Add trimmingSuffix, trimmingPrefix and mutating variants

Sources/Algorithms/Trim.swift

@@ -7,6 +7,51 @@
 //
 // See https://swift.org/LICENSE.txt for license information
 //
+
+//===----------------------------------------------------------------------===//
+// trimmingPrefix(while:)
+//===----------------------------------------------------------------------===//
+
+extension Collection {
+  /// Returns a `SubSequence` formed by discarding all elements at the start
+  /// of the collection which satisfy the given predicate.
+  ///
+  /// This example uses `trimmingPrefix(while:)` to get a substring without the white
+  /// space at the beginning of the string:
+  ///
+  ///     let myString = "  hello, world  "
+  ///     print(myString.trimmingPrefix(while: \.isWhitespace)) // "hello, world  "
+  ///
+  /// - 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 this collection.
+  ///
+  @inlinable
+  public func trimmingPrefix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows -> SubSequence {
+    let start = try endOfPrefix(while: predicate)
+    return self[start..<endIndex]
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// trimPrefix(toStartAt:)
+//===----------------------------------------------------------------------===//
+
+extension Collection where Self: RangeReplaceableCollection {
+  @inlinable
+  public mutating func trimPrefix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows {
+    replaceSubrange(startIndex..<endIndex, with: try trimmingPrefix(while: predicate))
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// trimming(toStartAt:) / trimmingSuffix(while:)
 //===----------------------------------------------------------------------===//
 
 extension BidirectionalCollection {
@@ -29,8 +74,49 @@ extension BidirectionalCollection {
   public func trimming(
     while predicate: (Element) throws -> Bool
   ) rethrows -> SubSequence {
-    let start = try endOfPrefix(while: predicate)
-    let end = try self[start...].startOfSuffix(while: predicate)
-    return self[start..<end]
+    return try trimmingPrefix(while: predicate).trimmingSuffix(while: predicate)
+  }
+
+  /// Returns a `SubSequence` formed by discarding all elements at the end
+  /// of the collection which satisfy the given predicate.
+  ///
+  /// This example uses `trimmingSuffix(while:)` to get a substring without the white
+  /// space at the end of the string:
+  ///
+  ///     let myString = "  hello, world  "
+  ///     print(myString.trimmingSuffix(while: \.isWhitespace)) // "  hello, world"
+  ///
+  /// - 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 this collection.
+  ///
+  @inlinable
+  public func trimmingSuffix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows -> SubSequence {
+    let end = try self[startIndex...].startOfSuffix(while: predicate)
+    return self[startIndex..<end]
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// trim(toStartAt:) / trimSuffix(while:)
+//===----------------------------------------------------------------------===//
+
+extension BidirectionalCollection where Self: RangeReplaceableCollection {
+  @inlinable
+  public mutating func trim(
+    while predicate: (Element) throws -> Bool
+  ) rethrows {
+    replaceSubrange(startIndex..<endIndex, with: try trimming(while: predicate))
+  }
+
+  @inlinable
+  public mutating func trimSuffix(
+    while predicate: (Element) throws -> Bool
+  ) rethrows {
+    replaceSubrange(startIndex..<endIndex, with: try trimmingSuffix(while: predicate))
   }
 }

Tests/SwiftAlgorithmsTests/TrimTests.swift

@@ -56,4 +56,14 @@ final class TrimTests: XCTestCase {
     let results_one = [2, 10, 12, 15, 20, 100].trimming { $0.isMultiple(of: 2) }
     XCTAssertEqual(results_one, [15])
   }
+
+  func testTrimmingPrefix() {
+      let results = [2, 10, 12, 15, 20, 100].trimmingPrefix { $0.isMultiple(of: 2) }
+      XCTAssertEqual(results, [15, 20, 100])
+  }
+
+  func testTrimmingSuffix() {
+      let results = [2, 10, 12, 15, 20, 100].trimmingSuffix { $0.isMultiple(of: 2) }
+      XCTAssertEqual(results, [2, 10, 12, 15])
+  }
 }