package main

import (
  "fmt"
  "flag"
  "os"
  "log"
  "strings"
)

func Fail(err error, format string, v ...any) {
  err_format := fmt.Sprintf("ERROR: %v; %s", err, format)
  log.Printf(err_format, v...)
  os.Exit(1)
}

func main() {
  var number bool

  flag.BoolVar(&number, "n", false, "Number all nonempty output lines, starting with 1")
  flag.BoolVar(&number, "number", false, "Number all nonempty output lines, starting with 1")
  flag.Parse()

  if flag.NArg() < 1 {
    log.Fatal("USAGE: cat [-n/--number] file0 [fileN]")
    os.Exit(1)
  }

  filename := flag.Arg(0)
  in_file, err := os.ReadFile(filename)

  if err != nil { Fail(err, "cannot open %s:", filename) }

  if(number) {
    count := 1
    for line := range strings.Lines(string(in_file)) {
      fmt.Printf("%0.4d: %s", count, line)
      count++
    }
  } else {
    fmt.Print(string(in_file))
  }
}