/* $Id$ */

/*
 * This file is part of OpenTTD.
 * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
 * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
 */

/** @file tile_map.cpp Global tile accessors. */

#include "stdafx.h"
#include "tile_map.h"

#include "safeguards.h"

/**
 * Returns the tile height for a coordinate outside map.  Such a height is
 * needed for painting the area outside map using completely black tiles.
 * The idea is descending to heightlevel 0 as fast as possible.
 * @param x The X-coordinate (same unit as TileX).
 * @param y The Y-coordinate (same unit as TileY).
 * @return The height in the same unit as TileHeight.
 */
uint TileHeightOutsideMap(int x, int y)
{
	/* In all cases: Descend to heightlevel 0 as fast as possible.
	 * So: If we are at the 0-side of the map (x<0 or y<0), we must
	 * subtract the distance to coordinate 0 from the heightlevel at
	 * coordinate 0.
	 * In other words: Subtract e.g. -x. If we are at the MapMax
	 * side of the map, we also need to subtract the distance to
	 * the edge of map, e.g. MapMaxX - x.
	 *
	 * NOTE: Assuming constant heightlevel outside map would be
	 * simpler here. However, then we run into painting problems,
	 * since whenever a heightlevel change at the map border occurs,
	 * we would need to repaint anything outside map.
	 * In contrast, by doing it this way, we can localize this change,
	 * which means we may assume constant heightlevel for all tiles
	 * at more than <heightlevel at map border> distance from the
	 * map border.
	 */
	if (x < 0) {
		if (y < 0) {
			return max((int)TileHeight(TileXY(0, 0)) - (-x) - (-y), 0);
		} else if (y < (int)MapMaxY()) {
			return max((int)TileHeight(TileXY(0, y)) - (-x), 0);
		} else {
			return max((int)TileHeight(TileXY(0, (int)MapMaxY())) - (-x) - (y - (int)MapMaxY()), 0);
		}
	} else if (x < (int)MapMaxX()) {
		if (y < 0) {
			return max((int)TileHeight(TileXY(x, 0)) - (-y), 0);
		} else if (y < (int)MapMaxY()) {
			return TileHeight(TileXY(x, y));
		} else {
			return max((int)TileHeight(TileXY(x, (int)MapMaxY())) - (y - (int)MapMaxY()), 0);
		}
	} else {
		if (y < 0) {
			return max((int)TileHeight(TileXY((int)MapMaxX(), 0)) - (x - (int)MapMaxX()) - (-y), 0);
		} else if (y < (int)MapMaxY()) {
			return max((int)TileHeight(TileXY((int)MapMaxX(), y)) - (x - (int)MapMaxX()), 0);
		} else {
			return max((int)TileHeight(TileXY((int)MapMaxX(), (int)MapMaxY())) - (x - (int)MapMaxX()) - (y - (int)MapMaxY()), 0);
		}
	}
}

/**
 * Get a tile's slope given the heigh of its four corners.
 * @param hnorth The height at the northern corner in the same unit as TileHeight.
 * @param hwest  The height at the western corner in the same unit as TileHeight.
 * @param heast  The height at the eastern corner in the same unit as TileHeight.
 * @param hsouth The height at the southern corner in the same unit as TileHeight.
 * @param[out] h The lowest height of the four corners.
 * @return The slope.
 */
static Slope GetTileSlopeGivenHeight(int hnorth, int hwest, int heast, int hsouth, int *h)
{
	/* Due to the fact that tiles must connect with each other without leaving gaps, the
	 * biggest difference in height between any corner and 'min' is between 0, 1, or 2.
	 *
	 * Also, there is at most 1 corner with height difference of 2.
	 */
	int hminnw = min(hnorth, hwest);
	int hmines = min(heast, hsouth);
	int hmin = min(hminnw, hmines);

	if (h != NULL) *h = hmin;

	int hmaxnw = max(hnorth, hwest);
	int hmaxes = max(heast, hsouth);
	int hmax = max(hmaxnw, hmaxes);

