implement script_num_to_hex

lib/bitcoin.py

@@ -152,6 +152,29 @@ def int_to_hex(i, length=1):
     s = "0"*(2*length - len(s)) + s
     return rev_hex(s)
 
+def script_num_to_hex(i):
+    """See CScriptNum in Bitcoin Core.
+    Encodes an integer as hex, to be used in script.
+
+    ported from https://github.com/bitcoin/bitcoin/blob/8cbc5c4be4be22aca228074f087a374a7ec38be8/src/script/script.h#L326
+    """
+    if i == 0:
+        return ''
+
+    result = bytearray()
+    neg = i < 0
+    absvalue = abs(i)
+    while absvalue > 0:
+        result.append(absvalue & 0xff)
+        absvalue >>= 8
+
+    if result[-1] & 0x80:
+        result.append(0x80 if neg else 0x00)
+    elif neg:
+        result[-1] |= 0x80
+
+    return bh2u(result)
+
 
 def var_int(i):
     # https://en.bitcoin.it/wiki/Protocol_specification#Variable_length_integer

lib/tests/test_bitcoin.py

@@ -11,7 +11,8 @@ from lib.bitcoin import (
     var_int, op_push, address_to_script, regenerate_key,
     verify_message, deserialize_privkey, serialize_privkey, is_segwit_address,
     is_b58_address, address_to_scripthash, is_minikey, is_compressed, is_xpub,
-    xpub_type, is_xprv, is_bip32_derivation, seed_type, EncodeBase58Check)
+    xpub_type, is_xprv, is_bip32_derivation, seed_type, EncodeBase58Check,
+    script_num_to_hex)
 from lib.util import bfh
 from lib import constants
 
@@ -150,6 +151,21 @@ class Test_bitcoin(unittest.TestCase):
         self.assertEqual(op_push(0x10000), '4e00000100')
         self.assertEqual(op_push(0x12345678), '4e78563412')
 
+    def test_script_num_to_hex(self):
+        # test vectors from https://github.com/btcsuite/btcd/blob/fdc2bc867bda6b351191b5872d2da8270df00d13/txscript/scriptnum.go#L77
+        self.assertEqual(script_num_to_hex(127), '7f')
+        self.assertEqual(script_num_to_hex(-127), 'ff')
+        self.assertEqual(script_num_to_hex(128), '8000')
+        self.assertEqual(script_num_to_hex(-128), '8080')
+        self.assertEqual(script_num_to_hex(129), '8100')
+        self.assertEqual(script_num_to_hex(-129), '8180')
+        self.assertEqual(script_num_to_hex(256), '0001')
+        self.assertEqual(script_num_to_hex(-256), '0081')
+        self.assertEqual(script_num_to_hex(32767), 'ff7f')
+        self.assertEqual(script_num_to_hex(-32767), 'ffff')
+        self.assertEqual(script_num_to_hex(32768), '008000')
+        self.assertEqual(script_num_to_hex(-32768), '008080')
+
     def test_address_to_script(self):
         # bech32 native segwit
         # test vectors from BIP-0173