Improve CalculateExpiry with Slope-based duration calculation

- Remove ICS NewCalendar() func
- TODO IsViewOnlyTerm

scrape.go

@@ -144,16 +144,17 @@ func ScrapeMajor(subject string) error {
 		}
 	}
 
+	term := Default(time.Now()).ToString()
+
 	// Calculate the expiry time for the scrape (1 hour for every 200 classes, random +-15%) with a minimum of 1 hour
 	var scrapeExpiry time.Duration
 	if totalClassCount == 0 {
 		scrapeExpiry = time.Hour * 12
 	} else {
-		scrapeExpiry = CalculateExpiry(totalClassCount, lo.Contains(PriorityMajors, subject))
+		scrapeExpiry = CalculateExpiry(term, totalClassCount, lo.Contains(PriorityMajors, subject))
 	}
 
 	// Mark the major as scraped
-	term := Default(time.Now()).ToString()
 	if totalClassCount == 0 {
 		totalClassCount = -1
 	}
@@ -166,27 +167,45 @@ func ScrapeMajor(subject string) error {
 }
 
 // CalculateExpiry calculates the expiry time until the next scrape for a major.
-func CalculateExpiry(count int, priority bool) time.Duration {
-	scrapeExpiry := time.Hour * time.Duration(count/100)
-	partial := scrapeExpiry.Seconds() * (rand.Float64() * 0.15) // Between 0 and 15% of the total
+// term is the term for which the relevant course is occurring within.
+// count is the number of courses that were scraped.
+// priority is a boolean indicating whether the major is a priority major.
+func CalculateExpiry(term string, count int, priority bool) time.Duration {
+	// Subjects with less than 50 classes have a reversed expiry (less classes, longer interval)
+	// 1 class => 12 hours, 49 classes => 1 hour
+	if count < 50 {
+		hours := Slope(Point{1, 12}, Point{49, 1}, float64(count)).Y
+		return time.Duration(hours * float64(time.Hour))
+	}
 
-	// Randomly add or subtract the partial (delta between -15% and 15%)
+	// An hour for every 100 classes
+	baseExpiry := time.Hour * time.Duration(count/100)
+
+	// If the subject is a priority, then the expiry is halved without variance
+	if priority {
+		return baseExpiry / 3
+	}
+
+	// If the term is considered "view only", then the expiry is multipled by 5
+	var expiry = baseExpiry
+	if IsViewOnlyTerm(term) {
+		expiry *= 5
+	}
+
+	// Add minor variance to the expiry
+	expiryVariance := baseExpiry.Seconds() * (rand.Float64() * 0.15) // Between 0 and 15% of the total
 	if rand.Intn(2) == 0 {
-		scrapeExpiry -= time.Duration(partial) * time.Second
+		expiry -= time.Duration(expiryVariance) * time.Second
 	} else {
-		scrapeExpiry += time.Duration(partial) * time.Second
+		expiry += time.Duration(expiryVariance) * time.Second
 	}
 
 	// Ensure the expiry is at least 1 hour with up to 15 extra minutes
-	if scrapeExpiry < time.Hour {
-		scrapeExpiry = time.Hour + time.Duration(rand.Intn(60*15))*time.Second
+	if expiry < time.Hour {
+		baseExpiry = time.Hour + time.Duration(rand.Intn(60*15))*time.Second
 	}
 
-	// If the subject is a priority, then the expiry is halved
-	if priority {
-		return scrapeExpiry / 3
-	}
-	return scrapeExpiry
+	return baseExpiry
 }
 
 // IntakeCourse stores a course in Redis.