptr};
+
+#[rustfmt::skip]
+static mut X_INIT: [u32; 32] = [
+    0x00000000, 0x5851f42d, 0xc0b18ccf, 0xcbb5f646,
+    0xc7033129, 0x30705b04, 0x20fd5db4, 0x9a8b7f78,
+    0x502959d8, 0xab894868, 0x6c0356a7, 0x88cdb7ff,
+    0xb477d43f, 0x70a3a52b, 0xa8e4baf1, 0xfd8341fc,
+    0x8ae16fd9, 0x742d2f7a, 0x0d1f0796, 0x76035e09,
+    0x40f7702c, 0x6fa72ca5, 0xaaa84157, 0x58a0df74,
+    0xc74a0364, 0xae533cc4, 0x04185faf, 0x6de3b115,
+    0x0cab8628, 0xf043bfa4, 0x398150e9, 0x37521657,
+];
+
+/* N needs to accommodate values up to 63, corresponding to the maximum
+ * state array size of 256 bytes. I and J must be able to accommodate
+ * values less than or equal to N. */
+pub static mut N: u8 = 31;
+pub static mut I: u8 = 3;
+pub static mut J: u8 = 0;
+
+/* Unlike in C, we can't take the address of the initializing array
+ * outside of a function. */
+pub static mut X_PTR: *mut u32 = ptr::null_mut();
+
+// To be called in any function that may read from X_PTR
+pub unsafe fn ensure_x_ptr_init() {
+    if X_PTR.is_null() {
+        X_PTR = &mut X_INIT[1];
+    }
+}
+
+pub fn lcg31_step(x: u32) -> u32 {
+    1103515245_u32.wrapping_mul(x).wrapping_add(12345_u32) & 0x7fffffff
+}
+
+pub fn lcg64_step(x: u64) -> u64 {
+    6364136223846793005_u64.wrapping_mul(x).wrapping_add(1_u64)
+}
+
+pub unsafe fn save_state() -> *mut u32 {
+    ensure_x_ptr_init();
+    *X_PTR.offset(-1) = (u32::from(N) << 16) | (u32::from(I) << 8) | u32::from(J);
+    X_PTR.offset(-1)
+}
+
+pub unsafe fn load_state(state_ptr: *mut u32) {
+    let prev_x = *state_ptr;
+    X_PTR = state_ptr.offset(1);
+
+    /* This calculation of N does not have a bit mask in the musl
+     * original, in principle resulting in a u16, but obtaining a value
+     * larger than 63 can probably be dismissed as pathological. */
+    N = u8::try_from((prev_x >> 16) & 0xff).unwrap();
+
+    // I and J calculations are straight from musl
+    I = u8::try_from((prev_x >> 8) & 0xff).unwrap();
+    J = u8::try_from(prev_x & 0xff).unwrap();
+}
+
+pub unsafe fn seed(seed: c_uint) {
+    ensure_x_ptr_init();
+
+    let mut s = seed as u64;
+
+    if N == 0 {
+        *X_PTR = s as u32;
+    } else {
+        I = if N == 31 || N == 7 { 3 } else { 1 };
+
+        J = 0;
+
+        for k in 0..usize::from(N) {
+            s = lcg64_step(s);
+
+            // Conversion will always succeed (value is a 32-bit right-
+            // shift of a 64-bit integer).
+            *X_PTR.add(k) = u32::try_from(s >> 32).unwrap();
+        }
+
+        // ensure X contains at least one odd number
+        *X_PTR |= 1;
+    }
+}