change --bytes to use block streaming instead of a single alloc->populate->write

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rng"
-version = "0.2.0"
+version = "0.2.1"
 authors = ["Avril <flanchan@cumallover.me>"]
 edition = "2018"
 
@@ -14,3 +14,9 @@ panic = "unwind"
 
 [dependencies]
 getrandom = "0.1"
+
+[features]
+# Dynamically allocate and populate all memory needed for `--bytes` at runtime instead of streaming in fixed size blocks.
+#
+# This can slightly improve total performance for large outputs, at the cost of a proportionally, arbitrary sized memory allocation along with a proportionally increasing wait while the entire area is populated.
+bytes-dynamic = []

src/main.rs

@@ -7,6 +7,8 @@ use getrandom::*;
 mod parse;
 mod r#impl;
 
+const BYTES_BUFFER_SIZE: usize = 4096;
+
 fn get<T: Default>() -> Result<T, Error>
 {
     let mut value: T = Default::default();
@@ -151,22 +153,40 @@ fn shuffle(args: &[String]) -> Result<(), Error>
 fn bytes(args: &[String]) -> Result<(), Error>
 {
     if args.len() < 1 {
-	usage(); 
+	usage();
     } else {
 	let num = parse::bytes(&args[0]).expect("Failed to parse number of bytes") as usize;
-	let mut mem = Vec::with_capacity(num);
-	
-	debug_assert_eq!(mem.capacity(), num);
-	// SAFETY: populating uninitialised memory with random data. There are no reads to the uninitialised data.
-	unsafe {
-	    mem.set_len(num);
-	    populate(&mut mem[..])?;
+
+	if cfg!(feature="bytes-dynamic") {
+	    let mut mem = Vec::with_capacity(num);
+	    
+	    debug_assert_eq!(mem.capacity(), num);
+	    // SAFETY: populating uninitialised memory with random data. There are no reads to the uninitialised data.
+	    unsafe {
+		mem.set_len(num);
+		populate(&mut mem[..])?;
+	    }
+	    debug_assert_eq!(num, mem.len());
+	    
+	    let stdout = std::io::stdout();
+	    
+	    use std::io::Write;
+	    stdout.lock().write_all(&mem[..]).expect("write error");
+	} else {
+	    let mut buffer = [0u8; BYTES_BUFFER_SIZE];
+	    let mut num = num;
+	    let stdout = std::io::stdout();
+	    let mut stdout = stdout.lock();
+	    
+	    while num > 0 {
+		let w = std::cmp::min(num, BYTES_BUFFER_SIZE);
+		getrandom(&mut buffer[..w])?;
+
+		use std::io::Write;
+		stdout.write_all(&buffer[..w]).expect("write error");
+		num -= w;
+	    }
 	}
-	debug_assert_eq!(num, mem.len());
-	
-	let stdout = std::io::stdout();
-	use std::io::Write;
-	stdout.lock().write_all(&mem[..]).expect("write error");
     }
     Ok(())
 }