Correctly set indexInParent in Markup.child(at:) (#45)

This fixes a recent regression where `Markup.child(at:)` began returning
markup with incorrect metadata resulting in out-of-bounds errors.

The `indexInParent` value of a child is unrelated to the parent's
`indexInParent`. This was missed initially because tests were only
checking for children of the first item where `indexInParent` would
be 0. A new test has been added that asserts `Markup.child(at:)` returns
correct values for children of nested items as well.

Author: ethan-kusters

Sources/Markdown/Base/Markup.swift

@@ -204,7 +204,7 @@ extension Markup {
                 childId: firstChildID.childId + siblingSubtreeCount
             )
 
-            childMetadata = MarkupMetadata(id: childID, indexInParent: indexInParent + position)
+            childMetadata = MarkupMetadata(id: childID, indexInParent: position)
         }
 
         let rawChild = raw.markup.child(at: position)

Tests/MarkdownTests/Base/MarkupTests.swift

@@ -348,6 +348,25 @@ final class MarkupTests: XCTestCase {
         }
     }
 
+    func testNestedChildAtPositionHasCorrectMetadata() throws {
+        let source = "This is some **bold** and *italic* text and a [multi-formatted *link* **to** `github`](github.com)."
+        
+        let document = Document(parsing: source)
+        let link = try XCTUnwrap(document.child(at: 0)?.child(at: 5) as? Link)
+        XCTAssertEqual(link.indexInParent, 5)
+        
+        for (index, sequencedChild) in link.children.enumerated() {
+            let indexedChild = try XCTUnwrap(link.child(at: index))
+            
+            let indexedChildMetadata = indexedChild.raw.metadata
+            let sequencedChildMetadata = sequencedChild.raw.metadata
+            
+            XCTAssertEqual(indexedChildMetadata.id, sequencedChildMetadata.id)
+            XCTAssertEqual(indexedChildMetadata.indexInParent, sequencedChildMetadata.indexInParent)
+            XCTAssertEqual(indexedChildMetadata.indexInParent, index)
+        }
+    }
+    
     func testChildAtPositionHasCorrectDataID() throws {
         let source = "This is a [*link*](github.com). And some **bold** and *italic* text."
 
@@ -361,6 +380,19 @@ final class MarkupTests: XCTestCase {
         }
     }
 
+    func testNestedChildAtPositionHasCorrectDataID() throws {
+        let source = "This is some **bold** and *italic* text and a [multi-formatted *link* **to** `github`](github.com)."
+        
+        let document = Document(parsing: source)
+        let link = try XCTUnwrap(document.child(at: 0)?.child(at: 5) as? Link)
+        
+        for (index, sequencedChild) in link.children.enumerated() {
+            let indexedChild = try XCTUnwrap(link.child(at: index))
+            
+            XCTAssertEqual(indexedChild._data.id, sequencedChild._data.id)
+        }
+    }
+    
     func assertEqualType<FirstType, SecondType>(
         _ first: FirstType,
         _ second: SecondType.Type,