Merge pull request #405 from apple/accidentally-quadratic-BigString-init

[BigString] Fix accidentally quadratic `BigString.init`

Author: lorentey

Benchmarks/Sources/Benchmarks/BigStringBenchmarks.swift

@@ -0,0 +1,108 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Collections open source project
+//
+// Copyright (c) 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
+//
+//===----------------------------------------------------------------------===//
+
+import CollectionsBenchmark
+import _RopeModule
+import Foundation
+
+let someLatinSymbols: [UnicodeScalar] = [
+  0x20 ..< 0x7f,
+  0xa1 ..< 0xad,
+  0xae ..< 0x2af,
+  0x300 ..< 0x370,
+  0x1e00 ..< 0x1eff,
+].flatMap {
+  $0.map { UnicodeScalar($0)! }
+}
+
+extension UnicodeScalar {
+  static func randomLatin(
+    using rng: inout some RandomNumberGenerator
+  ) -> Self {
+    someLatinSymbols.randomElement(using: &rng)!
+  }
+}
+
+extension String.UnicodeScalarView {
+  static func randomLatin(
+    runeCount: Int, using rng: inout some RandomNumberGenerator
+  ) -> Self {
+    var result = String.UnicodeScalarView()
+    for _ in 0 ..< runeCount {
+      result.append(UnicodeScalar.randomLatin(using: &rng))
+    }
+    return result
+  }
+}
+
+extension String {
+  static func randomLatin(
+    runeCount: Int, using rng: inout some RandomNumberGenerator
+  ) -> Self {
+    let text = String.UnicodeScalarView.randomLatin(
+      runeCount: runeCount, using: &rng)
+    return String(text)
+  }
+}
+
+struct NativeStringInput {
+  let value: String
+
+  init(runeCount: Int, using rng: inout some RandomNumberGenerator) {
+    self.value = String.randomLatin(runeCount: runeCount, using: &rng)
+  }
+}
+
+struct BridgedStringInput {
+  let value: String
+
+  init(runeCount: Int, using rng: inout some RandomNumberGenerator) {
+    let string = String.randomLatin(runeCount: runeCount, using: &rng)
+    let utf16 = Array(string.utf16)
+    let cocoa = utf16.withUnsafeBufferPointer {
+      NSString(characters: $0.baseAddress!, length: $0.count)
+    }
+    self.value = cocoa as String
+  }
+}
+
+
+extension Benchmark {
+  public mutating func addBigStringBenchmarks() {
+    guard #available(macOS 13.3, iOS 16.4, watchOS 9.4, tvOS 16.4, *) else {
+      return
+    }
+
+    self.registerInputGenerator(for: NativeStringInput.self) { c in
+      var rng = SystemRandomNumberGenerator()
+      return NativeStringInput(runeCount: c, using: &rng)
+    }
+
+    self.registerInputGenerator(for: BridgedStringInput.self) { c in
+      var rng = SystemRandomNumberGenerator()
+      return BridgedStringInput(runeCount: c, using: &rng)
+    }
+
+    self.addSimple(
+      title: "BigString init from native string",
+      input: NativeStringInput.self
+    ) { input in
+      blackHole(BigString(input.value))
+    }
+
+    self.addSimple(
+      title: "BigString init from bridged string",
+      input: BridgedStringInput.self
+    ) { input in
+      blackHole(BigString(input.value))
+    }
+  }
+}

Benchmarks/Sources/benchmark-tool/main.swift

@@ -37,6 +37,7 @@ benchmark.addHeapBenchmarks()
 benchmark.addBitSetBenchmarks()
 benchmark.addTreeSetBenchmarks()
 benchmark.addCppBenchmarks()
+benchmark.addBigStringBenchmarks()
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 benchmark.addFoundationBenchmarks()
 #endif

Sources/RopeModule/BigString/Basics/BigString+Ingester.swift

@@ -39,13 +39,19 @@ extension BigString {
     var state: _CharacterRecognizer
 
     init(_ input: Substring) {
-      self.input = input
-      self.start = input.startIndex
-      self.state = _CharacterRecognizer()
+      self.init(input, startState: _CharacterRecognizer())
     }
 
     init(_ input: Substring, startState: __owned _CharacterRecognizer) {
       self.input = input
+      // Prevent accidentally quadratic operation by ensuring that we have
+      // a native UTF-8 string.
+      // FIXME: This is wasteful: if `input` happens to be a bridged
+      // FIXME: NSString instance, then it temporarily allocates a full
+      // FIXME: copy of the (transcoded) input string, only to then copy
+      // FIXME: its pieces into the tree later.
+      // FIXME: We should have a direct ingester path for native UTF-16 data.
+      self.input.makeContiguousUTF8()
       self.start = input.startIndex
       self.state = startState
     }