From e69bd722905d97a709a9bcfe988b24e5538d1a95 Mon Sep 17 00:00:00 2001
From: Francis De Brabandere <francisdb@gmail.com>
Date: Sun, 1 Feb 2026 18:03:05 +0100
Subject: [PATCH] perf: optimize MiniFAT free sector management

Adds a worst case benchmark for MiniFAT write
---
 benches/benchmark.rs      | 26 ++++++++++++++++++++++++++
 src/internal/alloc.rs     |  3 +++
 src/internal/minialloc.rs | 31 ++++++++++++++++++++++++-------
 3 files changed, 53 insertions(+), 7 deletions(-)

diff --git a/benches/benchmark.rs b/benches/benchmark.rs
index 0fb4740..ca111d9 100644
--- a/benches/benchmark.rs
+++ b/benches/benchmark.rs
@@ -107,6 +107,32 @@ fn criterion_benchmark(c: &mut Criterion) {
     }
     small.finish();
 
+    // streams just below the MiniFAT cutoff
+    let mut cutoff = c.benchmark_group("write MiniFAT max streams");
+    let size = (4 * 1024) - 1;
+    let n = 500;
+    let total_bytes = (n * size) as u64;
+    cutoff.sample_size(10);
+    cutoff.throughput(Throughput::Bytes(total_bytes));
+
+    for (buf, label) in buffer_sizes {
+        cutoff.bench_with_input(
+            BenchmarkId::new("total", *label),
+            &size,
+            |b, &s| {
+                b.iter(|| {
+                    let out = write_many_streams(
+                        black_box(n),
+                        black_box(s),
+                        buf.map(black_box),
+                    );
+                    black_box(out);
+                })
+            },
+        );
+    }
+    cutoff.finish();
+
     // several medium streams with throughput reporting
     let mut medium = c.benchmark_group("write several medium streams");
     let size = 1024 * 1024usize;
diff --git a/src/internal/alloc.rs b/src/internal/alloc.rs
index 1621405..1fef5c0 100644
--- a/src/internal/alloc.rs
+++ b/src/internal/alloc.rs
@@ -342,6 +342,9 @@ impl<F: Write + Seek> Allocator<F> {
 
     /// Deallocates the specified sector.
     fn free_sector(&mut self, sector_id: u32) -> io::Result<()> {
+        if self.fat.get(sector_id as usize) == Some(&consts::FREE_SECTOR) {
+            invalid_input!("sector {} freed twice", sector_id);
+        }
         self.set_fat(sector_id, consts::FREE_SECTOR)?;
         self.free_sectors.push(sector_id);
         // TODO: Truncate FAT if last FAT sector is now all free.
diff --git a/src/internal/minialloc.rs b/src/internal/minialloc.rs
index e0a8bc9..2828bba 100644
--- a/src/internal/minialloc.rs
+++ b/src/internal/minialloc.rs
@@ -26,6 +26,7 @@ pub struct MiniAllocator<F> {
     directory: Directory<F>,
     minifat: Vec<u32>,
     minifat_start_sector: u32,
+    free_mini_sectors: Vec<u32>,
 }
 
 impl<F> MiniAllocator<F> {
@@ -35,8 +36,12 @@ impl<F> MiniAllocator<F> {
         minifat_start_sector: u32,
         validation: Validation,
     ) -> io::Result<MiniAllocator<F>> {
-        let mut minialloc =
-            MiniAllocator { directory, minifat, minifat_start_sector };
+        let mut minialloc = MiniAllocator {
+            directory,
+            minifat,
+            minifat_start_sector,
+            free_mini_sectors: Vec::new(),
+        };
         minialloc.validate(validation)?;
         Ok(minialloc)
     }
@@ -139,6 +144,13 @@ impl<F> MiniAllocator<F> {
                 pointees.insert(to_mini_sector);
             }
         }
+
+        self.free_mini_sectors.clear();
+        for (idx, &entry) in self.minifat.iter().enumerate() {
+            if entry == consts::FREE_SECTOR {
+                self.free_mini_sectors.push(idx as u32);
+            }
+        }
         Ok(())
     }
 }
@@ -237,11 +249,10 @@ impl<F: Write + Seek> MiniAllocator<F> {
     /// returns the new mini sector number.
     fn allocate_mini_sector(&mut self, value: u32) -> io::Result<u32> {
         // If there's an existing free mini sector, use that.
-        for mini_sector in 0..self.minifat.len() {
-            if self.minifat[mini_sector] == consts::FREE_SECTOR {
-                let mini_sector = mini_sector as u32;
-                self.set_minifat(mini_sector, value)?;
-                return Ok(mini_sector);
+        while let Some(free_idx) = self.free_mini_sectors.pop() {
+            if self.minifat[free_idx as usize] == consts::FREE_SECTOR {
+                self.set_minifat(free_idx, value)?;
+                return Ok(free_idx);
             }
         }
         // Otherwise, we need a new mini sector; if there's not room in the
@@ -305,7 +316,11 @@ impl<F: Write + Seek> MiniAllocator<F> {
 
     /// Deallocates the specified mini sector.
     fn free_mini_sector(&mut self, mini_sector: u32) -> io::Result<()> {
+        if self.minifat[mini_sector as usize] == consts::FREE_SECTOR {
+            invalid_input!("sector {} freed twice", mini_sector);
+        }
         self.set_minifat(mini_sector, consts::FREE_SECTOR)?;
+        self.free_mini_sectors.push(mini_sector);
         let mut mini_stream_len = self.directory.root_dir_entry().stream_len;
         debug_assert_eq!(mini_stream_len % consts::MINI_SECTOR_LEN as u64, 0);
         while self.minifat.last() == Some(&consts::FREE_SECTOR) {
@@ -313,6 +328,8 @@ impl<F: Write + Seek> MiniAllocator<F> {
             self.minifat.pop();
             // TODO: Truncate MiniFAT if last MiniFAT sector is now all free.
         }
+        let minifat_len = self.minifat.len();
+        self.free_mini_sectors.retain(|&idx| (idx as usize) < minifat_len);
 
         if mini_stream_len != self.directory.root_dir_entry().stream_len {
             self.directory.with_root_dir_entry_mut(|dir_entry| {