added some comments and polished up the code a bit

src/seed.rs

@@ -1,5 +1,6 @@
 use std::time::{SystemTime, UNIX_EPOCH};
 
+// Generate a random master seed from time and process id.
 pub fn random_seed() -> u64 {
     let nanos = SystemTime::now()
         .duration_since(UNIX_EPOCH)
@@ -9,10 +10,12 @@ pub fn random_seed() -> u64 {
     mix64(nanos ^ pid.rotate_left(17))
 }
 
+// Derive a deterministic seed for a specific random stream.
 pub fn derive_seed(master_seed: u64, stream_id: u64) -> u64 {
     mix64(master_seed ^ stream_id.wrapping_mul(0x9E37_79B9_7F4A_7C15))
 }
 
+// Mix bits for a 64-bit seed hash.
 fn mix64(mut x: u64) -> u64 {
     x = x.wrapping_add(0x9E37_79B9_7F4A_7C15);
     x = (x ^ (x >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);