bug: the WCA scramble generator produces moves but does not validate that consecutive moves are on the same face or opposite faces — invalid scrambles like "R R'" (self-cancelling) can be generated, producing a scramble shorter than 20 moves effectively

[prince-pokharna]

🐛 Problem Statement

CubeIt generates WCA-compliant scrambles to animate the 3D cube visualizer. WCA regulations (Article 4b) require that scrambles be randomly generated but also non-trivial — specifically:

• No two consecutive moves on the same face (e.g., R R is redundant — equivalent to R2)
• No two consecutive moves on opposite faces in the same axis that could merge (e.g., R L R is reducible)
• Scrambles must be effectively 20 moves of genuine state change

A naive random generator using Math.random() to pick from ['R', 'L', 'U', 'D', 'F', 'B'] + modifiers ['', "'", '2'] can produce:
R R' U2 U D D' F ...

Here R R' cancels to identity, U2 U reduces to U', D D' cancels — producing a scramble that's far shorter than 20 moves of actual state change. The 3D visualizer would then show incorrect scramble states.

💡 Proposed Fix

Add same-face and opposite-face filtering to the scramble generator:

// src/utils/scrambleGenerator.js

const FACES = ['U', 'D', 'R', 'L', 'F', 'B'];
const MODIFIERS = ['', "'", '2'];
const OPPOSITE_FACES = { U: 'D', D: 'U', R: 'L', L: 'R', F: 'B', B: 'F' };

export function generateScramble(length = 20) {
  const moves = [];
  let lastFace = null;
  let secondLastFace = null;

  while (moves.length < length) {
    // Filter: exclude same face and opposite face (if second-last was also on that axis)
    const availableFaces = FACES.filter(face => {
      if (face === lastFace) return false; // No same-face repeat
      // Avoid reducible three-move sequences: if last two were on same axis, skip opposite
      if (
        lastFace &&
        secondLastFace &&
        OPPOSITE_FACES[face] === lastFace &&
        OPPOSITE_FACES[face] === secondLastFace
      ) return false;
      return true;
    });

    const face = availableFaces[Math.floor(Math.random() * availableFaces.length)];
    const modifier = MODIFIERS[Math.floor(Math.random() * MODIFIERS.length)];

    moves.push(face + modifier);
    secondLastFace = lastFace;
    lastFace = face;
  }

  return moves.join(' ');
}

Add a unit test suite for the scramble generator:

// src/utils/scrambleGenerator.test.js

test('no consecutive same-face moves', () => {
  for (let i = 0; i < 1000; i++) {
    const scramble = generateScramble(20);
    const moves = scramble.split(' ');
    for (let j = 1; j < moves.length; j++) {
      const prevFace = moves[j - 1][0];
      const currFace = moves[j][0];
      expect(currFace).not.toBe(prevFace);
    }
  }
});

📁 Files to Modify

File	Change
src/utils/scrambleGenerator.js	Add face-filtering logic to generator
src/utils/scrambleGenerator.test.js	New — unit tests for scramble validity
package.json	Ensure Jest or Vitest test runner is configured

Suggested labels: bug, scramble-generator, wca-compliance

I would like to work on this. Could you please assign it to me?

[prince-pokharna]

Hey @vasanth642 ,
Please assign it to me under GSSoC 2026.
/claim /assign /claim-assignment