Modify gas price statistics (#2947)

* gas price distribution + median + tests

* put histogram in util

* use the util histogram

* remove the default gas price implementation

* histogram rpc

* fix empty corpus

* Add JS ethcore_gasPriceHistogram

* Fix typo (s/types/type/) & subsequent failing test

* Fix return type & formatting

* bucketBounds

* Add jsapi e2e test verification

util/src/lib.rs

@@ -144,6 +144,7 @@ pub mod semantic_version;
 pub mod log;
 pub mod path;
 pub mod snappy;
+pub mod stats;
 pub mod cache;
 mod timer;
 

util/src/stats.rs

@@ -0,0 +1,70 @@
+// Copyright 2015, 2016 Ethcore (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Statistical functions.
+
+use bigint::uint::*;
+
+/// Discretised histogram.
+#[derive(Debug, PartialEq)]
+pub struct Histogram {
+	/// Bounds of each bucket.
+	pub bucket_bounds: Vec<U256>,
+	/// Count within each bucket.
+	pub counts: Vec<u64>
+}
+
+impl Histogram {
+	/// Histogram if a sorted corpus is at least fills the buckets.
+	pub fn new(corpus: &[U256], bucket_number: usize) -> Option<Histogram> {
+		if corpus.len() < bucket_number { return None; }
+		let corpus_end = corpus.last().expect("there are at least bucket_number elements; qed").clone();
+		// If there are extremely few transactions, go from zero.
+		let corpus_start = corpus.first().expect("there are at least bucket_number elements; qed").clone();
+		let bucket_size = (corpus_end - corpus_start + 1.into()) / bucket_number.into();
+		let mut bucket_end = corpus_start + bucket_size;
+
+		let mut bucket_bounds = vec![corpus_start; bucket_number + 1];
+		let mut counts = vec![0; bucket_number];
+		let mut corpus_i = 0;
+		// Go through the corpus adding to buckets.
+		for bucket in 0..bucket_number {
+			while corpus[corpus_i] < bucket_end {
+				counts[bucket] += 1;
+				corpus_i += 1;
+			}
+			bucket_bounds[bucket + 1] = bucket_end;
+			bucket_end = bucket_end + bucket_size;
+		}
+		Some(Histogram { bucket_bounds: bucket_bounds, counts: counts })
+	}
+}
+
+
+#[cfg(test)]
+mod tests {
+	use bigint::uint::U256;
+	use super::Histogram;
+
+	#[test]
+	fn check_histogram() {
+		let hist = Histogram::new(&vec_into![643,689,1408,2000,2296,2512,4250,4320,4842,4958,5804,6065,6098,6354,7002,7145,7845,8589,8593,8895], 5).unwrap();
+		let correct_bounds: Vec<U256> = vec_into![643,2293,3943,5593,7243,8893];
+		assert_eq!(Histogram { bucket_bounds: correct_bounds, counts: vec![4,2,4,6,3] }, hist);
+
+		assert!(Histogram::new(&vec_into![1, 2], 5).is_none());
+	}
+}