<?php

/*
 *
 *  ____            _        _   __  __ _                  __  __ ____
 * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
 * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
 * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
 * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * @author PocketMine Team
 * @link http://www.pocketmine.net/
 *
 *
*/

declare(strict_types=1);

namespace pocketmine\math;

abstract class VoxelRayTrace{

	/**
	 * Performs a ray trace from the start position in the given direction, for a distance of $maxDistance. This
	 * returns a Generator which yields Vector3s containing the coordinates of voxels it passes through.
	 *
	 * @param Vector3 $start
	 * @param Vector3 $directionVector
	 * @param float   $maxDistance
	 *
	 * @return \Generator|Vector3[]
	 */
	public static function inDirection(Vector3 $start, Vector3 $directionVector, float $maxDistance) : \Generator{
		return self::betweenPoints($start, $start->add($directionVector->multiply($maxDistance)));
	}

	/**
	 * Performs a ray trace between the start and end coordinates. This returns a Generator which yields Vector3s
	 * containing the coordinates of voxels it passes through.
	 *
	 * This is an implementation of the algorithm described in the link below.
	 * @link http://www.cse.yorku.ca/~amana/research/grid.pdf
	 *
	 * @param Vector3 $start
	 * @param Vector3 $end
	 *
	 * @return \Generator|Vector3[]
	 */
	public static function betweenPoints(Vector3 $start, Vector3 $end) : \Generator{
		$currentBlock = $start->floor();

		$directionVector = $end->subtract($start)->normalize();
		if($directionVector->lengthSquared() <= 0){
			throw new \InvalidArgumentException("Start and end points are the same, giving a zero direction vector");
		}

		$radius = $start->distance($end);

		$stepX = $directionVector->x <=> 0;
		$stepY = $directionVector->y <=> 0;
		$stepZ = $directionVector->z <=> 0;

		//Initialize the step accumulation variables depending how far into the current block the start position is. If
		//the start position is on the corner of the block, these will be zero.
		$tMaxX = self::rayTraceDistanceToBoundary($start->x, $directionVector->x);
		$tMaxY = self::rayTraceDistanceToBoundary($start->y, $directionVector->y);
		$tMaxZ = self::rayTraceDistanceToBoundary($start->z, $directionVector->z);

		//The change in t on each axis when taking a step on that axis (always positive).
		$tDeltaX = $directionVector->x == 0 ? 0 : $stepX / $directionVector->x;
		$tDeltaY = $directionVector->y == 0 ? 0 : $stepY / $directionVector->y;
		$tDeltaZ = $directionVector->z == 0 ? 0 : $stepZ / $directionVector->z;

		while(true){
			yield $currentBlock;

			// tMaxX stores the t-value at which we cross a cube boundary along the
			// X axis, and similarly for Y and Z. Therefore, choosing the least tMax
			// chooses the closest cube boundary.
			if($tMaxX < $tMaxY and $tMaxX < $tMaxZ){
				if($tMaxX > $radius){
					break;
				}
				$currentBlock->x += $stepX;
				$tMaxX += $tDeltaX;
			}elseif($tMaxY < $tMaxZ){
				if($tMaxY > $radius){
					break;
				}
				$currentBlock->y += $stepY;
				$tMaxY += $tDeltaY;
			}else{
				if($tMaxZ > $radius){
					break;
				}
				$currentBlock->z += $stepZ;
				$tMaxZ += $tDeltaZ;
			}
		}
	}

	/**
	 * Returns the distance that must be travelled on an axis from the start point with the direction vector component to
	 * cross a block boundary.
	 *
	 * For example, given an X coordinate inside a block and the X component of a direction vector, will return the distance
	 * travelled by that direction component to reach a block with a different X coordinate.
	 *
	 * Find the smallest positive t such that s+t*ds is an integer.
	 *
	 * @param float $s Starting coordinate
	 * @param float $ds Direction vector component of the relevant axis
	 *
	 * @return float Distance along the ray trace that must be travelled to cross a boundary.
	 */
	private static function rayTraceDistanceToBoundary(float $s, float $ds) : float{
		if($ds == 0){
			return INF;
		}

		if($ds < 0){
			$s = -$s;
			$ds = -$ds;

			if(floor($s) == $s){ //exactly at coordinate, will leave the coordinate immediately by moving negatively
				return 0;
			}
		}

		// problem is now s+t*ds = 1
		return (1 - ($s - floor($s))) / $ds;
	}
}