memby/server/internal/recommend/magic_test.go

package recommend

import (
	"encoding/json"
	"testing"
)

func magicCandidate(id string, score float64, runtimeMinutes int) MagicCandidate {
	return MagicCandidate{ItemID: id, Title: id, Score: score, RuntimeMinutes: runtimeMinutes}
}

// The pool is kept between presses, so it has to survive the round trip that keeping it
// means. A field that lost its tag would show up as a button that quietly stopped weighing
// anything rather than as an error.
func TestMagicCandidateSurvivesBeingKept(t *testing.T) {
	pool := []MagicCandidate{{
		ItemID: "1", Title: "A Film", Score: 2.5, RuntimeMinutes: 104,
		Signals: []string{"unwatched"}, Reasons: []string{"Because you like Drama"},
	}}
	raw, err := json.Marshal(pool)
	if err != nil {
		t.Fatalf("marshal: %v", err)
	}
	var restored []MagicCandidate
	if err := json.Unmarshal(raw, &restored); err != nil {
		t.Fatalf("unmarshal: %v", err)
	}
	if len(restored) != 1 {
		t.Fatalf("pool length = %d, want 1", len(restored))
	}
	got, want := restored[0], pool[0]
	if got.ItemID != want.ItemID || got.Title != want.Title || got.Score != want.Score ||
		got.RuntimeMinutes != want.RuntimeMinutes ||
		len(got.Signals) != len(want.Signals) || len(got.Reasons) != len(want.Reasons) {
		t.Fatalf("pool did not survive being kept: %+v", got)
	}
}

func TestChooseMagicNeverReturnsAnExcludedTitle(t *testing.T) {
	pool := []MagicCandidate{
		magicCandidate("playing-now", 9, 0),
		magicCandidate("offered-before", 8, 0),
		magicCandidate("fresh", 1, 0),
	}
	for roll := 0.0; roll < 1; roll += 0.01 {
		selection, ok := ChooseMagic(pool, MagicOptions{
			ExcludeIDs: []string{"playing-now", " offered-before "},
			Roll:       roll,
		})
		if !ok {
			t.Fatalf("roll %.2f: expected a pick", roll)
		}
		if selection.ItemID != "fresh" {
			t.Fatalf("roll %.2f: drew an excluded title %q", roll, selection.ItemID)
		}
	}
}

// A household that has been offered everything the pool holds is the one case the button
// has no answer for, and it must say so rather than repeat itself.
func TestChooseMagicRefusesWhenEverythingIsExcluded(t *testing.T) {
	pool := []MagicCandidate{magicCandidate("only", 3, 0)}
	if _, ok := ChooseMagic(pool, MagicOptions{ExcludeIDs: []string{"only"}, Roll: 0.5}); ok {
		t.Fatal("expected no pick when the whole pool is excluded")
	}
	if _, ok := ChooseMagic(nil, MagicOptions{Roll: 0.5}); ok {
		t.Fatal("expected no pick from an empty pool")
	}
}

// The whole reason for drawing rather than sorting: pressing it twice must be able to give
// two answers, while merit still decides how many tickets each title holds.
func TestChooseMagicFavoursMeritWithoutBeingAForegoneConclusion(t *testing.T) {
	pool := make([]MagicCandidate, 0, 10)
	for index := 0; index < 10; index++ {
		pool = append(pool, magicCandidate(string(rune('a'+index)), float64(10-index), 0))
	}
	counts := map[string]int{}
	for roll := 0.0; roll < 1; roll += 0.001 {
		selection, ok := ChooseMagic(pool, MagicOptions{Roll: roll})
		if !ok {
			t.Fatalf("roll %.3f: expected a pick", roll)
		}
		counts[selection.ItemID]++
	}
	if len(counts) != len(pool) {
		t.Fatalf("every title should be reachable, got %d of %d", len(counts), len(pool))
	}
	if counts["a"] <= counts["j"] {
		t.Fatalf("the best title should hold the most tickets: %v", counts)
	}
}

// The pool is built once and asked more than one question, so the time budget cannot have
// been folded into it. The same pool has to answer "there is an hour" differently from
// "it is Saturday afternoon".
func TestChooseMagicAppliesTheTimeBudgetAtTheDraw(t *testing.T) {
	pool := []MagicCandidate{
		magicCandidate("epic", 1.0, 180),
		magicCandidate("short", 0.6, 85),
	}
	unhurried, ok := ChooseMagic(pool, MagicOptions{Roll: 0})
	if !ok || unhurried.ItemID != "epic" {
		t.Fatalf("with no limit the better title should lead, got %+v", unhurried)
	}
	rushed, ok := ChooseMagic(pool, MagicOptions{AvailableMinutes: 90, Roll: 0})
	if !ok || rushed.ItemID != "short" {
		t.Fatalf("with 90 minutes the one that fits should lead, got %+v", rushed)
	}
	if !hasSignal(rushed.Signals, "fits_time") {
		t.Fatalf("the fit should be reported as a signal: %v", rushed.Signals)
	}
	// And the pool itself must be unchanged by having been asked, or the second press
	// would inherit the first press's constraints.
	if pool[0].Score != 1.0 || len(pool[0].Signals) != 0 {
		t.Fatalf("the draw mutated the kept pool: %+v", pool[0])
	}
}

// Nothing recorded is not evidence either way: refusing those titles would quietly delete
// a slice of the library from the feature.
func TestMagicRuntimeAdjustmentStaysSilentWithoutEvidence(t *testing.T) {
	cases := []struct {
		name                             string
		runtimeMinutes, availableMinutes int
		wantSignal                       string
	}{
		{"no limit given", 200, 0, ""},
		{"no runtime recorded", 0, 60, ""},
		{"comfortably inside", 85, 90, "fits_time"},
		{"inside the slack", 95, 90, "fits_time"},
		{"past the slack", 101, 90, "too_long"},
	}
	for _, tc := range cases {
		t.Run(tc.name, func(t *testing.T) {
			_, signal := magicRuntimeAdjustment(tc.runtimeMinutes, tc.availableMinutes)
			if signal != tc.wantSignal {
				t.Fatalf("signal = %q, want %q", signal, tc.wantSignal)
			}
		})
	}
}

// Ties break by id so that the pool is reproducible even though the draw from it is not.
func TestSortMagicPoolIsReproducible(t *testing.T) {
	pool := []MagicCandidate{
		magicCandidate("z", 2, 0),
		magicCandidate("a", 2, 0),
		magicCandidate("m", 5, 0),
	}
	sortMagicPool(pool)
	got := []string{pool[0].ItemID, pool[1].ItemID, pool[2].ItemID}
	want := []string{"m", "a", "z"}
	for i := range want {
		if got[i] != want[i] {
			t.Fatalf("order = %v, want %v", got, want)
		}
	}
}

func hasSignal(signals []string, want string) bool {
	for _, signal := range signals {
		if signal == want {
			return true
		}
	}
	return false
}