Add riffle_shuffle

Author: webmaster128

packages/std/src/lib.rs

@@ -59,8 +59,8 @@ mod mock;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod testing {
     pub use crate::mock::{
-        mock_dependencies, mock_dependencies_with_balances, mock_env, BankQuerier, MockApi,
-        MockQuerier, MockQuerierCustomHandlerResult, MockStorage, StakingQuerier,
+        mock_dependencies, mock_dependencies_with_balances, mock_env, riffle_shuffle, BankQuerier,
+        MockApi, MockQuerier, MockQuerierCustomHandlerResult, MockStorage, StakingQuerier,
         MOCK_CONTRACT_ADDR,
     };
 }

packages/std/src/mock.rs

@@ -343,6 +343,25 @@ impl StakingQuerier {
     }
 }
 
+/// Performs a perfect shuffle (in shuffle)
+///
+/// https://en.wikipedia.org/wiki/Riffle_shuffle_permutation#Perfect_shuffles
+/// https://en.wikipedia.org/wiki/In_shuffle
+pub fn riffle_shuffle<T: Clone>(input: &[T]) -> Vec<T> {
+    assert!(
+        input.len() % 2 == 0,
+        "Method only defined for even number of elements"
+    );
+    let mid = input.len() / 2;
+    let (left, right) = input.split_at(mid);
+    let mut out = Vec::<T>::with_capacity(input.len());
+    for i in 0..mid {
+        out.push(right[i].clone());
+        out.push(left[i].clone());
+    }
+    out
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -610,4 +629,36 @@ mod test {
         let dels = get_delegator(&staking, user_c.clone(), val2.clone());
         assert_eq!(dels, Some(del2c.clone()));
     }
+
+    #[test]
+    fn riffle_shuffle_works() {
+        // Example from https://en.wikipedia.org/wiki/In_shuffle
+        let start = [0xA, 0x2, 0x3, 0x4, 0x5, 0x6];
+        let round1 = riffle_shuffle(&start);
+        assert_eq!(round1, [0x4, 0xA, 0x5, 0x2, 0x6, 0x3]);
+        let round2 = riffle_shuffle(&round1);
+        assert_eq!(round2, [0x2, 0x4, 0x6, 0xA, 0x3, 0x5]);
+        let round3 = riffle_shuffle(&round2);
+        assert_eq!(round3, start);
+
+        // For 14 elements, the original order is restored after 4 executions
+        // See https://en.wikipedia.org/wiki/In_shuffle#Mathematics and https://oeis.org/A002326
+        let original = [12, 33, 76, 576, 0, 44, 1, 14, 78, 99, 871212, -7, 2, -1];
+        let mut result = Vec::from(original);
+        for _ in 0..4 {
+            result = riffle_shuffle(&result);
+        }
+        assert_eq!(result, original);
+
+        // For 24 elements, the original order is restored after 20 executions
+        let original = [
+            7, 4, 2, 4656, 23, 45, 23, 1, 12, 76, 576, 0, 12, 1, 14, 78, 99, 12, 1212, 444, 31,
+            111, 424, 34,
+        ];
+        let mut result = Vec::from(original);
+        for _ in 0..20 {
+            result = riffle_shuffle(&result);
+        }
+        assert_eq!(result, original);
+    }
 }