fixing followup issue not resolves with #55 to fully resolve #54

Sources/Automerge/Codable/Encoding/AutomergeEncoderImpl.swift

@@ -65,23 +65,28 @@ final class AutomergeEncoderImpl {
                 }
             }
         case .Index:
-            precondition(highestUnkeyedIndexWritten != nil)
-            guard let highestUnkeyedIndexWritten else {
-                return
+            var highestIndexWritten: Int64 = -1
+            if let highestUnkeyedIndexWritten {
+                // If highestUnkeyedIndexWritten is nil, then a list/array was encoded
+                // with no items within it.
+                highestIndexWritten = Int64(highestUnkeyedIndexWritten)
             }
-            // Remove index elements that exist in this objectId beyond
-            // the max written during encode. (allow arrays to 'shrink')
-            var highestAutomergeIndex = document.length(obj: objectIdForContainer) - 1
-            while highestAutomergeIndex > highestUnkeyedIndexWritten {
+            let lengthOfAutomergeContainer = document.length(obj: objectIdForContainer)
+            if lengthOfAutomergeContainer > 0 {
+                var highestAutomergeIndex = Int64(lengthOfAutomergeContainer - 1)
+                // Remove index elements that exist in this objectId beyond
+                // the max written during encode. (allow arrays to 'shrink')
+                while highestAutomergeIndex > highestIndexWritten {
                 do {
-                    try document.delete(obj: objectIdForContainer, index: highestAutomergeIndex)
+                    try document.delete(obj: objectIdForContainer, index: UInt64(highestAutomergeIndex))
                     highestAutomergeIndex -= 1
                 } catch {
                     fatalError(
                         "Unable to delete index position \(highestAutomergeIndex) during post-encode cleanup: \(error)"
                     )
                 }
             }
+        }
         case .Value:
             return
         }

Tests/AutomergeTests/CodableTests/AutomergeArrayEncodeDecodeTests.swift

@@ -35,4 +35,32 @@ final class AutomergeArrayEncodeDecodeTests: XCTestCase {
         // ("StructWithArray(names: ["one", "one", "two"])") is not equal to
         // ("StructWithArray(names: ["one"])")
     }
+
+    func testEmptyArrayEncode() throws {
+        // illustrates https://github.com/automerge/automerge-swift/issues/54
+
+        struct StructWithArray: Codable, Equatable {
+            var names: [String] = []
+        }
+
+        let encoder = AutomergeEncoder(doc: doc)
+        let decoder = AutomergeDecoder(doc: doc)
+        var sample = StructWithArray()
+        try encoder.encode(sample)
+
+        sample.names.append("one")
+        sample.names.append("two")
+        try encoder.encode(sample)
+        let replica = try decoder.decode(StructWithArray.self)
+        XCTAssertEqual(replica, sample)
+
+        _ = sample.names.popLast()
+        _ = sample.names.popLast()
+        try encoder.encode(sample)
+        let secondReplica = try decoder.decode(StructWithArray.self)
+        XCTAssertEqual(secondReplica, sample)
+        // XCTAssertEqual failed:
+        // ("StructWithArray(names: ["one", "one", "two"])") is not equal to
+        // ("StructWithArray(names: ["one"])")
+    }
 }