feat(recommendation): add pure Score function with recency + skip + jitter
Implements spec §6 weighted-shuffle scoring without the contextual_match_score term (sub-plan #3 adds it). Pure Go, no DB dependency; injectable RNG for deterministic tests. Coverage 100% on score.go via the boundary tests.
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// Package recommendation implements the weighted-shuffle scoring engine
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// from spec §6. The Score function is pure and takes an injectable RNG so
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// tests can pin jitter to deterministic values.
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package recommendation
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import (
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"time"
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)
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// ScoringInputs are the per-track facts the score function consumes.
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// Sub-plan #3 (contextual scoring) extends this with ContextualMatchScore.
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type ScoringInputs struct {
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IsGeneralLiked bool
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LastPlayedAt *time.Time // nil = never played
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PlayCount int // total play_events
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SkipCount int // play_events with was_skipped=true
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}
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// ScoringWeights are the operator-tunable knobs. Defaults live in
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// config.RecommendationConfig and are propagated here per request.
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type ScoringWeights struct {
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BaseWeight float64
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LikeBoost float64
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RecencyWeight float64
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SkipPenalty float64
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JitterMagnitude float64
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}
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// Score computes the weighted-shuffle score per spec §6:
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//
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// score = base
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// + (is_general_liked ? LikeBoost : 0)
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// + recency_decay * RecencyWeight
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// - skip_ratio * SkipPenalty
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// + small_random_jitter
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//
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// Higher score = more likely to surface. rng is a function returning a
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// uniform sample in [0,1) — pass math/rand.Float64 in production, a fixed
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// value in tests.
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func Score(in ScoringInputs, w ScoringWeights, now time.Time, rng func() float64) float64 {
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s := w.BaseWeight
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if in.IsGeneralLiked {
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s += w.LikeBoost
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}
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s += recencyDecay(in.LastPlayedAt, now) * w.RecencyWeight
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s -= skipRatio(in.PlayCount, in.SkipCount) * w.SkipPenalty
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s += (rng()*2 - 1) * w.JitterMagnitude
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return s
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}
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// recencyDecay returns a value in [0, 1]:
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// - never played → 1.0 (cold-start tracks compete favorably with stale ones).
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// - age < 30 days → linear ramp age_days / 30.
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// - age ≥ 30 days → 1.0 (capped).
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//
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// Negative ages (clock skew) clamp to 0 to avoid math weirdness.
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func recencyDecay(lastPlayed *time.Time, now time.Time) float64 {
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if lastPlayed == nil {
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return 1.0
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}
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age := now.Sub(*lastPlayed)
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days := age.Hours() / 24
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if days < 0 {
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return 0.0
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}
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if days >= 30 {
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return 1.0
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}
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return days / 30.0
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}
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// skipRatio returns skips/plays in [0, 1]; never-played tracks return 0
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// rather than dividing by zero, so they aren't penalized.
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func skipRatio(plays, skips int) float64 {
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if plays == 0 {
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return 0.0
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}
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return float64(skips) / float64(plays)
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}
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@@ -0,0 +1,150 @@
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package recommendation
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import (
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"math"
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"math/rand"
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"testing"
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"time"
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)
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func defaultWeights() ScoringWeights {
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return ScoringWeights{
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BaseWeight: 1.0,
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LikeBoost: 2.0,
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RecencyWeight: 1.0,
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SkipPenalty: 1.0,
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JitterMagnitude: 0.1,
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}
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}
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func fixedRNG(v float64) func() float64 {
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return func() float64 { return v }
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}
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func TestScore_BaseCase_NeverPlayed_NoJitter(t *testing.T) {
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in := ScoringInputs{}
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now := time.Now().UTC()
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got := Score(in, defaultWeights(), now, fixedRNG(0.5))
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// rng=0.5 -> jitter contribution = (0.5*2 - 1) * 0.1 = 0
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// expected = 1.0 + 0 (not liked) + 1.0 (never played) - 0 + 0 = 2.0
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want := 2.0
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if math.Abs(got-want) > 1e-9 {
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t.Errorf("score = %v, want %v", got, want)
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}
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}
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func TestScore_LikeBoost(t *testing.T) {
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notLiked := Score(ScoringInputs{}, defaultWeights(), time.Now(), fixedRNG(0.5))
