Add Optional Error Context

.swiftformat

@@ -2,3 +2,7 @@
 --indentcase false
 --trimwhitespace always
 --ranges nospace
+--empty tuple
+--operatorfunc nospace
+--ifdef noindent
+--stripunusedargs closure-only

Sources/XMLCoder/Auxiliaries/XMLStackParser.swift

@@ -14,34 +14,58 @@ class _XMLStackParser: NSObject {
     var root: _XMLElement?
     private var stack: [_XMLElement] = []
 
-    static func parse(with data: Data) throws -> KeyedBox {
+    static func parse(with data: Data, errorContextLength length: UInt) throws -> KeyedBox {
         let parser = _XMLStackParser()
 
-        guard let node = try parser.parse(with: data) else {
-            throw DecodingError.dataCorrupted(DecodingError.Context(
-                codingPath: [],
-                debugDescription: "The given data could not be parsed into XML."
-            ))
-        }
+        let node = try parser.parse(with: data, errorContextLength: length)
 
         return node.flatten()
     }
 
-    func parse(with data: Data) throws -> _XMLElement? {
+    func parse(with data: Data, errorContextLength: UInt) throws -> _XMLElement {
         let xmlParser = XMLParser(data: data)
         xmlParser.delegate = self
 
-        guard xmlParser.parse() else {
-            if let error = xmlParser.parserError {
-                throw error
-            }
-            return nil
+        guard !xmlParser.parse(), root == nil else {
+            return root!
+        }
+
+        guard let error = xmlParser.parserError else {
+            throw DecodingError.dataCorrupted(DecodingError.Context(
+                codingPath: [],
+                debugDescription: "The given data could not be parsed into XML."
+            ))
+        }
+
+        guard errorContextLength > 0 else {
+            throw error
         }
 
-        return root
+        let string = String(data: data, encoding: .utf8) ?? ""
+        let lines = string.split(separator: "\n")
+        var errorPosition = 0
+        let offset = Int(errorContextLength / 2)
+        for i in 0..<xmlParser.lineNumber - 1 {
+            errorPosition += lines[i].count
+        }
+        errorPosition += xmlParser.columnNumber
+        let lowerBound = String.Index(encodedOffset: errorPosition - offset)
+        let upperBound = String.Index(encodedOffset: errorPosition + offset)
+
+        let context = string[lowerBound..<upperBound]
+
+        throw DecodingError.dataCorrupted(DecodingError.Context(
+            codingPath: [],
+            debugDescription: """
+            \(error.localizedDescription) \
+            at line \(xmlParser.lineNumber), column \(xmlParser.columnNumber):
+            `\(context)`
+            """,
+            underlyingError: error
+        ))
     }
 
-    func withCurrentElement(_ body: (inout _XMLElement) throws -> Void) rethrows {
+    func withCurrentElement(_ body: (inout _XMLElement) throws -> ()) rethrows {
         guard !stack.isEmpty else {
             return
         }
@@ -92,8 +116,4 @@ extension _XMLStackParser: XMLParserDelegate {
             currentElement.append(value: string)
         }
     }
-
-    func parser(_: XMLParser, parseErrorOccurred parseError: Error) {
-        print(parseError)
-    }
 }

Sources/XMLCoder/Box/NullBox.swift

@@ -24,7 +24,7 @@ extension NullBox: Box {
 extension NullBox: SimpleBox {}
 
 extension NullBox: Equatable {
-    static func == (_: NullBox, _: NullBox) -> Bool {
+    static func ==(_: NullBox, _: NullBox) -> Bool {
         return true
     }
 }

Sources/XMLCoder/Decoder/XMLDecoder.swift

@@ -214,6 +214,9 @@ open class XMLDecoder {
     /// Contextual user-provided information for use during decoding.
     open var userInfo: [CodingUserInfoKey: Any] = [:]
 
