HADataDecodable.swift

import Foundation

/// A type which can be decoded using our data type
///
/// - Note: This differs from `Decodable` intentionally; `Decodable` does not support `Any` types or JSON well when the
///         results are extremely dynamic. This limitation requires that we do it ourselves.
public protocol HADataDecodable: HADecodeTransformable {
    /// Create an instance from data
    /// One day, if Decodable can handle 'Any' types well, this can be init(decoder:).
    ///
    /// - Parameter data: The data to decode
    /// - Throws: When unable to decode
    init(data: HAData) throws
}

public extension HADataDecodable {
    /// Create a `HADataDecodable` instance via `.decode(…)` indirection
    /// - Parameter value: The value to convert to HAData for the init
    /// - Throws: When unable to decode
    /// - Returns: The decodable initialized with the given value
    static func decode(unknown value: Any) throws -> Self? {
        try .init(data: HAData(value: value))
    }
}

extension Array: HADataDecodable where Element: HADataDecodable {
    /// Construct an array of decodable elements
    /// - Parameter data: The data to decode
    /// - Throws: When unable to decode, e.g. the data isn't an array
    public init(data: HAData) throws {
        guard case let .array(array) = data else {
            throw HADataError.couldntTransform(key: "root")
        }

        try self.init(array.map { try Element(data: $0) })
    }
}

/// Parse error
public enum HADataError: Error, Equatable {
    /// The given key was missing
    case missingKey(String)
    /// The given key was present but the type could not be converted
    case incorrectType(key: String, expected: String, actual: String)
    /// The given key was present but couldn't be converted
    case couldntTransform(key: String)
}

public extension HAData {
    /// Convenience access to the dictionary case for a particular key, with an expected type
    ///
    /// - Parameter key: The key to look up in `dictionary` case
    /// - Throws: If the key was not present in the dictionary or the type was not the expected type or convertable
    /// - Returns: The value from the dictionary
    func decode<T>(_ key: String) throws -> T {
        guard case let .dictionary(dictionary) = self, let value = dictionary[key] else {
            throw HADataError.missingKey(key)
        }

        // Not the prettiest, but we need to super duper promise to the compiler that we're returning a good value
        if let type = T.self as? HADecodeTransformable.Type, let inside = try type.decode(unknown: value) as? T {
            return inside
        }

        if let value = value as? T {
            // Avoid full JSON cache when using JSONSerialization and referencing NSString
            if var valueString = value as? String {
                valueString.makeContiguousUTF8()
                return valueString as? T ?? value
            } else {
                return value
            }
        }

        throw HADataError.incorrectType(
            key: key,
            expected: String(describing: T.self),
            actual: String(describing: type(of: value))
        )
    }

    /// Convenience access to the dictionary case for a particular key, with an expected type, with a transform applied
    ///
    /// - Parameters:
    ///   - key: The key to look up in `dictionary` case
    ///   - transform: The transform to apply to the value, when found
    /// - Throws: If the key was not present in the dictionary or the type was not the expected type or the value
    ///           couldn't be transformed
    /// - Returns: The value from the dictionary
    func decode<Value, Transform>(_ key: String, transform: (Value) throws -> Transform?) throws -> Transform {
        let base: Value = try decode(key)

        guard let transformed = try transform(base) else {
            throw HADataError.couldntTransform(key: key)
        }

        return transformed
    }

    /// Convenience access to the dictionary case for a particular key, with an expected type
    ///
    /// - Parameters:
    ///   - key: The key to look up in `dictionary` case
    ///   - fallback: The fallback value to use if not found in the dictionary
    /// - Throws: If the inner fallback block throws
    /// - Returns: The value from the dictionary
    func decode<T>(_ key: String, fallback: @autoclosure () throws -> T) rethrows -> T {
        guard let value: T = try? decode(key) else {
            return try fallback()
        }

        return value
    }
}