A better implementation for `RationalU256`.

Description

use ckb_types::{core::RationalU256, u256, U256};
fn main() {
    let x = u256!("0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
    let x_rational = RationalU256::from_u256(x.clone());
    let numerator = 33u32;
    let denominator = 100u32;
    let y_rational = RationalU256::new(U256::from(numerator), U256::from(denominator));
    let z_rational = x_rational * y_rational; // panicked in this line!
    let z = z_rational.into_u256();
    println!("{:#x} = {:#x} * {}/{}", z, x, numerator, denominator);
}

Contributor guide

Tech stack

rust

Domain

backendblockchain

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

half day

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Rust programmingUnderstanding of U256 arithmeticFamiliarity with Nervos CKB codebase

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

40

Research direction

Investigate the multiplication implementation in `util/rational/src/lib.rs` lines 128-140. The current approach multiplies numerators and denominators directly, leading to overflow when both are large. Consider using a reduction step with GCD before multiplication to avoid overflow. Alternatively, use checked multiplication with fallback to a safe algorithm. See the referenced issue in ckb light client for prior discussion.