Adds "isSorted", "isSorted(by: )" and "allEqual" to Comparable Sequence

Sources/Algorithms/isSorted.swift

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+//===----------------------------------------------------------------------===//
+// isSorted()
+//===----------------------------------------------------------------------===//
+
+
+extension Sequence where Element: Comparable {
+    public func isSorted() -> Bool {
+        /// Returns Bool, indicating whether a sequence is sorted
+        /// into non-descending order.
+        ///
+        /// - Complexity: O(*n*), where *n* is the length of the sequence.
+
+        isSorted(by: <)
+    }
+
+    public func isSorted(by areInIncreasingOrder: (Element, Element) -> Bool) -> Bool {
+        /// Returns Bool, indicating whether a sequence is sorted using
+        /// the given predicate as the comparison between elements.
+        ///
+        /// - Complexity: O(*n*), where *n* is the length of the sequence.
+
+        var prev: Element?
+        for element in self {
+            if let p = prev, !areInIncreasingOrder(p, element) {
+                return false
+            }
+            prev = element
+        }
+        return true
+    }
+
+    public func allEqual() -> Bool {
+        /// Returns Bool, indicating whether all the
+        /// elements in sequence are equal to each other.
+        ///
+        /// - Complexity: O(*n*), where *n* is the length of the sequence.
+
+        isSorted(by: ==)
+    }
+}

Tests/SwiftAlgorithmsTests/IsSortedTests.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import Algorithms
+
+final class IsSortedTests: XCTestCase {
+
+  func testIsSorted() {
+    let a = 0...10
+    let b = (0...10).reversed()
+    let c = Array(repeating: 42, count: 10)
+    XCTAssertTrue(a.isSorted())
+    XCTAssertTrue(b.isSorted(by: >))
+    XCTAssertTrue(c.allEqual())
+  }
+}