<?php
/**
 * Reads what a Search Console change actually says.
 *
 * @package TurtleReport
 */

namespace TurtleReport\Attention;

defined( 'ABSPATH' ) || exit;

/**
 * Turns before/after totals and keyword rows into the few facts that tell
 * one kind of drop from another, and judges how much data stands behind
 * them. The Presenter writes sentences from this; nothing here guesses.
 *
 * Patterns, in order of precedence:
 *
 * - ranking:       average position got worse by 1+ (with enough impressions
 *                  to trust it), or keywords that slipped explain most of it;
 * - lost_searches: impressions fell 15%+ while average position improved 1+.
 *                  Dropping out of searches where it ranked low pushes the
 *                  average up, so "better position" here is not good news;
 * - demand:        impressions fell 15%+ and position held: shown for fewer
 *                  searches at about the same rank;
 * - snippet:       impressions held but click-through rate fell 15%+;
 * - mixed:         nothing stands out.
 */
final class Signals {

	/** Impressions needed in both periods before average position means anything. */
	const POSITION_MIN_IMPRESSIONS = 100;

	/** Keyword rows kept as evidence. */
	const ROWS = 8;

	/**
	 * Signals for a site or one page.
	 *
	 * @param array   $now  clicks, impr, ctr, pos (pos may be null).
	 * @param array   $prev Same, for the period before.
	 * @param array[] $rows Joined keyword rows (key, c, pc, i, pi, pos, ppos); may be empty.
	 * @return array
	 */
	public static function search( array $now, array $prev, array $rows = array() ) {
		$c  = (float) $now['clicks'];
		$pc = (float) $prev['clicks'];
		$i  = (float) $now['impr'];
		$pi = (float) $prev['impr'];

		$ctr  = $i > 0 ? $c / $i : 0.0;
		$pctr = $pi > 0 ? $pc / $pi : 0.0;
		$pos  = isset( $now['pos'] ) ? (float) $now['pos'] : null;
		$ppos = isset( $prev['pos'] ) ? (float) $prev['pos'] : null;

		$reliable = null !== $pos && null !== $ppos && $i >= self::POSITION_MIN_IMPRESSIONS && $pi >= self::POSITION_MIN_IMPRESSIONS;

		$out = array(
			'dc'        => Rules::change( $c, $pc ),
			'di'        => Rules::change( $i, $pi ),
			'ctr'       => $ctr,
			'pctr'      => $pctr,
			'dpts'      => ( $ctr - $pctr ) * 100,
			'dctr'      => Rules::change( $ctr, $pctr ),
			'now_pos'   => $pos,
			'prev_pos'  => $ppos,
			'dpos'      => $reliable ? round( $pos - $ppos, 1 ) : null,
			'reliable'  => $reliable,
			'pattern'   => 'mixed',
			'ctr_also'  => false,
			'top'       => null,
			'top_share' => 0,
			'vanished'  => array(
				'count' => 0,
				'impr'  => 0,
			),
			'slipped'   => array(),
			'snippet'   => array(),
			'rows'      => array(),
		);

