simple_portable_numeric

(en)Japanese ver is here.
(ja) この解説の日本語版はここ にあります。

Overview

simple_portable_numeric is an arbitrary-precision decimal and rational arithmetic library that gives identical results in Dart and Python.

Both versions are built only on arbitrary-precision integers (Dart BigInt, Python int) and follow one shared specification. Platform libraries such as Python's decimal (28 significant digits by default) are not used, so the number of digits, rounding and string format never differ between the languages.

Features

• PortableDecimal: exact + - * with no digit limit. Division always takes an explicit scale and rounding mode.
• PortableRational: exact + - * /, always kept reduced.
• Five rounding modes: halfEven, halfUp, down, floor, ceiling.
• Strict and stable string format, suitable for storing values as text (e.g. in a database).
• Equality by value (1.50 == 1.5), while the string keeps the scale ("1.50").
• Correctly rounded conversion to double, identical in both languages.
• No dependencies.
• Dart version available.
  → https://pub.dev/packages/simple_portable_numeric

Quick Start

from simple_portable_numeric import EnumRoundingMode, PortableDecimal, PortableRational

# Decimal
a = PortableDecimal.parse("1.50")
b = PortableDecimal.parse("2.3")
print(a + b)  # 3.80
print(a * b)  # 3.450
print(a == PortableDecimal.parse("1.5"))  # True
print(PortableDecimal.parse("1").divide(
    PortableDecimal.parse("3"), 5, EnumRoundingMode.halfEven))  # 0.33333
print(PortableDecimal.parse("2.5").set_scale(0, EnumRoundingMode.halfEven))  # 2

# Rational
r = PortableRational.parse("1/3") + PortableRational.parse("1/6")
print(r)  # 1/2
print(r.to_decimal(2, EnumRoundingMode.halfUp))  # 0.50
print(PortableRational.parse("0.75"))  # 3/4

# Conversion
print(PortableDecimal.parse("0.1").to_rational())  # 1/10
print(PortableRational.parse("1/3").to_float())  # 0.3333333333333333

Notes

• Strings must be plain notation such as "-12.340" or "3/4". Exponent notation ("1e3"), whitespace and non-ASCII digits are rejected.
• PortableDecimal and PortableRational are never mixed implicitly. Convert explicitly with to_rational() / to_decimal(scale, rounding).
• Conversion from double is not provided, because the string forms of doubles differ between languages.
• Hash values are consistent with equality, but are not the same across languages.

Support

This package is developed and maintained by me personally as an open-source project. For bug reports and feature requests, please use GitHub Issues.

If you need paid support, consulting, or custom development (e.g. priority support, design advice, or implementation help), please contact my company:

SimpleAppli Inc. https://simpleappli.com/en/index_en.html

About version control

The C part will be changed at the time of version upgrade. However, versions less than 1.0.0 may change the calculation results or the string format regardless of the following rules.

• Specification changes that change the calculation results or the string format.
  • C.X.X
• Adding methods, etc.
  • X.C.X
• Minor changes and bug fixes.
  • X.X.C

License

This software is released under the Apache-2.0 License, see LICENSE file.

Copyright 2026 Masahide Mori

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Trademarks

• “Dart” and “Flutter” are trademarks of Google LLC. This package is not developed or endorsed by Google LLC.

• “Python” is a trademark of the Python Software Foundation. This package is not affiliated with the Python Software Foundation.

• GitHub and the GitHub logo are trademarks of GitHub, Inc. This package is not affiliated with GitHub, Inc.