Add file priorities.

crates/librqbit/src/opened_file.rs

@@ -31,6 +31,20 @@ pub(crate) fn dummy_file() -> anyhow::Result<std::fs::File> {
         .with_context(|| format!("error opening {}", DEVNULL))
 }
 
+// Iterate file pieces in the following order: first, last, everything else from start to end.
+fn iter_piece_priorities(range: std::ops::Range<usize>) -> impl Iterator<Item = usize> {
+    // First and last of each file first, then the rest of pieces in that file.
+    let r = range;
+    use std::iter::once;
+
+    let first = once(r.start);
+    let last = once(r.start + r.len().overflowing_sub(1).0); // it's ok if it repeats, doesn't matter
+    let mid = r.clone().skip(1).take(r.len().overflowing_sub(2).0);
+
+    // The take(r.len()) is to not yield start/end pieces in case of 0 and 1 lengths.
+    first.chain(last).chain(mid).take(r.len())
+}
+
 impl OpenedFile {
     pub fn new(
         f: File,
@@ -94,4 +108,24 @@ impl OpenedFile {
         self.have.fetch_add(size, Ordering::Relaxed);
         size
     }
+
+    pub fn iter_piece_priorities(&self) -> impl Iterator<Item = usize> {
+        iter_piece_priorities(self.piece_range_usize())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::opened_file::iter_piece_priorities;
+
+    #[test]
+    fn test_iter_piece_priorities() {
+        let it = |r: std::ops::Range<usize>| -> Vec<usize> { iter_piece_priorities(r).collect() };
+        assert_eq!(it(0..0), Vec::<usize>::new());
+
+        assert_eq!(it(0..1), vec![0]);
+        assert_eq!(it(0..2), vec![0, 1]);
+        assert_eq!(it(0..3), vec![0, 2, 1]);
+        assert_eq!(it(0..4), vec![0, 3, 1, 2]);
+    }
 }