package main

import (
	"bufio"
	"fmt"
	"io"
	"log"
	"os/exec"
)

// Pajen represents an instance of the pajen.pl script running in infinite
// generation mode.
type Pajen struct {
	Chan chan string

	proc *exec.Cmd
	out  io.ReadCloser
}

// NewPajen starts the pajen script at the path given.
func NewPajen(path string) (*Pajen, error) {
	p := new(Pajen)

	p.proc = exec.Command("/usr/bin/perl", path, "-i")
	p.proc.Stdin = p

	pi, err := p.proc.StdoutPipe()
	if err != nil {
		return nil, fmt.Errorf("pajen: pipe pajen.pl: %w", err)
	}
	p.out = pi
	p.proc.Stderr = p.proc.Stdout

	p.Chan = make(chan string)

	if err := p.proc.Start(); err != nil {
		return nil, fmt.Errorf("pajen: start pajen.pl: %w", err)
	}

	go p.run()
	return p, nil
}

// Read feeds constant newlines to the program.
func (r *Pajen) Read(b []byte) (int, error) {
	if len(b) < 1 {
		return 0, nil
	}

	return copy(b, []byte("\n")), nil
}

func (r *Pajen) run() {
	sc := bufio.NewScanner(r.out)
	sc.Split(bufio.ScanLines)

	for sc.Scan() {
		l := sc.Text()
		if l != "" && l != "\n" {
			r.Chan <- l
		}
	}

	if sc.Err() != nil {
		log.Println("pajen: output read error: %s", sc.Err())
	}
}