Add acceptance/performance test with a program that calculates Fibonn…

…aci numbers.

Package.swift

@@ -22,6 +22,9 @@ let package = Package(
             dependencies: ["Syft", "Utility"]),
         .testTarget(
             name: "SongTests",
+            dependencies: ["Song"]),
+        .testTarget(
+            name: "SongPerformanceTests",
             dependencies: ["Song"])
     ]
 )

Tests/SongPerformanceTests/FibonacciTests.swift

@@ -0,0 +1,20 @@
+import XCTest
+import Song
+
+class FibonacciTests: XCTestCase {
+
+    func test() {
+        let code = """
+n.fib = n.fib(0, 1)
+0.fib(a, _) = a
+n.fib(a, b) When n > 0 = (n-1).fib(b, a+b)
+
+80.fib
+"""
+        measure {
+            assertNoThrow {
+                XCTAssertEqual(Expression.int(23416728348467685), try evaluate(code))
+            }
+        }
+    }
+}

Tests/SongPerformanceTests/PerformanceHelpers.swift

@@ -0,0 +1,35 @@
+import XCTest
+import Song
+
+func assertNoThrow(file: StaticString = #file, line: UInt = #line, _ closure: () throws -> Void) {
+    do {
+        try closure()
+    } catch {
+        XCTFail("\(error)", file: file, line: line)
+    }
+}
+
+func evaluate(_ code: String) throws -> Expression? {
+    let parser = makeParser()
+    let transformer = makeTransformer()
+    let lines = code.split(separator: "\n")
+    var context = Context()
+    var lastEval: Expression?
+    for line in lines {
+        let result = parser.parse(String(line))
+        let expression = try transformer.transform(result)
+        let evaluated = try expression.evaluate(context: context)
+        if case .closure(let name, _, _) = evaluated {
+            if let name = name {
+                context = extendContext(context: context, name: name, value: evaluated)
+            }
+        }
+        if case .assign(let variable, let value) = evaluated {
+            if case .name(let name) = variable {
+                context = extendContext(context: context, name: name, value: value)
+            }
+        }
+        lastEval = evaluated
+    }
+    return lastEval
+}