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use crate::error::{DbError, DbResult};
/// In-memory bitmap for tracking occupied/free slots.
///
/// Each bit represents one slot: 1 = occupied, 0 = free.
/// Backed by `Vec<u64>` — 64 slots per word.
pub struct Bitmap {
/// Bit storage: 1 = occupied, 0 = free.
words: Vec<u64>,
/// Total number of logical slots.
count: u32,
/// Number of currently occupied slots.
occupied: u32,
/// Index of the first word that *might* contain a free bit.
/// Invariant: all words before this index are `u64::MAX`.
first_free_hint: usize,
}
#[inline]
const fn words_for(bits: u32) -> usize {
bits.div_ceil(64) as usize
}
#[allow(dead_code)]
impl Bitmap {
/// Create a bitmap with `count` slots, all free.
pub fn new(count: u32) -> Self {
let n = words_for(count);
Self {
words: vec![0u64; n],
count,
occupied: 0,
first_free_hint: 0,
}
}
/// Find the first free slot, mark it occupied, and return its id.
///
/// Returns `Err(SlotsFull)` when every slot is occupied.
pub fn alloc(&mut self) -> DbResult<u32> {
let len = self.words.len();
for i in self.first_free_hint..len {
let word = self.words[i];
if word != u64::MAX {
let bit = (!word).trailing_zeros(); // first 0-bit
let slot_id = i as u32 * 64 + bit;
if slot_id >= self.count {
// Trailing bits beyond `count` — no real slot here.
break;
}
self.words[i] |= 1u64 << bit;
self.occupied += 1;
// If this word is now full, future scans can skip it.
if self.words[i] == u64::MAX {
self.first_free_hint = i + 1;
}
return Ok(slot_id);
}
}
Err(DbError::SlotsFull)
}
/// Mark `slot_id` as occupied.
///
/// # Panics
/// Panics if `slot_id >= count`.
pub fn set(&mut self, slot_id: u32) {
assert!(slot_id < self.count, "slot_id out of range");
let (wi, bit) = (slot_id as usize / 64, slot_id % 64);
let mask = 1u64 << bit;
if self.words[wi] & mask == 0 {
self.words[wi] |= mask;
self.occupied += 1;
}
}
/// Mark `slot_id` as free.
///
/// # Panics
/// Panics if `slot_id >= count`.
pub fn clear(&mut self, slot_id: u32) {
assert!(slot_id < self.count, "slot_id out of range");
let (wi, bit) = (slot_id as usize / 64, slot_id % 64);
let mask = 1u64 << bit;
if self.words[wi] & mask != 0 {
self.words[wi] &= !mask;
self.occupied -= 1;
if wi < self.first_free_hint {
self.first_free_hint = wi;
}
}
}
/// Returns `true` if `slot_id` is occupied.
///
/// # Panics
/// Panics if `slot_id >= count`.
pub fn is_set(&self, slot_id: u32) -> bool {
assert!(slot_id < self.count, "slot_id out of range");
let (wi, bit) = (slot_id as usize / 64, slot_id % 64);
self.words[wi] & (1u64 << bit) != 0
}
/// Number of currently occupied slots.
pub fn occupied(&self) -> u32 {
self.occupied
}
/// Total number of logical slots.
pub fn count(&self) -> u32 {
self.count
}
/// Returns `true` when every slot is occupied.
pub fn is_full(&self) -> bool {
self.occupied == self.count
}
/// Grow the bitmap to hold at least `new_count` slots.
///
/// # Panics
/// Panics if `new_count < count` (shrinking is not supported).
pub fn grow(&mut self, new_count: u32) {
assert!(
new_count >= self.count,
"cannot shrink bitmap: {} -> {}",
self.count,
new_count
);
let new_words = words_for(new_count);
self.words.resize(new_words, 0u64);
self.count = new_count;
}
/// Serialize the bitmap words as a byte slice (little-endian).
pub fn as_bytes(&self) -> &[u8] {
// SAFETY: `u64` has no padding; its natural alignment (8) is >= alignment of u8.
unsafe {
std::slice::from_raw_parts(
self.words.as_ptr() as *const u8,
self.words.len() * std::mem::size_of::<u64>(),
)
}
}
/// Build a bitmap by marking every slot whose `meta`'s status is OCCUPIED.
pub fn from_versions(versions: &[u32]) -> Self {
use crate::fixed::slot;
let mut b = Bitmap::new(versions.len() as u32);
for (i, &m) in versions.iter().enumerate() {
if slot::status_of(m) == slot::STATUS_OCCUPIED {
b.set(i as u32);
}
}
b
}
/// Deserialize a bitmap from raw bytes and a logical slot count.
///
/// Recomputes `occupied` and `first_free_hint` from the data.
