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//! BidiIterator: a bidirectional iterator that implements `DoubleEndedIterator`.
//!
//! Supports two modes:
//! - **Materialized**: pre-sorted entries for immediate bidirectional access
//! - **Lazy**: wraps a `DBIterator` for streaming forward iteration.
//! First `next_back()` uses `seek_to_last()` (O(log N)).
//! Subsequent `next_back()` calls use `db_iter.prev()` for O(1) memory streaming.
use std::mem;
use crate::iterator::db_iter::DBIterator;
/// A bidirectional iterator over (user_key, value) pairs.
pub struct BidiIterator {
inner: BidiInner,
}
enum BidiInner {
/// Pre-materialized entries with front/back cursors.
Materialized {
entries: Vec<(Vec<u8>, Vec<u8>)>,
front: usize,
back: usize,
},
/// Lazy streaming via DBIterator. Forward `next()` is zero-overhead
/// (no clone into consumed buffer). Backward access on first `next_back()`
/// uses `seek_to_last()` (O(log N)).
Lazy {
db_iter: Box<DBIterator>,
/// When true, skip backward support (pure forward streaming).
forward_only: bool,
/// Number of entries consumed via next() (for correct front cursor on materialize).
fwd_count: usize,
},
/// After first next_back() via seek_to_last(), subsequent next_back() calls
/// use db_iter.prev() for O(1) memory streaming backward.
LazyBackStarted {
db_iter: Box<DBIterator>,
/// Snapshot of db_iter.last_user_key() at the Lazy→LazyBackStarted transition.
/// Used for re-seeking forward on direction change.
last_fwd_key: Option<Vec<u8>>,
/// Last key returned by next_back(), used as forward stop boundary.
last_back_key: Option<Vec<u8>>,
},
/// After calling next() while in LazyBackStarted, the db_iter is re-seeked
/// forward with upper_bound set. Streams forward with O(1) memory.
/// next_back() returns None (forward-only after direction switch).
LazyFwdResumed { db_iter: Box<DBIterator> },
}
impl BidiIterator {
/// Create from a pre-sorted, deduplicated list of (user_key, value) pairs.
pub fn new(entries: Vec<(Vec<u8>, Vec<u8>)>) -> Self {
let len = entries.len();
Self {
inner: BidiInner::Materialized {
entries,
front: 0,
back: len,
},
}
}
/// Create a lazy streaming iterator from a DBIterator.
pub fn lazy(db_iter: DBIterator) -> Self {
Self {
inner: BidiInner::Lazy {
db_iter: Box::new(db_iter),
forward_only: false,
fwd_count: 0,
},
}
}
/// Create a forward-only lazy streaming iterator.
/// `next_back()` returns `None`.
pub fn lazy_forward(db_iter: DBIterator) -> Self {
Self {
inner: BidiInner::Lazy {
db_iter: Box::new(db_iter),
forward_only: true,
fwd_count: 0,
},
}
}
/// Number of remaining entries. Only accurate in materialized mode.
pub fn remaining(&self) -> usize {
match &self.inner {
BidiInner::Materialized { front, back, .. } => back.saturating_sub(*front),
_ => 0,
}
}
/// Whether the iterator has been exhausted.
pub fn is_empty(&self) -> bool {
match &self.inner {
BidiInner::Materialized { front, back, .. } => front >= back,
_ => false,
}
}
}
impl Iterator for BidiIterator {
type Item = (Vec<u8>, Vec<u8>);
fn next(&mut self) -> Option<Self::Item> {
match &mut self.inner {
BidiInner::Materialized {
entries,
front,
back,
} => {
if *front < *back {
let item = entries[*front].clone();
*front += 1;
Some(item)
} else {
None
}
}
BidiInner::Lazy {
db_iter, fwd_count, ..
} => {
let entry = db_iter.next()?;
*fwd_count += 1;
Some(entry)
}
BidiInner::LazyBackStarted { .. } => {
// Switch to streaming forward — O(1) memory, no collect().
let placeholder = BidiInner::Materialized {
entries: Vec::new(),
front: 0,
back: 0,
};
let old = mem::replace(&mut self.inner, placeholder);
if let BidiInner::LazyBackStarted {
mut db_iter,
last_fwd_key,
last_back_key,
..
