<?php

if (! function_exists('format_event_url')) {
    /**
     * format an event url with a given calendar ID and event ID
     */
    function format_event_url(string $eid, string $cid): string
    {
        return 'calendar/'.$cid.'/event/'.$eid;
    }
}

if (! function_exists('contrast_text_color')) {
    /**
     * Choose an accessible foreground (#fff or #000) against a HEX background.
     *
     * Rule set:
     *   1. Calculate WCAG contrast ratios for both black and white.
     *   2. Prefer the colour with the *higher* ratio.
     *   3. Override: if black wins but ratio < 5.5 AND white > 4, use white.
     *
     * @param  string $hex        Background colour (3- or 6-digit hex, with or without #)
     * @param  string $light      Return value for “white”   (default '#ffffff')
     * @param  string $dark       Return value for “black”   (default '#000000')
     * @return string
     */
    function contrast_text_color(string $hex, string $light = '#ffffff', string $dark = '#000000'): string
    {
        // --- normalise ----------------------------------------------------
        $hex = ltrim($hex, '#');
        if (strlen($hex) === 3) {                      // #abc → #aabbcc
            $hex = preg_replace('/./', '$0$0', $hex);
        }

        [$r, $g, $b] = [
            hexdec(substr($hex, 0, 2)) / 255,
            hexdec(substr($hex, 2, 2)) / 255,
            hexdec(substr($hex, 4, 2)) / 255,
        ];

        // --- convert sRGB → linear RGB -----------------------------------
        $linear = function (float $c): float {
            return $c <= 0.04045 ? $c / 12.92 : pow(($c + 0.055) / 1.055, 2.4);
        };

        $R = $linear($r);
        $G = $linear($g);
        $B = $linear($b);

        // --- relative luminance (ITU-R BT.709) ----------------------------
        $L_bg = 0.2126 * $R + 0.7152 * $G + 0.0722 * $B;       // 0–1

        // --- contrast ratios vs black (L=0) and white (L=1) ---------------
        $contrast_black = ($L_bg + 0.05) / 0.05;               // bg lighter than black
        $contrast_white = 1.05 / ($L_bg + 0.05);               // white vs bg

        // --- pick the winner ---------------------------------------------
        $useDark = $contrast_black >= $contrast_white;

        // override rule if dark is true but white "looks better"
        if ($useDark && $contrast_black < 5.5 && $contrast_white > 4) {
            $useDark = false; // switch to white
        }

        return $useDark ? $dark : $light;
    }
}