feat: add a function to read a "bloc / field size" in SSV memory

crates/sesam-vitale/src/main.rs

@@ -1,5 +1,6 @@
 mod ssvlib_demo;
 mod libssv;
+mod ssv_memory;
 
 fn main() {
     ssvlib_demo::demo();

crates/sesam-vitale/src/ssv_memory.rs

@@ -0,0 +1,119 @@
+/**
+ * Provide functions to manipulate raw memory from SSV library.
+ */
+
+// TODO : Est-ce qu'on pourrait/devrait définir un type custom pour représenter les tableaux de bytes ?
+
+#[derive(PartialEq, Debug)]
+pub struct ElementSize {
+    pub size: usize,
+    pub pad: usize,
+}
+
+#[derive(PartialEq, Debug)]
+pub enum MemoryParsingError<'a> {
+    MemoryIsEmpty(&'a str),
+    MemoryIsNotValid(&'a str),
+}
+
+pub fn read_element_size(bytes: &[u8]) -> Result<ElementSize, MemoryParsingError> {
+    /* Longueur:
+     *   - si le bit de poids fort du premier octet est à 0, la longueur est codée sur un octet
+     *   - si le bit de poids fort du premier octet est à 1, les 7 bits de poids faible codent le nombre d'octets utilisés pour coder la longueur
+     */
+    if bytes.len() == 0 {
+        return Err(MemoryParsingError::MemoryIsEmpty("Empty bytes input"));
+    }
+
+    let mut element_size = ElementSize { size: 0, pad: 1 };
+    if bytes[0] & 0b1000_0000 == 0 {
+        // Size coded on 1 byte
+        element_size.size = bytes[0] as usize;
+    } else {
+        // Size coded on N bytes
+        // N are the 7 lower bits of the first byte
+        let size_bytes_len = (bytes[0] & 0b0111_1111) as usize;
+        if size_bytes_len > bytes.len() - 1 {
+            return Err(MemoryParsingError::MemoryIsNotValid(
+                "Invalid memory: not enough bytes to read the size",
+            ));
+        } else if size_bytes_len > 4 {
+            return Err(MemoryParsingError::MemoryIsNotValid(
+                "Invalid memory: size is too big",
+            ));
+        }
+        let size_bytes = &bytes[1..1 + size_bytes_len];
+
+        // u32::from_be_bytes() requires a 4 bytes array
+        let mut padded_bytes = [0u8; 4];
+        padded_bytes[size_bytes_len..].copy_from_slice(&size_bytes);
+
+        element_size.size = u32::from_be_bytes(padded_bytes) as usize;
+        element_size.pad += size_bytes_len;
+    }
+    Ok(element_size)
+}
+
+#[cfg(test)]
+mod test_read_element_size {
+    use super::*;
+
+    #[test]
+    fn short_size() {
+        let bytes = [0b_0000_0001_u8];
+        let element_size = read_element_size(&bytes).unwrap();
+        assert_eq!(element_size.size, 1);
+        assert_eq!(element_size.pad, 1);
+
+        let bytes = [0b_0100_0000_u8];
+        let element_size = read_element_size(&bytes).unwrap();
+        assert_eq!(element_size.size, 64);
+        assert_eq!(element_size.pad, 1);
+    }
+
+    #[test]
+    fn long_size() {
+        let bytes = [0b_1000_0010_u8, 0b_0000_0001_u8, 0b_0100_0000_u8];
+        let element_size = read_element_size(&bytes).unwrap();
+        assert_eq!(element_size.size, 320);
+        assert_eq!(element_size.pad, 3);
+    }
+
+    #[test]
+    fn null_size() {
+        let bytes = [];
+        let toto = "toto";
+        // Expect an error
+        assert_eq!(
+            read_element_size(&bytes),
+            Err(MemoryParsingError::MemoryIsEmpty("Empty bytes input")),
+        );
+    }
+
+    #[test]
+    fn invalid_memory() {
+        let bytes = [0b_1000_0001_u8];
+        assert_eq!(
+            read_element_size(&bytes),
+            Err(MemoryParsingError::MemoryIsNotValid(
+                "Invalid memory: not enough bytes to read the size"
+            )),
+        );
+
+        let bytes = [0b_1000_0010_u8, 1];
+        assert_eq!(
+            read_element_size(&bytes),
+            Err(MemoryParsingError::MemoryIsNotValid(
+                "Invalid memory: not enough bytes to read the size"
+            )),
+        );
+
+        let bytes = [0b_1000_0101_u8, 1, 1, 1, 1, 1];
+        assert_eq!(
+            read_element_size(&bytes),
+            Err(MemoryParsingError::MemoryIsNotValid(
+                "Invalid memory: size is too big"
+            )),
+        );
+    }
+}