/* This file is the outcome of me starting to learn the Go programming
* language.  I figured I might as well follow the assignments I'm giving
* my students to learn C (http://www.cs.stevens.edu/~jschauma/631/), so
* here's a "trivial cp" implementation.  This can probably be goified
* significantly, but for what it's worth, it does what it's supposed to
* and provided a good practice exercise.
* - Jan Schaumann <jschauma@netmeister.org>, October 2011
*/

package main

import (
	"fmt"
	"os"
	"path"
	"syscall"
)

const _PROGNAME string	= "cp"

/* perform the actual copying */
func copy(src, dest string) {
	var (
		e, r, rfd, wfd int
		sb1, sb2 syscall.Stat_t
	)

	/* check if src exists */
	e = syscall.Stat(src, &sb1)
	if e != 0 {
		fmt.Fprintf(os.Stderr, "%s: %s: %s\n",
				_PROGNAME, src, syscall.Errstr(e))
		os.Exit(1)
	}

	/* check if src is a directory */
	if (syscall.S_IFDIR & sb1.Mode) != 0 {
		fmt.Fprintf(os.Stderr, "%s: %s is a directory\n",
				_PROGNAME, src)
		os.Exit(1)
	}

	/* if dest exists, make sure it's not the same file */
	e = syscall.Stat(dest, &sb2)
	if e == 0 && sb1.Ino == sb2.Ino {
		fmt.Fprintf(os.Stderr, "%s: %s and %s are identical (not copied)\n",
				_PROGNAME, src, dest)
		os.Exit(1)
	}

	rfd, e = syscall.Open(src, syscall.O_RDONLY, 0)
	if rfd == -1 {
		fmt.Fprintf(os.Stderr, "%s: %s: %s\n",
				_PROGNAME, src, syscall.Errstr(e))
		os.Exit(1)
	}
	defer syscall.Close(rfd)

	/* if dest is a directory, create the file under it */
	if (syscall.S_IFDIR & sb2.Mode) != 0 {
		dest = dest + "/" + path.Base(src)
	}

	wfd, e = syscall.Open(dest, syscall.O_WRONLY|syscall.O_CREAT|syscall.O_TRUNC, 0666)
	if wfd == -1 {
		fmt.Fprintf(os.Stderr, "%s: %s: %s\n",
				_PROGNAME, dest, syscall.Errstr(e))
		os.Exit(1)
	}
	defer syscall.Close(wfd)

	/* shortcut for zero-length source files */
	if (sb1.Size == 0) {
		return
	}

	var buf = make([]byte, sb1.Blksize)
	for {
		r, e = syscall.Read(rfd, buf)
		if r < 0 {
			fmt.Fprintf(os.Stderr, "%s: %s\n",
					_PROGNAME, syscall.Errstr(e))
			os.Exit(1)
		}
		if r == 0 {
			break
		}

		w, e2 := syscall.Write(wfd, buf[:r])
		if w != r || w == -1 {
			fmt.Fprintf(os.Stderr, "%s: %s\n",
					_PROGNAME, syscall.Errstr(e2))
			/* We differ from cp(1) here in that we abort on
			* error; cp(1) warns and moves on to the next
			* file.  This trivial version of cp only handles
			* one file, so we can exit. */
			os.Exit(1)
		}
	}
}

/* trivially copy a file to another file or into a directory */
func main() {

	if len(os.Args) != 3 {
		fmt.Fprintf(os.Stderr, "Usage: %s src dest\n", _PROGNAME)
		os.Exit(1)
	}

	argv := os.Args
	copy(argv[1], argv[2])
}