# Squash

> An independent browser reimplementation of squash on a full World Squash Federation singles court,
> with the ball's temperature as a first-class physical state: restitution rises as rallies heat the
> rubber, so the game alters its own dynamics as it is played.

URL: https://squash.skillsafe.ai/
Runs entirely in the browser.  No accounts, no network calls, no analytics, no language model.
WebGL2 is required; the renderer is hand-written, with no third-party library.

## What it implements

- The WSF international singles court to the millimetre: 9750 x 6400 mm, front-wall out-line
  4570 mm, back-wall out-line 2130 mm, service line 1780 mm, tin 480 mm (430 mm PSA option
  selectable), short line 4260 mm from the back wall, service boxes 1600 mm, lines 50 mm.
- Point-a-rally scoring to 11, two clear at 10-all, best of five (Rule 2).
- The serve, including that there is no second serve (Rule 5).
- Good return, down, out and not up (Rule 6.2 and Appendix 1).
- The full interference tree of Rule 8 and the ball-hitting-a-player rules of Rule 9.  Every call
  names the clause it came from.
- A thermal-mechanical ball model: time-temperature superposition for the rubber's loss, a separate
  amplitude term for large deformation, the enclosed air as a small lossless spring, and a
  two-node shell so the core and the surface can differ.

## The findings the app publishes

- Restitution rises from e = 0.354 at 23 C to e = 0.537 at 45 C at bounce-test speed.  98.2% of
  that gain is the rubber's loss falling; 2.4% is the enclosed air.  The folk explanation names the
  smaller mechanism, and Lewis, Arnold and Griffiths (2011) said as much experimentally in 2011.
- Calibrated on four points and then tested against five it never saw, the model is within 3.2% of
  Lewis, Arnold and Griffiths' independent drop-test table across 35-55 C.
- "Playable" is not a ball temperature.  It is a joint property of the ball and the shot.  The
  45 C of the WSF's hot bounce test is exactly the temperature at which a 29.1 m/s (105 km/h)
  length drive first carries to the back wall; at the 23 C of the cold test you would need
  59.4 m/s (214 km/h).
- A knock-up does NOT reach 45 C.  In the four minutes Rule 4.1 allows, a 20 m/s knock-up takes an
  18 C ball to 26.1 C, and the fixed point at that pace is in the low thirties.  Match play reaches
  about 36 C.  The conclusion survives a fourfold change in the convective coefficient.
- The ball's core runs hotter than its surface, so an infrared reading under-reads the temperature
  the bounce follows.
- The WSF ball specification bounds the cold rebound only from BELOW, in every category, so a ball
  that gets deader when heated meets it.  The double-yellow and single-yellow bands touch at 30%,
  so a ball can be legally both.  The green high-altitude ball shares the competition ball's hot
  band exactly.  Red and blue dots have no specification at all.  The test procedure is not
  published.
- The WSF rules contain no numeric threshold for any interference condition.  "Reasonable", "fair
  view", "every effort", "minimal" and "would have made a good return" are never quantified.  The
  2025 text does not use "in the opinion of the referee" anywhere; it replaced discretion with an
  enumerated tree whose branch conditions are still unquantified words.

## Files

- /CREDITS.txt  the line-by-line provenance register, 123 entries, and the sources that could not
  be opened.
- /LICENSE.txt  CC0 for this app; the WSF documents and the cited papers are not covered.