correctly update file size after write

fat-bits/src/slice_like.rs

@@ -0,0 +1,58 @@
+use std::fs::File;
+use std::io::{Read as _, Seek as _, SeekFrom, Write as _};
+
+pub trait SliceLike {
+    fn read_at_offset(&mut self, offset: u64, buf: &mut [u8]) -> std::io::Result<()>;
+
+    fn write_at_offset(&mut self, offset: u64, bytes: &[u8]) -> std::io::Result<()>;
+}
+
+impl SliceLike for &mut [u8] {
+    fn read_at_offset(&mut self, offset: u64, buf: &mut [u8]) -> std::io::Result<()> {
+        if offset as usize + buf.len() > self.len() {
+            return Err(std::io::Error::other(anyhow::anyhow!(
+                "reading {} bytes at offset {} is out of bounds for slice of len {}",
+                buf.len(),
+                offset,
+                self.len()
+            )));
+        }
+
+        buf.copy_from_slice(&self[offset as usize..][..buf.len()]);
+
+        Ok(())
+    }
+
+    fn write_at_offset(&mut self, offset: u64, bytes: &[u8]) -> std::io::Result<()> {
+        if offset as usize + bytes.len() > self.len() {
+            return Err(std::io::Error::other(anyhow::anyhow!(
+                "writing {} bytes at offset {} is out of bounds for slice of len {}",
+                bytes.len(),
+                offset,
+                self.len()
+            )));
+        }
+
+        self[offset as usize..][..bytes.len()].copy_from_slice(bytes);
+
+        Ok(())
+    }
+}
+
+impl SliceLike for File {
+    fn read_at_offset(&mut self, offset: u64, buf: &mut [u8]) -> std::io::Result<()> {
+        self.seek(SeekFrom::Start(offset))?;
+
+        self.read_exact(buf)?;
+
+        Ok(())
+    }
+
+    fn write_at_offset(&mut self, offset: u64, bytes: &[u8]) -> std::io::Result<()> {
+        self.seek(SeekFrom::Start(offset))?;
+
+        self.write_all(bytes)?;
+
+        Ok(())
+    }
+}