<?php

/**

 * Class for efficiently looking up and mapping string keys to string values, with limits.

 *

 * @package    WordPress

 * @since      6.6.0

 */

/**

 * WP_Token_Map class.

 *

 * Use this class in specific circumstances with a static set of lookup keys which map to

 * a static set of transformed values. For example, this class is used to map HTML named

 * character references to their equivalent UTF-8 values.

 *

 * This class works differently than code calling `in_array()` and other methods. It

 * internalizes lookup logic and provides helper interfaces to optimize lookup and

 * transformation. It provides a method for precomputing the lookup tables and storing

 * them as PHP source code.

 *

 * All tokens and substitutions must be shorter than 256 bytes.

 *

 * Example:

 *

 *     $smilies = WP_Token_Map::from_array( array(

 *         '8O' => '😯',

 *         ':(' => '🙁',

 *         ':)' => '🙂',

 *         ':?' => '😕',

 *      ) );

 *

 *      true  === $smilies->contains( ':)' );

 *      false === $smilies->contains( 'simile' );

 *

 *      '😕' === $smilies->read_token( 'Not sure :?.', 9, $length_of_smily_syntax );

 *      2    === $length_of_smily_syntax;

 *

 * ## Precomputing the Token Map.

 *

 * Creating the class involves some work sorting and organizing the tokens and their

 * replacement values. In order to skip this, it's possible for the class to export

 * its state and be used as actual PHP source code.

 *

 * Example:

 *

 *      // Export with four spaces as the indent, only for the sake of this docblock.

 *      // The default indent is a tab character.

 *      $indent = '    ';

 *      echo $smilies->precomputed_php_source_table( $indent );

 *

 *      // Output, to be pasted into a PHP source file:

 *      WP_Token_Map::from_precomputed_table(

 *          array(

 *              "storage_version" => "6.6.0",

 *              "key_length" => 2,

 *              "groups" => "",

 *              "long_words" => array(),

 *              "small_words" => "8O\x00:)\x00:(\x00:?\x00",

 *              "small_mappings" => array( "😯", "🙂", "🙁", "😕" )

 *          )

 *      );

 *

 * ## Large vs. small words.

 *

 * This class uses a short prefix called the "key" to optimize lookup of its tokens.

 * This means that some tokens may be shorter than or equal in length to that key.

 * Those words that are longer than the key are called "large" while those shorter

 * than or equal to the key length are called "small."

 *

 * This separation of large and small words is incidental to the way this class

 * optimizes lookup, and should be considered an internal implementation detail

 * of the class. It may still be important to be aware of it, however.

 *

 * ## Determining Key Length.

 *

 * The choice of the size of the key length should be based on the data being stored in

 * the token map. It should divide the data as evenly as possible, but should not create

 * so many groups that a large fraction of the groups only contain a single token.

 *

 * For the HTML5 named character references, a key length of 2 was found to provide a

 * sufficient spread and should be a good default for relatively large sets of tokens.

 *

 * However, for some data sets this might be too long. For example, a list of smilies

 * may be too small for a key length of 2. Perhaps 1 would be more appropriate. It's

 * best to experiment and determine empirically which values are appropriate.

 *

 * ## Generate Pre-Computed Source Code.

 *

 * Since the `WP_Token_Map` is designed for relatively static lookups, it can be

 * advantageous to precompute the values and instantiate a table that has already

 * sorted and grouped the tokens and built the lookup strings.

 *

 * This can be done with `WP_Token_Map::precomputed_php_source_table()`.

 *

 * Note that if there is a leading character that all tokens need, such as `&` for

 * HTML named character references, it can be beneficial to exclude this from the

 * token map. Instead, find occurrences of the leading character and then use the

 * token map to see if the following characters complete the token.

 *

 * Example:

 *

 *     $map = WP_Token_Map::from_array( array( 'simple_smile:' => '🙂', 'sob:' => '😭', 'soba:' => '🍜' ) );

 *     echo $map->precomputed_php_source_table();

 *     // Output

 *     WP_Token_Map::from_precomputed_table(

 *         array(

 *             "storage_version" => "6.6.0",

 *             "key_length" => 2,

 *             "groups" => "si\x00so\x00",

 *             "long_words" => array(

 *                 // simple_smile:[🙂].

