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33 lines
795 B
33 lines
795 B
// Copyright 2018, Aaron Cannon under the BSD license.
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// See the license file for details.
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// Package choose implements the N choose K formula, or the binomial coefficient
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// formula. See https://en.wikipedia.org/wiki/Binomial_coefficient for more.
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package choose
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// Choose calculates n choose k. Overflows are not detected, and Choose panics
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// if n >= k >= 0 is violated.
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func Choose(n, k int64) int64 {
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if k > n {
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panic("Choose: k > n")
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}
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if k < 0 {
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panic("Choose: k < 0")
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}
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if n <= 1 || k == 0 || n == k {
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return 1
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}
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if newK := n - k; newK < k {
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k = newK
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}
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if k == 1 {
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return n
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}
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// Our return value, and this allows us to skip the first iteration.
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ret := int64(n - k + 1)
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for i, j := ret+1, int64(2); j <= k; i, j = i+1, j+1 {
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ret = ret * i / j
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}
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return ret
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}
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