[RFC][DNM] Add isIdentical Method for Quick Comparisons to Data

Benchmarks/Benchmarks/Essentials/BenchmarkEssentials.swift

@@ -67,6 +67,15 @@ let benchmarks = {
         return box
     })
 
+    Benchmark("DataIdenticalEmpty", closure: { benchmark, box in
+        blackHole(box.d1.isIdentical(to: box.d2))
+    }, setup: { () -> TwoDatasBox in
+        let d1 = Data()
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
     Benchmark("DataEqualInline", closure: { benchmark, box in
         blackHole(box.d1 == box.d2)
     }, setup: { () -> TwoDatasBox in
@@ -75,6 +84,15 @@ let benchmarks = {
         let box = TwoDatasBox(d1: d1, d2: d2)
         return box
     })
+
+    Benchmark("DataIdenticalInline", closure: { benchmark, box in
+        blackHole(box.d1.isIdentical(to: box.d2))
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(12) // Less than size of InlineData.Buffer
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
 
     Benchmark("DataNotEqualInline", closure: { benchmark, box in
         blackHole(box.d1 != box.d2)
@@ -93,7 +111,16 @@ let benchmarks = {
         let box = TwoDatasBox(d1: d1, d2: d2)
         return box
     })
-
+
+    Benchmark("DataIdenticalLarge", closure: { benchmark, box in
+        blackHole(box.d1.isIdentical(to: box.d2))
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(1024 * 8)
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
     Benchmark("DataNotEqualLarge", closure: { benchmark, box in
         blackHole(box.d1 != box.d2)
     }, setup: { () -> TwoDatasBox in
@@ -112,6 +139,15 @@ let benchmarks = {
         return box
     })
 
+    Benchmark("DataIdenticalReallyLarge", closure: { benchmark, box in
+        blackHole(box.d1.isIdentical(to: box.d2))
+    }, setup: { () -> TwoDatasBox in
+        let d1 = createSomeData(1024 * 1024 * 8)
+        let d2 = d1
+        let box = TwoDatasBox(d1: d1, d2: d2)
+        return box
+    })
+
     Benchmark("DataNotEqualReallyLarge", closure: { benchmark, box in
         blackHole(box.d1 != box.d2)
     }, setup: { () -> TwoDatasBox in

Sources/FoundationEssentials/Data/Data.swift

@@ -2974,3 +2974,53 @@ extension Data : Codable {
         }
     }
 }
+
+extension Data {
+  /// Returns a boolean value indicating whether this data is identical to
+  /// `other`.
+  ///
+  /// Two data values are identical if there is no way to distinguish between
+  /// them.
+  ///
+  /// Comparing data this way includes comparing (normally) hidden
+  /// implementation details such as the memory location of any underlying
+  /// data storage object. Therefore, identical data are guaranteed to
+  /// compare equal with `==`, but not all equal data are considered
+  /// identical.
+  ///
+  /// - Performance: O(1)
+  @_alwaysEmitIntoClient
+  public func isIdentical(to other: Self) -> Bool {
+    // See if both are empty
+    switch (self._representation, other._representation) {
+    case (.empty, .empty):
+        return true
+    case (.inline, .inline), (.slice, .slice), (.large, .large):
+        // Continue on to checks below
+        break
+    default:
+        return false
+    }
+
+    let length1 = self.count
+    let length2 = other.count
+
+    // Unequal length data can never be equal
+    guard length1 == length2 else {
+        return false
+    }
+
+    if length1 > 0 {
+        return self.withUnsafeBytes { (b1: UnsafeRawBufferPointer) in
+            return other.withUnsafeBytes { (b2: UnsafeRawBufferPointer) in
+                // If they have the same base address and same count, it is equal
+                let b1Address = b1.baseAddress!
+                let b2Address = b2.baseAddress!
+
+                return b1Address == b2Address
+            }
+        }
+    }
+    return true
+  }
+}

Tests/FoundationEssentialsTests/DataTests.swift

@@ -46,6 +46,14 @@ extension Data {
     }
 }
 
+func createSomeData(_ length: Int) -> Data {
+    var d = Data(repeating: 42, count: length)
+    // Set a byte to be another value just so we know we have a unique pointer to the backing
+    // For maximum inefficiency in the not equal case, set the last byte
+    d[length - 1] = UInt8.random(in: UInt8.min..<UInt8.max)
+    return d
+}
+
 @Suite("Data")
 private final class DataTests {
 
@@ -197,6 +205,16 @@ private final class DataTests {
         #expect(d1 == d2, "Data should be equal")
     }
 
+    @Test func identical() {
+        let d1 = createSomeData(1024 * 8)
+        let d2 = createSomeData(1024 * 8)
+
+        #expect(d1.isIdentical(to: d1), "Data should be identical")
+        #expect(d2.isIdentical(to: d2), "Data should be identical")
+        #expect(!(d1.isIdentical(to: d2)), "Data should be identical")
+        #expect(!(d2.isIdentical(to: d1)), "Data should be identical")
+    }
+
     @Test func dataInSet() {
         let d1 = dataFrom("Hello")
         let d2 = dataFrom("Hello")