calc conway-chain.

main.go

package main

import (
	"fmt"
	"math/big"
)

// コンウェイのチェーン表記で a -> b -> c を計算する。
func chain(a big.Int, b big.Int, c big.Int) big.Int {
	if b.Cmp(big.NewInt(1)) == 0 { // b == 1
		return a
	}

	if c.Cmp(big.NewInt(1)) == 0 { // c == 1
		return *new(big.Int).Exp(&a, &b, nil) // a^b
	}

	// a -> (a -> (b-1) -> c) -> (c-1)
	return chain(
		a,
		chain(
			a,
			*new(big.Int).Add(&b, big.NewInt(-1)),
			c,
		),
		*new(big.Int).Add(&c, big.NewInt(-1)),
	)
}

func main() {
	var (
		a int64 = 3
		b int64 = 3
		c int64 = 2
	)
	result := chain(*big.NewInt(a), *big.NewInt(b), *big.NewInt(c))
	fmt.Println(result.String())
}