init?() now fails on nonsense input

Author: Reed Es

README.md

@@ -2,7 +2,7 @@
 
 Generate 'nice' numbers for label ticks over a range, such as for y-axis on a chart.
 
-Adapted from pseudo-code in *Graphics Gems, Volume 1* by Andrew S. Glassner (1995).
+Adapted from pseudo-code in *Graphics Gems, Volume 1* by Andrew S. Glassner (1995). See also Tufte (1983) for a discussion of topic.
 
 Available as an open source Swift library to be incorporated in other apps.
 
@@ -16,12 +16,15 @@ _SwiftNiceScale_ is part of the [OpenAlloc](https://github.com/openalloc) family
 let ns = NiceScale(105...543, desiredTicks: 5)
 
 print("nice range=\(ns.range)")
+
 => "nice range=100.0...600.0"
 
 print("tick interval=\(ns.tickInterval)")
+
 => "tick interval=100.0"
 
 print("labels=\(ns.tickValues)")
+
 => "labels=[100.0, 200.0, 300.0, 400.0, 500.0, 600.0]"
 ```
 
@@ -37,7 +40,9 @@ typealias ValueRange = ClosedRange<T>
 
 #### Initializer
 
-- `init(_ rawRange: NiceScale<T>.ValueRange, desiredTicks: Int)` - create a new `NiceScale` instance
+- `init?(_ rawRange: NiceScale<T>.ValueRange, desiredTicks: Int)` - create a new `NiceScale` instance
+
+Initialization will fail and return `nil` if provided nonsense parameters, such as a range with zero extent, or a desired tick count less than 2.
 
 The initialization values are also available as properties:
 

Sources/NiceScale.swift

@@ -26,7 +26,8 @@ public final class NiceScale<T: BinaryFloatingPoint & Real> {
     public let rawRange: ValueRange
     public let desiredTicks: Int
 
-    public init(_ rawRange: ValueRange, desiredTicks: Int = 10) {
+    public init?(_ rawRange: ValueRange, desiredTicks: Int = 10) {
+        guard rawRange.lowerBound < rawRange.upperBound, desiredTicks > 1 else { return nil }
         self.rawRange = rawRange
         self.desiredTicks = desiredTicks
     }

Tests/NiceScaleTests.swift

@@ -27,7 +27,7 @@ class NiceScaleTests: XCTestCase {
     typealias NS = NiceScale<Double>
 
     func testExample() {
-        let ns = NiceScale(105...543, desiredTicks: 5)
+        let ns = NiceScale(105...543, desiredTicks: 5)!
         XCTAssertEqual(0, ns.tickFractionDigits)
         XCTAssertEqual(100, ns.tickInterval)
         XCTAssertEqual(100.0...600.0, ns.range)
@@ -37,29 +37,29 @@ class NiceScaleTests: XCTestCase {
     }
 
     func testFractionDigits() {
-        let ns20 = NS(1...4, desiredTicks: 20)
+        let ns20 = NS(1...4, desiredTicks: 20)!
         XCTAssertEqual(1, ns20.tickFractionDigits)
 
-        let ns200 = NS(1...4, desiredTicks: 200)
+        let ns200 = NS(1...4, desiredTicks: 200)!
         XCTAssertEqual(2, ns200.tickFractionDigits)
     }
 
-    func testNoRange() {
-        let ns = NS(0...0)
-        XCTAssertFalse(ns.hasPositiveRange)
-        XCTAssertFalse(ns.hasNegativeRange)
-        XCTAssertEqual(0...0, ns.positiveRange)
-        XCTAssertEqual(0...0, ns.negativeRange)
-        XCTAssertEqual(0, ns.positiveExtent)
-        XCTAssertEqual(0, ns.negativeExtent)
+    func testBadInit() {
+        let ns0 = NS(0...0)
+        XCTAssertNil(ns0)
+        
+        let ns1 = NS(1...1)
+        XCTAssertNil(ns1)
+        
+        let ns2 = NS(0...1, desiredTicks: 0)
+        XCTAssertNil(ns2)
 
-        XCTAssertEqual(Double.infinity, ns.scaleToUnit(1000))
-        XCTAssertEqual(Double.infinity, ns.scaleToUnitPositive(1000))
-        XCTAssertEqual(-Double.infinity, ns.scaleToUnitNegative(1000))
+        let ns3 = NS(0...1, desiredTicks: 1)
+        XCTAssertNil(ns3)
     }
 
     func testPositiveRange() {
-        let ns = NS(10...100)
+        let ns = NS(10...100)!
         XCTAssertTrue(ns.hasPositiveRange)
         XCTAssertFalse(ns.hasNegativeRange)
         XCTAssertEqual(10...100, ns.positiveRange)
@@ -73,7 +73,7 @@ class NiceScaleTests: XCTestCase {
     }
 
     func testNegativeRange() {
-        let ns = NS(-100...(-10))
+        let ns = NS(-100...(-10))!
         XCTAssertFalse(ns.hasPositiveRange)
         XCTAssertTrue(ns.hasNegativeRange)
         XCTAssertEqual(0...0, ns.positiveRange)
@@ -87,7 +87,7 @@ class NiceScaleTests: XCTestCase {
     }
 
     func testBothRange() {
-        let ns = NS(-100...100)
+        let ns = NS(-100...100)!
         XCTAssertTrue(ns.hasPositiveRange)
         XCTAssertTrue(ns.hasNegativeRange)
         XCTAssertEqual(0...100, ns.positiveRange)