I should probably make more commits

src/iter.rs

@@ -0,0 +1,53 @@
+use std::io::Read;
+
+use crate::fat::Fatty;
+use crate::subslice::SubSlice;
+use crate::utils::replace;
+use crate::{FatFs, SliceLike};
+
+pub struct ClusterChainReader<'a, S: SliceLike> {
+    sub_slice: SubSlice<'a, S>,
+
+    next_cluster: Option<u32>,
+}
+
+impl<'a, S: SliceLike> ClusterChainReader<'a, S> {
+    pub fn new(fat_fs: &'a mut FatFs<S>, first_cluster: u32) -> ClusterChainReader<'a, S> {
+        let next_cluster = fat_fs.next_cluster(first_cluster).unwrap_or(None);
+
+        let sub_slice = fat_fs.cluster_as_subslice(first_cluster);
+
+        ClusterChainReader {
+            sub_slice,
+            next_cluster,
+        }
+    }
+
+    fn next_cluster(&mut self) -> bool {
+        let Some(next_cluster) = self.next_cluster else {
+            return false;
+        };
+
+        replace(&mut self.sub_slice, |sub_slice| {
+            let fat_fs = sub_slice.release();
+
+            self.next_cluster = fat_fs.next_cluster(next_cluster).unwrap_or(None);
+
+            fat_fs.cluster_as_subslice(next_cluster)
+        });
+
+        true
+    }
+}
+
+impl<'a, S: SliceLike> Read for ClusterChainReader<'a, S> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        if self.sub_slice.is_empty() {
+            if !self.next_cluster() {
+                return Ok(0);
+            }
+        }
+
+        self.sub_slice.read(buf)
+    }
+}