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//! Walking a [`Segment`] in order without allocating per entry: the
//! crate's [`Walker`] and the public, lending [`Scan`] built on it.
use std::cmp::Ordering;
use crate::seg_codec::{Codec, Form, nth_column};
use crate::seg_leaf::{Leaf, NIL, Probe};
use crate::seg_tree::{Pos, Tree};
use crate::segment::Segment;
use crate::value::IndexValue;
/// A position moving through a segment's entries, reading each one in
/// place in its leaf and decoding its key and value only when asked.
#[derive(Debug)]
pub(crate) struct Walker<'s> {
tree: &'s Tree,
codec: &'s Codec,
/// Values are the 8-byte head, so an entry is read without copying.
fixed: bool,
leaf: Option<&'s Leaf>,
slot: usize,
started: bool,
rev: bool,
/// Walking forward, where to stop: at the first entry not below the
/// bytes, or (`true`) past every entry they prefix.
until: Option<(Vec<u8>, bool)>,
/// The stop bytes as a head, for a fixed-width value.
until_head: u64,
head: u64,
rest: &'s [u8],
payload: &'s [u8],
/// The whole order key, assembled only for forms that need it.
e: Vec<u8>,
vlen: usize,
value: IndexValue,
value_ok: bool,
key: Vec<u8>,
key_ok: bool,
}
impl<'s> Walker<'s> {
pub(crate) fn new(seg: &'s Segment, from: Option<Pos>, rev: bool) -> Walker<'s> {
Walker {
tree: &seg.tree,
codec: &seg.codec,
fixed: matches!(seg.codec.form, Form::I64 | Form::F64),
leaf: from.map(|p| seg.tree.leaf(p.leaf)),
slot: from.map_or(0, |p| p.slot),
started: false,
rev,
until: None,
until_head: 0,
head: 0,
rest: &[],
payload: &[],
e: Vec::new(),
vlen: 0,
value: IndexValue::I64(0),
value_ok: false,
key: Vec::new(),
key_ok: false,
}
}
/// Stop at the first entry not below `v`, or with `inclusive` after
/// the last entry `v` prefixes.
pub(crate) fn until(mut self, v: Vec<u8>, inclusive: bool) -> Walker<'s> {
self.until_head = crate::seg_leaf::head_of(&v);
self.until = Some((v, inclusive));
self
}
/// Step to the next slot, crossing into the neighbouring leaf at an
/// edge; `false` past the end.
fn step(&mut self) -> bool {
let Some(l) = self.leaf else { return false };
if !self.started {
self.started = true;
return true;
}
if !self.rev {
if self.slot + 1 < l.len() {
self.slot += 1;
return true;
}
self.enter(l.next, false)
} else {
if self.slot > 0 {
self.slot -= 1;
return true;
}
self.enter(l.prev, true)
}
}
fn enter(&mut self, id: u32, at_end: bool) -> bool {
if id == NIL {
self.leaf = None;
return false;
}
let l = self.tree.leaf(id);
self.leaf = Some(l);
self.slot = if at_end { l.len() - 1 } else { 0 };
true
}
/// Move to the next entry; `false` when there is none.
pub(crate) fn advance(&mut self) -> bool {
if !self.step() {
return false;
}
let l = self.leaf.expect("stepped onto a leaf");
if let Some((u, inclusive)) = &self.until {
let stop = if self.fixed {
// the head is the value itself
let h = l.head(self.slot);
if *inclusive { h > self.until_head } else { h >= self.until_head }
} else if *inclusive {
l.cmp_at(&Probe::past(u), self.slot, &self.tree.ov) == Ordering::Less
} else {
l.cmp_at(&Probe::new(u), self.slot, &self.tree.ov) != Ordering::Greater
};
if stop {
self.leaf = None;
return false;
}
}
let t = l.tail(self.slot, &self.tree.ov);
(self.head, self.rest, self.payload) = (l.head(self.slot), t.rest, t.payload);
if !self.fixed {
self.e.clear();
self.e.extend_from_slice(&self.head.to_be_bytes()[..t.len.min(8)]);
self.e.extend_from_slice(t.rest);
self.vlen = t.vlen;
}
self.value_ok = false;
self.key_ok = false;
true
}
#[inline(always)]
pub(crate) fn value(&mut self) -> &IndexValue {
if !self.value_ok {
if self.fixed {
self.codec.value_into(&self.head.to_be_bytes(), &mut self.value);
} else {
self.codec.value_into(&self.e[..self.vlen], &mut self.value);
}
self.value_ok = true;
}
&self.value
}
#[inline(always)]
pub(crate) fn key(&mut self) -> &[u8] {
if !self.key_ok {
let h = if self.fixed { self.rest } else { &self.e[self.vlen..] };
self.codec.key_into(h, &mut self.key);
self.key_ok = true;
}
&self.key
}
/// Both at once, for callers that want the pair borrowed together.
