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//! Session-local free-space tracking for in-place editing (issue #21).
//!
//! [`EditSession`](crate::EditSession) writes by appending at end-of-file and,
//! on each commit, leaves the superseded object headers and any deleted-object
//! blocks behind as dead bytes. This module records those freed regions so a
//! later allocation can reuse them instead of growing the file, and so a run of
//! free space that reaches end-of-file can be truncated away.
//!
//! It is the in-memory half of HDF5's "free-space management". For a file opened
//! without persistence (the default) it is purely session-local: freed-but-
//! unreused space is invisible to other tools, exactly as the reference C
//! library's default `FSM_AGGR` strategy with persistence off leaves it. When the
//! file was created with `persist = true`, [`EditSession`](crate::EditSession)
//! seeds this list from the on-disk free-space managers (the `FSHD`/`FSSE` blocks
//! the File Space Info superblock-extension message points at) on open and writes
//! it back on each commit, so reuse spans sessions (see
//! [`free_space_manager`](crate::free_space_manager)).
//!
//! The structure is a sorted, fully coalesced list of disjoint `[addr, addr+len)`
//! regions. Every public operation preserves both invariants (sorted by address,
//! no two regions touching or overlapping), so the list is always in a canonical
//! form and `trailing_free` is a single comparison against the highest region.
/// A contiguous run of free bytes in the file, `[addr, addr + len)`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct FreeRegion {
addr: u64,
len: u64,
}
impl FreeRegion {
/// One past the last byte of the region.
fn end(&self) -> u64 {
self.addr + self.len
}
}
/// A sorted, coalesced set of free regions in a single file being edited.
///
/// Invariants, upheld by every method: regions are sorted by `addr`, are
/// non-empty, and never touch or overlap (any two that would are merged on
/// insertion). Allocation is best-fit to limit fragmentation.
#[derive(Debug, Default, Clone)]
pub(crate) struct FreeList {
/// Disjoint regions, sorted ascending by address and never adjacent.
regions: Vec<FreeRegion>,
}
impl FreeList {
/// An empty free list.
pub(crate) fn new() -> Self {
Self {
regions: Vec::new(),
}
}
/// Record `[addr, addr + len)` as free, merging it with any adjacent or
/// overlapping regions so the list stays canonical.
///
/// A zero-length free is a no-op. Overlapping an already-free region is a
/// caller bug (a double-free): in debug builds it panics; in release builds
/// the overlap is absorbed by the merge rather than corrupting the list.
pub(crate) fn free(&mut self, addr: u64, len: u64) {
if len == 0 {
return;
}
let new_end = addr + len;
// Find the first region that ends at or after `addr` — the leftmost one
// that could touch or overlap the freed range. Everything before it is
// strictly to the left with a gap and stays untouched.
let mut lo = 0;
while lo < self.regions.len() && self.regions[lo].end() < addr {
lo += 1;
}
// Find the end of the run of regions that touch or overlap `[addr,
// new_end)`: any region whose start is <= new_end is adjacent/overlapping
// and folds into the merged region.
let mut hi = lo;
let mut merged_addr = addr;
let mut merged_end = new_end;
while hi < self.regions.len() && self.regions[hi].addr <= merged_end {
debug_assert!(
self.regions[hi].addr >= new_end || self.regions[hi].end() <= addr,
"double-free: [{addr}, {new_end}) overlaps free region [{}, {})",
self.regions[hi].addr,
self.regions[hi].end()
);
merged_addr = merged_addr.min(self.regions[hi].addr);
merged_end = merged_end.max(self.regions[hi].end());
hi += 1;
}
let merged = FreeRegion {
addr: merged_addr,
len: merged_end - merged_addr,
};
self.regions.splice(lo..hi, [merged]);
}
/// Reserve `len` bytes from a free region, returning the address handed out,
/// or `None` if no single region is large enough.
///
/// Best-fit: the smallest region that fits, to keep large runs intact. The
/// allocation is taken from the low end of the chosen region; any remainder
/// stays free. `len` of 0 returns `None` (nothing to allocate).
pub(crate) fn alloc(&mut self, len: u64) -> Option<u64> {
if len == 0 {
return None;
}
let mut best: Option<usize> = None;
for (i, r) in self.regions.iter().enumerate() {
if r.len >= len && best.is_none_or(|b| r.len < self.regions[b].len) {
best = Some(i);
}
}
let i = best?;
let addr = self.regions[i].addr;
if self.regions[i].len == len {
self.regions.remove(i);
} else {
self.regions[i].addr += len;
self.regions[i].len -= len;
}
Some(addr)
}
/// The free regions as `(addr, len)` pairs, sorted ascending by address and
/// fully coalesced. Used to persist the free list to disk (issue #21).
pub(crate) fn sections(&self) -> Vec<(u64, u64)> {
self.regions.iter().map(|r| (r.addr, r.len)).collect()
}
/// If a free region ends exactly at `eof` (the current end-of-file), remove
/// it from the list and return its start address — the file can be truncated
/// to that address. Returns `None` if the highest free region does not reach
/// end-of-file.
