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Traffic then variant allocation: Traffic allocation admits one extra bucket · case 03

A 10 percent experiment enrols 11 percent of users.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Stage 1: t = crc32(salt + ":traffic:" + user) % 100; users with t >= traffic (1..100) are not in the experiment. Stage 2 uses an independent v = crc32(salt + ":variant:" + user) % 100 and walks weights ([arm, percent of allocated users], summing to 100) cumulatively, half-open. Return [in_experiment, arm or None].

Why this case matters

Separating traffic allocation from the variant split keeps arms balanced when traffic changes.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
traffic boundary excludes t equal to traffic	`[true, "b"]`	`[false, null]`	Failed

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