Add block_header_hash() function

sway-lib-std/src/block.sw

@@ -1,6 +1,15 @@
 //! Functionality for accessing block-related data.
 library;
 
+use ::assert::assert;
+use ::constants::ZERO_B256;
+use ::result::Result;
+use ::logging::log;
+
+enum BlockHashError {
+    BlockHeightTooHigh: (),
+}
+
 /// Get the current block height.
 pub fn height() -> u64 {
     asm(height) {
@@ -21,3 +30,51 @@ pub fn timestamp_of_block(block_height: u64) -> u64 {
         timestamp: u64
     }
 }
+
+/// Get the header hash of the block at height `block_height`
+pub fn block_header_hash(block_height: u64) -> Result<b256, BlockHashError> {
+
+    let mut header_hash = ZERO_B256;
+
+    asm(r1: __addr_of(header_hash), r2: block_height) {
+        bhsh r1 r2;
+    };
+
+    // `bhsh` returns b256(0) if the block is not found, so catch this and return an error
+    if header_hash == ZERO_B256 {
+        Result::Err(BlockHashError::BlockHeightTooHigh)
+    } else {
+        Result::Ok(header_hash)
+    }
+}
+
+////////////////////////////////////////////////////////////////////
+// Tests
+////////////////////////////////////////////////////////////////////
+
+#[test(should_revert)]
+fn test_block_header_hash_err_current_height() {
+    // Get the header hash of the current block. Each time this test runs, the block height will be 1. calling BHSH with a height >= current height will fail.
+    let mut hash = block_header_hash(height());
+    let correct_error = match hash {
+        Result::Ok(_) => false,
+        Result::Err(BlockHashError::BlockHeightTooHigh) => true,
+    };
+
+    assert(correct_error);
+}
+
+#[test(should_revert)]
+fn test_block_header_hash_err_future_height() {
+
+    // Try to get header hash of a block in the future
+    // The function should return a BlockHashError
+    let hash = block_header_hash(height() + 1);
+    let correct_error = match hash {
+        Result::Ok(_) => false,
+        Result::Err(BlockHashError::BlockHeightTooHigh) => true,
+    };
+
+    assert(correct_error);
+
+}