+    // The error context length
+    open var errorContextLength: UInt = 0
+
     /// Options set on the top-level encoder to pass down the decoding hierarchy.
     struct _Options {
         let dateDecodingStrategy: DateDecodingStrategy
@@ -247,15 +250,10 @@ open class XMLDecoder {
     /// - throws: `DecodingError.dataCorrupted` if values requested from the payload are corrupted, or if the given data is not valid XML.
     /// - throws: An error if any box throws an error during decoding.
     open func decode<T: Decodable>(_ type: T.Type, from data: Data) throws -> T {
-        let topLevel: Box
-        do {
-            topLevel = try _XMLStackParser.parse(with: data)
-        } catch {
-            throw DecodingError.dataCorrupted(DecodingError.Context(
-                codingPath: [],
-                debugDescription: "The given data was not valid XML.", underlyingError: error
-            ))
-        }
+        let topLevel: Box = try _XMLStackParser.parse(
+            with: data,
+            errorContextLength: errorContextLength
+        )
 
         let decoder = _XMLDecoder(referencing: topLevel, options: options)
 
@@ -289,6 +287,9 @@ class _XMLDecoder: Decoder {
         return options.userInfo
     }
 
+    // The error context lenght
+    open var errorContextLenght: UInt = 0
+
     // MARK: - Initialization
 
     /// Initializes `self` with the given top-level container and options.

Sources/XMLCoder/Encoder/XMLEncoder.swift

@@ -63,7 +63,7 @@ open class XMLEncoder {
         /// Encode the `Date` as a custom value encoded by the given closure.
         ///
         /// If the closure fails to encode a value into the given encoder, the encoder will encode an empty automatic container in its place.
-        case custom((Date, Encoder) throws -> Void)
+        case custom((Date, Encoder) throws -> ())
     }
 
     /// The strategy to use for encoding `String` values.
@@ -86,7 +86,7 @@ open class XMLEncoder {
         /// Encode the `Data` as a custom value encoded by the given closure.
         ///
         /// If the closure fails to encode a value into the given encoder, the encoder will encode an empty automatic container in its place.
-        case custom((Data, Encoder) throws -> Void)
+        case custom((Data, Encoder) throws -> ())
     }
 
     /// The strategy to use for non-XML-conforming floating-point values (IEEE 754 infinity and NaN).

Tests/XMLCoderTests/Auxiliary/XMLStackParserTests.swift

@@ -15,7 +15,8 @@ class XMLStackParserTests: XCTestCase {
         let xmlString = "<container><value>42</value></container>"
         let xmlData = xmlString.data(using: .utf8)!
 
-        let root: _XMLElement? = try parser.parse(with: xmlData)
+        let root: _XMLElement? = try parser.parse(with: xmlData,
+                                                  errorContextLength: 0)
 
         let expected = _XMLElement(
             key: "container",
@@ -37,6 +38,7 @@ class XMLStackParserTests: XCTestCase {
         let xmlString = "lorem ipsum"
         let xmlData = xmlString.data(using: .utf8)!
 
-        XCTAssertThrowsError(try parser.parse(with: xmlData))
+        XCTAssertThrowsError(try parser.parse(with: xmlData,
+                                              errorContextLength: 0))
     }
 }

Tests/XMLCoderTests/ErrorContextTest.swift

@@ -0,0 +1,52 @@
+//
+//  ErrorContextTest.swift
+//  XMLCoder
+//
+//  Created by Matvii Hodovaniuk on 12/27/18.
+//
+
+import Foundation
+import XCTest
+@testable import XMLCoder
+
+final class ErrorContextTest: XCTestCase {
+    struct Container: Codable {
+        let value: [String: Int]
+    }
+
+    func testErrorContext() {
+        let decoder = XMLDecoder()
+        decoder.errorContextLength = 8
+
+        let xmlString =
+            """
+            <container>
+                test1
+            </blah>
+            <container>
+                test2
+            </container>
+            """
+        let xmlData = xmlString.data(using: .utf8)!
+
+        XCTAssertThrowsError(try decoder.decode(Container.self,
+                                                from: xmlData)) { error in
+            guard case let DecodingError.dataCorrupted(ctx) = error,
+                let underlying = ctx.underlyingError else {
+                XCTAssert(false, "wrong error type thrown")
+                return
+            }
+
+            XCTAssertEqual(ctx.debugDescription, """
+            \(underlying.localizedDescription) \
+            at line 3, column 8:
+            `blah>
+            <c`
+            """)
+        }
+    }
+
+    static var allTests = [
+        ("testErrorContext", testErrorContext),
+    ]
+}

