Converting collections to query buidlers in Rila

Stage 1.php

<?php
class Collection {
	protected $item_type = 'Rila\\Item';
	public $args;
	protected $ids;

	public function __construct( $request = null ) {
		if( is_null( $request ) ) {
			# Allow simple initialization of child classes
			if( method_exists( $this, 'initialize' ) ) {
				$this->initialize();
			}

			return;
		}

		if( is_array( $request ) ) {
			$items_only = true;
			$ids_only   = true;
			$ids        = array();
			$k          = 0;

			foreach( $request as $i => $item ) {
				if( ! is_object( $item ) ) {
					$items_only = false;
				}

				if( ( $i !== $k ) || ! is_scalar( $item ) || ! intval( $item ) ) {
					$ids_only = false;
				} else {
					$ids[] = intval( $item );
				}

				$k++;
			}

			if( $ids_only ) {
				$this->ids = $ids;
			} elseif( $items_only ) {
				$this->items = array();
				foreach( $request as $item ) {
					$this->items[] = call_user_func( array( $this->item_type, 'factory' ),  $item );
				}
				$this->initialized = true;
			} else {
				$this->args = $request;
			}
		} else {
			$this->args = wp_parse_args( $request );
		}

		# Allow simple initialization of child classes
		if( method_exists( $this, 'initialize' ) ) {
			$this->initialize();
		}
	}

	protected function load() {
		$msg = "Generic collections cannot load from the database and can only be used with explicit data.";
		throw new \Exception( $msg );
	}

	public function where( $key, $value = null ) {
		if( $this->initialized ) {
			throw new \Exception( "Once a collection is initialized with data, it cannot be filtered from the database. Use ->filter() instead!" );
		}

		if( $value ) {
			$request = array( $key => $value );
		} else {
			$request = $key;
		}

		foreach( $request as $key => $value ) {
			$this->set( $key, $value );
			$method_name = 'set_' . $key;
		}

		return $this;
	}

	protected function set( $key, $value ) {
		$this->args[ $key ] = $value;
	}

	public function filter( $filters ) {
		if( ! $this->initialized ) {
			$this->load();
		}

		$filtered = array();

		foreach( $this->items as $item ) {
			$ok = true;

			foreach( $filters as $key => $value ) {
				if( $item->raw( $key ) != $value ) {
					$ok = false;
					break;
				}
			}

			if( $ok ) {
				$filtered[] = $item;
			}
		}

		if( empty( $filtered ) ) {
			return array();
		}

		$class_name = get_class( $this );
		return new $class_name( $filtered );
	}
}

Stage 2.php

<?php
class Collection {
	protected $item_type = 'Rila\\Item';
	public $args;
	protected $ids;

	public function __construct( $request = null ) {
		if( is_null( $request ) ) {
			# Allow simple initialization of child classes
			if( method_exists( $this, 'initialize' ) ) {
				$this->initialize();
			}

			return;
		}

		if( is_array( $request ) ) {
			$items_only = true;
			$ids_only   = true;
			$ids        = array();
			$k          = 0;

			foreach( $request as $i => $item ) {
				if( ! is_object( $item ) ) {
					$items_only = false;
				}

				if( ( $i !== $k ) || ! is_scalar( $item ) || ! intval( $item ) ) {
					$ids_only = false;
				} else {
					$ids[] = intval( $item );
				}

				$k++;
			}

			if( $ids_only ) {
				$this->ids = $ids;
			} elseif( $items_only ) {
				$this->items = array();
				foreach( $request as $item ) {
					$this->items[] = call_user_func( array( $this->item_type, 'factory' ),  $item );
				}
				$this->initialized = true;
			} else {
				$this->args = $request;
			}
		} else {
			$this->args = wp_parse_args( $request );
		}

		# Allow simple initialization of child classes
		if( method_exists( $this, 'initialize' ) ) {
			$this->initialize();
		}
	}

	protected function load() {
		$msg = "Generic collections cannot load from the database and can only be used with explicit data.";
		throw new \Exception( $msg );
	}

	public function where( $key, $value = null ) {
		if( $this->initialized ) {
			throw new \Exception( "Once a collection is initialized with data, it cannot be filtered from the database. Use ->filter() instead!" );
		}

		if( $value ) {
			$request = array( $key => $value );
		} else {
			$request = $key;
		}

		foreach( $request as $key => $value ) {
			$this->set( $key, $value );
			$method_name = 'set_' . $key;
		}

		return $this;
	}

	protected function set( $key, $value ) {
		$this->args[ $key ] = $value;
	}

	public function filter( $filters ) {
        $collection = $this->get();

        foreach( $filters as $key => $value ) {
            $collection = $collection->where( $key, $value );
        }

        return $collection;
	}
}

Stage 3.php

<?php
abstract class Query_Builder {
	protected $item_type = 'Rila\\Item';
	public $args;
	protected $ids;
    protected $items;

	public function __construct( $request = null ) {
        // If there is no request, `initialize` will directly load defaults.
		if( is_null( $request ) ) {
			return $this->maybe_initialize();
		}

        // Convert the request to an array, if a string is given (ex. posts_per_page=1&order=asc).
        if( is_string( $request ) ) {
            $request = wp_parse_args( $request );
        }

        // Check if the request is an array, which only contains IDs.
        $ids_only     = true;
        $linear_index = 0;
        foreach( $request as $internal_index => $element ) {
            if(
                $linear_index != $internal_index // Normal arrays should have proper indexes.
                || ! is_scalar( $element )       // IDs are always scalar
                || ! intval( $element )          // and can be converted to a positive integer.
            ) {
                $ids_only = false;
                break;
            }

            $linear_index++;
        }

        // Once confirmed, save the IDs as an indication of the `ids_only` mode and bail.
        if( $ids_only ) {
            $this->ids = array_map( 'intval', $request );
            return $this->maybe_initialize();
        }

        // Check if maybe the items are already loaded from the database
        $items_only = true;
        foreach( $request as $item ) {
            if( ! is_object( $Item ) ) {
                $items_only = false;
                break;
            }
        }

        // Should we just convert to a collection at this point ???
        if( $items_only ) {
            $this->items = $request;
            return $this->maybe_initialize();
        }
        
        // At this point, we can only have normal arguments
        $this->args = $request;
		$this->maybe_initialize();
	}

    /**
     * Checks if there is an `initialize` method and calls it.
     */
    protected function maybe_initialize() {
        if( method_exists( $this, 'initialize' ) ) {
			$this->initialize();
		}
    }

	protected function load() {
		$msg = "Generic collections cannot load from the database and can only be used with explicit data.";
		throw new \Exception( $msg );
	}

	public function where( $key, $value = null ) {
		if( $this->initialized ) {
			throw new \Exception( "Once a collection is initialized with data, it cannot be filtered from the database. Use ->filter() instead!" );
		}

		if( $value ) {
			$request = array( $key => $value );
		} else {
			$request = $key;
		}

		foreach( $request as $key => $value ) {
			$this->set( $key, $value );
		}

		return $this;
	}

	protected function set( $key, $value ) {
		$this->args[ $key ] = $value;

        return $this;
	}

	public function filter( $filters ) {
        $collection = $this->get();

        foreach( $filters as $key => $value ) {
            $collection = $collection->where( $key, $value );
        }

        return $collection;
	}
}