mathtools/cmd/shoelace.go

/*
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 -f FILE",
	Short: "Compute the area of a simple polygon from the vertex coordinates",
	Long: `Compute the area of a simple polygon from the vertex coordinates.

Put each point on its own line, with each coordinate separated by whitespace. For example, a file with
1 6
3 1
7 2
4 4
8 5
will output an area of 16.5.`,
	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
}