Refactor Nitrokey::execute_command to write to a buffer

To avoid unnecessary allocations and to simplify the code, this patch
refactors Nitrokey::execute_command to write the data to a slice instead
of returning it as an array.

1 files changed, 42 insertions, 38 deletions

diff --git a/src/device.rs b/src/device.rs
index 9ffc39a..a6bf9bf 100644
--- a/src/device.rs
+++ b/src/device.rs
@@ -10,6 +10,7 @@ use crate::util::TryFrom;
 const VID_CLAY_LOGIC: u16 = 0x20a0;
 const PID_NITROKEY_PRO: u16 = 0x4108;
 
+const COMMAND_LEN: usize = 53;
 const REPORT_LEN: usize = 64;
 const REPORT_DESCRIPTOR: &[u8] = &[
     0x05, 0x01, 0x09, 0x06, 0xA1, 0x01, 0x05, 0x07, 0x19, 0xE0, 0x29, 0xE7, 0x15, 0x00, 0x25, 0x01,
@@ -65,41 +66,41 @@ impl Nitrokey {
         }
     }
 
-    fn execute_command(
-        &self,
-        command_id: CommandId,
-        data: &[u8],
-    ) -> Result<[u8; 53], CommandStatus> {
-        match command_id {
+    fn execute_command(&self, command_id: CommandId, data: &[u8], buf: &mut [u8]) -> CommandStatus {
+        let data: &[u8] = match command_id {
             CommandId::GetStatus => {
-                let mut data = [0u8; 53];
-                data[0] = 1; // firmware_version_st.minor
-                data[1] = 0; // firmware_version_st.major
-                data[2] = 0; // card_serial[0]
-                data[3] = 0; // card_serial[1]
-                data[4] = 0; // card_serial[2]
-                data[5] = 0; // card_serial[3]
-                data[6] = 0; // numlock
-                data[7] = 0; // capslock
-                data[8] = 0; // enable_user_password
-                data[9] = 0; // delete_user_password
-                Ok(data)
+                &[
+                    1, // firmware_version_st.minor
+                    0, // firmware_version_st.major
+                    0, // card_serial[0]
+                    0, // card_serial[1]
+                    0, // card_serial[2]
+                    0, // card_serial[3]
+                    0, // numlock
+                    0, // capslock
+                    0, // enable_user_password
+                    0, // delete_user_password
+                ]
             }
             CommandId::ReadSlotName => {
                 assert!(data.len() > 1);
                 let slot_number = data[0];
                 if slot_number != 0x20 {
-                    Err(CommandStatus::SlotNotProgrammed)
-                } else {
-                    let mut data = [0u8; 53];
-                    data[0] = 0x74; // t
-                    data[1] = 0x65; // e
-                    data[2] = 0x73; // s
-                    data[3] = 0x74; // t
-                    Ok(data)
+                    return CommandStatus::SlotNotProgrammed;
                 }
+
+                &[
+                    0x74, // t
+                    0x65, // e
+                    0x73, // s
+                    0x74, // t
+                    0x00, // NULL
+                ]
             }
-        }
+        };
+        assert!(buf.len() >= data.len());
+        buf[..data.len()].copy_from_slice(data);
+        CommandStatus::Ok
     }
 }
 
@@ -121,18 +122,21 @@ impl HidDevice for Nitrokey {
             return Err(());
         }
 
+        const PREFIX_LEN: usize = 7;
+        const SUFFIX_LEN: usize = 4;
+        const PREFIX_END: usize = PREFIX_LEN;
+        const SUFFIX_START: usize = REPORT_LEN - SUFFIX_LEN;
+        let mut buf = [0; COMMAND_LEN];
+
         let device_status = DeviceStatus::Ok;
         let command_id = self.buf[0];
-        let result = if let Ok(command_id) = CommandId::try_from(command_id) {
-            self.execute_command(command_id, &self.buf[1..60])
+        let command_status = if let Ok(command_id) = CommandId::try_from(command_id) {
+            self.execute_command(command_id, &self.buf[1..60], &mut buf)
         } else {
-            Err(CommandStatus::UnknownCommand)
+            CommandStatus::UnknownCommand
         };
-        let (command_status, command_data) = result
-            .map(|d| (CommandStatus::Ok, d))
-            .unwrap_or_else(|status| (status, [0; 53]));
 
-        let pre_data: [u8; 7] = [
+        let pre_data: [u8; PREFIX_LEN] = [
             device_status.into(), // device status
             command_id,           // command_id
             self.buf[60],
@@ -141,13 +145,13 @@ impl HidDevice for Nitrokey {
             self.buf[63],          // last_command_crc
             command_status.into(), // last_command_status
         ];
-        let post_data: [u8; 4] = [
+        let post_data: [u8; SUFFIX_LEN] = [
             0x00, 0x00, 0x00, 0x01, // crc -- libnitrokey accepts any non-zero value
         ];
 
-        self.buf[0..7].copy_from_slice(&pre_data);
-        self.buf[7..7 + command_data.len()].copy_from_slice(&command_data);
-        self.buf[60..64].copy_from_slice(&post_data);
+        self.buf[..PREFIX_END].copy_from_slice(&pre_data);
+        self.buf[PREFIX_END..SUFFIX_START].copy_from_slice(&buf);
+        self.buf[SUFFIX_START..].copy_from_slice(&post_data);
         Ok(&self.buf)
     }