func checkStructuralEquivalence()

in Sources/markdown-tool/Commands/FormatCommand.swift [149:189]
27 lines of code
2 McCabe index (conditional complexity)

        func checkStructuralEquivalence(between original: Document,
                                        and formatted: Document,
                                        source: String) throws {
            struct FileHandlerOutputStream: TextOutputStream {
                private let fileHandle: FileHandle
                let encoding: String.Encoding

                init(_ fileHandle: FileHandle, encoding: String.Encoding = .utf8) {
                    self.fileHandle = fileHandle
                    self.encoding = encoding
                }

                mutating func write(_ string: String) {
                    if let data = string.data(using: encoding) {
                        fileHandle.write(data)
                    }
                }
            }

            var standardError = FileHandlerOutputStream(FileHandle.standardError, encoding: .utf8)

            guard preferredMaximumLineLength == nil else {
                print("Skipping structural equivalence check because --preferred-maximum-line-length was used, which can intentionally change document structure by inserting soft or hard line breaks.", to: &standardError)
                return
            }

            if !original.hasSameStructure(as: formatted) {
                print("Error: Formatted markup tree had different structure from the original!", to: &standardError)
                print("Please file a bug with the following information:", to: &standardError)
                print("Original source:", to: &standardError)
                print("```markdown", to: &standardError)
                print(source, to: &standardError)
                print("```", to: &standardError)
                print("Original structure:", to: &standardError)
                print(original.debugDescription(), to: &standardError)
                print("----------------", to: &standardError)
                print("Structure after formatting:", to: &standardError)
                print(formatted.debugDescription(), to: &standardError)
                throw Error.formattedOutputHasDifferentStructures
            }
        }