feat: finish p21. Implment mt19937.

.gitignore

@@ -1 +1,2 @@
 /target
+site

problems/p21/Cargo.toml

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+[package]
+name = "p21"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

problems/p21/src/main.cpp

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+#include <iostream>
+#include <random>
+
+int main() {
+  std::mt19937 gen(12345);
+  std::cout << gen() << std::endl; // 第一个输出
+  return 0;
+}

problems/p21/src/main.rs

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+// //創建一個長度為624的數組來存儲發生器的狀態
+// int[0..623] MT
+// int index = 0
+//
+// //初始化產生器，種子作為首項內容
+// function initialize_generator(int seed) {
+//     i := 0
+//     MT[0] := seed
+//     for i from 1 to 623 { // 走訪剩下的每個元素
+//         MT[i] := last 32 bits of(1812433253 * (MT[i-1] xor (right shift by 30 bits(MT[i-1]))) + i) // 1812433253 == 0x6c078965
+//     }
+// }
+//
+// // Extract a tempered pseudorandom number based on the index-th value,
+// // calling generate_numbers() every 624 numbers
+// function extract_number() {
+//     if index == 0 {
+//         generate_numbers()
+//     }
+//
+//     int y := MT[index]
+//     y := y xor (right shift by 11 bits(y))
+//     y := y xor (left shift by 7 bits(y) and (2636928640)) // 2636928640 == 0x9d2c5680
+//     y := y xor (left shift by 15 bits(y) and (4022730752)) // 4022730752 == 0xefc60000
+//     y := y xor (right shift by 18 bits(y))
+//
+//     index := (index + 1) mod 624
+//     return y
+// }
+//
+// // Generate an array of 624 untempered numbers
+// function generate_numbers() {
+//     for i from 0 to 623 {
+//         int y := (MT[i] & 0x80000000)                       // bit 31 (32nd bit) of MT[i]
+//                        + (MT[(i+1) mod 624] & 0x7fffffff)   // bits 0-30 (first 31 bits) of MT[...]
+//         MT[i] := MT[(i + 397) mod 624] xor (right shift by 1 bit(y))
+//         if (y mod 2) != 0 { // y is odd
+//             MT[i] := MT[i] xor (2567483615) // 2567483615 == 0x9908b0df
+//         }
+//     }
+// }
+
+struct MT19937 {
+    mt: [u32; 624],
+    index: usize,
+}
+
+impl MT19937 {
+    fn new(seed: u32) -> Self {
+        let mut mt = [0; 624];
+        mt[0] = seed;
+        for i in 1..624 {
+            mt[i] = 0x6c078965u32
+                .wrapping_mul(mt[i - 1] ^ (mt[i - 1] >> 30))
+                .wrapping_add(i as u32)
+        }
+        let index = 0;
+        MT19937 { mt, index }
+    }
+
+    fn extract_number(&mut self) -> u32 {
+        if self.index == 0 {
+            self.generate_numbers();
+        }
+        let mut result = self.mt[self.index];
+
+        result ^= result >> 11;
+        result ^= (result << 7) & 0x9d2c5680;
+        result ^= (result << 15) & 0xefc60000;
+        result ^= result >> 18;
+        self.index = (self.index + 1) % 624;
+
+        result
+    }
+
+    fn generate_numbers(&mut self) {
+        for i in 0..624 {
+            let y: u32 = (self.mt[i] & 0x80000000) + (self.mt[(i + 1) % 624] & 0x7fffffff);
+            self.mt[i] = self.mt[(i + 397) % 624] ^ (y >> 1);
+            if y % 2 != 0 {
+                self.mt[i] ^= 0x9908b0df;
+            }
+        }
+    }
+}
+
+fn main() {
+    // use C++ std::mt19937 to verify.
+    let mut mt19937 = MT19937::new(12345);
+    let result = mt19937.extract_number();
+
+    println!("{result}");
+}