1 // $G $F.go && $L $F.$A && ./$A.out
3 // Copyright 2009 The Go Authors. All rights reserved.
4 // Use of this source code is governed by a BSD-style
5 // license that can be found in the LICENSE file.
11 // -------------------------------------
18 func is_zero(x *Number) bool {
22 func add1(x *Number) *Number {
28 func sub1(x *Number) *Number {
32 func add(x, y *Number) *Number {
37 return add(add1(x), sub1(y))
40 func mul(x, y *Number) *Number {
41 if is_zero(x) || is_zero(y) {
45 return add(mul(x, sub1(y)), x)
48 func fact(n *Number) *Number {
53 return mul(fact(sub1(n)), n)
56 // -------------------------------------
57 // Helpers to generate/count Peano integers
59 func gen(n int) *Number {
61 return add1(gen(n - 1))
67 func count(x *Number) int {
72 return count(sub1(x)) + 1
75 func check(x *Number, expected int) {
78 print("error: found ", c, "; expected ", expected, "\n")
83 // -------------------------------------
84 // Test basic functionality
88 check(add1(zero()), 1)
91 check(add(gen(3), zero()), 3)
92 check(add(zero(), gen(4)), 4)
93 check(add(gen(3), gen(4)), 7)
95 check(mul(zero(), zero()), 0)
96 check(mul(gen(3), zero()), 0)
97 check(mul(zero(), gen(4)), 0)
98 check(mul(gen(3), add1(zero())), 3)
99 check(mul(add1(zero()), gen(4)), 4)
100 check(mul(gen(3), gen(4)), 12)
102 check(fact(zero()), 1)
103 check(fact(add1(zero())), 1)
104 check(fact(gen(5)), 120)
107 // -------------------------------------
110 var results = [...]int{
111 1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800,
116 for i := 0; i <= 9; i++ {
117 if f := count(fact(gen(i))); f != results[i] {
118 println("FAIL:", i, "!:", f, "!=", results[i])