 *                 "\x0bmple_smile:\x04🙂",

 *                 // soba:[🍜] sob:[😭].

 *                 "\x03ba:\x04🍜\x02b:\x04😭",

 *             ),

 *             "short_words" => "",

 *             "short_mappings" => array()

 *         }

 *     );

 *

 * This precomputed value can be stored directly in source code and will skip the

 * startup cost of generating the lookup strings. See `$html5_named_character_entities`.

 *

 * Note that any updates to the precomputed format should update the storage version

 * constant. It would also be best to provide an update function to take older known

 * versions and upgrade them in place when loading into `from_precomputed_table()`.

 *

 * ## Future Direction.

 *

 * It may be viable to dynamically increase the length limits such that there's no need to impose them.

 * The limit appears because of the packing structure, which indicates how many bytes each segment of

 * text in the lookup tables spans. If, however, care were taken to track the longest word length, then

 * the packing structure could change its representation to allow for that. Each additional byte storing

 * length, however, increases the memory overhead and lookup runtime.

 *

 * An alternative approach could be to borrow the UTF-8 variable-length encoding and store lengths of less

 * than 127 as a single byte with the high bit unset, storing longer lengths as the combination of

 * continuation bytes.

 *

 * Since it has not been shown during the development of this class that longer strings are required, this

 * update is deferred until such a need is clear.

 *

 * @since 6.6.0

 */

class WP_Token_Map {

	/**

	 * Denotes the version of the code which produces pre-computed source tables.

	 *

	 * This version will be used not only to verify pre-computed data, but also

	 * to upgrade pre-computed data from older versions. Choosing a name that

	 * corresponds to the WordPress release will help people identify where an

	 * old copy of data came from.

	 */

	const STORAGE_VERSION = '6.6.0-trunk';

	/**

	 * Maximum length for each key and each transformed value in the table (in bytes).

	 *

	 * @since 6.6.0

	 */

	const MAX_LENGTH = 256;

	/**

	 * How many bytes of each key are used to form a group key for lookup.

	 * This also determines whether a word is considered short or long.

	 *

	 * @since 6.6.0

	 *

	 * @var int

	 */

	private $key_length = 2;

	/**

	 * Stores an optimized form of the word set, where words are grouped

	 * by a prefix of the `$key_length` and then collapsed into a string.

	 *

	 * In each group, the keys and lookups form a packed data structure.

	 * The keys in the string are stripped of their "group key," which is

	 * the prefix of length `$this->key_length` shared by all of the items

	 * in the group. Each word in the string is prefixed by a single byte

	 * whose raw unsigned integer value represents how many bytes follow.

	 *

	 *     ┌────────────────┬───────────────┬─────────────────┬────────┐

	 *     │ Length of rest │ Rest of key   │ Length of value │ Value  │

	 *     │ of key (bytes) │               │ (bytes)         │        │

	 *     ├────────────────┼───────────────┼─────────────────┼────────┤

	 *     │ 0x08           │ nterDot;      │ 0x02            │ ·      │

	 *     └────────────────┴───────────────┴─────────────────┴────────┘

	 *

	 * In this example, the key `CenterDot;` has a group key `Ce`, leaving

	 * eight bytes for the rest of the key, `nterDot;`, and two bytes for

	 * the transformed value `·` (or U+B7 or "\xC2\xB7").

	 *

	 * Example:

	 *

	 *    // Stores array( 'CenterDot;' => '·', 'Cedilla;' => '¸' ).

	 *    $groups      = "Ce\x00";

	 *    $large_words = array( "\x08nterDot;\x02·\x06dilla;\x02¸" )

	 *

	 * The prefixes appear in the `$groups` string, each followed by a null

	 * byte. This makes for quick lookup of where in the group string the key

	 * is found, and then a simple division converts that offset into the index

	 * in the `$large_words` array where the group string is to be found.

