add shoelace command

cmd/shoelace.go

@@ -0,0 +1,123 @@
+/*
+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 (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+var shoelaceFile string
+
+type point struct {
+	x float64
+	y float64
+}
+
+func splitLines(file *os.File) []string {
+	slice := make([]string, 0)
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		slice = append(slice, scanner.Text())
+	}
+
+	return slice
+}
+
+func readFromFile(filepath string) ([]point, error) {
+	points := make([]point, 0)
+	file, err := os.Open(filepath)
+	if err != nil {
+		return points, err
+	}
+	defer file.Close()
+
+	lines := splitLines(file)
+	for _, line := range lines {
+		fields := strings.Fields(line)
+
+		x, err := strconv.ParseFloat(fields[0], 64)
+		if err != nil {
+			return points, err
+		}
+
+		y, err := strconv.ParseFloat(fields[1], 64)
+		if err != nil {
+			return points, err
+		}
+
+		points = append(points, point{x, y})
+	}
+
+	return points, nil
+}
+
+func area(points []point) float64 {
+	total := float64(0)
+	n := len(points)
+	for i, p := range points {
+		q := points[(i+1)%n]
+		total += (p.y + q.y) * (p.x - q.x)
+	}
+
+	return total / 2
+}
+
+func shoelace(cmd *cobra.Command, args []string) {
+	var coordinates []point
+	var err error
+	if shoelaceFile != "" {
+		coordinates, err = readFromFile(shoelaceFile)
+		if err != nil {
+			cobra.CheckErr(err)
+		}
+	} else {
+		cobra.CheckErr("filename required")
+	}
+
+	fmt.Println(area(coordinates))
+}
+
+// shoelaceCmd represents the shoelace command
+var shoelaceCmd = &cobra.Command{
+	Use:   "shoelace",
+	Short: "Compute the area of a simple polygon from the vertex coordinates",
+	Long:  `Compute the area of a simple polygon from the vertex coordinates.`,
+	Run:   shoelace,
+}
+
+func init() {
+	rootCmd.AddCommand(shoelaceCmd)
+
+	// Here you will define your flags and configuration settings.
+
+	// Cobra supports Persistent Flags which will work for this command
+	// and all subcommands, e.g.:
+	// shoelaceCmd.PersistentFlags().String("foo", "", "A help for foo")
+
+	// Cobra supports local flags which will only run when this command
+	// is called directly, e.g.:
+	// shoelaceCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
+
+	shoelaceCmd.Flags().StringVarP(&shoelaceFile, "file", "f", "", "file with coordinates")
+	// TODO: add flag for inputting coordinates from cli
+}