} = old
{
// Re-seek forward past the last consumed forward key.
match &last_fwd_key {
Some(key) => {
db_iter.seek(key);
// Skip last_fwd_key itself (already returned via next()).
if db_iter.valid() && db_iter.key() == Some(key.as_slice()) {
db_iter.advance();
}
}
None => {
// Never called next() before backward mode — start from beginning.
db_iter.seek_to_first();
}
}
// Set upper bound to stop before already-returned backward entries.
if let Some(ref back_key) = last_back_key {
db_iter.set_upper_bound(back_key.clone());
}
// Stream forward — no materialization needed.
self.inner = BidiInner::LazyFwdResumed { db_iter };
self.next()
} else {
unreachable!()
}
}
BidiInner::LazyFwdResumed { db_iter } => db_iter.next(),
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
match &self.inner {
BidiInner::Materialized { front, back, .. } => {
let r = back.saturating_sub(*front);
(r, Some(r))
}
_ => (0, None),
}
}
}
impl DoubleEndedIterator for BidiIterator {
fn next_back(&mut self) -> Option<Self::Item> {
match &mut self.inner {
BidiInner::Materialized {
entries,
front,
back,
} => {
if *back > *front {
*back -= 1;
Some(entries[*back].clone())
} else {
None
}
}
BidiInner::Lazy { forward_only, .. } if *forward_only => None,
BidiInner::Lazy { .. } => {
// First next_back(): use seek_to_last() for O(log N) access.
// This is the hot path for vsdb's `last()` = `iter().next_back()`.
let placeholder = BidiInner::Materialized {
entries: Vec::new(),
front: 0,
back: 0,
};
let old = mem::replace(&mut self.inner, placeholder);
let BidiInner::Lazy { mut db_iter, .. } = old else {
unreachable!()
};
// Snapshot last_user_key before seek_to_last destroys it
let last_fwd_key = db_iter.last_user_key().map(|k| k.to_vec());
db_iter.seek_to_last();
if !db_iter.valid() {
return None;
}
let k = db_iter.key()?.to_vec();
let v = db_iter.value()?.to_vec();
self.inner = BidiInner::LazyBackStarted {
db_iter,
last_fwd_key,
last_back_key: Some(k.clone()),
};
Some((k, v))
}
BidiInner::LazyFwdResumed { .. } => {
// Need backward access — materialize remaining forward window
// (bounded by upper_bound already set on db_iter).
let placeholder = BidiInner::Materialized {
entries: Vec::new(),
front: 0,
back: 0,
};
let old = mem::replace(&mut self.inner, placeholder);
let BidiInner::LazyFwdResumed { mut db_iter } = old else {
unreachable!()
};
let entries: Vec<(Vec<u8>, Vec<u8>)> = db_iter.by_ref().collect();
let len = entries.len();
self.inner = BidiInner::Materialized {
entries,
front: 0,
back: len,
};
self.next_back()
}
BidiInner::LazyBackStarted {
db_iter,
last_fwd_key,
last_back_key,
} => {
// Second+ next_back(): stream backward via db_iter.prev().
// O(1) memory — no materialization needed.
db_iter.prev();
if db_iter.valid() {
let k = db_iter.key()?.to_vec();
let v = db_iter.value()?.to_vec();
// Stop if backward cursor has crossed the forward frontier.
// Keep last_back_key unchanged — it's the correct upper bound
// for a subsequent next() materialization.
if let Some(fk) = last_fwd_key.as_deref()
&& k.as_slice() <= fk
{
return None;
}
*last_back_key = Some(k.clone());
Some((k, v))
} else {
// Keep last_back_key — it's the correct upper bound for a
// subsequent next() so it won't re-stream consumed entries.
