implement to and from be bytes for stored type

lib-bms-protocol/src/types.rs

@@ -46,12 +46,16 @@
 
 const U26_VALUE_MASK: u32 = 0xFFFFFFc0;
 const U26_MAX_VALUE: u32 = 0x03FFFFFF;
+const U26_HEMMING_MASK: u32 = 0x0000003e;
 
+#[derive(Debug, PartialEq)]
 pub enum U26Error {
+    InvalidCodeWord,
+    IncorrectibleError,
     Overflow,
 }
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 /// this type is used for storing data on memory devices the concern here is mostly protecting against memory corruption
 /// du to faulty memory cells. Here one bit errors are the most probabl cause of errors so this types uses a build in
 /// hemming code for one mit error correction.
@@ -68,6 +72,38 @@ impl Stored_U26 {
         let hemming_code = calc_hemming(value);
         Self((value << 6) | ((hemming_code & 0x3F) << 1) as u32)
     }
+
+    pub fn to_be_bytes(&self) -> [u8; 4] {
+        self.0.to_be_bytes()
+    }
+
+    pub fn from_be_bytes(bytes: [u8;4]) -> Result<Self, U26Error> {
+        let raw_u32 = u32::from_be_bytes(bytes);
+        // the last bit is expetced to be 0
+        if raw_u32 & 0x1 != 0x0 {
+            return Err(U26Error::InvalidCodeWord);
+        }
+        let value = (raw_u32 & U26_VALUE_MASK) >> 6;
+        let hemming = ((raw_u32 & U26_HEMMING_MASK) >> 1) as u8;
+        match check_hemming(value, hemming) {
+            Ok(()) => Ok(
+                Stored_U26(raw_u32)
+            ),
+            Err(correction_data) => {
+                let (dc, hc) = correction_data.calc_correction_data();
+                if check_hemming(value ^ dc, hemming ^ hc).is_err() {
+                    // this case will not occur in our case because
+                    // or codeword length corresponds to 31 and thus a perfect hemming
+                    // code, flipping the correct bit will always produce a valid
+                    // codeword even though the data was not the orginal stored data,
+                    // to secure against this a crc is required
+                    Err(U26Error::IncorrectibleError)
+                } else {
+                    Ok(Self::new(value ^ dc))
+                }
+            },
+        }
+    }
 }
 
 impl From<Stored_U26> for u32 {
@@ -276,7 +312,9 @@ pub fn check_hemming(data: u32, hemming: u8) -> Result<(), HemmingCorrectionValu
 mod base_type_tests {
     use std::println;
 
-    use super::{calc_hemming, check_hemming, U26_MAX_VALUE};
+    use crate::types::U26Error;
+
+    use super::{calc_hemming, check_hemming, Stored_U26, U26_MAX_VALUE};
 
     #[test]
     fn hemming_code_generation() {
@@ -345,12 +383,12 @@ mod base_type_tests {
     }
 
     #[test]
-    fn hemming_dual_bit_error_detection() {
-        let data = 0x123456;
-        let hemming = calc_hemming(data);
-        let bit_error = 0x101;
-        let correction = check_hemming(data ^ bit_error, hemming).unwrap_err();
-        let (dc, hc) = correction.calc_correction_data();
-        assert!(check_hemming(data ^ dc, hemming ^ hc).is_err())
+    fn stored_u26_to_from_bytes() {
+        let data: Stored_U26 = 0x123456.try_into().unwrap();
+        let mut bytes = data.to_be_bytes();
+        assert_eq!(Stored_U26::from_be_bytes(bytes).unwrap(), data);
+        // introduce 1 bit error
+        bytes[2] = bytes[2] ^ 0x01;
+        assert_eq!(Stored_U26::from_be_bytes(bytes).unwrap(), data);
     }
 }