69 lines
1.6 KiB
Go
69 lines
1.6 KiB
Go
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/*
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Copyright © 2025 filifa
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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package lib
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import (
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"math/big"
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)
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// TODO: implement both of these with dynamic programming
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func Stirling1(n, k *big.Int) *big.Int {
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if n.Cmp(big.NewInt(0)) == 0 && k.Cmp(big.NewInt(0)) == 0 {
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return big.NewInt(1)
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}
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if n.Cmp(big.NewInt(0)) == 0 || k.Cmp(big.NewInt(0)) == 0 {
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return big.NewInt(0)
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}
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newN := new(big.Int).Set(n)
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newN.Sub(newN, big.NewInt(1))
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result := Stirling1(newN, k)
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result.Mul(result, newN)
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result.Neg(result)
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newK := new(big.Int).Set(k)
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newK.Sub(newK, big.NewInt(1))
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result.Add(result, Stirling1(newN, newK))
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return result
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}
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func Stirling2(n, k *big.Int) *big.Int {
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if n.Cmp(k) == 0 {
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return big.NewInt(1)
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}
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if n.Cmp(big.NewInt(0)) == 0 || k.Cmp(big.NewInt(0)) == 0 {
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return big.NewInt(0)
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}
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newN := new(big.Int).Set(n)
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newN.Sub(newN, big.NewInt(1))
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result := Stirling2(newN, k)
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result.Mul(result, k)
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newK := new(big.Int).Set(k)
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newK.Sub(newK, big.NewInt(1))
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result.Add(result, Stirling2(newN, newK))
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return result
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}
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