[RFC PATCH 12/24] erofs: add directory entry data structure in Rust

[Yiyang Wu <toolmanp@tlmp.cc>]

This patch adds DirentDesc and DirCollection in Rust.
It will later be used as helper to read_dir and lookup operations.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/rust/erofs_sys.rs     |  1 +
 fs/erofs/rust/erofs_sys/dir.rs | 98 ++++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+)
 create mode 100644 fs/erofs/rust/erofs_sys/dir.rs

diff --git a/fs/erofs/rust/erofs_sys.rs b/fs/erofs/rust/erofs_sys.rs
index 15ed65866097..65dc563986c3 100644
--- a/fs/erofs/rust/erofs_sys.rs
+++ b/fs/erofs/rust/erofs_sys.rs
@@ -26,6 +26,7 @@
 pub(crate) mod alloc_helper;
 pub(crate) mod data;
 pub(crate) mod devices;
+pub(crate) mod dir;
 pub(crate) mod errnos;
 pub(crate) mod inode;
 pub(crate) mod map;
diff --git a/fs/erofs/rust/erofs_sys/dir.rs b/fs/erofs/rust/erofs_sys/dir.rs
new file mode 100644
index 000000000000..d4255582b7c0
--- /dev/null
+++ b/fs/erofs/rust/erofs_sys/dir.rs
@@ -0,0 +1,98 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+/// On-disk Directory Descriptor Format for EROFS
+/// Documented on [EROFS Directory](https://erofs.docs.kernel.org/en/latest/core_ondisk.html#directories)
+use core::mem::size_of;
+
+#[repr(C, packed)]
+#[derive(Debug, Clone, Copy)]
+pub(crate) struct DirentDesc {
+    pub(crate) nid: u64,
+    pub(crate) nameoff: u16,
+    pub(crate) file_type: u8,
+    pub(crate) reserved: u8,
+}
+
+/// In memory representation of a real directory entry.
+#[derive(Debug, Clone, Copy)]
+pub(crate) struct Dirent<'a> {
+    pub(crate) desc: DirentDesc,
+    pub(crate) name: &'a [u8],
+}
+
+impl From<[u8; size_of::<DirentDesc>()]> for DirentDesc {
+    fn from(data: [u8; size_of::<DirentDesc>()]) -> Self {
+        Self {
+            nid: u64::from_le_bytes([
+                data[0], data[1], data[2], data[3], data[4], data[5], data[6], data[7],
+            ]),
+            nameoff: u16::from_le_bytes([data[8], data[9]]),
+            file_type: data[10],
+            reserved: data[11],
+        }
+    }
+}
+
+/// Create a collection of directory entries from a buffer.
+/// This is a helper struct to iterate over directory entries.
+pub(crate) struct DirCollection<'a> {
+    data: &'a [u8],
+    offset: usize,
+    total: usize,
+}
+
+impl<'a> DirCollection<'a> {
+    pub(crate) fn new(buffer: &'a [u8]) -> Self {
+        let desc: &DirentDesc = unsafe { &*(buffer.as_ptr() as *const DirentDesc) };
+        Self {
+            data: buffer,
+            offset: 0,
+            total: desc.nameoff as usize / core::mem::size_of::<DirentDesc>(),
+        }
+    }
+    pub(crate) fn dirent(&self, index: usize) -> Option<Dirent<'a>> {
+        let descs: &'a [[u8; size_of::<DirentDesc>()]] =
+            unsafe { core::slice::from_raw_parts(self.data.as_ptr().cast(), self.total) };
+        if index >= self.total {
+            None
+        } else if index == self.total - 1 {
+            let desc = DirentDesc::from(descs[index]);
+            let len = self.data.len() - desc.nameoff as usize;
+            Some(Dirent {
+                desc,
+                name: &self.data[desc.nameoff as usize..(desc.nameoff as usize) + len],
+            })
+        } else {
+            let desc = DirentDesc::from(descs[index]);
+            let next_desc = DirentDesc::from(descs[index + 1]);
+            let len = (next_desc.nameoff - desc.nameoff) as usize;
+            Some(Dirent {
+                desc,
+                name: &self.data[desc.nameoff as usize..(desc.nameoff as usize) + len],
+            })
+        }
+    }
+    pub(crate) fn skip_dir(&mut self, offset: usize) {
+        self.offset += offset;
+    }
+    pub(crate) fn total(&self) -> usize {
+        self.total
+    }
+}
+
+impl<'a> Iterator for DirCollection<'a> {
+    type Item = Dirent<'a>;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.dirent(self.offset).map(|x| {
+            self.offset += 1;
+            x
+        })
+    }
+}
+
+impl<'a> Dirent<'a> {
+    pub(crate) fn dirname(&self) -> &'a [u8] {
+        self.name
+    }
+}
-- 
2.46.0