集合函数：阶乘、排列和组合

set-operation.reb

REBOL []

factorial: func [ n [ integer! ]][
	factorial-util n 1
]

factorial-util: func [ n [ integer! ] init [ integer! ]][
	if any [ n = 0 n = 1] [ return init ]
	return factorial-util n - 1 init * n
]

permutation: func [ n [ integer! ] m [ integer! ] /local result ][
	if m > n [ return "error" ]
	if m = 1 [ return n ]
	result: 1
	for i 1 m 1 [
		result: result * ( n - i + 1 )
	]
	
	result
]

permutation2: func [ n [ integer! ] m [ integer! ] ][
	permutation2-util n m 1
]

permutation2-util: func [ n [ integer! ] m [ integer! ] init [ integer! ]][
	if m > n [ return "error"]
	if m = 0 [ return init ]
	return permutation2-util n - 1 m - 1 n * init 
]

combination: func [ n [ integer! ] m [ integer! ]][
	if m > n [ return 0 ]
	if m > ( n / 2 + 1) [ return combination n n - m ]
	;print [permutation n m " / " factorial m]
	return ( permutation n m ) / (factorial m)
]