[RFC PATCH 22/24] erofs: add skippable iters in Rust

[Yiyang Wu <toolmanp@tlmp.cc>]

This patch introduce self-owned skippable data iterators in Rust.
This iterators will be used to access extended attributes later.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/rust/erofs_sys/data/raw_iters.rs | 121 ++++++++++++++++++++++
 1 file changed, 121 insertions(+)

diff --git a/fs/erofs/rust/erofs_sys/data/raw_iters.rs b/fs/erofs/rust/erofs_sys/data/raw_iters.rs
index 8f3bd250d252..f1ff0a251596 100644
--- a/fs/erofs/rust/erofs_sys/data/raw_iters.rs
+++ b/fs/erofs/rust/erofs_sys/data/raw_iters.rs
@@ -4,3 +4,124 @@
 pub(crate) mod ref_iter;
 mod traits;
 pub(crate) use traits::*;
+
+use super::*;
+use alloc::boxed::Box;
+
+/// Represents a skippable continuous buffer iterator. This is used primarily for reading the
+/// extended attributes. Since the key-value is flattened out in its original format.
+pub(crate) struct SkippableContinuousIter<'a> {
+    iter: Box<dyn ContinuousBufferIter<'a> + 'a>,
+    data: RefBuffer<'a>,
+    cur: usize,
+}
+
+fn cmp_with_cursor_move(
+    lhs: &[u8],
+    rhs: &[u8],
+    lhs_cur: &mut usize,
+    rhs_cur: &mut usize,
+    len: usize,
+) -> bool {
+    let result = lhs[*lhs_cur..(*lhs_cur + len)] == rhs[*rhs_cur..(*rhs_cur + len)];
+    *lhs_cur += len;
+    *rhs_cur += len;
+    result
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub(crate) enum SkipCmpError {
+    PosixError(Errno),
+    NotEqual(Off),
+}
+
+impl From<Errno> for SkipCmpError {
+    fn from(e: Errno) -> Self {
+        SkipCmpError::PosixError(e)
+    }
+}
+
+impl<'a> SkippableContinuousIter<'a> {
+    pub(crate) fn try_new(
+        mut iter: Box<dyn ContinuousBufferIter<'a> + 'a>,
+    ) -> PosixResult<Option<Self>> {
+        if iter.eof() {
+            return Ok(None);
+        }
+        let data = iter.next().unwrap()?;
+        Ok(Some(Self { iter, data, cur: 0 }))
+    }
+    pub(crate) fn skip(&mut self, offset: Off) -> PosixResult<()> {
+        let dlen = self.data.content().len() - self.cur;
+        if offset as usize <= dlen {
+            self.cur += offset as usize;
+        } else {
+            self.cur = 0;
+            self.iter.advance_off(dlen as Off);
+            self.data = self.iter.next().unwrap()?;
+        }
+        Ok(())
+    }
+
+    pub(crate) fn read(&mut self, buf: &mut [u8]) -> PosixResult<()> {
+        let mut dlen = self.data.content().len() - self.cur;
+        let mut bcur = 0_usize;
+        let blen = buf.len();
+        if dlen != 0 && dlen >= blen {
+            buf.clone_from_slice(&self.data.content()[self.cur..(self.cur + blen)]);
+            self.cur += blen;
+        } else {
+            buf[bcur..(bcur + dlen)].copy_from_slice(&self.data.content()[self.cur..]);
+            bcur += dlen;
+            while bcur < blen {
+                self.cur = 0;
+                self.data = self.iter.next().unwrap()?;
+                dlen = self.data.content().len();
+                if dlen >= blen - bcur {
+                    buf[bcur..].copy_from_slice(&self.data.content()[..(blen - bcur)]);
+                    self.cur = blen - bcur;
+                    return Ok(());
+                } else {
+                    buf[bcur..(bcur + dlen)].copy_from_slice(self.data.content());
+                    bcur += dlen;
+                }
+            }
+        }
+        Ok(())
+    }
+
+    pub(crate) fn try_cmp(&mut self, buf: &[u8]) -> Result<(), SkipCmpError> {
+        let dlen = self.data.content().len() - self.cur;
+        let blen = buf.len();
+        let mut bcur = 0_usize;
+
+        if dlen != 0 && dlen >= blen {
+            if cmp_with_cursor_move(self.data.content(), buf, &mut self.cur, &mut bcur, blen) {
+                Ok(())
+            } else {
+                Err(SkipCmpError::NotEqual(bcur as Off))
+            }
+        } else {
+            if dlen != 0 {
+                let clen = dlen.min(blen);
+                if !cmp_with_cursor_move(self.data.content(), buf, &mut self.cur, &mut bcur, clen) {
+                    return Err(SkipCmpError::NotEqual(bcur as Off));
+                }
+            }
+            while bcur < blen {
+                self.cur = 0;
+                self.data = self.iter.next().unwrap()?;
+                let dlen = self.data.content().len();
+                let clen = dlen.min(blen - bcur);
+                if !cmp_with_cursor_move(self.data.content(), buf, &mut self.cur, &mut bcur, clen) {
+                    return Err(SkipCmpError::NotEqual(bcur as Off));
+                }
+            }
+
+            Ok(())
+        }
+    }
+    pub(crate) fn eof(&self) -> bool {
+        self.data.content().len() - self.cur == 0 && self.iter.eof()
+    }
+}
-- 
2.46.0