Fix isCoinbase

Signed-off-by: James Zuccon <zuccon@gmail.com>

packages/primitives/src/transaction.ts

@@ -108,24 +108,22 @@ export class Transaction {
   }
 
   public isCoinbase(): boolean {
-    // TODO: This definitely needs checking.
-    // TODO: You're a moron Jim. outpointTransactionHash is 0x00 x 32 for Coinbase.
-
-    // A coinbase transaction MUST only have one input.
-    if (this.getInputs().length > 1) {
+    // A coinbase transaction MUST only have one input
+    if (this.getInputs().length !== 1) {
       return false;
     }
 
-    // Get the input's unlocking bytecode.
-    const unlockingBytecode = this.getInputs()[0].unlockingBytecode;
+    const input = this.getInputs()[0];
 
-    // Get the firt byte.
-    const firstByte = unlockingBytecode[0];
+    // Check if the outpoint transaction hash is all zeros (32 bytes of 0x00)
+    const isOutpointZero = input.outpointTransactionHash.every(
+      (byte) => byte === 0x00
+    );
 
-    if (firstByte >= 0x01 && firstByte <= 0x4b) {
-      return true;
-    }
+    // Check if the outpoint index is 0xFFFFFFFF (or -1 in signed 32-bit integer representation)
+    const isOutpointIndexMax = input.outpointIndex === 0xffffffff;
 
-    return false;
+    // Both conditions must be true for a coinbase transaction
+    return isOutpointZero && isOutpointIndexMax;
   }
 }