#[inline(always)]
pub(crate) fn pair(&mut self) -> (&IndexValue, &[u8]) {
self.value();
self.key();
(&self.value, &self.key)
}
pub(crate) fn payload(&self) -> &'s [u8] {
self.payload
}
/// Stored column `field` of the current entry, decoded into `buf`.
pub(crate) fn column<'b>(&self, field: usize, buf: &'b mut Vec<u8>) -> Option<&'b [u8]>
where
's: 'b,
{
if field >= self.codec.arity {
return None;
}
nth_column(self.payload, field).bytes(buf)
}
/// Every stored column of the current entry.
pub(crate) fn row(&self) -> Vec<Option<Vec<u8>>> {
let mut at = 0;
(0..self.codec.arity)
.map(|_| crate::seg_codec::column(self.payload, &mut at).into_vec())
.collect()
}
}
/// An ordered walk over a [`Segment`], lending each entry.
///
/// The segment stores entries encoded, so a walk decodes each one into
/// buffers it owns and lends them until the next call — no allocation per
/// entry. That is why this is not an `Iterator`.
///
/// ```
/// use kevy_index::{IndexValue, Segment, SortOrder};
/// let mut s = Segment::with_values(1);
/// s.apply_with_values(b"b", None, Some(IndexValue::I64(2)), &[Some(b"x")]);
/// s.apply_with_values(b"a", None, Some(IndexValue::I64(1)), &[None]);
/// let mut scan = s.scan(None, SortOrder::Desc);
/// let mut seen = Vec::new();
/// while let Some((v, k)) = scan.next_entry() {
/// seen.push((v.clone(), k.to_vec(), scan.stored_row()));
/// }
/// assert_eq!(seen[0], (IndexValue::I64(2), b"b".to_vec(), vec![Some(b"x".to_vec())]));
/// assert_eq!(seen[1], (IndexValue::I64(1), b"a".to_vec(), vec![None]));
/// ```
#[derive(Debug)]
pub struct Scan<'s> {
w: Walker<'s>,
}
impl<'s> Scan<'s> {
pub(crate) fn new(w: Walker<'s>) -> Scan<'s> {
Scan { w }
}
/// The next entry as `(value, key)`, or `None` at the end.
///
/// ```
/// use kevy_index::{IndexValue, Segment, SortOrder};
/// let mut s = Segment::new();
/// s.apply(b"k", None, Some(IndexValue::I64(5)));
/// let mut scan = s.scan(None, SortOrder::Asc);
/// assert_eq!(scan.next_entry(), Some((&IndexValue::I64(5), &b"k"[..])));
/// assert_eq!(scan.next_entry(), None);
/// ```
pub fn next_entry(&mut self) -> Option<(&IndexValue, &[u8])> {
if !self.w.advance() {
return None;
}
Some(self.w.pair())
}
/// The stored `VALUES` of the entry [`Scan::next_entry`] last
/// returned, in declared order (empty when the index declares none).
///
/// ```
/// use kevy_index::{IndexValue, Segment, SortOrder};
/// let mut s = Segment::with_values(2);
/// s.apply_with_values(b"k", None, Some(IndexValue::I64(5)), &[Some(b"eu"), None]);
/// let mut scan = s.scan(None, SortOrder::Asc);
/// scan.next_entry();
/// assert_eq!(scan.stored_row(), [Some(b"eu".to_vec()), None]);
/// ```
pub fn stored_row(&self) -> Vec<Option<Vec<u8>>> {
self.w.row()
}
}