Fix delta calculation when previous value is zero and current value is negative

Author: rachelmcr

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

@@ -127,8 +127,10 @@ extension StatsDataTextFormatter {
             return DeltaPercentage(value: 0) // Missing or equal values: 0% change
         }
 
-        guard previousValue > 0 else {
-            return DeltaPercentage(value: 1) // Previous value was 0: 100% change
+        // If the previous value was 0, return a 100% or -100% change
+        guard previousValue != 0 else {
+            let deltaValue: Decimal = currentValue > 0 ? 1 : -1
+            return DeltaPercentage(value: deltaValue)
         }
 
         return DeltaPercentage(value: (currentValue - previousValue) / previousValue)

WooCommerce/WooCommerceTests/ViewRelated/Dashboard/StatsDataTextFormatterTests.swift

@@ -307,7 +307,7 @@ final class StatsDataTextFormatterTests: XCTestCase {
         XCTAssertEqual(delta.direction, .zero)
     }
 
-    func test_createDeltaPercentage_returns_100_percent_change_when_previous_value_is_zero() {
+    func test_createDeltaPercentage_returns_positive_100_percent_change_when_previous_value_is_zero() {
         // Given
         let previousValue: Double = 0
         let currentValue: Double = 10
@@ -320,6 +320,19 @@ final class StatsDataTextFormatterTests: XCTestCase {
         XCTAssertEqual(delta.direction, .positive)
     }
 
+    func test_createDeltaPercentage_returns_negative_100_percent_change_when_previous_value_is_zero() {
+        // Given
+        let previousValue: Double = 0
+        let currentValue: Double = -10
+
+        // When
+        let delta = StatsDataTextFormatter.createDeltaPercentage(from: previousValue, to: currentValue)
+
+        // Then
+        XCTAssertEqual(delta.string, "-100%")
+        XCTAssertEqual(delta.direction, .negative)
+    }
+
     func test_createDeltaPercentage_returns_negative_100_percent_change_when_current_value_is_zero() {
         // Given
         let previousValue: Double = 10