Add StdRng test setting the counter

Author: dhardy

src/rngs/std.rs

@@ -348,4 +348,53 @@ mod test {
 
         assert_eq!(rng.0.get_word_pos(), 32);
     }
+
+    #[test]
+    fn test_chacha_counter() {
+        // Source: none
+        // Test: all zero key and IV, block set to u32::MAX, rounds 12, 256-bit key
+
+        let seed = [0u8; 32];
+        let mut rng = StdRng::from_seed(seed);
+        let block = u32::MAX;
+        let words_per_block = 16;
+        rng.0.set_word_pos((block as u128) * words_per_block);
+
+        let mut expected = [0u32; 12];
+        // Note: four blocks get generated by set_word_pos
+        expected[8] = block.wrapping_add(4);
+        expected[9] = 1;
+        assert_eq!(rng_state(&rng), &expected);
+
+        let mut results = [0u8; 64];
+        rng.fill_bytes(&mut results);
+        #[rustfmt::skip]
+        let expected = [
+            0xd7, 0xa6, 0xaf, 0x50, 0xf1, 0xc9, 0x2a, 0x29,
+            0x48, 0x42, 0x52, 0xbb, 0xfc, 0xe2, 0x06, 0xf1,
+            0x7d, 0x01, 0xdd, 0x13, 0x95, 0x30, 0xa3, 0x83,
+            0x0a, 0xb5, 0x83, 0xc1, 0xf6, 0x2e, 0x03, 0x12,
+            0x82, 0x93, 0x61, 0xa1, 0x9a, 0x8a, 0x95, 0x6c,
+            0xed, 0xea, 0x38, 0x04, 0x30, 0xff, 0x93, 0x2c,
+            0xd0, 0x52, 0xdb, 0x5e, 0x94, 0x77, 0x83, 0x50,
+            0x58, 0xb8, 0x0a, 0x27, 0x24, 0x06, 0xfc, 0x74,
+        ];
+        assert_eq!(results, expected);
+
+        rng.fill_bytes(&mut results);
+        #[rustfmt::skip]
+        let expected = [
+            0xcc, 0x7b, 0x53, 0xdc, 0x11, 0x89, 0x4d, 0x26,
+            0x24, 0x05, 0x81, 0xb8, 0xa8, 0xf4, 0xf4, 0xe5,
+            0xaf, 0x40, 0x67, 0x05, 0x80, 0x12, 0x23, 0xb1,
+            0x3f, 0x82, 0x1f, 0xdc, 0xcb, 0xa6, 0xa6, 0x18,
+            0x8a, 0x63, 0xf8, 0xd3, 0xdc, 0x83, 0xcc, 0xbc,
+            0xed, 0x45, 0x1f, 0x4b, 0xa4, 0xe0, 0xda, 0xab,
+            0x22, 0x8a, 0xbb, 0x0d, 0x74, 0x39, 0xcc, 0x67,
+            0xe5, 0x0d, 0xf7, 0x12, 0x9f, 0x64, 0x6b, 0xad,
+        ];
+        assert_eq!(results, expected);
+
+        assert_eq!(rng.0.get_word_pos(), (block as u128) * words_per_block + 32);
+    }
 }