[feat] add next_prev_timestamps function implementation

src/validation.cairo

@@ -73,8 +73,18 @@ fn validate_timestamp(self: @ChainState, block: @Block) -> Result<(), ByteArray>
 }
 
 fn next_prev_timestamps(self: @ChainState, block: @Block) -> Span<u32> {
-    // TODO: implement
-    *self.prev_timestamps
+    let mut timestamps = ArrayTrait::<u32>::new();
+    timestamps.append(*block.header.time);
+    //copy the 10 most recent previous timestamps from the chain
+    let mut prev_timestamps = *self.prev_timestamps;
+    let mut index = 1;
+    loop {
+        if index > 10 {
+            break timestamps.span();
+        }
+        timestamps.append(*prev_timestamps.at(index));
+        index += 1;
+    }
 }
 
 fn compute_total_work(current_total_work: u256, target: u256) -> u256 {
@@ -209,7 +219,7 @@ mod tests {
     use raito::state::{Header, Transaction, TxIn, TxOut};
     use super::{
         validate_timestamp, validate_proof_of_work, compute_block_reward, compute_total_work,
-        compute_work_from_target, shr, shl, Block, ChainState, UtreexoState,
+        compute_work_from_target, shr, shl, Block, ChainState, UtreexoState, next_prev_timestamps
     };
 
 
@@ -350,4 +360,26 @@ mod tests {
         let last_reward = compute_block_reward(max_halvings * block_height);
         assert_eq!(last_reward, 0);
     }
+
+    #[test]
+    fn test_next_prev_timstamps() {
+        let chain_state = ChainState {
+            block_height: 1,
+            total_work: 1,
+            best_block_hash: 1,
+            current_target: 1,
+            epoch_start_time: 1,
+            prev_timestamps: array![0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10].span(),
+            utreexo_state: UtreexoState { roots: array![].span() },
+        };
+        let block = Block {
+            header: Header { version: 1, time: 12, nonce: 1, bits: 1 },
+            txs: ArrayTrait::new().span(),
+        };
+        let next_prev_timestamps = next_prev_timestamps(@chain_state, @block);
+        assert(*next_prev_timestamps.at(0) == 12, 'Failed to compute');
+        assert(*next_prev_timestamps.at(6) == 6, 'Failed to compute');
+        assert(*next_prev_timestamps.at(8) == 8, 'Failed to compute');
+        assert(*next_prev_timestamps.at(9) == 9, 'Failed to compute');
+    }
 }