Merge #14816: Add CScriptNum decode python implementation in functional suite

2012d4df2 Add CScriptNum decode python implementation in functional suite (Gregory Sanders)

Pull request description:

  I needed this for reasons and thought it'd be good to upsteam it.

Tree-SHA512: 6ea89fa2a5f5a7759ba722f2b4ed5cd6423ebfff4e83ac8b8b5c935e6aa479684e626c5f41fa020816d2a9079a99af5564e30808594d5c13e3b51ec9b474926d

test/functional/test_framework/script.py

@@ -385,6 +385,22 @@ class CScriptNum:
             r[-1] |= 0x80
         return bytes([len(r)]) + r
 
+    @staticmethod
+    def decode(vch):
+        result = 0
+        # We assume valid push_size and minimal encoding
+        value = vch[1:]
+        if len(value) == 0:
+            return result
+        for i, byte in enumerate(value):
+            result |= int(byte) << 8*i
+        if value[-1] >= 0x80:
+            # Mask for all but the highest result bit
+            num_mask = (2**(len(value)*8) - 1) >> 1
+            result &= num_mask
+            result *= -1
+        return result
+
 
 class CScript(bytes):
     """Serialized script