Fix Transit_I26 implementation and complete protocol conversion

lib-bms-protocol/src/lib.rs

@@ -185,7 +185,8 @@ impl BmsReadable for BatteryState {
             lifetime_capacity_mah: Transit_U26::from_be_bytes(lifetime_capacity_mah)?.into(),
             remaining_capacity_mah: Transit_U26::from_be_bytes(remaining_capacity_mah)?.into(),
             current_mv: Transit_U26::from_be_bytes(current_mv)?.into(),
-            temperature_celcius: i32::from_be_bytes(temperature_celcius),
+            temperature_celcius: crate::types::Transit_I26::from_be_bytes(temperature_celcius)?
+                .into(),
             health_percent: Transit_U26::from_be_bytes(health_percent)?.into(),
         })
     }

lib-bms-protocol/src/types.rs

@@ -165,7 +165,7 @@ impl Transit_U26 {
 
 impl From<Transit_U26> for u32 {
     fn from(value: Transit_U26) -> Self {
-        value.0 & U26_VALUE_MASK >> 6
+        (value.0 & U26_VALUE_MASK) >> 6
     }
 }
 
@@ -181,6 +181,58 @@ impl TryFrom<u32> for Transit_U26 {
     }
 }
 
+#[derive(Debug, PartialEq)]
+/// this type is used for transmitting signed 26-bit integer data
+/// It wraps the Transit_U26 type to provide i32 support
+/// The i32 value is converted to u32 by casting, preserving the bit pattern
+/// This ensures proper sign extension when converting between signed and unsigned
+/// Valid range: -33554432 to 33554431 (26-bit signed)
+pub struct Transit_I26(Transit_U26);
+
+impl Transit_I26 {
+    pub fn new(value: i32) -> Result<Self, U26Error> {
+        // Check if value is within 26-bit signed range
+        if value < -0x02000000 || value > 0x01FFFFFF {
+            return Err(U26Error::Overflow);
+        }
+        // Extract only the lower 26 bits
+        let u26_val = (value as u32) & 0x03FFFFFF;
+        let transit_u26 = Transit_U26::try_from(u26_val)?;
+        Ok(Self(transit_u26))
+    }
+
+    pub fn from_be_bytes(bytes: [u8; 4]) -> Result<Self, U26Error> {
+        let transit_u26 = Transit_U26::from_be_bytes(bytes)?;
+        Ok(Self(transit_u26))
+    }
+
+    pub fn to_be_bytes(&self) -> [u8; 4] {
+        self.0.to_be_bytes()
+    }
+}
+
+impl From<Transit_I26> for i32 {
+    fn from(value: Transit_I26) -> Self {
+        // Convert u32 back to i32 by casting, preserving the exact bit pattern
+        let u32_val: u32 = value.0.into();
+        // Interpret as signed 26-bit integer
+        if u32_val & 0x02000000 != 0 {
+            // Sign-extend to 32 bits
+            (u32_val as i32) | 0xFC000000u32 as i32
+        } else {
+            u32_val as i32
+        }
+    }
+}
+
+impl TryFrom<i32> for Transit_I26 {
+    type Error = U26Error;
+
+    fn try_from(value: i32) -> Result<Self, Self::Error> {
+        Self::new(value)
+    }
+}
+
 // only calc hemming code for the first 24 data bits including the 5 hemming bits
 //  pos  |     | p1  | p2  | p3  | p4  | p5  |
 // 00001 | p1  |  o  |     |     |     |     |
@@ -388,7 +440,8 @@ pub fn check_crc6(data: u32, crc: u8) -> bool {
 #[cfg(test)]
 mod base_type_tests {
     use super::{
-        Stored_U26, Transit_U26, U26_MAX_VALUE, calc_crc6, calc_hemming, check_crc6, check_hemming,
+        calc_crc6, calc_hemming, check_crc6, check_hemming, Stored_U26, Transit_I26, Transit_U26,
+        U26_MAX_VALUE,
     };
     use crate::types::U26Error;
 
@@ -515,4 +568,52 @@ mod base_type_tests {
             assert!(!check_crc6(corrupted_data, original_crc));
         }
     }
+
+    #[test]
+    fn transit_i26_positive_values() {
+        let data = Transit_I26::new(12345).unwrap();
+        let bytes = data.to_be_bytes();
+        let decoded = Transit_I26::from_be_bytes(bytes).unwrap();
+        assert_eq!(i32::from(decoded), 12345);
+    }
+
+    #[test]
+    fn transit_i26_negative_values() {
+        let data = Transit_I26::new(-12345).unwrap();
+        let bytes = data.to_be_bytes();
+        let decoded = Transit_I26::from_be_bytes(bytes).unwrap();
+        assert_eq!(i32::from(decoded), -12345);
+    }
+
+    #[test]
+    fn transit_i26_zero() {
+        let data = Transit_I26::new(0).unwrap();
+        let bytes = data.to_be_bytes();
+        let decoded = Transit_I26::from_be_bytes(bytes).unwrap();
+        assert_eq!(i32::from(decoded), 0);
+    }
+
+    #[test]
+    fn transit_i26_max_positive() {
+        let data = Transit_I26::new(0x01FFFFFF).unwrap();
+        let bytes = data.to_be_bytes();
+        let decoded = Transit_I26::from_be_bytes(bytes).unwrap();
+        assert_eq!(i32::from(decoded), 0x01FFFFFF as i32);
+    }
+
+    #[test]
+    fn transit_i26_max_negative() {
+        let data = Transit_I26::new(-0x01FFFFFF).unwrap();
+        let bytes = data.to_be_bytes();
+        let decoded = Transit_I26::from_be_bytes(bytes).unwrap();
+        assert_eq!(i32::from(decoded), -0x01FFFFFF as i32);
+    }
+
+    #[test]
+    fn transit_i26_checksum_error() {
+        let data = Transit_I26::new(12345).unwrap();
+        let mut bytes = data.to_be_bytes();
+        bytes[1] ^= 0x01; // Corrupt one byte
+        assert!(Transit_I26::from_be_bytes(bytes).is_err());
+    }
 }