Use `ArrayVec` over heap-allocated `Vec` for some structured values.

This greatly reduces the number of heap allocations when traversing an index.

3 files changed, 15 insertions, 8 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 90cfc1a..047e301 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,6 +7,7 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+arrayvec = { version = "0.7.1", default-features = false }
 binread = { version = "2.1.1", features = ["const_generics"], default-features = false }
 byteorder = { version = "1.4.3", default-features = false }
 bitflags = "1.2.1"
@@ -17,4 +18,4 @@ memoffset = "0.6.4"
 
 [features]
 default = ["std"]
-std = ["binread/std", "byteorder/std"]
+std = ["arrayvec/std", "binread/std", "byteorder/std"]
diff --git a/src/structured_values/file_name.rs b/src/structured_values/file_name.rs
index 0fdac97..24dbbde 100644
--- a/src/structured_values/file_name.rs
+++ b/src/structured_values/file_name.rs
@@ -10,7 +10,7 @@ use crate::structured_values::{
     NtfsFileAttributeFlags, NtfsStructuredValue, NtfsStructuredValueFromSlice,
 };
 use crate::time::NtfsTime;
-use alloc::vec::Vec;
+use arrayvec::ArrayVec;
 use binread::io::Cursor;
 use binread::{BinRead, BinReaderExt};
 use core::mem;
@@ -22,6 +22,10 @@ const FILE_NAME_HEADER_SIZE: usize = 66;
 /// The smallest FileName attribute has a name containing just a single character.
 const FILE_NAME_MIN_SIZE: usize = FILE_NAME_HEADER_SIZE + mem::size_of::<u16>();
 
+/// The "name" stored in the FileName attribute has an `u8` length field specifying the number of UTF-16 code points.
+/// Hence, the name occupies up to 510 bytes.
+const NAME_MAX_SIZE: usize = (u8::MAX as usize) * mem::size_of::<u16>();
+
 #[allow(unused)]
 #[derive(BinRead, Clone, Debug)]
 struct FileNameHeader {
@@ -50,7 +54,7 @@ pub enum NtfsFileNamespace {
 #[derive(Clone, Debug)]
 pub struct NtfsFileName {
     header: FileNameHeader,
-    name: Vec<u8>,
+    name: ArrayVec<u8, NAME_MAX_SIZE>,
 }
 
 impl NtfsFileName {
@@ -104,7 +108,7 @@ impl NtfsFileName {
         debug_assert!(self.name.is_empty());
         let start = FILE_NAME_HEADER_SIZE;
         let end = start + self.name_length();
-        self.name.extend_from_slice(&data[start..end]);
+        self.name.try_extend_from_slice(&data[start..end]).unwrap();
     }
 
     fn validate_name_length(&self, data_size: usize, position: u64) -> Result<()> {
@@ -154,7 +158,7 @@ impl<'s> NtfsStructuredValueFromSlice<'s> for NtfsFileName {
 
         let mut file_name = Self {
             header,
-            name: Vec::new(),
+            name: ArrayVec::new(),
         };
         file_name.validate_name_length(slice.len(), position)?;
         file_name.validate_namespace(position)?;
diff --git a/src/structured_values/volume_name.rs b/src/structured_values/volume_name.rs
index 77b2c8e..91ca1a2 100644
--- a/src/structured_values/volume_name.rs
+++ b/src/structured_values/volume_name.rs
@@ -5,7 +5,7 @@ use crate::attribute::NtfsAttributeType;
 use crate::error::{NtfsError, Result};
 use crate::string::NtfsString;
 use crate::structured_values::{NtfsStructuredValue, NtfsStructuredValueFromSlice};
-use alloc::vec::Vec;
+use arrayvec::ArrayVec;
 use core::mem;
 
 /// The smallest VolumeName attribute has a name containing just a single character.
@@ -16,7 +16,7 @@ const VOLUME_NAME_MAX_SIZE: usize = 128 * mem::size_of::<u16>();
 
 #[derive(Clone, Debug)]
 pub struct NtfsVolumeName {
-    name: Vec<u8>,
+    name: ArrayVec<u8, VOLUME_NAME_MAX_SIZE>,
 }
 
 impl NtfsVolumeName {
@@ -55,7 +55,9 @@ impl<'s> NtfsStructuredValueFromSlice<'s> for NtfsVolumeName {
             });
         }
 
-        let name = slice.to_vec();
+        let mut name = ArrayVec::new();
+        name.try_extend_from_slice(slice).unwrap();
+
         Ok(Self { name })
     }
 }