feat(server/m7-352): system mix helpers (tieBreakHash, pickSeedArtists)

internal/playlists/seed_selection_test.go

@@ -0,0 +1,47 @@
+package playlists
+
+import (
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+// pickSeedArtistsFromRows is the pure helper that turns sqlc rows into
+// the seed list. The DB-level query is exercised in system_test.go's
+// integration test; this unit test pins the post-fetch logic.
+
+func TestPickSeedArtistsFromRows_PreservesOrder(t *testing.T) {
+	mk := func(b byte, score int64) seedArtistRow {
+		return seedArtistRow{
+			ArtistID: pgtype.UUID{Bytes: [16]byte{b}, Valid: true},
+			Score:    score,
+		}
+	}
+	// Caller (SQL) already orders + limits; helper preserves order.
+	rows := []seedArtistRow{mk(1, 100), mk(2, 50), mk(3, 25)}
+	got := pickSeedArtistsFromRows(rows)
+	if len(got) != 3 {
+		t.Fatalf("len: got %d, want 3", len(got))
+	}
+	if got[0].Bytes[0] != 1 || got[1].Bytes[0] != 2 || got[2].Bytes[0] != 3 {
+		t.Errorf("expected [1,2,3] preserving input order; got %v", got)
+	}
+}
+
+func TestPickSeedArtistsFromRows_FewerThanThree(t *testing.T) {
+	mk := func(b byte, score int64) seedArtistRow {
+		return seedArtistRow{ArtistID: pgtype.UUID{Bytes: [16]byte{b}, Valid: true}, Score: score}
+	}
+	rows := []seedArtistRow{mk(1, 100)}
+	got := pickSeedArtistsFromRows(rows)
+	if len(got) != 1 {
+		t.Errorf("len: got %d, want 1", len(got))
+	}
+}
+
+func TestPickSeedArtistsFromRows_Empty(t *testing.T) {
+	got := pickSeedArtistsFromRows(nil)
+	if len(got) != 0 {
+		t.Errorf("empty input should yield empty output; got %v", got)
+	}
+}