///
/// Because the list is coalesced, at most one region can end at `eof`, and it
/// is the last one.
pub(crate) fn take_trailing(&mut self, eof: u64) -> Option<u64> {
match self.regions.last() {
Some(last) if last.end() == eof => {
let addr = last.addr;
self.regions.pop();
Some(addr)
}
_ => None,
}
}
}
#[cfg(test)]
mod tests {
use super::*;
/// Expose the canonical region list as `(addr, len)` pairs for assertions.
fn regions(fl: &FreeList) -> Vec<(u64, u64)> {
fl.regions.iter().map(|r| (r.addr, r.len)).collect()
}
#[test]
fn free_into_empty_list() {
let mut fl = FreeList::new();
fl.free(100, 50);
assert_eq!(regions(&fl), [(100, 50)]);
}
#[test]
fn zero_length_free_is_noop() {
let mut fl = FreeList::new();
fl.free(100, 0);
assert!(regions(&fl).is_empty());
}
#[test]
fn disjoint_frees_stay_sorted() {
let mut fl = FreeList::new();
fl.free(300, 10);
fl.free(100, 10);
fl.free(200, 10);
assert_eq!(regions(&fl), [(100, 10), (200, 10), (300, 10)]);
}
#[test]
fn coalesce_with_right_neighbor() {
let mut fl = FreeList::new();
fl.free(200, 50); // [200, 250)
fl.free(150, 50); // [150, 200) touches left edge of the above
assert_eq!(regions(&fl), [(150, 100)]);
}
#[test]
fn coalesce_with_left_neighbor() {
let mut fl = FreeList::new();
fl.free(150, 50); // [150, 200)
fl.free(200, 50); // [200, 250) touches right edge of the above
assert_eq!(regions(&fl), [(150, 100)]);
}
#[test]
fn coalesce_bridges_gap_between_two() {
let mut fl = FreeList::new();
fl.free(100, 50); // [100, 150)
fl.free(250, 50); // [250, 300)
fl.free(150, 100); // [150, 250) bridges the two
assert_eq!(regions(&fl), [(100, 200)]);
}
#[test]
fn no_coalesce_when_gap_remains() {
let mut fl = FreeList::new();
fl.free(100, 50); // [100, 150)
fl.free(151, 50); // [151, 201) one byte gap
assert_eq!(regions(&fl), [(100, 50), (151, 50)]);
}
#[test]
fn alloc_best_fit_chooses_smallest_sufficient() {
let mut fl = FreeList::new();
fl.free(0, 100); // big
fl.free(200, 30); // exact-ish, smallest that fits 30
fl.free(400, 60); // medium
let addr = fl.alloc(30).unwrap();
assert_eq!(addr, 200);
// The 30-region is consumed exactly; the others remain.
assert_eq!(regions(&fl), [(0, 100), (400, 60)]);
}
#[test]
fn alloc_splits_remainder() {
let mut fl = FreeList::new();
fl.free(1000, 100);
let addr = fl.alloc(40).unwrap();
assert_eq!(addr, 1000);
assert_eq!(regions(&fl), [(1040, 60)]);
}
#[test]
fn alloc_none_when_nothing_fits() {
let mut fl = FreeList::new();
fl.free(0, 10);
fl.free(100, 20);
assert!(fl.alloc(50).is_none());
// List is unchanged on a failed allocation.
assert_eq!(regions(&fl), [(0, 10), (100, 20)]);
}
#[test]
fn alloc_zero_returns_none() {
let mut fl = FreeList::new();
fl.free(0, 100);
assert!(fl.alloc(0).is_none());
}
#[test]
fn alloc_then_free_roundtrips() {
let mut fl = FreeList::new();
fl.free(0, 100);
let a = fl.alloc(40).unwrap();
fl.free(a, 40); // give it back
assert_eq!(regions(&fl), [(0, 100)]); // coalesced back to whole
}
#[test]
fn take_trailing_at_eof() {
let mut fl = FreeList::new();
fl.free(500, 100); // [500, 600)
let cut = fl.take_trailing(600);
assert_eq!(cut, Some(500));
assert!(regions(&fl).is_empty());
}
#[test]
fn take_trailing_none_when_not_at_eof() {
let mut fl = FreeList::new();
fl.free(500, 100); // [500, 600)
assert_eq!(fl.take_trailing(700), None); // live bytes between 600 and 700
assert_eq!(regions(&fl), [(500, 100)]); // unchanged
}
#[test]
fn take_trailing_only_cuts_the_tail_region() {
let mut fl = FreeList::new();
fl.free(100, 50); // interior hole [100, 150)
fl.free(500, 100); // trailing [500, 600)
let cut = fl.take_trailing(600);
assert_eq!(cut, Some(500));
assert_eq!(regions(&fl), [(100, 50)]); // interior hole preserved
}
#[test]
fn take_trailing_empty_list() {
let mut fl = FreeList::new();
assert_eq!(fl.take_trailing(0), None);
}
}