	 *

	 * This lookup data structure is designed to optimize cache locality and

	 * minimize indirect memory reads when matching strings in the set.

	 *

	 * @since 6.6.0

	 *

	 * @var array

	 */

	private $large_words = array();

	/**

	 * Stores the group keys for sequential string lookup.

	 *

	 * The offset into this string where the group key appears corresponds with the index

	 * into the group array where the rest of the group string appears. This is an optimization

	 * to improve cache locality while searching and minimize indirect memory accesses.

	 *

	 * @since 6.6.0

	 *

	 * @var string

	 */

	private $groups = '';

	/**

	 * Stores an optimized row of small words, where every entry is

	 * `$this->key_size + 1` bytes long and zero-extended.

	 *

	 * This packing allows for direct lookup of a short word followed

	 * by the null byte, if extended to `$this->key_size + 1`.

	 *

	 * Example:

	 *

	 *     // Stores array( 'GT', 'LT', 'gt', 'lt' ).

	 *     "GT\x00LT\x00gt\x00lt\x00"

	 *

	 * @since 6.6.0

	 *

	 * @var string

	 */

	private $small_words = '';

	/**

	 * Replacements for the small words, in the same order they appear.

	 *

	 * With the position of a small word it's possible to index the translation

	 * directly, as its position in the `$small_words` string corresponds to

	 * the index of the replacement in the `$small_mapping` array.

	 *

	 * Example:

	 *

	 *     array( '>', '<', '>', '<' )

	 *

	 * @since 6.6.0

	 *

	 * @var string[]

	 */

	private $small_mappings = array();

	/**

	 * Create a token map using an associative array of key/value pairs as the input.

	 *

	 * Example:

	 *

	 *     $smilies = WP_Token_Map::from_array( array(

	 *          '8O' => '😯',

	 *          ':(' => '🙁',

	 *          ':)' => '🙂',

	 *          ':?' => '😕',

	 *       ) );

	 *

	 * @since 6.6.0

	 *

	 * @param array $mappings   The keys transform into the values, both are strings.

	 * @param int   $key_length Determines the group key length. Leave at the default value

	 *                          of 2 unless there's an empirical reason to change it.

	 *

	 * @return WP_Token_Map|null Token map, unless unable to create it.

	 */

	public static function from_array( array $mappings, int $key_length = 2 ): ?WP_Token_Map {

		$map             = new WP_Token_Map();

		$map->key_length = $key_length;

		// Start by grouping words.

		$groups = array();

		$shorts = array();

		foreach ( $mappings as $word => $mapping ) {

			if (

				self::MAX_LENGTH <= strlen( $word ) ||

				self::MAX_LENGTH <= strlen( $mapping )

			) {

				_doing_it_wrong(

					__METHOD__,

					sprintf(

						/* translators: 1: maximum byte length (a count) */

						__( 'Token Map tokens and substitutions must all be shorter than %1$d bytes.' ),

						self::MAX_LENGTH

					),

					'6.6.0'

				);

				return null;

			}

			$length = strlen( $word );

			if ( $key_length >= $length ) {

				$shorts[] = $word;

			} else {

				$group = substr( $word, 0, $key_length );

				if ( ! isset( $groups[ $group ] ) ) {

					$groups[ $group ] = array();

				}

				$groups[ $group ][] = array( substr( $word, $key_length ), $mapping );

			}

		}

		/*

		 * Sort the words to ensure that no smaller substring of a match masks the full match.

		 * For example, `Cap` should not match before `CapitalDifferentialD`.

