From f9aade002a3a034e93cb86a1e4f86c65a886090c Mon Sep 17 00:00:00 2001
From: Jeremy Schonfeld <jschonfeld@apple.com>
Date: Wed, 21 Sep 2022 16:44:55 -0700
Subject: [PATCH] Fix logic error in AttributedString.Runs.== for empty
 collections

---
 .../AttributedString/AttributedString.swift   | 29 +++++++++++++++----
 .../Tests/TestAttributedString.swift          | 14 +++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/Sources/Foundation/AttributedString/AttributedString.swift b/Sources/Foundation/AttributedString/AttributedString.swift
index 9ee0191f01..06a53e031f 100644
--- a/Sources/Foundation/AttributedString/AttributedString.swift
+++ b/Sources/Foundation/AttributedString/AttributedString.swift
@@ -974,18 +974,35 @@ public struct AttributedString : AttributedStringProtocol {
         public static func == (lhs: Runs, rhs: Runs) -> Bool {
             let lhsSlice = lhs._guts.runs[lhs._startingRunIndex ..< lhs._endingRunIndex]
             let rhsSlice = rhs._guts.runs[rhs._startingRunIndex ..< rhs._endingRunIndex]
-            guard lhsSlice.count == rhs.count else {
+            
+            // If there are different numbers of runs, they aren't equal
+            guard lhsSlice.count == rhsSlice.count else {
                 return false
             }
-            // Compare all inner runs (their lengths will not be limited by _range)
-            if lhsSlice.count > 2 && !lhsSlice[lhsSlice.startIndex + 1 ..< lhsSlice.endIndex - 1].elementsEqual(rhsSlice[rhsSlice.startIndex + 1 ..< rhsSlice.endIndex - 1]) {
+            
+            let runCount = lhsSlice.count
+            
+            // Empty slices are always equal
+            guard runCount > 0 else {
+                return true
+            }
+            
+            // Compare the first run (clamping their ranges) since we know each has at least one run
+            if lhs._guts.run(at: lhs.startIndex, clampedBy: lhs._range) != rhs._guts.run(at: rhs.startIndex, clampedBy: rhs._range) {
                 return false
             }
-            // If the inner runs are equivalent, check the first and last runs with clamped ranges
-            if lhsSlice.count > 1 && lhs._guts.run(at: Index(rangeIndex: lhs._endingRunIndex - 1), clampedBy: lhs._range) != rhs._guts.run(at: Index(rangeIndex: rhs._endingRunIndex - 1), clampedBy: rhs._range) {
+            
+            // Compare all inner runs if they exist without needing to clamp ranges
+            if runCount > 2 && !lhsSlice[lhsSlice.startIndex + 1 ..< lhsSlice.endIndex - 1].elementsEqual(rhsSlice[rhsSlice.startIndex + 1 ..< rhsSlice.endIndex - 1]) {
                 return false
             }
-            return lhs._guts.run(at: lhs.startIndex, clampedBy: lhs._range) == rhs._guts.run(at: rhs.startIndex, clampedBy: rhs._range)
+            
+            // If there are more than one run (so we didn't already check this as the first run), check the last run (clamping its range)
+            if runCount > 1 && lhs._guts.run(at: Index(rangeIndex: lhs._endingRunIndex - 1), clampedBy: lhs._range) != rhs._guts.run(at: Index(rangeIndex: rhs._endingRunIndex - 1), clampedBy: rhs._range) {
+                return false
+            }
+            
+            return true
         }
         
         public var description: String {
diff --git a/Tests/Foundation/Tests/TestAttributedString.swift b/Tests/Foundation/Tests/TestAttributedString.swift
index 8896642121..ee8c5768ac 100644
--- a/Tests/Foundation/Tests/TestAttributedString.swift
+++ b/Tests/Foundation/Tests/TestAttributedString.swift
@@ -989,6 +989,19 @@ E {
             XCTAssertFalse(desc.isEmpty)
         }
     }
+    
+    func testSubstringEquality() {
+        let str = AttributedString("")
+        let range = str.startIndex ..< str.endIndex
+        XCTAssertEqual(str[range], str[range])
+        
+        let str2 = "A" + AttributedString("A", attributes: .init().testInt(2))
+        let substringA = str2[str2.startIndex ..< str2.index(afterCharacter: str2.startIndex)]
+        let substringB = str2[str2.index(afterCharacter: str2.startIndex) ..< str2.endIndex]
+        XCTAssertNotEqual(substringA, substringB)
+        XCTAssertEqual(substringA, substringA)
+        XCTAssertEqual(substringB, substringB)
+    }
 
     // MARK: - Coding Tests
     
@@ -2196,6 +2209,7 @@ E {
             ("testSubstringBase", testSubstringBase),
             ("testSubstringGetAttribute", testSubstringGetAttribute),
             ("testSubstringDescription", testSubstringDescription),
+            ("testSubstringEquality", testSubstringEquality),
             ("testJSONEncoding", testJSONEncoding),
             ("testDecodingThenConvertingToNSAttributedString", testDecodingThenConvertingToNSAttributedString),
             ("testCustomAttributeCoding", testCustomAttributeCoding),