Fun with rating stars

Author: Paul King
Published: 2023-01-25 12:00AM

Star Ratings

A recent tweet showed some C# code for producing a String of stars that might be used when displaying ratings on a website:

Image of original tweet

Let’s have a look at several ways to do the same thing in Groovy.

def rating0(percentage) {
    int stars = Math.ceil(percentage * 10)
    ("🔵" * stars).padRight(10, "⚪")
}

def rating1(percentage) {
    int stars = Math.ceil(percentage * 10)
    "🔵" * stars + "⚪" * (10-stars)
}

def rating2(percentage) {
    int skip = 10 - Math.ceil(percentage * 10)
    "🔵🔵🔵🔵🔵🔵🔵🔵🔵🔵⚪⚪⚪⚪⚪⚪⚪⚪⚪⚪"[skip..<10+skip]
}

def rating3(percentage) {
    switch(percentage) {
        case 0             -> "⚪⚪⚪⚪⚪⚪⚪⚪⚪⚪"
        case { it <= 0.1 } -> "🔵⚪⚪⚪⚪⚪⚪⚪⚪⚪"
        case { it <= 0.2 } -> "🔵🔵⚪⚪⚪⚪⚪⚪⚪⚪"
        case { it <= 0.3 } -> "🔵🔵🔵⚪⚪⚪⚪⚪⚪⚪"
        case { it <= 0.4 } -> "🔵🔵🔵🔵⚪⚪⚪⚪⚪⚪"
        case { it <= 0.5 } -> "🔵🔵🔵🔵🔵⚪⚪⚪⚪⚪"
        case { it <= 0.6 } -> "🔵🔵🔵🔵🔵🔵⚪⚪⚪⚪"
        case { it <= 0.7 } -> "🔵🔵🔵🔵🔵🔵🔵⚪⚪⚪"
        case { it <= 0.8 } -> "🔵🔵🔵🔵🔵🔵🔵🔵⚪⚪"
        case { it <= 0.9 } -> "🔵🔵🔵🔵🔵🔵🔵🔵🔵⚪"
        default            -> "🔵🔵🔵🔵🔵🔵🔵🔵🔵🔵"
    }
}

If you want you can test the various edge cases:

for (i in 0..3)
    for (j in [0, 0.09, 0.1, 0.11, 0.9, 1])
        println "rating$i"(j)

Increasing Robustness

The code examples here assume that the input is in the range 0 <= percentage <= 1. There are several tweaks we could do to guard against inputs outside those ranges.

We could simply add an assert, e.g.:

def rating0b(percentage) {
    assert percentage >= 0 && percentage <= 1
    int stars = Math.ceil(percentage * 10)
    ("🔵" * stars).padRight(10, "⚪")
}

Or, if we wanted to not fail, tweak some of our conditions, e.g. for rating3, instead of case 0, we could use case { it ⇐ 0 }.

We could push the checks into our types by making a special class, Percent say, which only permitted the allowed values:

final class Percent {
    final Double value
    Percent(Double value) {
        assert value >= 0 && value <= 1
        this.value = value
    }
}

And we could optionally also use some metaprogramming to provide a custom isCase method:

Double.metaClass.isCase = { Percent p -> delegate >= p.value }

Which means we could tweak the rating method to be:

def rating3b(Percent p) {
    switch(p) {
        case 0.0d -> "⚪⚪⚪⚪⚪⚪⚪⚪⚪⚪"
        case 0.1d -> "🔵⚪⚪⚪⚪⚪⚪⚪⚪⚪"
        case 0.2d -> "🔵🔵⚪⚪⚪⚪⚪⚪⚪⚪"
        case 0.3d -> "🔵🔵🔵⚪⚪⚪⚪⚪⚪⚪"
        case 0.4d -> "🔵🔵🔵🔵⚪⚪⚪⚪⚪⚪"
        case 0.5d -> "🔵🔵🔵🔵🔵⚪⚪⚪⚪⚪"
        case 0.6d -> "🔵🔵🔵🔵🔵🔵⚪⚪⚪⚪"
        case 0.7d -> "🔵🔵🔵🔵🔵🔵🔵⚪⚪⚪"
        case 0.8d -> "🔵🔵🔵🔵🔵🔵🔵🔵⚪⚪"
        case 0.9d -> "🔵🔵🔵🔵🔵🔵🔵🔵🔵⚪"
        default   -> "🔵🔵🔵🔵🔵🔵🔵🔵🔵🔵"
    }
}

We can be fancier here using @EqualsAndHashCode on the Percent class and/or using @Delegate on the value property, depending on how rich in functionality we wanted the Percent instances to be.

Alternatively, we could use a design-by-contract approach:

@Requires({ percentage >= 0 && percentage <= 1 })
def rating1b(percentage) {
    int stars = Math.ceil(percentage * 10)
    "🔵" * stars + "⚪" * (10-stars)
}

References

Original tweet