<?php


namespace Vizyt\Support;

defined( 'ABSPATH' ) || exit;

/**
 * Colour maths for theme CSS: mixing, alpha, WCAG contrast.
 * Hex in (3 or 6 digits), lowercase #rrggbb out.
 */
class Color {

    const WHITE = '#ffffff';
    const INK   = '#16181d';

    /**
     * @return int[] [ r, g, b ]; black for anything that is not a hex colour.
     */
    public static function rgb( string $hex ): array {
        $hex = trim( $hex );
        if ( '#' === substr( $hex, 0, 1 ) ) {
            $hex = substr( $hex, 1 );
        }
        if ( 3 === strlen( $hex ) ) {
            $hex = $hex[0] . $hex[0] . $hex[1] . $hex[1] . $hex[2] . $hex[2];
        }
        if ( ! preg_match( '/^[0-9a-fA-F]{6}$/', $hex ) ) {
            return [ 0, 0, 0 ];
        }

        return [ (int) hexdec( substr( $hex, 0, 2 ) ), (int) hexdec( substr( $hex, 2, 2 ) ), (int) hexdec( substr( $hex, 4, 2 ) ) ];
    }

    /**
     * Move $a towards $b by $weight (0 = $a, 1 = $b).
     */
    public static function mix( string $a, string $b, float $weight ): string {
        $ca  = self::rgb( $a );
        $cb  = self::rgb( $b );
        $out = [];
        for ( $i = 0; $i < 3; $i++ ) {
            $out[] = (int) round( $ca[ $i ] * ( 1 - $weight ) + $cb[ $i ] * $weight );
        }

        return vsprintf( '#%02x%02x%02x', $out );
    }

    public static function rgba( string $hex, float $alpha ): string {
        [ $r, $g, $b ] = self::rgb( $hex );
        $a             = rtrim( rtrim( number_format( $alpha, 2, '.', '' ), '0' ), '.' );

        return sprintf( 'rgba(%d,%d,%d,%s)', $r, $g, $b, $a );
    }

    /**
     * WCAG 2 relative luminance.
     */
    public static function luminance( string $hex ): float {
        $weights = [ 0.2126, 0.7152, 0.0722 ];
        $sum     = 0.0;
        foreach ( self::rgb( $hex ) as $i => $channel ) {
            $c    = $channel / 255;
            $c    = $c <= 0.03928 ? $c / 12.92 : ( ( $c + 0.055 ) / 1.055 ) ** 2.4;
            $sum += $weights[ $i ] * $c;
        }

        return $sum;
    }

    public static function contrast( string $a, string $b ): float {
        $la = self::luminance( $a );
        $lb = self::luminance( $b );

        return ( max( $la, $lb ) + 0.05 ) / ( min( $la, $lb ) + 0.05 );
    }

    /**
     * White or ink text, whichever reads better on $background.
     */
    public static function readable_on( string $background ): string {
        return self::contrast( self::WHITE, $background ) >= self::contrast( self::INK, $background ) ? self::WHITE : self::INK;
    }
}