FA-69960 / Map projection transforms / Member archive

Bing-style quadkey encoding of a tile: zoom zero key · case 05

The world tile at zoom 0 gets a one-character key that collides with a zoom-1 tile.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input [z, col, row] with xyz rows. For zoom 0 return the empty string. If col or row is outside [0, 2**z - 1] return None. Otherwise emit z digits from the most significant bit down: digit = (1 if col bit set) + (2 if row bit set).

Why this case matters

Quadkeys are used as cache keys and spatial index prefixes; a wrong digit order breaks prefix containment.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
boundary #12	`"0"`	`""`	Failed

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