Make a micro optimization for whitespace trimming of strings (#945)

Author: d-ronnqvist

Sources/SwiftDocC/Model/Rendering/Content/Extensions/RenderTermLists.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2024 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
@@ -286,22 +286,20 @@ extension RenderInlineContent {
 }
 
 extension String {
-    
-    /// The result of removing whitespace from the beginning of the string.
+    /// Returns a new string made by removing whitespace from the beginning of the string.
     func removingLeadingWhitespace() -> String {
-        var trimmedString = self
-        while trimmedString.first?.isWhitespace == true {
-            trimmedString = String(trimmedString.dropFirst())
+        guard let index = self.firstIndex(where: { !$0.isWhitespace }) else { return "" }
+        if index == startIndex {
+            // Avoid a potential copy if the string doesn't have any leading whitespace.
+            return self
         }
-        return trimmedString
+        return String(self[index...])
     }
 
-    /// The result of removing whitespace from the end of the string.
+    /// Returns a new string made by removing whitespace from the end of the string.
     func removingTrailingWhitespace() -> String {
-        var trimmedString = self
-        while trimmedString.last?.isWhitespace == true {
-            trimmedString = String(trimmedString.dropLast())
-        }
-        return trimmedString
+        guard let index = self.lastIndex(where: { !$0.isWhitespace }) else { return "" }
+        // It's not worth checking if the index is at the end because `index(before: endIndex)` is slower than not checking.
+        return String(self[...index])
     }
 }

Tests/SwiftDocCTests/Utility/String+WhitespaceTests.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2024 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
@@ -37,5 +37,25 @@ class String_WhitespaceTests: XCTestCase {
         XCTAssertEqual("Words: with, various. punctuation! as - separators".replacingWhitespaceAndPunctuation(with: "-"),
                        "Words-with-various-punctuation-as-separators")
     }
+    
+    func testTrimmingWhitespace() {
+        XCTAssertEqual("".removingLeadingWhitespace(), "")
+        XCTAssertEqual("".removingTrailingWhitespace(), "")
+        
+        XCTAssertEqual("ABC".removingLeadingWhitespace(), "ABC")
+        XCTAssertEqual("ABC".removingTrailingWhitespace(), "ABC")
+        
+        XCTAssertEqual("\t ABC\t ".removingLeadingWhitespace(), "ABC\t ", "Removing leading whitespace doesn't remove trailing whitespace")
+        XCTAssertEqual("\t ABC\t ".removingTrailingWhitespace(), "\t ABC", "Removing trailing whitespace doesn't remove leading whitespace")
+        
+        XCTAssertEqual("\t ".removingLeadingWhitespace(), "", "All characters are whitespace")
+        XCTAssertEqual("\t ".removingTrailingWhitespace(), "", "All characters are whitespace")
+        
+        XCTAssertEqual("\n ABC \n".removingLeadingWhitespace(), "ABC \n", "Newline is considered a whitespace character")
+        XCTAssertEqual("\n ABC \n".removingTrailingWhitespace(), "\n ABC", "Newline is considered a whitespace character")
+        
+        XCTAssertEqual("start ABC end".removingLeadingWhitespace(), "start ABC end", "Only removes whitespace at the start and end of the string")
+        XCTAssertEqual("start ABC end".removingTrailingWhitespace(), "start ABC end", "Only removes whitespace at the start and end of the string")
+    }
 }