The Move That Quartered Crits — JavaScript Bug Hunt

Briefing

Modelled on the Focus Energy bug in Pokémon Red and Blue (1996). The move was meant to raise the user's chance of landing a critical hit, but the game code shifted the critical-hit value the wrong way, so using it actually made critical hits less likely. The bug shipped in the Generation I games.

This battle engine computes a critical-hit threshold from the attacker's base speed. Its Focus Energy branch shifts in the wrong direction.

Fix critThreshold so Focus Energy raises the threshold.

Bug report

BUG-FOCUS · Priority: Medium · Reported by: competitive players

critThreshold(baseSpeed, focused):

• the threshold starts at floor(baseSpeed / 2)
• Focus Energy (focused = true) multiplies it by 4 (shift LEFT by rules.FOCUS_SHIFT)
• the result is capped at rules.MAX_THRESHOLD (255)

isCritical(baseSpeed, focused, roll) is true when roll (0-255) < threshold.

Observed: a base-speed-100 attacker has threshold 50 normally but 12 after using Focus Energy.

Logs

[battle] crit threshold speed=100 focused=false -> 50
[battle] crit threshold speed=100 focused=true -> 12

The code as shipped

src/battle/critical.js (editable)

var rules = require("./rules");

exports.critThreshold = function (baseSpeed, focused) {
  var threshold = Math.floor(baseSpeed / 2);
  if (focused) threshold = threshold >> rules.FOCUS_SHIFT;
  return Math.min(threshold, rules.MAX_THRESHOLD);
};

exports.isCritical = function (baseSpeed, focused, roll) {
  return roll < exports.critThreshold(baseSpeed, focused);
};

Read-only context: src/battle/rules.js.