Gas price statistics. (#1291)

* Gas price statistics.

Affects eth_gasPrice.
Added ethcore_gasPriceStatistics.

Closes #1265

* Fix a bug in eth_gasPrice

* Fix tests.

* Revert minor alteration.

* Tests for gas_price_statistics.

- Tests;
- Additional infrastructure for generating test blocks with
transactions.

ethcore/src/client/client.rs

@@ -770,8 +770,7 @@ impl<V> MiningBlockChainClient for Client<V> where V: Verifier {
 			author,
 			gas_floor_target,
 			extra_data,
-		).expect("OpenBlock::new only fails if parent state root invalid. State root of best block's header is never invalid. \
-		         Therefore creating an OpenBlock with the best block's header will not fail.");
+		).expect("OpenBlock::new only fails if parent state root invalid; state root of best block's header is never invalid; qed");
 
 		// Add uncles
 		self.chain

ethcore/src/client/mod.rs

@@ -34,6 +34,7 @@ pub use env_info::{LastHashes, EnvInfo};
 use util::bytes::Bytes;
 use util::hash::{Address, H256, H2048};
 use util::numbers::U256;
+use util::Itertools;
 use blockchain::TreeRoute;
 use block_queue::BlockQueueInfo;
 use block::OpenBlock;
@@ -41,6 +42,7 @@ use header::{BlockNumber, Header};
 use transaction::{LocalizedTransaction, SignedTransaction};
 use log_entry::LocalizedLogEntry;
 use filter::Filter;
+use views::BlockView;
 use error::{ImportResult, ExecutionError};
 use receipt::LocalizedReceipt;
 use trace::LocalizedTrace;
@@ -193,6 +195,32 @@ pub trait BlockChainClient : Sync + Send {
 
 	/// list all transactions
 	fn all_transactions(&self) -> Vec<SignedTransaction>;
+
+	/// Get the gas price distribution.
+	fn gas_price_statistics(&self, sample_size: usize, distribution_size: usize) -> Result<Vec<U256>, ()> {
+		let mut h = self.chain_info().best_block_hash;
+		let mut corpus = Vec::new();
+		for _ in 0..sample_size {
+			let block_bytes = self.block(BlockID::Hash(h)).expect("h is either the best_block_hash or an ancestor; qed");
+			let block = BlockView::new(&block_bytes);
+			let header = block.header_view();
+			if header.number() == 0 {
+				break;
+			}
+			block.transaction_views().iter().foreach(|t| corpus.push(t.gas_price()));
+			h = header.parent_hash().clone();
+		}
+		corpus.sort();
+		let n = corpus.len();
+		if n > 0 {
+			Ok((0..(distribution_size + 1))
+				.map(|i| corpus[i * (n - 1) / distribution_size])
+				.collect::<Vec<_>>()
+			)
+		} else {
+			Err(())
+		}
+	}
 }
 
 /// Extended client interface used for mining