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move stirling methods to public package

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filifa
2025-11-15 19:34:49 -05:00
parent 85229bffe1
commit 73c750d317
2 changed files with 1 additions and 1 deletions
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82
internal/lib/stirling.go
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@@ -1,82 +0,0 @@
/*
Copyright © 2025 filifa
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package lib
import (
"math/big"
)
// given a slice where vals[i] = Stirling1(i+k-1, k-1) for a given k, nextStirling1 updates the slice so vals[i] = Stirling1(i+k, k) using the property that Stirling1(n, k) = -(n-1)*Stirling1(n-1, k) + Stirling1(n-1, k-1)
func nextStirling1(k int, vals []*big.Int) {
for i := 1; i < len(vals); i++ {
n := int64(k + i - 1)
v := big.NewInt(-n)
v.Mul(v, vals[i-1])
vals[i].Add(vals[i], v)
}
}
/*
Stirling1 computes Stirling numbers of the first kind.
*/
func Stirling1(n, k int) *big.Int {
if k > n {
return big.NewInt(0)
}
vals := make([]*big.Int, n-k+1)
for i := range vals {
vals[i] = big.NewInt(0)
}
vals[0] = big.NewInt(1)
for i := 1; i <= k; i++ {
nextStirling1(i, vals)
}
return vals[n-k]
}
// given a slice where vals[i] = Stirling2(i+k-1, k-1) for a given k, nextStirling2 updates the slice so vals[i] = Stirling2(i+k, k) using the property that Stirling2(n, k) = k*Stirling2(n-1, k) + Stirling2(n-1, k-1)
func nextStirling2(k int64, vals []*big.Int) {
for i := 1; i < len(vals); i++ {
v := big.NewInt(k)
v.Mul(v, vals[i-1])
vals[i].Add(vals[i], v)
}
}
/*
Stirling2 computes Stirling numbers of the second kind.
*/
func Stirling2(n, k int) *big.Int {
if k > n {
return big.NewInt(0)
}
vals := make([]*big.Int, n-k+1)
for i := range vals {
vals[i] = big.NewInt(0)
}
vals[0] = big.NewInt(1)
for i := 1; i <= k; i++ {
nextStirling2(int64(i), vals)
}
return vals[n-k]
}