None
}
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
fn make_entries(keys: &[&[u8]]) -> Vec<(Vec<u8>, Vec<u8>)> {
keys.iter()
.map(|k| {
(
k.to_vec(),
format!("val_{}", String::from_utf8_lossy(k)).into_bytes(),
)
})
.collect()
}
#[test]
fn test_forward_only() {
let entries = make_entries(&[b"a", b"b", b"c"]);
let mut it = BidiIterator::new(entries);
assert_eq!(it.next().unwrap().0, b"a");
assert_eq!(it.next().unwrap().0, b"b");
assert_eq!(it.next().unwrap().0, b"c");
assert!(it.next().is_none());
}
#[test]
fn test_reverse_only() {
let entries = make_entries(&[b"a", b"b", b"c"]);
let mut it = BidiIterator::new(entries);
assert_eq!(it.next_back().unwrap().0, b"c");
assert_eq!(it.next_back().unwrap().0, b"b");
assert_eq!(it.next_back().unwrap().0, b"a");
assert!(it.next_back().is_none());
}
#[test]
fn test_interleaved() {
let entries = make_entries(&[b"a", b"b", b"c", b"d"]);
let mut it = BidiIterator::new(entries);
assert_eq!(it.next().unwrap().0, b"a");
assert_eq!(it.next_back().unwrap().0, b"d");
assert_eq!(it.next().unwrap().0, b"b");
assert_eq!(it.next_back().unwrap().0, b"c");
assert!(it.next().is_none());
assert!(it.next_back().is_none());
}
#[test]
fn test_cursor_crossing() {
let entries = make_entries(&[b"a", b"b", b"c", b"d", b"e"]);
let mut it = BidiIterator::new(entries);
assert_eq!(it.next().unwrap().0, b"a");
assert_eq!(it.next().unwrap().0, b"b");
assert_eq!(it.next_back().unwrap().0, b"e");
assert_eq!(it.next_back().unwrap().0, b"d");
assert_eq!(it.remaining(), 1);
assert_eq!(it.next().unwrap().0, b"c");
assert!(it.next().is_none());
assert!(it.next_back().is_none());
}
#[test]
fn test_empty() {
let mut it = BidiIterator::new(vec![]);
assert!(it.is_empty());
assert!(it.next().is_none());
assert!(it.next_back().is_none());
}
#[test]
fn test_single_entry() {
let entries = make_entries(&[b"x"]);
let mut it = BidiIterator::new(entries.clone());
assert_eq!(it.next().unwrap().0, b"x");
assert!(it.next().is_none());
let mut it = BidiIterator::new(entries);
assert_eq!(it.next_back().unwrap().0, b"x");
assert!(it.next_back().is_none());
}
#[test]
fn test_exact_size() {
let entries = make_entries(&[b"a", b"b", b"c"]);
let mut it = BidiIterator::new(entries);
assert_eq!(it.remaining(), 3);
it.next();
assert_eq!(it.remaining(), 2);
it.next_back();
assert_eq!(it.remaining(), 1);
}
// Helper: create a lazy BidiIterator from user keys using DBIterator internals.
fn make_lazy_bidi(keys: &[&[u8]]) -> BidiIterator {
use crate::types::{InternalKey, ValueType};
let mut entries: Vec<(Vec<u8>, Vec<u8>)> = keys
.iter()
.enumerate()
.map(|(i, k)| {
let ik = InternalKey::new(k, (i + 1) as u64, ValueType::Value);
(ik.into_bytes(), format!("val_{}", i).into_bytes())
})
.collect();
entries.sort_by(|(a, _), (b, _)| crate::types::compare_internal_key(a, b));
let db_iter = DBIterator::new(vec![entries], 100);
BidiIterator::lazy(db_iter)
}
#[test]
fn test_lazy_forward_backward_forward() {
// Bug 5 regression: mixed forward+backward must NOT OOM.
let mut it = make_lazy_bidi(&[b"a", b"b", b"c", b"d", b"e", b"f"]);
// Forward: consume a, b, c
assert_eq!(it.next().unwrap().0, b"a");
assert_eq!(it.next().unwrap().0, b"b");
assert_eq!(it.next().unwrap().0, b"c");
// Backward: consume f, e
assert_eq!(it.next_back().unwrap().0, b"f");
assert_eq!(it.next_back().unwrap().0, b"e");
// Forward again (re-seek): should get d (between c and e)
assert_eq!(it.next().unwrap().0, b"d");
// d is the last entry between front (c) and back (e), so next should be None
assert!(it.next().is_none());
}
#[test]
fn test_lazy_back_then_forward_immediately() {
// next_back() first (no prior next()), then next() to end.
let mut it = make_lazy_bidi(&[b"a", b"b", b"c", b"d"]);
// Back first: get d
assert_eq!(it.next_back().unwrap().0, b"d");
// Forward: should start from beginning, stop before d
assert_eq!(it.next().unwrap().0, b"a");
assert_eq!(it.next().unwrap().0, b"b");
assert_eq!(it.next().unwrap().0, b"c");
assert!(it.next().is_none());
}
#[test]
fn test_lazy_interleaved_exhaustion() {
// Alternate next/next_back until both sides meet.
let mut it = make_lazy_bidi(&[b"a", b"b", b"c", b"d"]);
assert_eq!(it.next().unwrap().0, b"a");
assert_eq!(it.next_back().unwrap().0, b"d");
assert_eq!(it.next_back().unwrap().0, b"c");
// Now forward should get b, then stop (upper bound is c)
assert_eq!(it.next().unwrap().0, b"b");
assert!(it.next().is_none());
}
}