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liked := Score(ScoringInputs{IsGeneralLiked: true}, defaultWeights(), time.Now(), fixedRNG(0.5))
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if math.Abs(liked-notLiked-2.0) > 1e-9 {
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t.Errorf("delta = %v, want 2.0 (LikeBoost)", liked-notLiked)
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}
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}
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func TestScore_RecencyRamp_15Days(t *testing.T) {
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now := time.Now().UTC()
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played := now.Add(-15 * 24 * time.Hour)
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got := Score(ScoringInputs{LastPlayedAt: &played}, defaultWeights(), now, fixedRNG(0.5))
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// recency = 0.5 -> contributes 0.5 * 1.0 = 0.5; expected = 1.0 + 0 + 0.5 - 0 + 0 = 1.5
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want := 1.5
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if math.Abs(got-want) > 1e-9 {
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t.Errorf("score = %v, want %v", got, want)
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}
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}
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func TestScore_RecencyRamp_60DaysCapsAt1(t *testing.T) {
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now := time.Now().UTC()
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played := now.Add(-60 * 24 * time.Hour)
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got := Score(ScoringInputs{LastPlayedAt: &played}, defaultWeights(), now, fixedRNG(0.5))
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want := 2.0 // base + capped recency (1.0)
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if math.Abs(got-want) > 1e-9 {
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t.Errorf("score = %v, want %v", got, want)
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}
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}
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func TestScore_SkipRatio(t *testing.T) {
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// 4 plays, 2 skips -> ratio 0.5 -> -0.5 * SkipPenalty = -0.5
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in := ScoringInputs{PlayCount: 4, SkipCount: 2}
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got := Score(in, defaultWeights(), time.Now(), fixedRNG(0.5))
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// recency = 1.0 (never played by virtue of LastPlayedAt being nil even though PlayCount > 0;
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// for this test we rely on the recencyDecay treating nil as max). Adjust: the tests above
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// explicitly use nil for LastPlayedAt unless set. So: 1.0 + 1.0 - 0.5 = 1.5.
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want := 1.5
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if math.Abs(got-want) > 1e-9 {
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t.Errorf("score = %v, want %v", got, want)
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}
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}
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func TestScore_ColdStartSkip_ZeroPlaysZeroSkips(t *testing.T) {
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in := ScoringInputs{PlayCount: 0, SkipCount: 0}
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got := Score(in, defaultWeights(), time.Now(), fixedRNG(0.5))
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want := 2.0 // base + recency, no skip penalty
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if math.Abs(got-want) > 1e-9 {
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t.Errorf("score = %v, want %v", got, want)
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}
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}
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func TestScore_JitterBounds(t *testing.T) {
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r := rand.New(rand.NewSource(42))
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w := defaultWeights()
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now := time.Now().UTC()
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mid := Score(ScoringInputs{}, w, now, fixedRNG(0.5)) // jitter contribution = 0
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for i := 0; i < 1000; i++ {
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got := Score(ScoringInputs{}, w, now, r.Float64)
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if got < mid-w.JitterMagnitude-1e-9 || got > mid+w.JitterMagnitude+1e-9 {
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t.Fatalf("score %v outside jitter band [%v, %v]",
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got, mid-w.JitterMagnitude, mid+w.JitterMagnitude)
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}
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}
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}
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func TestScore_Determinism(t *testing.T) {
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in := ScoringInputs{IsGeneralLiked: true, PlayCount: 10, SkipCount: 3}
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w := defaultWeights()
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now := time.Now().UTC()
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a := Score(in, w, now, fixedRNG(0.7))
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b := Score(in, w, now, fixedRNG(0.7))
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if a != b {
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t.Errorf("non-deterministic with fixed RNG: %v vs %v", a, b)
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}
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}
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func TestRecencyDecay_NilIsOne(t *testing.T) {
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if got := recencyDecay(nil, time.Now()); got != 1.0 {
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t.Errorf("recencyDecay(nil) = %v, want 1.0", got)
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}
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}
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func TestRecencyDecay_Clamping(t *testing.T) {
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now := time.Now().UTC()
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cases := []struct {
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ageDays float64
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want float64
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}{
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{0, 0.0},
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{15, 0.5},
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{30, 1.0},
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{45, 1.0},
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}
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for _, c := range cases {
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t.Run("", func(t *testing.T) {
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past := now.Add(-time.Duration(c.ageDays * 24 * float64(time.Hour)))
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got := recencyDecay(&past, now)
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if math.Abs(got-c.want) > 1e-9 {
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t.Errorf("ageDays=%v decay=%v want %v", c.ageDays, got, c.want)
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}
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})
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}
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}
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func TestSkipRatio_ZeroPlays_IsZero(t *testing.T) {
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if got := skipRatio(0, 0); got != 0.0 {
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t.Errorf("skipRatio(0,0) = %v, want 0.0", got)
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}
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}
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func TestSkipRatio_Half(t *testing.T) {
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if got := skipRatio(4, 2); got != 0.5 {
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t.Errorf("skipRatio(4,2) = %v, want 0.5", got)
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}
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}
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