Implement parsing grayscale colors as white(value) or white(value, alpha)

Sources/LibMakeColors/Model/Color.swift

@@ -19,6 +19,14 @@ struct Color: CustomStringConvertible, Equatable {
         alpha = array.count == 4 ? array[3] : 0xFF
     }
 
+    init(white array: [UInt8]) {
+        precondition(array.count == 1 || array.count == 2)
+        red = array[0]
+        green = array[0]
+        blue = array[0]
+        alpha = array.count == 2 ? array[1] : 0xFF
+    }
+
     var description: String {
         let alphaSuffix = alpha != 0xFF ? String(format: "%02X", alpha) : ""
         return String(format: "#%02X%02X%02X%@", red, green, blue, alphaSuffix)

Sources/LibMakeColors/Model/Scanner+ColorParser.swift

@@ -27,6 +27,10 @@ extension Scanner {
             return Color(components)
         }
 
+        if string("white"), let arguments = argumentList(min: 1, max: 2) {
+            return Color(white: arguments)
+        }
+
         return nil
     }
 
@@ -91,6 +95,26 @@ extension Scanner {
         return result
     }
 
+    // swiftlint:disable:next discouraged_optional_collection
+    func argumentList(_ count: Int) -> [UInt8]? {
+        argumentList(min: count, max: count)
+    }
+
+    // swiftlint:disable:next discouraged_optional_collection
+    func argumentList(min: Int, max: Int? = nil) -> [UInt8]? {
+        let max = max ?? Int.max
+        guard
+            string("("),
+            let arguments = commaSeparated(),
+            string(")"),
+            (min...max) ~= arguments.count
+        else {
+            return nil
+        }
+
+        return arguments
+    }
+
     // swiftlint:disable:next discouraged_optional_collection
     func commaSeparated() -> [UInt8]? {
         var result: [UInt8] = []

Tests/MakeColorsTests/ColorParserTest.swift

@@ -43,4 +43,28 @@ final class ColorParserTest: XCTestCase {
         let color = scanner.color()
         XCTAssertEqual(Color(red: 1, green: 2, blue: 3, alpha: 4), color)
     }
+
+    func testScanningWhite() throws {
+        let scanner = Scanner(string: "white(255)")
+        let color = scanner.color()
+        XCTAssertEqual(Color(red: 255, green: 255, blue: 255, alpha: 255), color)
+    }
+
+    func testScanningWhiteWithAlpha() throws {
+        let scanner = Scanner(string: "white(255, 128)")
+        let color = scanner.color()
+        XCTAssertEqual(Color(red: 255, green: 255, blue: 255, alpha: 128), color)
+    }
+
+    func testWhiteFailsWithoutArguments() throws {
+        let scanner = Scanner(string: "white()")
+        let color = scanner.color()
+        XCTAssertNil(color)
+    }
+
+    func testWhiteFailsWith3Arguments() throws {
+        let scanner = Scanner(string: "white(1,2,3)")
+        let color = scanner.color()
+        XCTAssertNil(color)
+    }
 }