fix Buckets helpers to not throw fatal error (#41)

* fix Buckets to not throw fatal error

* fix compiler warning

* simplify linear buckets creation

Co-authored-by: Jari (LotU) <j.koopman@jarict.nl>

Author: avolokhov

Sources/Prometheus/MetricTypes/Histogram.swift

@@ -39,12 +39,7 @@ public struct Buckets: ExpressibleByArrayLiteral {
     ///     - count: Amount of buckets to generate, should be larger than zero. The +Inf bucket is not included in this count.
     public static func linear(start: Double, width: Double, count: Int) -> Buckets {
         assert(count >= 1, "Bucket.linear needs a count larger than 1")
-        var arr = [Double]()
-        var s = start
-        for x in 0..<count {
-            arr[x] = s
-            s += width
-        }
+        let arr = (0..<count).map { Double(start) + Double($0) * Double(width) }
         return Buckets(arr)
     }
 
@@ -60,8 +55,8 @@ public struct Buckets: ExpressibleByArrayLiteral {
         assert(factor > 1, "Bucket.exponential needs a factor larger than 1")
         var arr = [Double]()
         var s = start
-        for x in 0..<count {
-            arr[x] = s
+        for _ in 0..<count {
+            arr.append(s)
             s *= factor
         }
         return Buckets(arr)

Tests/SwiftPrometheusTests/BucketsTests.swift

@@ -0,0 +1,13 @@
+import XCTest
+import Prometheus
+class BucketsTests: XCTestCase {
+    func testExponentialDoesNotThrow() {
+        let buckets = Buckets.exponential(start: 1, factor: 2, count: 4)
+        XCTAssertEqual([1.0, 2.0, 4.0, 8.0, Double.greatestFiniteMagnitude], buckets.buckets)
+    }
+
+    func testLinearDoesNotThrow() {
+        let buckets = Buckets.linear(start: 1, width: 20, count: 4)
+        XCTAssertEqual([1, 21, 41, 61, Double.greatestFiniteMagnitude], buckets.buckets)
+    }
+}