`Strategy::Frag` panics with a subtract overflow on the `i16` engine

[DRMacIver]

Summary

The last of the eight overlays in this program panics. Every coordinate fits i16.

Steps to reproduce

use i_float::int::point::IntPoint;
use i_overlay::core::fill_rule::FillRule;
use i_overlay::core::overlay::Overlay;
use i_overlay::core::overlay_rule::OverlayRule;
use i_overlay::core::solver::{Solver, Strategy};

const PTS: [(i32, i32); 3] = [(0, 0), (0, 9854), (129, -23169)];

fn main() {
    let c32: Vec<Vec<IntPoint<i32>>> =
        vec![PTS.iter().map(|&(x, y)| IntPoint::new(x, y)).collect()];
    let c16: Vec<Vec<IntPoint<i16>>> =
        vec![PTS.iter().map(|&(x, y)| IntPoint::new(x as i16, y as i16)).collect()];

    for (name, strategy) in [
        ("List", Strategy::List),
        ("Tree", Strategy::Tree),
        ("Auto", Strategy::Auto),
        ("Frag", Strategy::Frag),
    ] {
        let solver = Solver { strategy, ..Default::default() };

        let r = Overlay::<i32>::with_contours_custom(&c32, &[], Default::default(), solver)
            .overlay(OverlayRule::Subject, FillRule::EvenOdd);
        let pts: Vec<Vec<(i32, i32)>> =
            r.iter().flatten().map(|c| c.iter().map(|p| (p.x, p.y)).collect()).collect();
        println!("i32 engine, {}: {:?}", name, pts);

        let r = Overlay::<i16>::with_contours_custom(&c16, &[], Default::default(), solver)
            .overlay(OverlayRule::Subject, FillRule::EvenOdd);
        let pts: Vec<Vec<(i16, i16)>> =
            r.iter().flatten().map(|c| c.iter().map(|p| (p.x, p.y)).collect()).collect();
        println!("i16 engine, {}: {:?}", name, pts);
    }
}

Expected behavior

Strategy::Frag on the i16 engine to return the same contour as the other three strategies do, and as it does on a release build.

Actual behavior

On a debug build:

i32 engine, List: [[(0, 9854), (0, 0), (129, -23169)]]
i16 engine, List: [[(0, 9854), (0, 0), (129, -23169)]]
i32 engine, Tree: [[(0, 9854), (0, 0), (129, -23169)]]
i16 engine, Tree: [[(0, 9854), (0, 0), (129, -23169)]]
i32 engine, Auto: [[(0, 9854), (0, 0), (129, -23169)]]
i16 engine, Auto: [[(0, 9854), (0, 0), (129, -23169)]]
i32 engine, Frag: [[(0, 9854), (0, 0), (129, -23169)]]

thread 'main' panicked at src/split/grid_layout.rs:146:29:
attempt to subtract with overflow

Frag on the i32 engine and every other strategy on the i16 engine return the same contour. On a release build there is no panic and the i16 Frag overlay prints that contour too.

Environment

• iOverlay version: 8.1.0, and current main (0022b34)
• Rust version: 1.98.0 (88d9e12ae 2026-08-18)
• OS: macOS 26.6.2, aarch64

Additional context

The reproduction also depends on i_float for IntPoint.

BTW, this bug was found using hegel. Happy to contribute the tests if you're interested.

[NailxSharipov]

This case is expectable.

i16: -32_767...32_768 (-2^15...2^15)

the library use operation like a + b or a - b without extending dimension that why the correct coordinate is in range

i16: −16_384...16_383 (-2^14...2^14)

which is already valid for float convertation but not properly verified/documented for int api

[NailxSharipov]

Add int range explanation in #90