<?php
/**
 * Country codes, made readable.
 *
 * @package AdPix\Analytics
 */

declare( strict_types = 1 );

namespace AdPix\Analytics\Service;

defined( 'ABSPATH' ) || exit;

/**
 * Turns the two-letter codes the geography report returns into something a person can read.
 *
 * The report answers in ISO 3166-1 alpha-2 — "DE", "IR", "AE" — which is exactly right for a machine
 * and no use at all on a screen. No list of country names is shipped here: two hundred and fifty
 * names in three languages is a translation burden that would never be kept current, and both
 * sources below already have the names, already translated, already maintained by somebody whose
 * job that is.
 *
 * The flag is derived arithmetically rather than looked up. Regional indicator symbols are simply
 * the letters offset into their own Unicode block, so any two-letter code produces its own flag with
 * no table to go stale — and where a system has no flag glyphs it falls back to showing the letters,
 * which is the information anyway.
 */
final class Countries {

	/**
	 * Codepoint of the first regional indicator symbol, which stands for "A".
	 */
	private const INDICATOR_BASE = 0x1F1E6;

	/**
	 * Names already resolved this request.
	 *
	 * @var array<string,string>
	 */
	private $names = array();

	/**
	 * WooCommerce's country list, loaded at most once.
	 *
	 * @var array<string,string>|null
	 */
	private $fallback;

	/**
	 * The readable name of a country.
	 *
	 * @param string $code Two-letter code, in any case.
	 * @return string The name, or the code itself when nothing can name it.
	 */
	public function name( string $code ): string {
		$code = strtoupper( trim( $code ) );

		if ( ! $this->is_code( $code ) ) {
			// The report uses this for traffic it could not place, and "(not set)" is more honest on a
			// table row than an empty cell that reads as a rendering fault.
			return '' === $code ? __( '(not set)', 'adpix-analytics' ) : $code;
		}

		if ( isset( $this->names[ $code ] ) ) {
			return $this->names[ $code ];
		}

		$this->names[ $code ] = $this->resolve( $code );

		return $this->names[ $code ];
	}

	/**
	 * The flag for a country code.
	 *
	 * @param string $code Two-letter code, in any case.
	 * @return string Empty when the code is not a country code.
	 */
	public function flag( string $code ): string {
		$code = strtoupper( trim( $code ) );

		if ( ! $this->is_code( $code ) ) {
			return '';
		}

		$flag = '';

		for ( $i = 0; $i < 2; $i++ ) {
			$flag .= $this->indicator( ord( $code[ $i ] ) - ord( 'A' ) );
		}

		return $flag;
	}

	/**
	 * Find a name for a code.
	 *
	 * @param string $code Upper-case two-letter code.
	 * @return string
	 */
	private function resolve( string $code ): string {
		// PHP's own internationalisation data, when the host has it: names in the admin's language,
		// current with CLDR, costing nothing to ship.
		if ( class_exists( '\Locale' ) && method_exists( '\Locale', 'getDisplayRegion' ) ) {
			$name = \Locale::getDisplayRegion( '-' . $code, str_replace( '_', '-', get_user_locale() ) );

			if ( is_string( $name ) && '' !== $name && $name !== $code ) {
				return $name;
			}
		}

		// Otherwise WooCommerce, which most sites running this plugin's commerce tracking already have,
		// and which translates its list through the same locale the admin is being read in.
		$fallback = $this->woocommerce_countries();

		if ( isset( $fallback[ $code ] ) ) {
			return $fallback[ $code ];
		}

		return $code;
	}

	/**
	 * WooCommerce's translated country list, if WooCommerce is here.
	 *
	 * @return array<string,string>
	 */
	private function woocommerce_countries(): array {
		if ( null !== $this->fallback ) {
			return $this->fallback;
		}

		$this->fallback = array();

		if ( ! function_exists( 'WC' ) ) {
			return $this->fallback;
		}

		$wc = WC();

		if ( ! is_object( $wc ) || ! isset( $wc->countries ) || ! is_object( $wc->countries ) ) {
			return $this->fallback;
		}

		$countries = $wc->countries->get_countries();

		if ( is_array( $countries ) ) {
			$this->fallback = array_map( 'strval', $countries );
		}

		return $this->fallback;
	}

	/**
	 * Whether a string is shaped like an ISO 3166-1 alpha-2 code.
	 *
	 * @param string $code Candidate.
	 * @return bool
	 */
	private function is_code( string $code ): bool {
		return 1 === preg_match( '/^[A-Z]{2}$/', $code );
	}

	/**
	 * One regional indicator symbol, as UTF-8.
	 *
	 * Encoded by hand because the alternative, mb_chr(), needs an extension that is not guaranteed,
	 * and this is four lines of a format that has not changed since 1996.
	 *
	 * @param int $offset Letter index, zero for "A".
	 * @return string
	 */
	private function indicator( int $offset ): string {
		$codepoint = self::INDICATOR_BASE + $offset;

		return chr( 0xF0 | ( $codepoint >> 18 ) )
			. chr( 0x80 | ( ( $codepoint >> 12 ) & 0x3F ) )
			. chr( 0x80 | ( ( $codepoint >> 6 ) & 0x3F ) )
			. chr( 0x80 | ( $codepoint & 0x3F ) );
	}
}