Add exclusive eager version of reductions(into:_:)

apple · Nov 29, 2020 · 7cbff5c · 7cbff5c
1 parent 25665e4
commit 7cbff5c
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Showing 2 changed files with 21 additions and 3 deletions.
diff --git a/Sources/Algorithms/Reductions.swift b/Sources/Algorithms/Reductions.swift
@@ -47,14 +47,24 @@ extension Sequence {
     _ transform: (Result, Element) throws -> Result
   ) rethrows -> [Result] {
 
+    var result = initial
+    return try reductions(into: &result) { result, element in
+      result = try transform(result, element)
+    }
+  }
+
+  public func reductions<Result>(
+    into initial: inout Result,
+    _ transform: (inout Result, Element) throws -> Void
+  ) rethrows -> [Result] {
+
     var output = [Result]()
     output.reserveCapacity(underestimatedCount + 1)
     output.append(initial)
 
-    var result = initial
     for element in self {
-      result = try transform(result, element)
-      output.append(result)
+      try transform(&initial, element)
+      output.append(initial)
     }
 
     return output

diff --git a/Tests/SwiftAlgorithmsTests/ReductionsTests.swift b/Tests/SwiftAlgorithmsTests/ReductionsTests.swift
@@ -35,6 +35,14 @@ final class ReductionsTests: XCTestCase {
     XCTAssertEqual([1].reductions(0, +), [0, 1])
     XCTAssertEqual(EmptyCollection<Int>().reductions(0, +), [0])
 
+    var value0 = 0
+    var value1 = 0
+    var value2 = 0
+    func add(lhs: inout Int, rhs: Int) { lhs += rhs }
+    XCTAssertEqual([1, 2, 3, 4].reductions(into: &value0, add), [0, 1, 3, 6, 10])
+    XCTAssertEqual([1].reductions(into: &value1, add), [0, 1])
+    XCTAssertEqual(EmptyCollection<Int>().reductions(into: &value2, add), [0])
+
     XCTAssertNoThrow(try [].reductions(0) { _, _ in throw TestError() })
     XCTAssertThrowsError(try [1].reductions(0) { _, _ in throw TestError() })
   }