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.
14 // -------------------------------------
22 func is_zero(x *Number) bool {
27 func add1(x *Number) *Number {
34 func sub1(x *Number) *Number {
39 func add(x, y *Number) *Number{
44 return add(add1(x), sub1(y));
48 func mul(x, y *Number) *Number {
49 if is_zero(x) || is_zero(y){
53 return add(mul(x, sub1(y)), x);
57 func fact(n *Number) *Number {
62 return mul(fact(sub1(n)), n);
66 // -------------------------------------
67 // Helpers to generate/count Peano integers
69 func gen(n int) *Number {
71 return add1(gen(n - 1));
78 func count(x *Number) int {
83 return count(sub1(x)) + 1;
87 func check(x *Number, expected int) {
90 panic "error: found ", c, "; expected ", expected, "\n";
95 // -------------------------------------
96 // Test basic functionality
100 check(add1(zero()), 1);
103 check(add(gen(3), zero()), 3);
104 check(add(zero(), gen(4)), 4);
105 check(add(gen(3), gen(4)), 7);
107 check(mul(zero(), zero()), 0);
108 check(mul(gen(3), zero()), 0);
109 check(mul(zero(), gen(4)), 0);
110 check(mul(gen(3), add1(zero())), 3);
111 check(mul(add1(zero()), gen(4)), 4);
112 check(mul(gen(3), gen(4)), 12);
114 check(fact(zero()), 1);
115 check(fact(add1(zero())), 1);
116 check(fact(gen(5)), 120);
120 // -------------------------------------
127 for i := 0; i <= 10; i++ {
128 print i, "! = ", count(fact(gen(i))), "\n";