add stirling subcommand

cmd/stirling.go

@@ -0,0 +1,91 @@
+/*
+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 cmd
+
+import (
+	"fmt"
+	"math/big"
+
+	"github.com/spf13/cobra"
+	"scm.dairydemon.net/filifa/mathtools/internal/lib"
+)
+
+var firstKind bool
+var secondKind bool
+var stirlingTop string
+var stirlingBottom string
+var stirlingUnsigned bool
+
+func stirling(cmd *cobra.Command, args []string) {
+	n, ok := new(big.Int).SetString(stirlingTop, 10)
+	if !ok {
+		cobra.CheckErr("invalid input " + stirlingTop)
+	}
+
+	k, ok := new(big.Int).SetString(stirlingBottom, 10)
+	if !ok {
+		cobra.CheckErr("invalid input " + stirlingBottom)
+	}
+
+	var result *big.Int
+	if firstKind {
+		result = lib.Stirling1(n, k)
+	} else if secondKind {
+		result = lib.Stirling2(n, k)
+	}
+
+	if stirlingUnsigned {
+		result.Abs(result)
+	}
+
+	fmt.Println(result)
+}
+
+// stirlingCmd represents the stirling command
+var stirlingCmd = &cobra.Command{
+	Use:   "stirling",
+	Short: "Compute the Stirling numbers",
+	Long:  `Compute the Stirling numbers.`,
+	Run:   stirling,
+}
+
+func init() {
+	rootCmd.AddCommand(stirlingCmd)
+
+	// Here you will define your flags and configuration settings.
+
+	// Cobra supports Persistent Flags which will work for this command
+	// and all subcommands, e.g.:
+	// stirlingCmd.PersistentFlags().String("foo", "", "A help for foo")
+
+	// Cobra supports local flags which will only run when this command
+	// is called directly, e.g.:
+	// stirlingCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
+
+	stirlingCmd.Flags().BoolVarP(&firstKind, "first", "1", false, "Compute Stirling numbers of the first kind")
+	stirlingCmd.Flags().BoolVarP(&secondKind, "second", "2", false, "Compute Stirling numbers of the second kind")
+
+	stirlingCmd.MarkFlagsMutuallyExclusive("first", "second")
+	stirlingCmd.MarkFlagsOneRequired("first", "second")
+
+	stirlingCmd.Flags().StringVarP(&stirlingTop, "n", "n", "", "n")
+	stirlingCmd.Flags().StringVarP(&stirlingBottom, "k", "k", "", "k")
+	stirlingCmd.MarkFlagRequired("n")
+	stirlingCmd.MarkFlagRequired("k")
+
+	stirlingCmd.Flags().BoolVar(&stirlingUnsigned, "unsigned", false, "output the absolute value of the number (only relevant for first kind)")
+}

internal/lib/stirling.go

@@ -0,0 +1,68 @@
+/*
+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"
+)
+
+// TODO: implement both of these with dynamic programming
+
+func Stirling1(n, k *big.Int) *big.Int {
+	if n.Cmp(big.NewInt(0)) == 0 && k.Cmp(big.NewInt(0)) == 0 {
+		return big.NewInt(1)
+	}
+
+	if n.Cmp(big.NewInt(0)) == 0 || k.Cmp(big.NewInt(0)) == 0 {
+		return big.NewInt(0)
+	}
+
+	newN := new(big.Int).Set(n)
+	newN.Sub(newN, big.NewInt(1))
+
+	result := Stirling1(newN, k)
+	result.Mul(result, newN)
+	result.Neg(result)
+
+	newK := new(big.Int).Set(k)
+	newK.Sub(newK, big.NewInt(1))
+
+	result.Add(result, Stirling1(newN, newK))
+	return result
+}
+
+func Stirling2(n, k *big.Int) *big.Int {
+	if n.Cmp(k) == 0 {
+		return big.NewInt(1)
+	}
+
+	if n.Cmp(big.NewInt(0)) == 0 || k.Cmp(big.NewInt(0)) == 0 {
+		return big.NewInt(0)
+	}
+
+	newN := new(big.Int).Set(n)
+	newN.Sub(newN, big.NewInt(1))
+
+	result := Stirling2(newN, k)
+	result.Mul(result, k)
+
+	newK := new(big.Int).Set(k)
+	newK.Sub(newK, big.NewInt(1))
+
+	result.Add(result, Stirling2(newN, newK))
+	return result
+}