		// Keyword level: where impressions and clicks went.
		$impr_lost  = 0.0;
		$click_lost = 0.0;
		$slipped    = array();
		$snippet    = array();
		foreach ( $rows as $row ) {
			$impr_lost  += max( 0, $row['pi'] - $row['i'] );
			$click_lost += max( 0, $row['pc'] - $row['c'] );
			if ( $row['pi'] >= 20 && 0.0 === (float) $row['i'] ) {
				++$out['vanished']['count'];
				$out['vanished']['impr'] += $row['pi'];
			}
			if ( $row['pi'] >= 50 && $row['i'] > 0 && $row['pos'] - $row['ppos'] >= 3 ) {
				$slipped[] = $row;
			}
			if ( $row['pi'] >= 100 && $row['i'] >= 0.8 * $row['pi'] && $row['pc'] - $row['c'] >= 5 && $row['pi'] > 0 && $row['i'] > 0 && ( $row['c'] / $row['i'] ) < 0.7 * ( $row['pc'] / $row['pi'] ) ) {
				$snippet[] = $row;
			}
		}
		$by_impr = $rows;
		usort(
			$by_impr,
			static function ( $a, $b ) {
				return ( $b['pi'] - $b['i'] ) <=> ( $a['pi'] - $a['i'] );
			}
		);
		$by_click = $rows;
		usort(
			$by_click,
			static function ( $a, $b ) {
				return ( $b['pc'] - $b['c'] ) <=> ( $a['pc'] - $a['c'] );
			}
		);
		$lost_by_slip = 0.0;
		foreach ( $slipped as $row ) {
			$lost_by_slip += max( 0, $row['pc'] - $row['c'] );
		}
		$sort_lost = static function ( $a, $b ) {
			return ( $b['pc'] - $b['c'] ) <=> ( $a['pc'] - $a['c'] );
		};
		usort( $slipped, $sort_lost );
		usort( $snippet, $sort_lost );
		$out['slipped'] = array_slice( $slipped, 0, 3 );
		$out['snippet'] = array_slice( $snippet, 0, 3 );

		// The keyword that explains the most: by impressions when visibility fell, else by clicks.
		$visibility = null !== $out['di'] && $out['di'] <= -15;
		$lead       = $visibility ? ( $by_impr ? $by_impr[0] : null ) : ( $by_click ? $by_click[0] : null );
		if ( $lead && ( $visibility ? $lead['pi'] > $lead['i'] : $lead['pc'] > $lead['c'] ) ) {
			$out['top']       = $lead;
			$total            = $visibility ? max( $impr_lost, $pi - $i ) : max( $click_lost, $pc - $c );
			$out['top_share'] = $total > 0 ? round( ( $visibility ? $lead['pi'] - $lead['i'] : $lead['pc'] - $lead['c'] ) / $total * 100 ) : 0;
		}

		// Evidence rows: the biggest losers by clicks, then by impressions, without repeats.
		$keep = array();
		foreach ( array_merge( array_slice( $by_click, 0, 5 ), array_slice( $by_impr, 0, 5 ) ) as $row ) {
			if ( ( $row['pc'] > $row['c'] || $row['pi'] > $row['i'] ) && ! isset( $keep[ $row['key'] ] ) && count( $keep ) < self::ROWS ) {
				$keep[ $row['key'] ] = $row;
			}
		}
		$out['rows'] = array_values( $keep );

		// Pattern, in the precedence order listed on the class.
		$dpos = $out['dpos'];
		if ( ( null !== $dpos && $dpos >= 1.0 ) || ( count( $slipped ) >= 2 && $click_lost > 0 && $lost_by_slip / $click_lost >= 0.4 ) ) {
			$out['pattern'] = 'ranking';
		} elseif ( $visibility && null !== $dpos && $dpos <= -1.0 ) {
			$out['pattern'] = 'lost_searches';
		} elseif ( $visibility ) {
			$out['pattern'] = 'demand';
		} elseif ( ( null === $out['di'] || $out['di'] > -10 ) && null !== $out['dctr'] && $out['dctr'] <= -15 && $i >= 500 ) {
			$out['pattern'] = 'snippet';
		}
		$out['ctr_also'] = 'snippet' !== $out['pattern'] && null !== $out['dctr'] && $out['dctr'] <= -20 && $out['dpts'] <= -0.3 && $i >= 500;
		return $out;
	}

	/**
	 * How much data stands behind a change.
	 *
	 * @param float $before Main number before (clicks, leads or events).
	 * @param array $levels [strong, moderate] floors.
	 * @return string strong|moderate|early
	 */
	public static function confidence( $before, array $levels ) {
		if ( $before >= $levels[0] ) {
			return 'strong';
		}
		return $before >= $levels[1] ? 'moderate' : 'early';
	}
}