add convergents subcommand

cmd/convergents.go

@@ -0,0 +1,98 @@
+/*
+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"
+	"log"
+	"math/big"
+
+	"github.com/spf13/cobra"
+)
+
+func convergents(cmd *cobra.Command, args []string) {
+	hprev := big.NewInt(0)
+	kprev := big.NewInt(1)
+
+	h := big.NewInt(1)
+	k := big.NewInt(0)
+
+	a0, ok := new(big.Int).SetString(args[0], 10)
+	if !ok {
+		log.Fatal("invalid input " + args[0])
+	}
+
+	denoms := make([]big.Int, len(args[1:]))
+	for i, d := range args[1:] {
+		_, ok = denoms[i].SetString(d, 10)
+		if !ok {
+			log.Fatal("invalid input " + d)
+		}
+	}
+
+	tmp := new(big.Int)
+	tmp.Mul(a0, h)
+	tmp.Add(tmp, hprev)
+	hprev.Set(h)
+	h.Set(tmp)
+
+	tmp.Mul(a0, k)
+	tmp.Add(tmp, kprev)
+	kprev.Set(k)
+	k.Set(tmp)
+
+	fmt.Println(h, k)
+
+	for i := 0; true; i = (i + 1) % len(denoms) {
+		a := &denoms[i]
+
+		tmp.Mul(a, h)
+		tmp.Add(tmp, hprev)
+		hprev.Set(h)
+		h.Set(tmp)
+
+		tmp.Mul(a, k)
+		tmp.Add(tmp, kprev)
+		kprev.Set(k)
+		k.Set(tmp)
+
+		fmt.Println(h, k)
+	}
+}
+
+// convergentsCmd represents the convergents command
+var convergentsCmd = &cobra.Command{
+	Use:   "convergents N N [N ...]",
+	Short: "Compute convergents of a periodic continued fraction",
+	Long:  `Compute convergents of a periodic continued fraction.`,
+	Args:  cobra.MinimumNArgs(2),
+	Run:   convergents,
+}
+
+func init() {
+	rootCmd.AddCommand(convergentsCmd)
+
+	// Here you will define your flags and configuration settings.
+
+	// Cobra supports Persistent Flags which will work for this command
+	// and all subcommands, e.g.:
+	// convergentsCmd.PersistentFlags().String("foo", "", "A help for foo")
+
+	// Cobra supports local flags which will only run when this command
+	// is called directly, e.g.:
+	// convergentsCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
+}