deviation() returns a spurious 1 for degenerate (zero-area) polygons

[mrshannon]

Summary

deviation() — the helper for verifying a triangulation's correctness — returns 1 ("100% wrong") for a valid, correctly-triangulated degenerate polygon (a zero-area / collinear ring). earcut triangulates such a ring to zero triangles, which is correct, yet deviation() reports the result as maximally wrong.

Reproduction

import earcut, {deviation} from 'earcut';

// four collinear points on the line y = 2x → a zero-area ring
const polygon = [0.1, 0.2, 1.3, 2.6, 2.5, 5.0, 3.7, 7.4];

const triangles = earcut(polygon);                    // []  (correct: zero-area ring, nothing to fill)
console.log(triangles.length);                        // 0
console.log(deviation(polygon, null, 2, triangles));  // 1   ← expected 0

Expected: 0 (the triangulation is correct).
Actual: 1.

Root cause

deviation() computes a relative error and special-cases the zero-area case with an exact comparison:

return polygonArea === 0 && trianglesArea === 0 ? 0 :
    Math.abs((trianglesArea - polygonArea) / polygonArea);

polygonArea comes from the non-robust shoelace sum. For a zero-area ring the true area is exactly 0, but the shoelace adds several large terms that cancel, and floating-point rounding leaves a tiny residual (~1.78e-15 for the example) instead of 0. So:

• polygonArea === 0 is false (the value is 1.78e-15),
• the guard is skipped, and
• it evaluates |0 − 1.78e-15| / 1.78e-15 = 1.

It is data-dependent: an axis-aligned collinear ring cancels to exactly 0 (the guard fires, returns 0), but a non-axis-aligned / decimal one leaves a residual and returns 1. That is why the existing degenerate fixtures (degenerate, empty-square, …) don't catch it — they all happen to cancel to exact 0.

Impact

deviation() is the sanctioned way to verify earcut's (deliberately non-guaranteed) output. Pipelines that gate on deviation(...) > threshold hard-fail on valid degenerate input.

Suggested fix

Replace the exact polygonArea === 0 test with a scale-relative floating-point tolerance - treat |polygonArea| within its rounding-error bound as zero. See #209