Implement `NtfsIndexAllocation::record_from_vcn` to aid index traversals

2 files changed, 33 insertions, 1 deletions

diff --git a/src/error.rs b/src/error.rs
index 98923f5..b7f753a 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -86,6 +86,8 @@ pub enum NtfsError {
         position: u64,
         ty: NtfsAttributeType,
     },
+    /// The requested Virtual Cluster Number (VCN) {requested_vcn} leads to a record with VCN {record_vcn}
+    VcnMismatch { requested_vcn: Vcn, record_vcn: Vcn },
     /// The Virtual Cluster Number (VCN) {vcn} is too big to be processed
     VcnTooBig { vcn: Vcn },
 }
diff --git a/src/structured_values/index_allocation.rs b/src/structured_values/index_allocation.rs
index e826590..582faec 100644
--- a/src/structured_values/index_allocation.rs
+++ b/src/structured_values/index_allocation.rs
@@ -2,12 +2,13 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 use crate::attribute_value::NtfsAttributeValue;
-use crate::error::Result;
+use crate::error::{NtfsError, Result};
 use crate::index_record::NtfsIndexRecord;
 use crate::ntfs::Ntfs;
 use crate::structured_values::index_root::NtfsIndexRoot;
 use crate::structured_values::NewNtfsStructuredValue;
 use crate::traits::NtfsReadSeek;
+use crate::types::Vcn;
 use binread::io::{Read, Seek, SeekFrom};
 use core::iter::FusedIterator;
 
@@ -22,6 +23,35 @@ impl<'n> NtfsIndexAllocation<'n> {
         let index_record_size = index_root.index_record_size();
         NtfsIndexRecords::new(self.ntfs, self.value.clone(), index_record_size)
     }
+
+    pub fn record_from_vcn<T>(
+        &self,
+        fs: &mut T,
+        index_root: &NtfsIndexRoot<'n>,
+        vcn: Vcn,
+    ) -> Result<NtfsIndexRecord<'n>>
+    where
+        T: Read + Seek,
+    {
+        // Seek to the byte offset of the given VCN.
+        let mut value = self.value.clone();
+        let offset = vcn.offset(self.ntfs)?;
+        value.seek(fs, SeekFrom::Current(offset))?;
+
+        // Get the record.
+        let index_record_size = index_root.index_record_size();
+        let record = NtfsIndexRecord::new(self.ntfs, fs, value, index_record_size)?;
+
+        // Validate that the VCN in the record is the requested one.
+        if record.vcn() != vcn {
+            return Err(NtfsError::VcnMismatch {
+                requested_vcn: vcn,
+                record_vcn: record.vcn(),
+            });
+        }
+
+        Ok(record)
+    }
 }
 
 impl<'n> NewNtfsStructuredValue<'n> for NtfsIndexAllocation<'n> {