refactor: use better PRNG

Author: kanru

src/dictionary/trie.rs

@@ -1,4 +1,5 @@
 use std::{
+    cell::Cell,
     cmp::Ordering,
     collections::VecDeque,
     error::Error,
@@ -1150,10 +1151,7 @@ impl DictionaryBuilder for TrieBuilder {
 
     fn build(&mut self, path: &Path) -> Result<(), BuildDictionaryError> {
         let mut tmpname = path.to_path_buf();
-        let pseudo_random = SystemTime::now()
-            .duration_since(SystemTime::UNIX_EPOCH)
-            .map(|du| du.subsec_micros())
-            .unwrap_or_default();
+        let pseudo_random = rand();
         tmpname.set_file_name(format!("chewing-{pseudo_random}.dat"));
         let database = File::create(&tmpname)?;
         let mut writer = BufWriter::new(&database);
@@ -1165,6 +1163,41 @@ impl DictionaryBuilder for TrieBuilder {
     }
 }
 
+// xoshiro256** PRNG
+//
+// Ref: <https://en.wikipedia.org/wiki/Xorshift#xoshiro256**>
+fn rand() -> u64 {
+    thread_local! {
+        static PRNG_STATE: Cell<[u64; 4]> = SystemTime::now()
+            .duration_since(SystemTime::UNIX_EPOCH)
+            .map(|du| {
+                Cell::new([
+                    du.as_secs(),
+                    du.subsec_millis() as u64,
+                    du.subsec_micros() as u64,
+                    du.subsec_nanos() as u64,
+                ])
+            })
+            .unwrap_or_default();
+    }
+    fn rol64(x: u64, k: u32) -> u64 {
+        x.wrapping_shl(k) | x.wrapping_shr(64 - k)
+    }
+    PRNG_STATE.with(|state| {
+        let mut s = state.get();
+        let result = rol64(s[1].wrapping_mul(5), 7).wrapping_mul(9);
+        let t = s[1].wrapping_shl(17);
+        s[2] ^= s[0];
+        s[3] ^= s[1];
+        s[1] ^= s[2];
+        s[0] ^= s[3];
+        s[2] ^= t;
+        s[3] = rol64(s[3], 45);
+        state.set(s);
+        result
+    })
+}
+
 impl Default for TrieBuilder {
     fn default() -> Self {
         Self::new()