Fix parsing root relationships worksheet paths (#123)

Resolve #121

Sources/CoreXLSX/Relationships.swift

@@ -110,4 +110,8 @@ public struct Relationship: Codable, Equatable {
   public let id: String
   public let type: SchemaType
   public let target: String
+
+  func path(from root: String) -> String {
+    return Path(target).isRoot ? target : "\(root)/\(target)"
+  }
 }

Sources/CoreXLSX/XLSXFile.swift

@@ -248,7 +248,7 @@ public class XLSXFile {
         sheet.name.flatMap { (sheet.relationship, $0) }
       })
 
-      return worksheets.map { (name: sheetIDs[$0.id], path: "\(pathPrefix)/\($0.target)") }
+      return worksheets.map { (name: sheetIDs[$0.id], path: $0.path(from: pathPrefix)) }
     }
   }
 
@@ -265,7 +265,7 @@ public class XLSXFile {
       // storing that path in `pathPrefix`
       let pathPrefix = path.components.dropLast().joined(separator: "/")
 
-      return worksheets.map { "\(pathPrefix)/\($0.target)" }
+      return worksheets.map { $0.path(from: pathPrefix) }
     }
   }
 

Tests/CoreXLSXTests/XLSXFile.swift

@@ -205,4 +205,15 @@ final class CoreXLSXTests: XCTestCase {
       try file.cellsInWorksheet(at: sheetPath, columns: columnReferences)
     XCTAssertEqual(allCells, cellsFromAllColumns)
   }
+
+  func testRootRelationships() throws {
+    guard let file =
+      XLSXFile(filepath: "\(currentWorkingPath)/root_relationships.xlsx") else {
+      XCTFail("failed to open the file")
+      return
+    }
+
+    XCTAssertEqual(try file.parseDocumentPaths(), ["xl/workbook.xml"])
+    XCTAssertEqual(try file.parseWorksheetPaths(), ["/\(sheetPath)"])
+  }
 }