FrankC01 · October 20, 2021 10:38 · Oct 20, 2021
diff --git a/main.rs b/main.rs
@@ -0,0 +1,139 @@
+// Used to map pointers into data array
+use arrayref::{array_mut_ref, array_ref, array_refs, mut_array_refs};
+use borsh::{BorshDeserialize, BorshSerialize};
+use std::iter::repeat;
+
+const INITIALIZED_BYTES: usize = 1;
+const VECTOR_CHUNK_LENGTH: usize = 4;
+const VECTOR_CHUNK_BYTES: usize = 5116;
+const DATA_ACCOUNT_STATE_SPACE: usize =
+    INITIALIZED_BYTES + VECTOR_CHUNK_LENGTH + VECTOR_CHUNK_BYTES;
+
+#[derive(Debug, BorshDeserialize, BorshSerialize, Default)]
+struct FauxStructure {
+    somestring: String,
+    somecount: u32,
+}
+
+#[derive(Debug, BorshDeserialize, BorshSerialize, Default)]
+struct FauxProgram {
+    initialized: bool,
+    data_vec: Vec<FauxStructure>,
+}
+
+impl FauxProgram {
+    /// Add a new FauxStructure to the vector
+    fn add_struct(&mut self, mystruct: FauxStructure) {
+        self.data_vec.push(mystruct);
+        self.drop_first()
+    }
+    /// Drops the first element if there are 2 or more
+    fn drop_first(&mut self) {
+        // If I have more than one entry
+        // remove the 0th
+        if self.data_vec.len() > 2 {
+            self.data_vec.remove(0);
+        }
+        // Do nothing
+        else {
+        }
+    }
+
+    /// Emulate Pack
+    #[allow(clippy::ptr_offset_with_cast)]
+    fn pack_into_slice(&self, dst: &mut [u8]) {
+        // Setup a pointer to the overall data
+        let dst = array_mut_ref![dst, 0, DATA_ACCOUNT_STATE_SPACE];
+        // Convert array to three (3) reference pointers
+        let (is_initialized_dst, vec_len, vec_dst) = mut_array_refs![
+            dst,
+            INITIALIZED_BYTES,
+            VECTOR_CHUNK_LENGTH,
+            VECTOR_CHUNK_BYTES
+        ];
+
+        is_initialized_dst[0] = self.initialized as u8;
+        // Serialize the vector using Borsh
+        let data_ser = self.data_vec.try_to_vec().unwrap();
+        // Get the resulting length in bytes
+        let data_len = data_ser.len();
+        // Store the length (used in later unpacking/deserializing)
+        vec_len[..].copy_from_slice(&(data_len as u32).to_le_bytes());
+        // Store the data
+        vec_dst[..data_len].copy_from_slice(&data_ser);
+    }
+
+    /// Emulate Unpack
+    #[allow(clippy::ptr_offset_with_cast)]
+    fn unpack_from_slice(src: &[u8]) -> Self {
+        let src = array_ref![src, 0, DATA_ACCOUNT_STATE_SPACE];
+        let (is_initialized_src, vec_len, vec_src) = array_refs![
+            src,
+            INITIALIZED_BYTES,
+            VECTOR_CHUNK_LENGTH,
+            VECTOR_CHUNK_BYTES
+        ];
+        let is_initialized = match is_initialized_src {
+            [0] => false,
+            [1] => true,
+            _ => panic!(),
+        };
+        // Setup default empty vector if first use
+        let mut vec_dser = Vec::<FauxStructure>::new();
+        let data_length = u32::from_le_bytes(*vec_len) as usize;
+        // Has data, deseriaalize to vector of structures
+        if data_length > 0 {
+            vec_dser = Vec::<FauxStructure>::try_from_slice(&vec_src[0..data_length]).unwrap()
+        }
+        // Return instance of encapsulating structure
+        Self {
+            initialized: is_initialized,
+            data_vec: vec_dser,
+        }
+    }
+}
+fn main() {
+    // Setup the 'emulated' account data space
+    let mut faux_data: Vec<u8> = repeat(0u8).take(DATA_ACCOUNT_STATE_SPACE).collect();
+    // Emulate unpacking from an accounts data area
+    let mut account_state = FauxProgram::unpack_from_slice(&faux_data);
+    println!("Account initial state T0 = {:?}", account_state);
+    // Should always initialize the state before instruction data
+    account_state.initialized = true;
+    // Add 1st fake structure destined to reside in account's data space
+    account_state.add_struct(FauxStructure {
+        somestring: String::from("first"),
+        somecount: 1u32,
+    });
+    println!("Account state T1 (added first) = {:?}", account_state);
+    // Add 2nd fake structure destined to reside in account's data space
+    account_state.add_struct(FauxStructure {
+        somestring: String::from("second"),
+        somecount: 2u32,
+    });
+    println!(
+        "Account state T2 (added second and packing) = {:?}",
+        account_state
+    );
+    // Emulate packing into account data space
+    account_state.pack_into_slice(&mut faux_data);
+    // Emulate unpacking from an accounts data area
+    let mut account_state_next = FauxProgram::unpack_from_slice(&faux_data);
+    println!(
+        "Account state T3 (after unpacked) = {:?}",
+        account_state_next
+    );
+    // Add 3rd fake structure destined to resode in account's data space
+    account_state_next.add_struct(FauxStructure {
+        somestring: String::from("third"),
+        somecount: 3u32,
+    });
+    println!(
+        "Account state T4 (added third and packing) = {:?}",
+        account_state
+    );
+    // Emulate packing into account data space
+    account_state_next.pack_into_slice(&mut faux_data);
+    let account_state_last = FauxProgram::unpack_from_slice(&faux_data);
+    println!("Account state end mock = {:?}", account_state_last);
+}
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