	Slope r = SLOPE_FLAT;

	if (hnorth != hmin) r |= SLOPE_N;
	if (hwest  != hmin) r |= SLOPE_W;
	if (heast  != hmin) r |= SLOPE_E;
	if (hsouth != hmin) r |= SLOPE_S;

	if (hmax - hmin == 2) r |= SLOPE_STEEP;

	return r;
}

/**
 * Return the slope of a given tile inside the map.
 * @param tile Tile to compute slope of
 * @param h    If not \c NULL, pointer to storage of z height
 * @return Slope of the tile, except for the HALFTILE part
 */
Slope GetTileSlope(TileIndex tile, int *h)
{
	assert(tile < MapSize());

	uint x = TileX(tile);
	uint y = TileY(tile);
	if (x == MapMaxX() || y == MapMaxY()) {
		if (h != NULL) *h = TileHeight(tile);
		return SLOPE_FLAT;
	}

	int hnorth = TileHeight(tile);                    // Height of the North corner.
	int hwest  = TileHeight(tile + TileDiffXY(1, 0)); // Height of the West corner.
	int heast  = TileHeight(tile + TileDiffXY(0, 1)); // Height of the East corner.
	int hsouth = TileHeight(tile + TileDiffXY(1, 1)); // Height of the South corner.

	return GetTileSlopeGivenHeight(hnorth, hwest, heast, hsouth, h);
}

/**
 * Return the slope of a given tile outside the map.
 *
 * @param x X-coordinate of the tile outside to compute height of.
 * @param y Y-coordinate of the tile outside to compute height of.
 * @param h    If not \c NULL, pointer to storage of z height.
 * @return Slope of the tile outside map, except for the HALFTILE part.
 */
Slope GetTilePixelSlopeOutsideMap(int x, int y, int *h)
{
	int hnorth = TileHeightOutsideMap(x,     y);     // N corner.
	int hwest  = TileHeightOutsideMap(x + 1, y);     // W corner.
	int heast  = TileHeightOutsideMap(x,     y + 1); // E corner.
	int hsouth = TileHeightOutsideMap(x + 1, y + 1); // S corner.

	Slope s = GetTileSlopeGivenHeight(hnorth, hwest, heast, hsouth, h);
	if (h != NULL) *h *= TILE_HEIGHT;
	return s;
}

/**
 * Check if a given tile is flat
 * @param tile Tile to check
 * @param h If not \c NULL, pointer to storage of z height (only if tile is flat)
 * @return Whether the tile is flat
 */
bool IsTileFlat(TileIndex tile, int *h)
{
	assert(tile < MapSize());

	if (!IsInnerTile(tile)) {
		if (h != NULL) *h = TileHeight(tile);
		return true;
	}

	uint z = TileHeight(tile);
	if (TileHeight(tile + TileDiffXY(1, 0)) != z) return false;
	if (TileHeight(tile + TileDiffXY(0, 1)) != z) return false;
	if (TileHeight(tile + TileDiffXY(1, 1)) != z) return false;

	if (h != NULL) *h = z;
	return true;
}

/**
 * Get bottom height of the tile
 * @param tile Tile to compute height of
 * @return Minimum height of the tile
 */
int GetTileZ(TileIndex tile)
{
	if (TileX(tile) == MapMaxX() || TileY(tile) == MapMaxY()) return 0;

	int h =    TileHeight(tile);                     // N corner
	h = min(h, TileHeight(tile + TileDiffXY(1, 0))); // W corner
	h = min(h, TileHeight(tile + TileDiffXY(0, 1))); // E corner
	h = min(h, TileHeight(tile + TileDiffXY(1, 1))); // S corner

	return h;
}

/**
 * Get top height of the tile inside the map.
 * @param t Tile to compute height of
 * @return Maximum height of the tile
 */
int GetTileMaxZ(TileIndex t)
{
	if (TileX(t) == MapMaxX() || TileY(t) == MapMaxY()) return TileHeightOutsideMap(TileX(t), TileY(t));

	int h =         TileHeight(t);                     // N corner
	h = max<int>(h, TileHeight(t + TileDiffXY(1, 0))); // W corner
	h = max<int>(h, TileHeight(t + TileDiffXY(0, 1))); // E corner
	h = max<int>(h, TileHeight(t + TileDiffXY(1, 1))); // S corner

	return h;
}