FLIP 341: Add 128-bit fixed-point types to Cadence (#342)

* FLIP for adding 128-bit fixed-point types in Cadence

* Add reason behind selecting 24 as the scaling factor

* Add detailed explanation on choosing a 24 scale

Co-authored-by: Dieter Shirley <dete@dapperlabs.com>

* Update the explanation

Co-authored-by: Dieter Shirley <dete@dapperlabs.com>

* Accept the FLIP

---------

Co-authored-by: Dieter Shirley <dete@dapperlabs.com>

Author: SupunS

cadence/20250815-128-bit-fixed-point-types.md

@@ -0,0 +1,90 @@
+---
+status: accepted 
+flip: 341
+authors: Supun Setunga (supun.setunga@flowfoundation.org)
+sponsor: Dieter Shirley (dete@flowfoundation.com)
+updated: 2025-08-29
+---
+
+# FLIP 341: Add 128-bit Fixed-point Types to Cadence
+
+## Objective
+
+The objective is to add a set of 128-bit wide fixed-point types, `Fix128` and `UFix128` to Cadence.
+
+## Motivation
+
+Cadence currently only supports 64-bit wide decimal fixed-point types, `Fix64` and `UFix64`, 
+having the ranges `-92233720368.54775808` through `92233720368.54775807` and `0.0` through `184467440737.09551615`
+respectively.
+
+However, there could be requirements to have a much higher precision than what `Fix64` and `UFix64` provide.
+For example, applications that handle financial data would require higher precision for arithmetic operations.
+Though this can be achieved with a custom fixed-point implementation using integers (128-bit length, 256-bit length,
+or even unbounded integers), it is not only a lot of work for developers to implement such a type,
+but also would be very inefficient.
+
+## User Benefit
+
+Developers can use fixed-point values in Cadence for high-precision arithmetics, efficiently and conveniently.
+
+## Design Proposal
+
+Add `Fix128` and `UFix128` to Cadence.
+
+The proposal is to use a scale of 24, and therefore a scaling factor of 1e-24.
+
+Other blockchain ecosystems (most notably Ethereum) frequently use scale factors (usually called "decimals", per ERC-20)
+of 18 (used by ETH itself), 24 (used for internal calculations in MakerDAO), and 27 (used internally by Compound and Aave
+for interest calculations).
+Although 27 is the most commonly used value internally by defi protocols, the value of 24 has a precedent in Maker, and
+introduces a type that can hold values in the trillions (extreme, but plausible values for very large financial calculations).
+Note that all `UFix64` and `Fix64` values can be converted to the equivalent 128-bit types without any loss of precision or range.
+
+A scaling factor of 24 provides a very high precision for fractional values, while also leaving large enough upper and 
+lower bounds sufficient for most real-world use cases (which typically will involve internal calculations that convert
+back to the 64-bit types).
+
+The new fixed-point type can accommodate values in the following ranges:
+- `Fix128`: `-170141183460469.231731687303715884105728` through `170141183460469.231731687303715884105727`
+- `UFix128`: `0.0` through `340282366920938.463463374607431768211455`
+
+### Drawbacks
+
+None.
+
+### Performance Implications
+
+None.
+
+### Dependencies
+
+None.
+
+### Engineering Impact
+
+Implementing a new and efficient fixed-point representation is non-trivial.
+
+### Compatibility
+
+Given this is a new feature addition, there's no impact on backward compatibility.
+
+### User Impact
+
+Given this is a new feature addition, there's no impact for existing users.
+
+## Related Issues
+
+None.
+
+## Implementation
+- `Fix128` - https://github.com/onflow/cadence/pull/4131
+- `UFix128` - https://github.com/onflow/cadence/pull/4147
+
+## Prior Art
+
+Most programing languages do not have built-in types for 128-bit fixed-point values, but are provided either
+as a standard library or are available as third-party libraries. e.g:
+- Python: https://docs.python.org/3/library/decimal.html
+- Rust: https://docs.rs/fixed/latest/fixed/index.html
+