SwiftLint/Source/SwiftLintFramework/Rules/NoGroupingExtensionRule.swift

import Foundation
import SourceKittenFramework

public struct NoGroupingExtensionRule: OptInRule, ConfigurationProviderRule {
    public var configuration = SeverityConfiguration(.warning)

    public init() {}

    public static let description = RuleDescription(
        identifier: "no_grouping_extension",
        name: "No Grouping Extension",
        description: "Extensions shouldn't be used to group code within the same source file.",
        kind: .idiomatic,
        nonTriggeringExamples: [
            "protocol Food {}\nextension Food {}\n",
            "class Apples {}\nextension Oranges {}\n"
        ],
        triggeringExamples: [
            "enum Fruit {}\n↓extension Fruit {}\n",
            "↓extension Tea: Error {}\nstruct Tea {}\n",
            "class Ham { class Spam {}}\n↓extension Ham.Spam {}\n",
            "extension External { struct Gotcha {}}\n↓extension External.Gotcha {}\n"
        ]
    )

    public func validate(file: File) -> [StyleViolation] {
        let collector = NamespaceCollector(dictionary: file.structure.dictionary)
        let elements = collector.findAllElements(of: [.class, .enum, .struct, .extension])

        let susceptibleNames = Set(elements.compactMap { $0.kind != .extension ? $0.name : nil })

        return elements
            .filter { $0.kind == .extension && susceptibleNames.contains($0.name) }
            .map {
                StyleViolation(ruleDescription: type(of: self).description,
                               severity: configuration.severity,
                               location: Location(file: file, byteOffset: $0.offset))
            }
    }
}