add validate_target

Scarb.toml

@@ -4,3 +4,6 @@ version = "0.1.0"
 edition = "2023_11"
 
 [dependencies]
+
+[dev-dependencies]
+cairo_test = "2.7.0"

src/state.cairo

@@ -31,7 +31,7 @@ pub struct Block {
 /// Block header
 /// https://developer.bitcoin.org/reference/block_chain.html#block-headers
 #[derive(Drop, Copy)]
-struct Header {
+pub struct Header {
     /// The version of the block.
     pub version: u32,
     /// The hash of the previous block in the blockchain.

src/validation.cairo

@@ -35,8 +35,11 @@ fn validate_proof_of_work(self: @ChainState, block: @Block) -> Result<(), ByteAr
 }
 
 fn validate_target(self: @ChainState, block: @Block) -> Result<(), ByteArray> {
-    // TODO: implement
-    Result::Ok(())
+    if self.current_target == block.header.bits {
+        Result::Ok(())
+    } else {
+        Result::Err("Target is {block.header.bits}. Expected {self.current_target}")
+    }
 }
 
 fn validate_timestamp(self: @ChainState, block: @Block) -> Result<(), ByteArray> {
@@ -63,3 +66,43 @@ fn validate_merkle_root(self: @ChainState, block: @Block) -> Result<(), ByteArra
     // TODO: implement
     Result::Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use core::result::ResultTrait;
+    use super::validate_target;
+    use super::{Block, ChainState};
+    use super::super::state::{Header, Transaction, TxIn, TxOut};
+    use core::array::{Span};
+
+    #[test]
+    fn test_validate_target() {
+        let mut chain_state = ChainState {
+            block_height: 1,
+            total_work: 1,
+            best_block_hash: 1,
+            current_target: 1,
+            epoch_start_time: 1,
+            prev_timestamps: array![1, 2, 3, 4, 5].span(),
+        };
+        let mut block = Block {
+            header: Header {
+                version: 1, prev_block_hash: 1, merkle_root_hash: 1, time: 1, bits: 1, nonce: 1,
+            },
+            txs: ArrayTrait::new().span(),
+        };
+
+        let result = validate_target(@chain_state, @block);
+        assert(result.is_ok(), 'Expected target to be valid');
+
+        chain_state.current_target = 2;
+        block.header.bits = 1;
+        let result = validate_target(@chain_state, @block);
+        assert(result.is_err(), 'Expected target to be invalid');
+
+        chain_state.current_target = 1;
+        block.header.bits = 2;
+        let result = validate_target(@chain_state, @block);
+        assert(result.is_err(), 'Expected target to be invalid');
+    }
+}