add suffix:while method

Sources/Algorithms/Suffix.swift

@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2021 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
+//
+//===----------------------------------------------------------------------===//
+
+//===----------------------------------------------------------------------===//
+// Suffix(while:)
+//===----------------------------------------------------------------------===//
+
+extension BidirectionalCollection {
+  /// Returns a subsequence containing the elements from the end until
+  /// `predicate` returns `false` and skipping the remaining elements.
+  ///
+  /// - Parameter predicate: A closure that takes an element of the
+  ///   sequence as its argument and returns `true` if the element should
+  ///   be included or `false` if it should be excluded. Once the predicate
+  ///   returns `false` it will not be called again.
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the collection.
+  public func suffix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows -> SubSequence {
+    let start = startIndex
+    var result = endIndex
+    while result != start {
+      let previous = index(before: result)
+      guard try predicate(self[previous]) else { break }
+      result = previous
+    }
+    return self[result...]
+  }
+}

Tests/SwiftAlgorithmsTests/SuffixTests.swift

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2021 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 SuffixTests: XCTestCase {
+  func testSuffix() {
+    let a = 0...10
+    XCTAssertEqualSequences(a.suffix(while: { $0 > 5 }), (6...10))
+    XCTAssertEqualSequences(a.suffix(while: { $0 > 10 }), [])
+    XCTAssertEqualSequences(a.suffix(while: { $0 > 9 }), [10])
+    XCTAssertEqualSequences(a.suffix(while: { $0 > -1 }), (0...10))
+
+    let empty: [Int] = []
+    XCTAssertEqualSequences(empty.suffix(while: { $0 > 10 }), [])
+  }
+}