Merge pull request #10976 from d-ronnqvist/reduce-with-inout

[stdlib] Implement SE-0171: Reduce with inout

Author: Dave Abrahams

benchmark/CMakeLists.txt

@@ -90,6 +90,7 @@ set(SWIFT_BENCH_MODULES
     single-source/RGBHistogram
     single-source/RangeAssignment
     single-source/RecursiveOwnedParameter
+    single-source/ReduceInto
     single-source/ReversedCollections
     single-source/SetTests
     single-source/SevenBoom

benchmark/single-source/ReduceInto.swift

@@ -0,0 +1,112 @@
+//===--- ReduceInto.swift -------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 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
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import TestsUtils
+import Foundation
+
+// Sum
+
+@inline(never)
+public func run_SumUsingReduce(_ N: Int) {
+  let numbers = [Int](0..<1000)
+  
+  var c = 0
+  for _ in 1...N*1000 {
+    c = c &+ numbers.reduce(0) { (acc: Int, num: Int) -> Int in
+      acc &+ num
+    }
+  }
+  CheckResults(c != 0)
+}
+
+@inline(never)
+public func run_SumUsingReduceInto(_ N: Int) {
+  let numbers = [Int](0..<1000)
+  
+  var c = 0
+  for _ in 1...N*1000 {
+    c = c &+ numbers.reduce(into: 0) { (acc: inout Int, num: Int) in
+      acc = acc &+ num
+    }
+  }
+  CheckResults(c != 0)
+}
+
+// Filter
+
+@inline(never)
+public func run_FilterEvenUsingReduce(_ N: Int) {
+  let numbers = [Int](0..<100)
+  
+  var c = 0
+  for _ in 1...N*100 {
+    let a = numbers.reduce([]) { (acc: [Int], num: Int) -> [Int] in
+      var a = acc
+      if num % 2 == 0 {
+        a.append(num)
+      }
+      return a
+    }
+    c = c &+ a.count
+  }
+  CheckResults(c != 0)
+}
+
+@inline(never)
+public func run_FilterEvenUsingReduceInto(_ N: Int) {
+  let numbers = [Int](0..<100)
+  
+  var c = 0
+  for _ in 1...N*100 {
+    let a = numbers.reduce(into: []) { (acc: inout [Int], num: Int) in
+      if num % 2 == 0 {
+        acc.append(num)
+      }
+    }
+    c = c &+ a.count
+  }
+  CheckResults(c != 0)
+}
+
+// Frequencies
+
+@inline(never)
+public func run_FrequenciesUsingReduce(_ N: Int) {
+  let s = "thequickbrownfoxjumpsoverthelazydogusingasmanycharacteraspossible123456789"
+  
+  var c = 0
+  for _ in 1...N*100 {
+    let a = s.reduce([:]) {
+      (acc: [Character: Int], c: Character) -> [Character: Int] in
+      var d = acc
+      d[c, default: 0] += 1
+      return d
+    }
+    c = c &+ a.count
+  }
+  CheckResults(c != 0)
+}
+
+@inline(never)
+public func run_FrequenciesUsingReduceInto(_ N: Int) {
+  let s = "thequickbrownfoxjumpsoverthelazydogusingasmanycharacteraspossible123456789"
+  
+  var c = 0
+  for _ in 1...N*100 {
+    let a = s.reduce(into: [:]) {
+      (acc: inout [Character: Int], c: Character) in
+      acc[c, default: 0] += 1
+    }
+    c = c &+ a.count
+  }
+  CheckResults(c != 0)
+}

benchmark/utils/main.swift

@@ -95,6 +95,7 @@ import RC4
 import RGBHistogram
 import RangeAssignment
 import RecursiveOwnedParameter
+import ReduceInto
 import ReversedCollections
 import SetTests
 import SevenBoom
@@ -273,6 +274,10 @@ addTo(&precommitTests, "EqualSubstringString", run_EqualSubstringString)
 addTo(&precommitTests, "EqualSubstringSubstring", run_EqualSubstringSubstring)
 addTo(&precommitTests, "EqualSubstringSubstringGenericEquatable", run_EqualSubstringSubstringGenericEquatable)
 addTo(&precommitTests, "ErrorHandling", run_ErrorHandling)
+addTo(&precommitTests, "FilterEvenUsingReduce", run_FilterEvenUsingReduce)
+addTo(&precommitTests, "FilterEvenUsingReduceInto", run_FilterEvenUsingReduceInto)
+addTo(&precommitTests, "FrequenciesUsingReduce", run_FrequenciesUsingReduce)
+addTo(&precommitTests, "FrequenciesUsingReduceInto", run_FrequenciesUsingReduceInto)
 addTo(&precommitTests, "Hanoi", run_Hanoi)
 addTo(&precommitTests, "HashTest", run_HashTest)
 addTo(&precommitTests, "Histogram", run_Histogram)
@@ -443,6 +448,8 @@ addTo(&precommitTests, "SuffixCountableRange", run_SuffixCountableRange)
 addTo(&precommitTests, "SuffixCountableRangeLazy", run_SuffixCountableRangeLazy)
 addTo(&precommitTests, "SuffixSequence", run_SuffixSequence)
 addTo(&precommitTests, "SuffixSequenceLazy", run_SuffixSequenceLazy)
+addTo(&precommitTests, "SumUsingReduce", run_SumUsingReduce)
+addTo(&precommitTests, "SumUsingReduceInto", run_SumUsingReduceInto)
 addTo(&precommitTests, "SuperChars", run_SuperChars)
 addTo(&precommitTests, "TwoSum", run_TwoSum)
 addTo(&precommitTests, "TypeFlood", run_TypeFlood)

