X-Mirror Rotation

When prefabs are X-mirrored (X coordinates flipped), rotation indices must be corrected so blocks face the right direction in the mirrored coordinate space.

Source: index.html lines ~1112–1123

X_MIRROR_ROT Lookup Table

const X_MIRROR_ROT = [
//  input:  0  1  2  3    (Up)
    0, 3, 2, 1,
//  input:  4  5  6  7    (Down)
    4, 7, 6, 5,
//  input:  8  9  10 11   (North)
    8, 11, 10, 9,
//  input: 12 13 14 15    (South)
    12, 15, 14, 13,
//  input: 16 17 18 19    (East → West)
    20, 23, 22, 21,
//  input: 20 21 22 23    (West → East)
    16, 19, 18, 17,
];

Usage: mirroredRotation = X_MIRROR_ROT[originalRotation]

Logic

Two transformations happen when mirroring across X:

1. Spin Reversal (within face groups)

Within each group of 4, indices 1 and 3 swap (90° ↔ 270°). Index 0 and 2 stay (0° and 180° are symmetric under X-mirror).

Original	Mirrored	Why
0 (0°)	0 (0°)	Symmetric
1 (90°)	3 (270°)	Mirror flips CW ↔ CCW
2 (180°)	2 (180°)	Symmetric
3 (270°)	1 (90°)	Mirror flips CW ↔ CCW

2. East ↔ West Swap

The East group (16–19) and West group (20–23) swap entirely because mirroring across X flips the X-axis direction:

Original	Mirrored
16 (East 0°)	20 (West 0°) then spin-reversed → 20
17 (East 90°)	23 (West 270°)
20 (West 0°)	16 (East 0°)
21 (West 90°)	19 (East 270°)

Up, Down, North, and South face groups stay in their own group (only spin is reversed).