JSON.ObjectDecoder.swift

extension JSON {
    /// A thin wrapper around a native Swift dictionary providing an efficient decoding
    /// interface for a JSON object.
    @frozen public struct ObjectDecoder<CodingKey>
        where CodingKey: RawRepresentable<String> & Hashable & Sendable {
        public var index: [CodingKey: JSON.Node]

        @inlinable public init(_ index: [CodingKey: JSON.Node] = [:]) {
            self.index = index
        }
    }
}
extension JSON.ObjectDecoder: JSONDecodable {
    @inlinable public init(json: borrowing JSON.Node) throws {
        try self.init(indexing: try .init(json: json))
    }
}
extension JSON.ObjectDecoder where CodingKey: RawRepresentable<String> {
    @inlinable public init(indexing object: borrowing JSON.Object) throws {
        self.init(.init(minimumCapacity: object.count))
        for field: JSON.FieldDecoder<String> in copy object {
            guard let key: CodingKey = .init(rawValue: field.key) else {
                continue
            }
            if case _? = self.index.updateValue(field.value, forKey: key) {
                throw JSON.ObjectKeyError<CodingKey>.duplicate(key)
            }
        }
    }
}
extension JSON.ObjectDecoder {
    @inlinable public __consuming func single() throws -> JSON.FieldDecoder<CodingKey> {
        guard let (key, value): (CodingKey, JSON.Node) = self.index.first else {
            throw JSON.SingleKeyError<CodingKey>.none
        }
        if self.index.count == 1 {
            return .init(key: key, value: value)
        } else {
            throw JSON.SingleKeyError<CodingKey>.multiple
        }
    }

    @inlinable public subscript(key: CodingKey) -> JSON.OptionalDecoder<CodingKey> {
        .init(key: key, value: self.index[key])
    }
    @inlinable public subscript(key: CodingKey) -> JSON.FieldDecoder<CodingKey>? {
        self.index[key].map { .init(key: key, value: $0) }
    }
}