old interface: Added more efficient flow of output `stdout` object(s) through main branches. Internal buffer is flushed into lock before it is dropped in the `-` case, & only the non-locked handle is flushed in the cli-string case.

TODO: We should move this into `master` too really. No reason not to.

Fortune for rematch's current commit: Curse − 凶

src/main.rs

@@ -4,6 +4,52 @@ mod re;
 mod text;
 mod args;
 
+/// Run an expression on an named value with a result type `Result<T, U>`.
+/// Where `T` and `U` have *the same API surface* for the duration of the provided expression.
+///
+/// # Example
+/// If there is a value `let mut value: Result<T, U>`, where `T: Write` & `U: BufWrite`;
+/// the expression `value.flush()` is valid for both `T` and `U`.
+/// Therefore, it can be simplified to be called as so: `unwrap_either(mut value => value.flush())`.
+///
+/// # Reference capture vs. `move` capture.
+/// Note that by default, the identified value is **moved** *into* the expression.
+/// The type of reference can be controlled by appending `ref`, `mut`, or `ref mut` to the ident.
+///
+/// Identifier capture table:
+/// - **none** ~default~ - Capture by move, value is immutable in expression.
+/// - `mut` - Capture by move, value is mutable in expression.
+/// - `ref` - Capture by ref, value is immutable (`&value`) in expression.
+/// - `ref mut` - Capture by mutable ref, value is mutable (`&mut value`) in expression. (__NOTE__: `value` must be defined as mutable to take a mutable reference of it.)
+///
+/// Essentially the same rules as any `match` branch pattern.
+macro_rules! unwrap_either {
+    ($res:ident => $($rest:tt)+) => {
+        match $res {
+	    Ok(ref mut $res) => $($rest)+,
+	    Err(ref mut $res) => $($rest)+,
+	}
+    };
+    (ref mut $res:ident => $($rest:tt)+) => {
+        match $res {
+	    Ok(ref mut $res) => $($rest)+,
+	    Err(ref mut $res) => $($rest)+,
+	}
+    };
+    (ref $res:ident => $($rest:tt)+) => {
+        match $res {
+	    Ok(ref $res) => $($rest)+,
+	    Err(ref $res) => $($rest)+,
+	}
+    };
+    (mut $res:ident => $($rest:tt)+) => {
+        match $res {
+	    Ok(mut $res) => $($rest)+,
+	    Err(mut $res) => $($rest)+,
+	}
+    };
+}
+
 use color_eyre::{
     eyre::{
         self,
@@ -175,15 +221,16 @@ fn main() -> eyre::Result<()>
 		.wrap_err("Invalid group index specified")?
 	};
 
-	//TODO: XXX: How to handle multiple groups in `stdin_lines()` case?
-	//let group = groups[0]; //args[3].parse().expect("Invalid group number.");
-
 	use std::io::Write;
 	let mut stdout = std::io::stdout();
-	
+
-	if &text[..] == "-" {
+	// Take the kind of `stdout` used (locked & buffered, or not locked & buffered) ..
+	let stdout = if &text[..] == "-" {
+	    // Lock the output for the duration of the read lines.
+	    // Buffer the output in program memory to make processing a bit faster (i.e. the segmented 'write' operations in `print_groups()`, which may be called many times here) & not have to wait on write lines for no reason (since we're already waiting on read lines.)
 	    let mut stdout = std::io::BufWriter::new(stdout.lock());
-	    text::stdin_lines(move |text| -> eyre::Result<bool> {
+	    
+	    text::stdin_lines(|text| -> eyre::Result<bool> {
 		match re.exec(&text)? {
 		    Some(g) /*if g.len() > group*/ => // NOTE: This check branch has now been moved into `print_groups()`
 			print_groups(&mut stdout, &g, &groups, &print_cfg)?, //println!("{}", &g[group]),
@@ -191,13 +238,21 @@ fn main() -> eyre::Result<()>
 		}
 		Ok(true)
 	    })?;
+
+	    // Return the buffer to the main block to be flushed to output (see above & below.)
+	    Some(stdout)
 	} else {
 	    match re.exec(&text)? {
 		Some(g) /*if g.len() > group*/ => print_groups(&mut stdout, &g[..], &groups, print_cfg)?,//println!("{}", &g.nth(group).unwrap().map(|x| x.as_ref()).unwrap_or("")),
 		_ => (),
 	    }
-	}
+
-	stdout.flush().unwrap();
+	    // As there is no internal buffer used, there is no reason to pass it to be flushed by the program when finished like the `-` case above.
+	    None
+	}.ok_or_else(move || stdout); // NOTE ^: Instead, we have it flush the non-memory-buffered output handle before exiting the program.
+	
+	// and .. Ensure the stream (and buffer, if used) is flushed (then dropped.)
+	unwrap_either!(mut stdout => stdout.flush()).unwrap();
     }
     Ok(())
 }