Introduce Witness struct mainly to improve ser/de performance while keeping most usability.

Witness struct is in place of the Vec<Vec<u8>> we have before this commit.

from_vec() and to_vec() methods are provided to switch between this type and Vec<Vec<u8>>

Moreover, implementation of Default, Iterator and others allows to have similar behaviour but
using a single Vec prevent many allocations during deserialization which in turns results in
better performance, even 20% better perfomance on recent block.

last() and second_to_last() allows to access respective element without going through costly Vec
transformation

Author: RCasatta

src/blockdata/block.rs

@@ -199,9 +199,10 @@ impl Block {
                         o.script_pubkey[0..6] == [0x6a, 0x24, 0xaa, 0x21, 0xa9, 0xed] }) {
                     let commitment = WitnessCommitment::from_slice(&coinbase.output[pos].script_pubkey.as_bytes()[6..38]).unwrap();
                     // witness reserved value is in coinbase input witness
-                    if coinbase.input[0].witness.len() == 1 && coinbase.input[0].witness[0].len() == 32 {
+                    let witness_vec: Vec<_> = coinbase.input[0].witness.iter().collect();
+                    if witness_vec.len() == 1 && witness_vec[0].len() == 32 {
                         match self.witness_root() {
-                            Some(witness_root) => return commitment == Self::compute_witness_commitment(&witness_root, coinbase.input[0].witness[0].as_slice()),
+                            Some(witness_root) => return commitment == Self::compute_witness_commitment(&witness_root, witness_vec[0]),
                             None => return false,
                         }
                     }

src/blockdata/constants.rs

@@ -29,6 +29,7 @@ use blockdata::opcodes;
 use blockdata::script;
 use blockdata::transaction::{OutPoint, Transaction, TxOut, TxIn};
 use blockdata::block::{Block, BlockHeader};
+use blockdata::witness::Witness;
 use network::constants::Network;
 use util::uint::Uint256;
 
@@ -93,7 +94,7 @@ fn bitcoin_genesis_tx() -> Transaction {
         previous_output: OutPoint::null(),
         script_sig: in_script,
         sequence: MAX_SEQUENCE,
-        witness: vec![],
+        witness: Witness::default(),
     });
 
     // Outputs

src/blockdata/mod.rs

@@ -23,4 +23,5 @@ pub mod opcodes;
 pub mod script;
 pub mod transaction;
 pub mod block;
+pub mod witness;
 

src/blockdata/transaction.rs

@@ -36,6 +36,7 @@ use util::endian;
 use blockdata::constants::WITNESS_SCALE_FACTOR;
 #[cfg(feature="bitcoinconsensus")] use blockdata::script;
 use blockdata::script::Script;
+use blockdata::witness::Witness;
 use consensus::{encode, Decodable, Encodable};
 use consensus::encode::MAX_VEC_SIZE;
 use hash_types::{SigHash, Txid, Wtxid};
@@ -197,7 +198,7 @@ pub struct TxIn {
     /// Encodable/Decodable, as it is (de)serialized at the end of the full
     /// Transaction. It *is* (de)serialized with the rest of the TxIn in other
     /// (de)serialization routines.
-    pub witness: Vec<Vec<u8>>
+    pub witness: Witness
 }
 
 impl Default for TxIn {
@@ -206,7 +207,7 @@ impl Default for TxIn {
             previous_output: OutPoint::default(),
             script_sig: Script::new(),
             sequence: u32::max_value(),
-            witness: Vec::new(),
+            witness: Witness::default(),
         }
     }
 }
@@ -280,7 +281,7 @@ impl Transaction {
         let cloned_tx = Transaction {
             version: self.version,
             lock_time: self.lock_time,
-            input: self.input.iter().map(|txin| TxIn { script_sig: Script::new(), witness: vec![], .. *txin }).collect(),
+            input: self.input.iter().map(|txin| TxIn { script_sig: Script::new(), witness: Witness::default(), .. *txin }).collect(),
             output: self.output.clone(),
         };
         cloned_tx.txid().into()
@@ -357,7 +358,7 @@ impl Transaction {
                 previous_output: self.input[input_index].previous_output,
                 script_sig: script_pubkey.clone(),
                 sequence: self.input[input_index].sequence,
-                witness: vec![],
+                witness: Witness::default(),
             }];
         } else {
             tx.input = Vec::with_capacity(self.input.len());
@@ -366,7 +367,7 @@ impl Transaction {
                     previous_output: input.previous_output,
                     script_sig: if n == input_index { script_pubkey.clone() } else { Script::new() },
                     sequence: if n != input_index && (sighash == EcdsaSigHashType::Single || sighash == EcdsaSigHashType::None) { 0 } else { input.sequence },
-                    witness: vec![],
+                    witness: Witness::default(),
                 });
             }
         }
@@ -473,10 +474,7 @@ impl Transaction {
                 input.script_sig.len());
             if !input.witness.is_empty() {
                 inputs_with_witnesses += 1;
-                input_weight += VarInt(input.witness.len() as u64).len();
-                for elem in &input.witness {
-                    input_weight += VarInt(elem.len() as u64).len() + elem.len();
-                }
+                input_weight += input.witness.serialized_len();
             }
         }
         let mut output_size = 0;
@@ -578,7 +576,7 @@ impl Decodable for TxIn {
             previous_output: Decodable::consensus_decode(&mut d)?,
             script_sig: Decodable::consensus_decode(&mut d)?,
             sequence: Decodable::consensus_decode(d)?,
-            witness: vec![],
+            witness: Witness::default(),
         })
     }
 }
@@ -1433,6 +1431,7 @@ mod tests {
         use hashes::hex::FromHex;
         use std::collections::HashMap;
         use blockdata::script;
+        use blockdata::witness::Witness;
 
         // a random recent segwit transaction from blockchain using both old and segwit inputs
         let mut spending: Transaction = deserialize(Vec::from_hex("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")
@@ -1471,7 +1470,9 @@ mod tests {
         }).is_err());
 
         // test that we get a failure if we corrupt a signature
-        spending.input[1].witness[0][10] = 42;
+        let mut witness: Vec<_> = spending.input[1].witness.to_vec();
+        witness[0][10] = 42;
+        spending.input[1].witness = Witness::from_vec(witness);
         match spending.verify(|point: &OutPoint| {
             if let Some(tx) = spent3.remove(&point.txid) {
                 return tx.output.get(point.vout as usize).cloned();