///
/// # Panics
/// Panics if `data.len()` is not a multiple of 8 or does not have
/// enough words for `count`.
pub fn from_bytes(data: &[u8], count: u32) -> Self {
assert!(
data.len().is_multiple_of(8),
"data length must be a multiple of 8"
);
let n_words = data.len() / 8;
assert!(
n_words >= words_for(count),
"not enough words for {count} slots"
);
let mut words = vec![0u64; n_words];
// SAFETY: copying aligned-enough bytes into Vec<u64>.
unsafe {
std::ptr::copy_nonoverlapping(data.as_ptr(), words.as_mut_ptr() as *mut u8, data.len());
}
// Mask out any trailing bits beyond `count` so they don't
// pollute the occupied counter.
let tail = count % 64;
if tail != 0 {
let last = words_for(count) - 1;
words[last] &= (1u64 << tail) - 1;
}
let mut occupied: u32 = 0;
let mut first_free_hint: usize = n_words; // assume full
for (i, &w) in words.iter().enumerate() {
occupied += w.count_ones();
if first_free_hint == n_words && w != u64::MAX {
first_free_hint = i;
}
}
// If all words are full but occupied < count, the last word has
// free tail bits — handle that edge via the general hint logic:
// `first_free_hint` is already set correctly because the masked
// last word won't be MAX when tail bits exist.
Self {
words,
count,
occupied,
first_free_hint,
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_from_versions() {
use crate::fixed::slot::{STATUS_DELETED, STATUS_FREE, STATUS_OCCUPIED, pack_meta};
let v = vec![
pack_meta(STATUS_OCCUPIED, 1),
pack_meta(STATUS_DELETED, 2),
pack_meta(STATUS_FREE, 0),
pack_meta(STATUS_OCCUPIED, 5),
];
let b = Bitmap::from_versions(&v);
assert!(b.is_set(0));
assert!(!b.is_set(1));
assert!(!b.is_set(2));
assert!(b.is_set(3));
assert_eq!(b.occupied(), 2);
}
#[test]
fn test_alloc_sequential() {
let mut bm = Bitmap::new(128);
for i in 0..128 {
assert_eq!(bm.alloc().unwrap(), i);
}
assert!(bm.is_full());
assert_eq!(bm.occupied(), 128);
assert!(bm.alloc().is_err());
}
#[test]
fn test_free_and_realloc() {
let mut bm = Bitmap::new(64);
let a = bm.alloc().unwrap();
let b = bm.alloc().unwrap();
let c = bm.alloc().unwrap();
assert_eq!((a, b, c), (0, 1, 2));
// Free the middle slot.
bm.clear(b);
assert_eq!(bm.occupied(), 2);
assert!(!bm.is_set(b));
// Next alloc should reuse slot 1.
let reused = bm.alloc().unwrap();
assert_eq!(reused, b);
assert_eq!(bm.occupied(), 3);
}
#[test]
fn test_grow() {
let mut bm = Bitmap::new(64);
for _ in 0..64 {
bm.alloc().unwrap();
}
assert!(bm.is_full());
bm.grow(128);
assert!(!bm.is_full());
assert_eq!(bm.count(), 128);
let slot = bm.alloc().unwrap();
assert_eq!(slot, 64);
}
#[test]
fn test_serde_roundtrip() {
let mut bm = Bitmap::new(200);
bm.set(0);
bm.set(63);
bm.set(64);
bm.set(127);
bm.set(199);
let bytes = bm.as_bytes().to_vec();
let restored = Bitmap::from_bytes(&bytes, 200);
assert_eq!(restored.count(), 200);
assert_eq!(restored.occupied(), 5);
assert!(restored.is_set(0));
assert!(restored.is_set(63));
assert!(restored.is_set(64));
assert!(restored.is_set(127));
assert!(restored.is_set(199));
assert!(!restored.is_set(1));
assert!(!restored.is_set(100));
}
#[test]
fn test_set_idempotent() {
let mut bm = Bitmap::new(64);
bm.set(5);
bm.set(5);
assert_eq!(bm.occupied(), 1);
}
#[test]
fn test_clear_idempotent() {
let mut bm = Bitmap::new(64);
bm.set(5);
bm.clear(5);
bm.clear(5);
assert_eq!(bm.occupied(), 0);
}
#[test]
#[should_panic(expected = "slot_id out of range")]
fn test_set_out_of_range() {
let mut bm = Bitmap::new(64);
bm.set(64);
}
#[test]
fn test_non_aligned_count() {
// Count not a multiple of 64.
let mut bm = Bitmap::new(100);
for i in 0..100 {
assert_eq!(bm.alloc().unwrap(), i);
}
assert!(bm.is_full());
assert!(bm.alloc().is_err());
}
}