move stream width related into unit

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/utils/stream/band.rs

@@ -0,0 +1,26 @@
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+/// Stream concurrency factor; this is a live value.
+static WIDTH: AtomicUsize = AtomicUsize::new(32);
+
+/// Practicable limits on the stream width
+pub const WIDTH_LIMIT: (usize, usize) = (1, 1024);
+
+/// Sets the live concurrency factor. The first return value is the previous
+/// width which was replaced. The second return value is the value which was set
+/// after any applied limits.
+pub fn set_width(width: usize) -> (usize, usize) {
+	let width = width.clamp(WIDTH_LIMIT.0, WIDTH_LIMIT.1);
+	(WIDTH.swap(width, Ordering::Relaxed), width)
+}
+
+/// Used by stream operations where the concurrency factor hasn't been manually
+/// supplied by the caller (most uses). Instead we provide a default value which
+/// is adjusted at startup for the specific system and also dynamically.
+#[inline]
+pub fn automatic_width() -> usize {
+	let width = WIDTH.load(Ordering::Relaxed);
+	debug_assert!(width >= WIDTH_LIMIT.0, "WIDTH should not be zero");
+	debug_assert!(width <= WIDTH_LIMIT.1, "WIDTH is probably too large");
+	width
+}