Add benchmark for expression (#58)

* Add benchmark for expression

There are 3 phases: generate strings, parsing, executing.

The string generation phase is as follows:

Only the 4 basic math (two argument aka binary) operations are used,
it starts with a pool of nodes (initially a node is just a variable),
and randomly combines 2 nodes until only one node remains.

Parsing and executing are straightforward uses of the Expression class.

* Split out expression generator to Python script.

benchmarks/math/expression_gen.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+import json
+import random
+
+NUM_EXPRESSIONS = 20
+NUM_VARS = 3000  # Should be kept in sync with NUM_VARS in expression.gd
+
+def _var_names():
+	rv = []
+	for i in range(NUM_VARS):
+		rv.append("x" + str(i))
+	return rv
+
+def _var_values():
+	rv = []
+	for i in range (NUM_VARS):
+		rv.append((i+1)*10)
+	return rv
+
+def _combine(nodes, ia, ib, op):
+	na = nodes[ia]
+	nb = nodes[ib]
+	del nodes[ib]
+	del nodes[ia]
+	nodes.append("(" + str(na) + " " + op + " " + str(nb) + ")")
+
+def _generate_string():
+	nodes = []
+	operators = ["+", "-", "*", "/"]
+
+	nodes += _var_names()
+
+	while len(nodes) > 1:
+		ia = random.randrange(0, len(nodes))
+		ib = random.randrange(0, len(nodes))
+		io = random.randrange(0, len(operators))
+		op = operators[io]
+
+		if ia == ib:
+			ib = ia+1
+			if ib == len(nodes):
+				ib = 0
+
+		if ia < ib:
+			_combine(nodes, ia, ib, op)
+		elif ib < ia:
+			_combine(nodes, ib, ia, op)
+
+	return nodes[0]
+
+def _generate_strings():
+	random.seed(234)
+	rv = []
+	for i in range(NUM_EXPRESSIONS):
+		rv.append(_generate_string())
+	return rv
+
+
+strings = _generate_strings()
+print("const EXPRESSIONS = ", json.dumps(strings, indent=4).replace('    ', '\t'))