Add helpers for calculating the delta percentage for stats

Author: rachelmcr

WooCommerce/Classes/ViewRelated/Dashboard/Factories/StatsV4DataHelper.swift

@@ -83,6 +83,30 @@ final class StatsV4DataHelper {
         }
     }
 
+    // MARK: Delta Calculations
+
+    /// Creates the text showing the percent change from the previous `Decimal` value to the current `Decimal` value
+    ///
+    static func createDeltaText(from previousValue: Decimal, to currentValue: Decimal, locale: Locale = Locale.current) -> String {
+        let numberFormatter = NumberFormatter()
+        numberFormatter.numberStyle = .percent
+        numberFormatter.positivePrefix = numberFormatter.plusSign
+        numberFormatter.locale = locale
+
+        guard previousValue > 0 else {
+            return numberFormatter.string(from: 1) ?? "+100%"
+        }
+
+        let deltaValue = ((currentValue - previousValue) / previousValue)
+        return numberFormatter.string(from: deltaValue as NSNumber) ?? Constants.placeholderText
+    }
+
+    /// Creates the text showing the percent change from the previous `Double` value to the current `Double` value
+    ///
+    static func createDeltaText(from previousValue: Double, to currentValue: Double, locale: Locale = Locale.current) -> String {
+        createDeltaText(from: Decimal(previousValue), to: Decimal(currentValue))
+    }
+
     // MARK: Stats Intervals
 
     /// Returns the order stats intervals, ordered by date.

WooCommerce/WooCommerceTests/ViewRelated/Dashboard/StatsV4DataHelperTests.swift

@@ -9,6 +9,7 @@ final class StatsV4DataHelperTests: XCTestCase {
 
     private let currencyFormatter = CurrencyFormatter(currencySettings: CurrencySettings()) // Default is US
     private let currencyCode = CurrencySettings().currencyCode
+    private let locale = Locale(identifier: "en_US")
 
     // MARK: Revenue Stats
 
@@ -200,4 +201,54 @@ final class StatsV4DataHelperTests: XCTestCase {
         // Then
         XCTAssertEqual(conversionRate, "10%")
     }
+
+    // MARK: Delta Calculations
+
+    func test_createDeltaText_returns_expected_positive_text() {
+        // Given
+        let previousValue: Double = 100
+        let currentValue: Double = 150
+
+        // When
+        let deltaText = StatsV4DataHelper.createDeltaText(from: previousValue, to: currentValue, locale: locale)
+
+        // Then
+        XCTAssertEqual(deltaText, "+50%")
+    }
+
+    func test_createDeltaText_returns_expected_negative_text() {
+        // Given
+        let previousValue: Double = 150
+        let currentValue: Double = 100
+
+        // When
+        let deltaText = StatsV4DataHelper.createDeltaText(from: previousValue, to: currentValue, locale: locale)
+
+        // Then
+        XCTAssertEqual(deltaText, "-33%")
+    }
+
+    func test_createDeltaText_returns_100_percent_change_when_previous_value_is_zero() {
+        // Given
+        let previousValue: Double = 0
+        let currentValue: Double = 10
+
+        // When
+        let deltaText = StatsV4DataHelper.createDeltaText(from: previousValue, to: currentValue, locale: locale)
+
+        // Then
+        XCTAssertEqual(deltaText, "+100%")
+    }
+
+    func test_createDeltaText_returns_negative_100_percent_change_when_current_value_is_zero() {
+        // Given
+        let previousValue: Double = 10
+        let currentValue: Double = 0
+
+        // When
+        let deltaText = StatsV4DataHelper.createDeltaText(from: previousValue, to: currentValue, locale: locale)
+
+        // Then
+        XCTAssertEqual(deltaText, "-100%")
+    }
 }