		 */

		usort( $shorts, 'WP_Token_Map::longest_first_then_alphabetical' );

		foreach ( $groups as $group_key => $group ) {

			usort(

				$groups[ $group_key ],

				static function ( array $a, array $b ): int {

					return self::longest_first_then_alphabetical( $a[0], $b[0] );

				}

			);

		}

		// Finally construct the optimized lookups.

		foreach ( $shorts as $word ) {

			$map->small_words     .= str_pad( $word, $key_length + 1, "\x00", STR_PAD_RIGHT );

			$map->small_mappings[] = $mappings[ $word ];

		}

		$group_keys = array_keys( $groups );

		sort( $group_keys );

		foreach ( $group_keys as $group ) {

			$map->groups .= "{$group}\x00";

			$group_string = '';

			foreach ( $groups[ $group ] as $group_word ) {

				list( $word, $mapping ) = $group_word;

				$word_length    = pack( 'C', strlen( $word ) );

				$mapping_length = pack( 'C', strlen( $mapping ) );

				$group_string  .= "{$word_length}{$word}{$mapping_length}{$mapping}";

			}

			$map->large_words[] = $group_string;

		}

		return $map;

	}

	/**

	 * Creates a token map from a pre-computed table.

	 * This skips the initialization cost of generating the table.

	 *

	 * This function should only be used to load data created with

	 * WP_Token_Map::precomputed_php_source_tag().

	 *

	 * @since 6.6.0

	 *

	 * @param array $state {

	 *     Stores pre-computed state for directly loading into a Token Map.

	 *

	 *     @type string $storage_version Which version of the code produced this state.

	 *     @type int    $key_length      Group key length.

	 *     @type string $groups          Group lookup index.

	 *     @type array  $large_words     Large word groups and packed strings.

	 *     @type string $small_words     Small words packed string.

	 *     @type array  $small_mappings  Small word mappings.

	 * }

	 *

	 * @return WP_Token_Map Map with precomputed data loaded.

	 */

	public static function from_precomputed_table( $state ): ?WP_Token_Map {

		$has_necessary_state = isset(

			$state['storage_version'],

			$state['key_length'],

			$state['groups'],

			$state['large_words'],

			$state['small_words'],

			$state['small_mappings']

		);

		if ( ! $has_necessary_state ) {

			_doing_it_wrong(

				__METHOD__,

				__( 'Missing required inputs to pre-computed WP_Token_Map.' ),

				'6.6.0'

			);

			return null;

		}

		if ( self::STORAGE_VERSION !== $state['storage_version'] ) {

			_doing_it_wrong(

				__METHOD__,

				/* translators: 1: version string, 2: version string. */

				sprintf( __( 'Loaded version \'%1$s\' incompatible with expected version \'%2$s\'.' ), $state['storage_version'], self::STORAGE_VERSION ),

				'6.6.0'

			);

			return null;

		}

		$map = new WP_Token_Map();

		$map->key_length     = $state['key_length'];

		$map->groups         = $state['groups'];

		$map->large_words    = $state['large_words'];

		$map->small_words    = $state['small_words'];

		$map->small_mappings = $state['small_mappings'];

		return $map;

	}

	/**

	 * Indicates if a given word is a lookup key in the map.

	 *

	 * Example:

	 *

	 *     true  === $smilies->contains( ':)' );

	 *     false === $smilies->contains( 'simile' );

	 *

	 * @since 6.6.0

	 *

	 * @param string $word             Determine if this word is a lookup key in the map.

	 * @param string $case_sensitivity Optional. Pass 'ascii-case-insensitive' to ignore ASCII case when matching. Default 'case-sensitive'.

	 * @return bool Whether there's an entry for the given word in the map.

	 */

	public function contains( string $word, string $case_sensitivity = 'case-sensitive' ): bool {

		$ignore_case = 'ascii-case-insensitive' === $case_sensitivity;

		if ( $this->key_length >= strlen( $word ) ) {

			if ( 0 === strlen( $this->small_words ) ) {

				return false;

			}

			$term    = str_pad( $word, $this->key_length + 1, "\x00", STR_PAD_RIGHT );

			$word_at = $ignore_case ? stripos( $this->small_words, $term ) : strpos( $this->small_words, $term );

			if ( false === $word_at ) {

				return false;

			}

			return true;

		}

		$group_key = substr( $word, 0, $this->key_length );

		$group_at  = $ignore_case ? stripos( $this->groups, $group_key ) : strpos( $this->groups, $group_key );

		if ( false === $group_at ) {

			return false;

		}