Add BidirectionalCollection.trimmed

Sources/Algorithms/Trim.swift

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+extension BidirectionalCollection {
+
+  /// Returns a `SubSequence` formed by discarding all elements at the start and end of the Collection
+  /// which satisfy the given predicate.
+  ///
+  /// Trimming is a common operation in computing, often referring to removing leading and trailing whitespace
+  /// from a string[1].
+  ///
+  ///  ```
+  ///  let myString = "  hello, world  "
+  ///  print(myString.trimmed(where: \.isWhitespace)) // "hello, world"
+  ///  ```
+  ///
+  /// [1]: https://en.wikipedia.org/wiki/Trimming_(computer_programming)
+  ///
+  /// - parameters:
+  ///    - predicate:  A closure which determines if the element should be omitted from the resulting slice.
+  ///
+  /// - complexity: `O(n)`, where `n` is the length of the Collection.
+  ///
+  @inlinable
+  public func trimmed(where predicate: (Element) throws -> Bool) rethrows -> SubSequence {
+
+    // Consume elements from the front.
+    let sliceStart = try firstIndex(where: { try predicate($0) == false }) ?? endIndex
+    // sliceEnd is the index _after_ the last index to match the predicate.
+    var sliceEnd = endIndex
+    while sliceStart != sliceEnd {
+      let idxBeforeSliceEnd = index(before: sliceEnd)
+      guard try predicate(self[idxBeforeSliceEnd]) else {
+        return self[sliceStart..<sliceEnd]
+      }
+      sliceEnd = idxBeforeSliceEnd
+    }
+    return self[Range(uncheckedBounds: (sliceStart, sliceStart))]  // Trimmed everything.
+  }
+}

Tests/SwiftAlgorithmsTests/TrimTests.swift

@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 TrimTests: XCTestCase {
+
+  func testEmpty() {
+    let results_empty = ([] as [Int]).trimmed { $0.isMultiple(of: 2) }
+    XCTAssertEqual(results_empty, [])
+  }
+
+  func testNoMatch() {
+    // No match (nothing trimmed).
+    let results_nomatch = [1, 3, 5, 7, 9, 11, 13, 15].trimmed { $0.isMultiple(of: 2) }
+    XCTAssertEqual(results_nomatch, [1, 3, 5, 7, 9, 11, 13, 15])
+  }
+
+  func testNoTailMatch() {
+    // No tail match (only trim head).
+    let results_notailmatch = [1, 3, 5, 7, 9, 11, 13, 15].trimmed { $0 < 10 }
+    XCTAssertEqual(results_notailmatch, [11, 13, 15])
+  }
+
+  func testNoHeadMatch() {
+    // No head match (only trim tail).
+    let results_noheadmatch = [1, 3, 5, 7, 9, 11, 13, 15].trimmed { $0 > 10 }
+    XCTAssertEqual(results_noheadmatch, [1, 3, 5, 7, 9])
+  }
+
+  func testBothEndsMatch() {
+    // Both ends match, some string of >1 elements do not match (return that string).
+    let results = [2, 10, 11, 15, 20, 21, 100].trimmed(where: { $0.isMultiple(of: 2) })
+    XCTAssertEqual(results, [11, 15, 20, 21])
+  }
+
+  func testEverythingMatches() {
+    // Everything matches (trim everything).
+    let results_allmatch = [1, 3, 5, 7, 9, 11, 13, 15].trimmed { _ in true }
+    XCTAssertEqual(results_allmatch, [])
+  }
+
+  func testEverythingButOneMatches() {
+    // Both ends match, one element does not match (trim everything except that element).
+    let results_onematch = [2, 10, 12, 15, 20, 100].trimmed { $0.isMultiple(of: 2) }
+    XCTAssertEqual(results_onematch, [15])
+  }
+}