SwiftLint/Source/SwiftLintBuiltInRules/Rules/Idiomatic/FunctionDefaultParameterAtEndRule.swift

import SwiftSyntax

struct FunctionDefaultParameterAtEndRule: SwiftSyntaxRule, ConfigurationProviderRule, OptInRule {
    var configuration = SeverityConfiguration<Self>(.warning)

    static let description = RuleDescription(
        identifier: "function_default_parameter_at_end",
        name: "Function Default Parameter at End",
        description: "Prefer to locate parameters with defaults toward the end of the parameter list",
        kind: .idiomatic,
        nonTriggeringExamples: [
            Example("func foo(baz: String, bar: Int = 0) {}"),
            Example("func foo(x: String, y: Int = 0, z: CGFloat = 0) {}"),
            Example("func foo(bar: String, baz: Int = 0, z: () -> Void) {}"),
            Example("func foo(bar: String, z: () -> Void, baz: Int = 0) {}"),
            Example("func foo(bar: Int = 0) {}"),
            Example("func foo() {}"),
            Example("""
            class A: B {
              override func foo(bar: Int = 0, baz: String) {}
            """),
            Example("func foo(bar: Int = 0, completion: @escaping CompletionHandler) {}"),
            Example("""
            func foo(a: Int, b: CGFloat = 0) {
              let block = { (error: Error?) in }
            }
            """),
            Example("""
            func foo(a: String, b: String? = nil,
                     c: String? = nil, d: @escaping AlertActionHandler = { _ in }) {}
            """),
            Example("override init?(for date: Date = Date(), coordinate: CLLocationCoordinate2D) {}"),
            Example("""
            func handleNotification(_ userInfo: NSDictionary,
                                    userInteraction: Bool = false,
                                    completionHandler: ((UIBackgroundFetchResult) -> Void)?) {}
            """),
            Example("""
            func write(withoutNotifying tokens: [NotificationToken] =  {}, _ block: (() throws -> Int)) {}
            """),
            Example("""
            func expect<T>(file: String = #file, _ expression: @autoclosure () -> (() throws -> T)) -> Expectation<T> {}
            """, excludeFromDocumentation: true)
        ],
        triggeringExamples: [
            Example("↓func foo(bar: Int = 0, baz: String) {}"),
            Example("private ↓func foo(bar: Int = 0, baz: String) {}"),
            Example("public ↓init?(for date: Date = Date(), coordinate: CLLocationCoordinate2D) {}")
        ]
    )

    func makeVisitor(file: SwiftLintFile) -> ViolationsSyntaxVisitor {
        Visitor(viewMode: .sourceAccurate)
    }
}

private extension FunctionDefaultParameterAtEndRule {
    final class Visitor: ViolationsSyntaxVisitor {
        override func visitPost(_ node: FunctionDeclSyntax) {
            guard !node.modifiers.containsOverride, node.signature.containsViolation else {
                return
            }

            violations.append(node.funcKeyword.positionAfterSkippingLeadingTrivia)
        }

        override func visitPost(_ node: InitializerDeclSyntax) {
            guard !node.modifiers.containsOverride, node.signature.containsViolation else {
                return
            }

            violations.append(node.initKeyword.positionAfterSkippingLeadingTrivia)
        }
    }
}

private extension FunctionSignatureSyntax {
    var containsViolation: Bool {
        let params = input.parameterList.filter { param in
            !param.isClosure
        }

        guard params.isNotEmpty else {
            return false
        }

        let defaultParams = params.filter { param in
            param.defaultArgument != nil
        }
        guard defaultParams.isNotEmpty else {
            return false
        }

        let lastParameters = params.suffix(defaultParams.count)
        let lastParametersWithDefaultValue = lastParameters.filter { param in
            param.defaultArgument != nil
        }

        return lastParameters.count != lastParametersWithDefaultValue.count
    }
}

private extension FunctionParameterSyntax {
    var isClosure: Bool {
        if isEscaping || type.is(FunctionTypeSyntax.self) {
            return true
        }

        if let optionalType = type.as(OptionalTypeSyntax.self),
           let tuple = optionalType.wrappedType.as(TupleTypeSyntax.self) {
            return tuple.elements.onlyElement?.type.as(FunctionTypeSyntax.self) != nil
        }

        if let tuple = type.as(TupleTypeSyntax.self) {
            return tuple.elements.onlyElement?.type.as(FunctionTypeSyntax.self) != nil
        }

        if let attrType = type.as(AttributedTypeSyntax.self) {
            return attrType.baseType.is(FunctionTypeSyntax.self)
        }

        return false
    }

    var isEscaping: Bool {
        guard let attrType = type.as(AttributedTypeSyntax.self) else {
            return false
        }

        return attrType.attributes.contains(attributeNamed: "escaping")
    }
}