test: introduce output amount (de)compression routines

Author: theStack

test/functional/feature_framework_unit_tests.py

@@ -18,6 +18,7 @@
     "address",
     "crypto.bip324_cipher",
     "blocktools",
+    "compressor",
     "crypto.chacha20",
     "crypto.ellswift",
     "key",

test/functional/test_framework/compressor.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+# Copyright (c) 2025-present The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Routines for compressing transaction output amounts and scripts."""
+import unittest
+
+from .messages import COIN
+
+
+def compress_amount(n):
+    if n == 0:
+        return 0
+    e = 0
+    while ((n % 10) == 0) and (e < 9):
+        n //= 10
+        e += 1
+    if e < 9:
+        d = n % 10
+        assert (d >= 1 and d <= 9)
+        n //= 10
+        return 1 + (n*9 + d - 1)*10 + e
+    else:
+        return 1 + (n - 1)*10 + 9
+
+
+def decompress_amount(x):
+    if x == 0:
+        return 0
+    x -= 1
+    e = x % 10
+    x //= 10
+    n = 0
+    if e < 9:
+        d = (x % 9) + 1
+        x //= 9
+        n = x * 10 + d
+    else:
+        n = x + 1
+    while e > 0:
+        n *= 10
+        e -= 1
+    return n
+
+
+class TestFrameworkCompressor(unittest.TestCase):
+    def test_amount_compress_decompress(self):
+        def check_amount(amount, expected_compressed):
+            self.assertEqual(compress_amount(amount), expected_compressed)
+            self.assertEqual(decompress_amount(expected_compressed), amount)
+
+        # test cases from compress_tests.cpp:compress_amounts
+        check_amount(0, 0x0)
+        check_amount(1, 0x1)
+        check_amount(1000000, 0x7)
+        check_amount(COIN, 0x9)
+        check_amount(50*COIN, 0x32)
+        check_amount(21000000*COIN, 0x1406f40)