XMLCoder.xcodeproj/project.pbxproj

@@ -69,6 +69,7 @@
 		BF9457F521CBB6BC005ACFDE /* DecimalTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = BF9457EA21CBB6BC005ACFDE /* DecimalTests.swift */; };
 		BF9457F621CBB6BC005ACFDE /* KeyedTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = BF9457EB21CBB6BC005ACFDE /* KeyedTests.swift */; };
 		BF9457F721CBB6BC005ACFDE /* DataTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = BF9457EC21CBB6BC005ACFDE /* DataTests.swift */; };
+		D1E0C85321D8E65E0042A261 /* ErrorContextTest.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1E0C85121D8E6540042A261 /* ErrorContextTest.swift */; };
 		D1FC040521C7EF8200065B43 /* RJISample.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1FC040421C7EF8200065B43 /* RJISample.swift */; };
 		OBJ_48 /* DecodingErrorExtension.swift in Sources */ = {isa = PBXBuildFile; fileRef = OBJ_10 /* DecodingErrorExtension.swift */; };
 		OBJ_49 /* XMLDecoder.swift in Sources */ = {isa = PBXBuildFile; fileRef = OBJ_11 /* XMLDecoder.swift */; };
@@ -161,6 +162,7 @@
 		BF9457EA21CBB6BC005ACFDE /* DecimalTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DecimalTests.swift; sourceTree = "<group>"; };
 		BF9457EB21CBB6BC005ACFDE /* KeyedTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = KeyedTests.swift; sourceTree = "<group>"; };
 		BF9457EC21CBB6BC005ACFDE /* DataTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DataTests.swift; sourceTree = "<group>"; };
+		D1E0C85121D8E6540042A261 /* ErrorContextTest.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ErrorContextTest.swift; sourceTree = "<group>"; };
 		D1FC040421C7EF8200065B43 /* RJISample.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = RJISample.swift; sourceTree = "<group>"; };
 		OBJ_10 /* DecodingErrorExtension.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = DecodingErrorExtension.swift; sourceTree = "<group>"; };
 		OBJ_11 /* XMLDecoder.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = XMLDecoder.swift; sourceTree = "<group>"; };
@@ -322,6 +324,7 @@
 				OBJ_29 /* BreakfastTest.swift */,
 				OBJ_30 /* CDCatalog.swift */,
 				OBJ_31 /* CDTest.swift */,
+				D1E0C85121D8E6540042A261 /* ErrorContextTest.swift */,
 				OBJ_33 /* NodeEncodingStrategyTests.swift */,
 				OBJ_34 /* NoteTest.swift */,
 				OBJ_35 /* PlantCatalog.swift */,
@@ -551,6 +554,7 @@
 				BF9457CD21CBB516005ACFDE /* FloatBoxTests.swift in Sources */,
 				BF9457F621CBB6BC005ACFDE /* KeyedTests.swift in Sources */,
 				BF9457C821CBB516005ACFDE /* BoolBoxTests.swift in Sources */,
+				D1E0C85321D8E65E0042A261 /* ErrorContextTest.swift in Sources */,
 				BF9457F421CBB6BC005ACFDE /* UIntTests.swift in Sources */,
 				OBJ_89 /* RJITest.swift in Sources */,
 				BF9457F121CBB6BC005ACFDE /* FloatTests.swift in Sources */,