Begin work on v2.0.0

The original release of mmv took filenames as command line arguments,
opened them in an editor, and then used the saved changes to rename
files.  This commit begins work on a new version of mmv where files are
provided via the standard input and the command line arguments specify a
process to spawn.  The spawned process reads command line arguments from
the standard input, processes them, and prints new names to the standard
output.  Those new names represent the new file names.  Here are a few
example usages, some more useful than others.

Reverse file names:

    $ ls * | mmv rev

Edit file names in your editor (v1.0.0 behavior):

    $ ls * | mmv vipe

Number movies so they’re automatically sorted:

    $ ls movie1 movie2 ... movieN \
          | mmv awk '{ printf "%02d-%s\n", NR, $0 }'

1 files changed, 54 insertions, 24 deletions

diff --git a/src/main.rs b/src/main.rs
index 71052a2..3272041 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,5 +1,6 @@
 mod encoded_string;
 mod error;
+mod flags;
 mod main_result;
 
 use std::{
@@ -7,9 +8,9 @@ use std::{
 	env,
 	fs,
 	hash::Hash,
-	io::{self, BufRead, BufReader, BufWriter, Write, Seek, SeekFrom},
+	io::{self, BufRead, BufReader, Write},
 	path::Path,
-	process::Command
+	process::{Command, Stdio}
 };
 
 use {
@@ -19,6 +20,8 @@ use {
 };
 
 use {
+	flags::Flags,
+	getopt::{Opt, Parser},
 	itertools::Itertools,
 	tempfile::{Builder, NamedTempFile, TempDir}
 };
@@ -28,44 +31,71 @@ fn main() -> MainResult {
 }
 
 fn work() -> Result<(), Error> {
-	let old_files = env::args().skip(1).collect::<Vec<String>>();
-	if old_files.is_empty() {
-		return Err(Error::NoArgs);
+	let mut argv = env::args().collect::<Vec<String>>();
+	let mut flags = Flags { ..Default::default() };
+	let mut opts = Parser::new(&argv, ":0a");
+
+	loop {
+		match opts.next().transpose() {
+			Ok(v) => match v {
+				None => break,
+				Some(opt) => match opt {
+					Opt('0', None) => flags.nul = true,
+					Opt('e', None) => flags.encode = true,
+					Opt('i', None) => flags.individual = true,
+					_ => { return Err(Error::BadArgs); }
+				}
+			},
+			Err(_) => { return Err(Error::BadArgs); }
+		}
+	}
+
+	let rest = argv.split_off(opts.index());
+	let cmd  = rest.get(0).ok_or(Error::BadArgs)?;
+	let args = &rest[1..];
+
+	let old_files: Vec<_> = io::stdin()
+		.lines()
+		.collect::<Result<_, _>>()
+		.unwrap();
+	if old_files.iter().any(|s| s.is_empty()) {
+		return Err(Error::BadArgs);
 	}
 
 	let dups = duplicate_elements(old_files.clone());
 	if !dups.is_empty() {
-		return Err(Error::DuplicateElems(dups));
+		return Err(Error::DupInputElems(dups));
 	}
 
-	let mut tmpfile = NamedTempFile::new()?;
-	let mut writer = BufWriter::new(&tmpfile);
-
-	old_files.iter().try_for_each(|f| encode_to_file(&mut writer, f))?;
-	writer.flush()?;
-	drop(writer);
+	let mut child = Command::new(cmd)
+		.args(args)
+		.stdin(Stdio::piped())
+		.stdout(Stdio::piped())
+		.spawn()
+		.map_err(|e| Error::SpawnFailed(cmd.to_owned(), e))?;
 
-	let editor = env::var("EDITOR")
-		.ok()
-		.filter(|e| !e.is_empty())
-		.ok_or(Error::NoEditor)?;
+	{
+		let mut stdin = child.stdin.take().expect("Failed to open stdin");
+		old_files.iter().try_for_each(|f| writeln!(stdin, "{f}"))?;
+	}
 
-	Command::new(&editor)
-		.arg(tmpfile.path().as_os_str())
-		.spawn()
-		.map_err(|err| Error::SpawnFailed(editor, err))?
-		.wait()?;
+	let output = child.wait_with_output()?;
+	if !output.status.success() {
+		return Err(Error::Nop);
+	}
 
-	tmpfile.seek(SeekFrom::Start(0))?;
+	let new_files = String::from_utf8(output.stdout)?
+		.lines()
+		.map(|s| s.to_string())
+		.collect::<Vec<String>>();
 
-	let new_files = decode_from_file(&tmpfile)?;
 	if old_files.len() != new_files.len() {
 		return Err(Error::BadLengths);
 	}
 
 	let dups = duplicate_elements(new_files.iter().cloned().collect::<Vec<_>>());
 	if !dups.is_empty() {
-		return Err(Error::DuplicateElems(dups));
+		return Err(Error::DupOutputElems(dups));
 	}
 
 	let tmpdir = Builder::new().prefix("mmv").tempdir()?;