Inside Bitcoin

1#!/usr/bin/env python3
2# Copyright (c) 2025-present The Bitcoin Core developers
3# Distributed under the MIT software license, see the accompanying
4# file COPYING or http://www.opensource.org/licenses/mit-license.php.
5"""Routines for compressing transaction output amounts and scripts."""
6import unittest
7
8from .messages import COIN
9
10
11def compress_amount(n):
12    if n == 0:
13        return 0
14    e = 0
15    while ((n % 10) == 0) and (e < 9):
16        n //= 10
17        e += 1
18    if e < 9:
19        d = n % 10
20        assert (d >= 1 and d <= 9)
21        n //= 10
22        return 1 + (n*9 + d - 1)*10 + e
23    else:
24        return 1 + (n - 1)*10 + 9
25
26
27def decompress_amount(x):
28    if x == 0:
29        return 0
30    x -= 1
31    e = x % 10
32    x //= 10
33    n = 0
34    if e < 9:
35        d = (x % 9) + 1
36        x //= 9
37        n = x * 10 + d
38    else:
39        n = x + 1
40    while e > 0:
41        n *= 10
42        e -= 1
43    return n
44
45
46class TestFrameworkCompressor(unittest.TestCase):
47    def test_amount_compress_decompress(self):
48        def check_amount(amount, expected_compressed):
49            self.assertEqual(compress_amount(amount), expected_compressed)
50            self.assertEqual(decompress_amount(expected_compressed), amount)
51
52        # test cases from compress_tests.cpp:compress_amounts
53        check_amount(0, 0x0)
54        check_amount(1, 0x1)
55        check_amount(1000000, 0x7)
56        check_amount(COIN, 0x9)
57        check_amount(50*COIN, 0x32)
58        check_amount(21000000*COIN, 0x1406f40)
59