core, consensus: pluggable consensus engines (#3817)

This commit adds pluggable consensus engines to go-ethereum. In short, it
introduces a generic consensus interface, and refactors the entire codebase to
use this interface.

consensus/ethash/algorithm_go1.7.go

@@ -0,0 +1,47 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library 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 Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// +build !go1.8
+
+package ethash
+
+// cacheSize calculates and returns the size of the ethash verification cache that
+// belongs to a certain block number. The cache size grows linearly, however, we
+// always take the highest prime below the linearly growing threshold in order to
+// reduce the risk of accidental regularities leading to cyclic behavior.
+func cacheSize(block uint64) uint64 {
+	// If we have a pre-generated value, use that
+	epoch := int(block / epochLength)
+	if epoch < len(cacheSizes) {
+		return cacheSizes[epoch]
+	}
+	// We don't have a way to verify primes fast before Go 1.8
+	panic("fast prime testing unsupported in Go < 1.8")
+}
+
+// datasetSize calculates and returns the size of the ethash mining dataset that
+// belongs to a certain block number. The dataset size grows linearly, however, we
+// always take the highest prime below the linearly growing threshold in order to
+// reduce the risk of accidental regularities leading to cyclic behavior.
+func datasetSize(block uint64) uint64 {
+	// If we have a pre-generated value, use that
+	epoch := int(block / epochLength)
+	if epoch < len(datasetSizes) {
+		return datasetSizes[epoch]
+	}
+	// We don't have a way to verify primes fast before Go 1.8
+	panic("fast prime testing unsupported in Go < 1.8")
+}