Replaced hard tier boundaries with additive score adjustments:
Artist tiers: exact +12, starts-with +6, substring +0, none -10
Album tiers: credit-exact +12, credit-contains +8,
title-exact +4, title-contains +0, none -5
A sufficiently popular lower-tier result can now overcome an
unpopular exact match. The effective gap between tier 0 and tier 1
is 6 points on a 0-100 scale, requiring roughly a 4-5x popularity
difference to overcome — matching the intuition that 'slightly more
popular near-match loses to exact, much more popular near-match wins.'
Also added library bonus (+10M) to the slow path (boostWithPopularity)
so library artists rank highly regardless of which reranking path
is used. Previously only the index fast path applied this bonus.
yonlu
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