// Grep searches the input file for lines containing the given pattern and
// prints these lines. It is a simplified version of the Unix grep command.
package main

import (
	"bufio"
	"fmt"
	"io"
	"log"
	"os"
	"strings"
)

const prog = "grep"

func init() {
	log.SetPrefix(prog + ": ")
	log.SetFlags(0) // no extra info in log messages
}

func main() {
	if len(os.Args) != 3 {
		log.Fatalf("usage: %s PATTERN FILE\n", prog)
	}
	pattern := os.Args[1]
	file, err := os.Open(os.Args[2])
	if err != nil {
		log.Fatalln(err)
	}
	defer file.Close()

	if grep(pattern, file) != nil {
		log.Println(err)
	}
}

// grep prints all lines that contain the pattern.
func grep(pattern string, file *os.File) error {
	reader := bufio.NewReader(file)
	for {
		switch line, err := reader.ReadString('\n'); err {
		case nil:
			grepLine(pattern, line)
		case io.EOF:
			if len(line) > 0 { // The final line of the file doesn't end with '\n'.
				grepLine(pattern, line)
			}
			return nil
		default:
			return err
		}
	}
	panic("internal error")
}

// grepLine prints the line if it contains the pattern.
func grepLine(pattern, line string) {
	if strings.Contains(line, pattern) {
		fmt.Print(line)
	}
}