Implement AFC file seek (#28)

* Implement AFC file seek

* refactored to be more readable

idevice/src/services/afc/file.rs

@@ -1,5 +1,7 @@
 // Jackson Coxson
 
+use std::io::SeekFrom;
+
 use crate::IdeviceError;
 
 use super::{
@@ -19,28 +21,76 @@ pub struct FileDescriptor<'a> {
 }
 
 impl FileDescriptor<'_> {
-    /// Closes the file descriptor
-    pub async fn close(self) -> Result<(), IdeviceError> {
-        let header_payload = self.fd.to_le_bytes().to_vec();
+    /// Generic helper to send an AFC packet and read the response
+    async fn send_packet(
+        &mut self,
+        opcode: AfcOpcode,
+        header_payload: Vec<u8>,
+        payload: Vec<u8>,
+    ) -> Result<AfcPacket, IdeviceError> {
         let header_len = header_payload.len() as u64 + AfcPacketHeader::LEN;
-
         let header = AfcPacketHeader {
             magic: super::MAGIC,
-            entire_len: header_len,
+            entire_len: header_len + payload.len() as u64,
             header_payload_len: header_len,
             packet_num: self.client.package_number,
-            operation: AfcOpcode::FileClose,
+            operation: opcode,
         };
         self.client.package_number += 1;
 
         let packet = AfcPacket {
             header,
             header_payload,
-            payload: Vec::new(),
+            payload,
         };
 
         self.client.send(packet).await?;
-        self.client.read().await?;
+        self.client.read().await
+    }
+
+    /// Returns the current cursor position for the file
+    pub async fn seek_tell(&mut self) -> Result<u64, IdeviceError> {
+        let header_payload = self.fd.to_le_bytes().to_vec();
+        let res = self
+            .send_packet(AfcOpcode::FileTell, header_payload, Vec::new())
+            .await?;
+
+        let cur_pos = res
+            .header_payload
+            .get(..8)
+            .ok_or(IdeviceError::UnexpectedResponse)?
+            .try_into()
+            .map(u64::from_le_bytes)
+            .map_err(|_| IdeviceError::UnexpectedResponse)?;
+
+        Ok(cur_pos)
+    }
+
+    /// Moves the file cursor
+    pub async fn seek(&mut self, pos: SeekFrom) -> Result<(), IdeviceError> {
+        let (offset, whence) = match pos {
+            SeekFrom::Start(off) => (off as i64, 0),
+            SeekFrom::Current(off) => (off, 1),
+            SeekFrom::End(off) => (off, 2),
+        };
+
+        let mut header_payload = Vec::new();
+        header_payload.extend(self.fd.to_le_bytes());
+        header_payload.extend((whence as u64).to_le_bytes());
+        header_payload.extend(offset.to_le_bytes());
+
+        self.send_packet(AfcOpcode::FileSeek, header_payload, Vec::new())
+            .await?;
+
+        Ok(())
+    }
+
+    /// Closes the file descriptor
+    pub async fn close(mut self) -> Result<(), IdeviceError> {
+        let header_payload = self.fd.to_le_bytes().to_vec();
+
+        self.send_packet(AfcOpcode::FileClose, header_payload, Vec::new())
+            .await?;
         Ok(())
     }
 
@@ -49,32 +99,18 @@ impl FileDescriptor<'_> {
     /// # Returns
     /// A vector containing the file's data
     pub async fn read(&mut self) -> Result<Vec<u8>, IdeviceError> {
-        // Get the file size first
-        let mut bytes_left = self.client.get_file_info(&self.path).await?.size;
+        let seek_pos = self.seek_tell().await? as usize;
+        let file_info = self.client.get_file_info(&self.path).await?;
+        let mut bytes_left = file_info.size.saturating_sub(seek_pos);
         let mut collected_bytes = Vec::with_capacity(bytes_left);
 
         while bytes_left > 0 {
             let mut header_payload = self.fd.to_le_bytes().to_vec();
             header_payload.extend_from_slice(&MAX_TRANSFER.to_le_bytes());
-            let header_len = header_payload.len() as u64 + AfcPacketHeader::LEN;
+            let res = self
+                .send_packet(AfcOpcode::Read, header_payload, Vec::new())
+                .await?;
 
-            let header = AfcPacketHeader {
-                magic: super::MAGIC,
-                entire_len: header_len,
-                header_payload_len: header_len,
-                packet_num: self.client.package_number,
-                operation: AfcOpcode::Read,
-            };
-            self.client.package_number += 1;
-
-            let packet = AfcPacket {
-                header,
-                header_payload,
-                payload: Vec::new(),
-            };
-
-            self.client.send(packet).await?;
-            let res = self.client.read().await?;
             bytes_left -= res.payload.len();
             collected_bytes.extend(res.payload);
         }
@@ -87,29 +123,10 @@ impl FileDescriptor<'_> {
     /// # Arguments
     /// * `bytes` - Data to write to the file
     pub async fn write(&mut self, bytes: &[u8]) -> Result<(), IdeviceError> {
-        let chunks = bytes.chunks(MAX_TRANSFER as usize);
-
-        for chunk in chunks {
+        for chunk in bytes.chunks(MAX_TRANSFER as usize) {
             let header_payload = self.fd.to_le_bytes().to_vec();
-            let header_len = header_payload.len() as u64 + AfcPacketHeader::LEN;
-
-            let header = AfcPacketHeader {
-                magic: super::MAGIC,
-                entire_len: header_len + chunk.len() as u64,
-                header_payload_len: header_len,
-                packet_num: self.client.package_number,
-                operation: AfcOpcode::Write,
-            };
-            self.client.package_number += 1;
-
-            let packet = AfcPacket {
-                header,
-                header_payload,
-                payload: chunk.to_vec(),
-            };
-
-            self.client.send(packet).await?;
-            self.client.read().await?;
+            self.send_packet(AfcOpcode::Write, header_payload, chunk.to_vec())
+                .await?;
         }
         Ok(())
     }