/**
 * ChildrenStorage - Low-level storage for structural children
 *
 * Handles the 'children:' prefix for storing container child names.
 *
 * Key format: children:<parent_path>
 * Value: Serialized array of child names
 *
 * @version 0.1
 * @since 0.1
 */
namespace Implexus.Storage.LowLevel {

/**
 * Low-level storage for structural children.
 *
 * This class provides type-safe operations for storing and retrieving
 * structural child names using the 'children:' key prefix.
 */
public class ChildrenStorage : Object {
    
    /**
     * Key prefix for children entries.
     */
    private const string PREFIX = "children:";
    
    /**
     * The underlying Dbm storage.
     */
    private Dbm _dbm;
    
    /**
     * Creates a new ChildrenStorage with the given Dbm backend.
     *
     * @param dbm The Dbm backend to use for storage
     */
    public ChildrenStorage(Dbm dbm) {
        _dbm = dbm;
    }
    
    /**
     * Adds a child name to a parent's children set.
     *
     * @param parent The parent entity path
     * @param child_name The name of the child to add
     * @throws StorageError if the operation fails
     */
    public void add_child(Core.EntityPath parent, string child_name) throws StorageError {
        var children = load_children_set(parent);
        if (!children.contains(child_name)) {
            children.add(child_name);
            save_children_set(parent, children);
        }
    }
    
    /**
     * Removes a child name from a parent's children set.
     *
     * @param parent The parent entity path
     * @param child_name The name of the child to remove
     * @throws StorageError if the operation fails
     */
    public void remove_child(Core.EntityPath parent, string child_name) throws StorageError {
        var children = load_children_set(parent);
        if (children.contains(child_name)) {
            children.remove(child_name);
            save_children_set(parent, children);
        }
    }
    
    /**
     * Checks if a parent has a specific child.
     *
     * @param parent The parent entity path
     * @param child_name The name of the child to check
     * @return True if the child exists
     */
    public bool has_child(Core.EntityPath parent, string child_name) {
        var children = load_children_set(parent);
        return children.contains(child_name);
    }
    
    /**
     * Gets all child names for a parent.
     *
     * @param parent The parent entity path
     * @return An enumerable of child names
     */
    public Invercargill.Enumerable<string> get_children(Core.EntityPath parent) {
        var children = load_children_set(parent);
        return children.as_enumerable();
    }
    
    /**
     * Deletes all children for a parent.
     *
     * @param parent The parent entity path
     * @throws StorageError if the operation fails
     */
    public void delete(Core.EntityPath parent) throws StorageError {
        string key = PREFIX + parent.to_string();
        try {
            _dbm.delete(key);
        } catch (StorageError e) {
            // Key doesn't exist, that's fine
        }
    }
    
    /**
     * Loads the children set for a parent.
     *
     * @param parent The parent entity path
     * @return A vector of child names (empty if not found)
     */
    private Invercargill.DataStructures.Vector<string> load_children_set(Core.EntityPath parent) {
        string key = PREFIX + parent.to_string();
        var result = new Invercargill.DataStructures.Vector<string>();
        var data = _dbm.get(key);
        
        if (data == null) {
            return result;
        }
        
        var reader = new ElementReader((!) data);
        try {
            var element = reader.read_element();
            if (element.is_null()) {
                return result;
            }
            
            // The children set is stored as an array of strings
            var array = element.as<Invercargill.Enumerable<Invercargill.Element>>();
            foreach (var child_element in array) {
                if (!child_element.is_null()) {
                    string child_name = child_element.as<string>();
                    if (!result.contains(child_name)) {
                        result.add(child_name);
                    }
                }
            }
        } catch (Invercargill.ElementError e) {
            warning("Failed to read children set: %s", e.message);
        }
        
        return result;
    }
    
    /**
     * Saves the children set for a parent.
     *
     * @param parent The parent entity path
     * @param children The set of child names to save
     * @throws StorageError if the operation fails
     */
    private void save_children_set(Core.EntityPath parent, Invercargill.DataStructures.Vector<string> children) throws StorageError {
        string key = PREFIX + parent.to_string();
        
        uint count = children.count();
        if (count == 0) {
            try {
                _dbm.delete(key);
            } catch (StorageError e) {
                // Key doesn't exist, that's fine
            }
            return;
        }
        
        // Store as array of strings
        var array = new Invercargill.DataStructures.Vector<Invercargill.Element>();
        foreach (var child_name in children) {
            var element = new Invercargill.NativeElement<string>(child_name);
            array.add(element);
        }
        
        var writer = new ElementWriter();
        writer.write_element(new Invercargill.NativeElement<Invercargill.Enumerable<Invercargill.Element>>(array));
        _dbm.set(key, writer.to_binary_data());
    }
}

} // namespace Implexus.Storage.LowLevel