<?php
/**
 * Colour contrast of inline styles (F-A11Y-02).
 *
 * @package ForensixDigital
 */

declare(strict_types=1);

namespace ForensixDigital\Services\Scan;

defined( 'ABSPATH' ) || exit;

/**
 * Reads a text colour and a background colour written in one style attribute and works out the
 * contrast ratio of the WCAG formula. Only colours that can be read with certainty are judged:
 * hex values, rgb() and a short list of colour names. Anything else (variables, gradients,
 * colours set in a stylesheet) is left alone, because guessing would give false alarms.
 */
final class Contrast {

	public const NORMAL = 4.5;
	public const LARGE  = 3.0;

	private const NAMES = array(
		'black'   => array( 0, 0, 0 ),
		'white'   => array( 255, 255, 255 ),
		'red'     => array( 255, 0, 0 ),
		'green'   => array( 0, 128, 0 ),
		'blue'    => array( 0, 0, 255 ),
		'yellow'  => array( 255, 255, 0 ),
		'orange'  => array( 255, 165, 0 ),
		'gray'    => array( 128, 128, 128 ),
		'grey'    => array( 128, 128, 128 ),
		'silver'  => array( 192, 192, 192 ),
		'navy'    => array( 0, 0, 128 ),
		'purple'  => array( 128, 0, 128 ),
		'lime'    => array( 0, 255, 0 ),
		'aqua'    => array( 0, 255, 255 ),
		'cyan'    => array( 0, 255, 255 ),
		'maroon'  => array( 128, 0, 0 ),
		'teal'    => array( 0, 128, 128 ),
		'olive'   => array( 128, 128, 0 ),
		'fuchsia' => array( 255, 0, 255 ),
		'magenta' => array( 255, 0, 255 ),
		'pink'    => array( 255, 192, 203 ),
	);

	/**
	 * Reads one colour value.
	 *
	 * @param string $value CSS colour, e.g. "#777", "#777777", "rgb(119,119,119)" or "white".
	 * @return array{0: int, 1: int, 2: int}|null Red, green, blue; null when not readable or see-through.
	 */
	public static function parse( string $value ): ?array {
		$value = strtolower( trim( $value ) );
		$value = trim( (string) preg_replace( '/\s*!important$/', '', $value ) );
		if ( isset( self::NAMES[ $value ] ) ) {
			return self::NAMES[ $value ];
		}
		if ( 1 === preg_match( '/^#([0-9a-f]{3})$/', $value, $m ) ) {
			return array( (int) hexdec( $m[1][0] . $m[1][0] ), (int) hexdec( $m[1][1] . $m[1][1] ), (int) hexdec( $m[1][2] . $m[1][2] ) );
		}
		if ( 1 === preg_match( '/^#([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})$/', $value, $m ) ) {
			return array( (int) hexdec( $m[1] ), (int) hexdec( $m[2] ), (int) hexdec( $m[3] ) );
		}
		if ( 1 === preg_match( '/^rgb\(\s*(\d{1,3})\s*,\s*(\d{1,3})\s*,\s*(\d{1,3})\s*\)$/', $value, $m ) ) {
			return array( min( 255, (int) $m[1] ), min( 255, (int) $m[2] ), min( 255, (int) $m[3] ) );
		}
		return null;
	}

	/**
	 * Contrast ratio between two colours, 1 to 21.
	 *
	 * @param array{0: int, 1: int, 2: int} $a First colour.
	 * @param array{0: int, 1: int, 2: int} $b Second colour.
	 */
	public static function ratio( array $a, array $b ): float {
		$la = self::luminance( $a );
		$lb = self::luminance( $b );
		return ( max( $la, $lb ) + 0.05 ) / ( min( $la, $lb ) + 0.05 );
	}

	/**
	 * Relative luminance of a colour.
	 *
	 * @param array{0: int, 1: int, 2: int} $rgb Colour.
	 */
	private static function luminance( array $rgb ): float {
		$channels = array();
		foreach ( $rgb as $value ) {
			$c          = $value / 255;
			$channels[] = $c <= 0.03928 ? $c / 12.92 : ( ( $c + 0.055 ) / 1.055 ) ** 2.4;
		}
		return 0.2126 * $channels[0] + 0.7152 * $channels[1] + 0.0722 * $channels[2];
	}

	/**
	 * Judges the text and background colours of a style attribute.
	 *
	 * @param string $style Style attribute.
	 * @return array{ratio: float, text: string, background: string, limit: float}|null Null when the pair is fine or not readable.
	 */
	public static function check_style( string $style ): ?array {
		$props = array();
		foreach ( explode( ';', $style ) as $declaration ) {
			$parts = explode( ':', $declaration, 2 );
			if ( 2 === count( $parts ) ) {
				$props[ strtolower( trim( $parts[0] ) ) ] = trim( $parts[1] );
			}
		}
		$text = isset( $props['color'] ) ? self::parse( $props['color'] ) : null;
		$back = self::parse( $props['background-color'] ?? ( str_contains( $props['background'] ?? '', ' ' ) ? '' : ( $props['background'] ?? '' ) ) );
		if ( null === $text || null === $back ) {
			return null;
		}
		$large = 1 === preg_match( '/^(\d+(?:\.\d+)?)px$/', $props['font-size'] ?? '', $m ) && (float) $m[1] >= 24.0;
		$limit = $large ? self::LARGE : self::NORMAL;
		$ratio = self::ratio( $text, $back );
		return $ratio < $limit ? array(
			'ratio'      => round( $ratio, 2 ),
			'text'       => $props['color'],
			'background' => $props['background-color'] ?? $props['background'],
			'limit'      => $limit,
		) : null;
	}
}