Edge Cases in a Waist-to-Height Ratio Calculator: Units, Rounding, and Boundaries
Waist-to-height ratio (WHtR) looks like the easiest calculator you could write: divide one number by another. It's also a good example of how a "trivial" calculator goes wrong in production. Users type centimeters into an inches box. Floating point makes a value that should sit exactly on a limit land a hair above it. A display rounded to two decimals disagrees with the pass/fail check underneath…
The waist-to-height ratio (WHtR) calculator demonstrates how a simple tool can encounter issues in a real-world setting. Users input measurements in various units, leading to unexpected behavior when the calculator is used in production.
A JavaScript module is presented as an example, illustrating the challenges faced when handling different units and rounding methods. The module's core function converts input values to a single unit (inches) for all calculations, ensuring consistency throughout the process.
A key aspect of the calculator is the comparison against a threshold limit. A small value, called EPS, is used to account for floating-point inaccuracies that can cause values on the limit to be slightly over or under the threshold. This ensures that the calculator's results remain stable despite minor fluctuations in decimal places.
The module also includes a function for determining the maximum waist measurement allowed for a given height and limit. This function rounds down to the nearest tape resolution, which is set to 0.5 inches in the provided examples.
The code includes several test cases to validate the calculator's functionality, ensuring that it correctly handles different input units and bound values. These tests cover scenarios where the ratio is exactly at the limit, slightly above the limit, and when the input units are mixed. Additionally, the code demonstrates how the module handles invalid inputs, throwing appropriate errors when necessary.
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