stdlib/public/core/SequenceAlgorithms.swift.gyb

@@ -596,6 +596,61 @@ extension Sequence {
     }
     return accumulator
   }
+  
+  /// Returns the result of combining the elements of the sequence using the
+  /// given closure.
+  ///
+  /// Use the `reduce(into:_:)` method to produce a single value from the
+  /// elements of an entire sequence. For example, you can use this method on an
+  /// array of integers to filter adjacent equal entries or count frequencies.
+  ///
+  /// This method is preferred over `reduce(_:_:)` for efficiency when the
+  /// result is a copy-on-write type, for example an Array or a Dictionary.
+  ///
+  /// The `updateAccumulatingResult` closure is called sequentially with a
+  /// mutable accumulating value initialized to `initialResult` and each element
+  /// of the sequence. This example shows how to build a dictionary of letter
+  /// frequencies of a string.
+  ///
+  ///     let letters = "abracadabra"
+  ///     let letterCount = letters.reduce(into: [:]) { counts, letter in
+  ///         counts[letter, default: 0] += 1
+  ///     }
+  ///     // letterCount == ["a": 5, "b": 2, "r": 2, "c": 1, "d": 1]
+  ///
+  /// When `letters.reduce(into:_:)` is called, the following steps occur:
+  ///
+  /// 1. The `updateAccumulatingResult` closure is called with the initial
+  ///    accumulating value---`[:]` in this case---and the first character of
+  ///    `letters`, modifying the accumulating value by setting `1` for the key
+  ///    `"a"`.
+  /// 2. The closure is called again repeatedly with the updated accumulating
+  ///    value and each element of the sequence.
+  /// 3. When the sequence is exhausted, the accumulating value is returned to
+  ///    the caller.
+  ///
+  /// If the sequence has no elements, `updateAccumulatingResult` is never
+  /// executed and `initialResult` is the result of the call to
+  /// `reduce(into:_:)`.
+  ///
+  /// - Parameters:
+  ///   - initialResult: The value to use as the initial accumulating value.
+  ///   - updateAccumulatingResult: A closure that updates the accumulating
+  ///     value with an element of the sequence.
+  /// - Returns: The final accumulated value. If the sequence has no elements,
+  ///   the result is `initialResult`.
+  @_inlineable
+  public func reduce<Result>(
+    into initialResult: Result,
+    _ updateAccumulatingResult:
+      (_ partialResult: inout Result, Element) throws -> ()
+  ) rethrows -> Result {
+    var accumulator = initialResult
+    for element in self {
+      try updateAccumulatingResult(&accumulator, element)
+    }
+    return accumulator
+  }
 }
 
 //===----------------------------------------------------------------------===//

test/Constraints/closures.swift

@@ -164,7 +164,7 @@ func testMap() {
 }
 
 // <rdar://problem/22414757> "UnresolvedDot" "in wrong phase" assertion from verifier
-[].reduce { $0 + $1 }  // expected-error {{missing argument for parameter #1 in call}}
+[].reduce { $0 + $1 }  // expected-error {{cannot invoke 'reduce' with an argument list of type '((_, _) -> _)'}}
 
 
 

test/stdlib/Reduce.swift

@@ -0,0 +1,90 @@
+//===--- Reduce.swift - tests for the two reduce variants -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 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
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+// CHECK: testing...
+print("testing...")
+
+// Test the examples from the documentation of reduce(_:_:) and reduce(into:_:)
+
+let numbers = [1, 2, 3, 4]
+let numberSum = numbers.reduce(0, { x, y in
+  x + y
+})
+// CHECK-NEXT: 10
+print(numberSum)
+
+let letters = "abracadabra"
+let letterCount = letters.reduce(into: [:]) { counts, letter in
+  counts[letter, default: 0] += 1
+}
+// CHECK-NEXT: ["a", "b", "c", "d", "r"]
+print(letterCount.keys.sorted())
+print(letterCount["a"]!) // CHECK: 5
+print(letterCount["b"]!) // CHECK: 2
+print(letterCount["c"]!) // CHECK: 1
+print(letterCount["d"]!) // CHECK: 1
+print(letterCount["r"]!) // CHECK: 2
+
+
+// Test the two reduce methods with different levels of inference
+let numbers2 = Array(2..<7)
+
+// Test reduce(_:_:)
+// CHECK-NEXT: 20
+let sum1 = numbers2.reduce(0) { (x: Int, y: Int) -> Int in x + y }
+print(sum1)
+
+// CHECK-NEXT: 20
+let sum2 = numbers2.reduce(0) { (x, y) in x + y }
+print(sum2)
+
+// CHECK-NEXT: 20
+let sum3 = numbers2.reduce(0) { $0 + $1 }
+print(sum3)
+
+// CHECK-NEXT: 20
+let sum4 = numbers2.reduce(0, +)
+print(sum4)
+
+// Test reduce(into:_:)
+// CHECK-NEXT: 20
+let sum5 = numbers2.reduce(into: 0) { (x: inout Int, y: Int) in x += y }
+print(sum5)
+
+// CHECK-NEXT: 20
+let sum6 = numbers2.reduce(into: 0) { x, y in x += y }
+print(sum6)
+
+// CHECK-NEXT: 20
+let sum7 = numbers2.reduce(into: 0) { $0 += $1 }
+print(sum7)
+
+// CHECK-NEXT: 20
+let sum8 = numbers2.reduce(into: 0, +=)
+print(sum8)
+
+
+// Test that the initial value remains unmodified
+var original = [0, 1]
+let result = numbers2.reduce(into: original) { acc, x in
+  acc.append(x)
+}
+// CHECK-NEXT: [0, 1]
+print(original)
+// CHECK-NEXT: [0, 1, 2, 3, 4, 5, 6]
+print(result)
+
+
+// CHECK: all done.
+print("all done.")