/**
 * Draw canal or river edges.
 * @param canal  True if canal edges should be drawn, false for river edges.
 * @param offset Sprite offset.
 * @param tile   Tile to draw.
 */
static void DrawWaterEdges(bool canal, uint offset, TileIndex tile)
{
	CanalFeature feature;
	SpriteID base = 0;
	if (canal) {
		feature = CF_DIKES;
		base = GetCanalSprite(CF_DIKES, tile);
		if (base == 0) base = SPR_CANAL_DIKES_BASE;
	} else {
		feature = CF_RIVER_EDGE;
		base = GetCanalSprite(CF_RIVER_EDGE, tile);
		if (base == 0) return; // Don't draw if no sprites provided.
	}

	uint wa;

	/* determine the edges around with water. */
	wa  = IsWateredTile(TILE_ADDXY(tile, -1,  0), DIR_SW) << 0;  // NE
	wa += IsWateredTile(TILE_ADDXY(tile,  0,  1), DIR_NW) << 1;  // SE
	wa += IsWateredTile(TILE_ADDXY(tile,  1,  0), DIR_NE) << 2;  // SW
	wa += IsWateredTile(TILE_ADDXY(tile,  0, -1), DIR_SE) << 3;  // NW

	Slope tileh = GetTileSlope(tile);
	if (!(wa & 1)) {
		DrawWaterSprite(base, offset, feature, tile);
		if (tileh == SLOPE_N) DrawWaterSprite(base, offset + 48, feature, tile);
		if (tileh == SLOPE_E) DrawWaterSprite(base, offset + 12, feature, tile);
	}
	if (!(wa & 2)) {
		DrawWaterSprite(base, offset + 1, feature, tile);
		if (tileh == SLOPE_E) DrawWaterSprite(base, offset + 25, feature, tile);
		if (tileh == SLOPE_S) DrawWaterSprite(base, offset + 37, feature, tile);
	}
	if (!(wa & 4)) {
		DrawWaterSprite(base, offset + 2, feature, tile);
		if (tileh == SLOPE_S) DrawWaterSprite(base, offset + 14, feature, tile);
		if (tileh == SLOPE_W) DrawWaterSprite(base, offset + 50, feature, tile);
	}
	if (!(wa & 8)) {
		DrawWaterSprite(base, offset + 3, feature, tile);
		if (tileh == SLOPE_W) DrawWaterSprite(base, offset + 39, feature, tile);
		if (tileh == SLOPE_N) DrawWaterSprite(base, offset + 27, feature, tile);
	}

	/* right corner */
	switch (wa & 0x03) {
		case 0: DrawWaterSprite(base, offset + 4, feature, tile); break;
		case 3: if (!IsWateredTile(TILE_ADDXY(tile, -1, 1), DIR_W)) DrawWaterSprite(base, offset + 8, feature, tile); break;
	}

	/* bottom corner */
	switch (wa & 0x06) {
		case 0: DrawWaterSprite(base, offset + 5, feature, tile); break;
		case 6: if (!IsWateredTile(TILE_ADDXY(tile, 1, 1), DIR_N)) DrawWaterSprite(base, offset + 9, feature, tile); break;
	}

	/* left corner */
	switch (wa & 0x0C) {
		case  0: DrawWaterSprite(base, offset + 6, feature, tile); break;
		case 12: if (!IsWateredTile(TILE_ADDXY(tile, 1, -1), DIR_E)) DrawWaterSprite(base, offset + 10, feature, tile); break;
	}

	/* upper corner */
	switch (wa & 0x09) {
		case 0: DrawWaterSprite(base, offset + 7, feature, tile); break;
		case 9: if (!IsWateredTile(TILE_ADDXY(tile, -1, -1), DIR_S)) DrawWaterSprite(base, offset + 11, feature, tile); break;
	}
}


static void DrawRiverWater(const TileInfo *ti)
{
	SpriteID image = SPR_FLAT_WATER_TILE;
	uint     offset = 0;
	uint     edges_offset = 0;

	if (ti->tileh != SLOPE_FLAT || HasBit(_water_feature[CF_RIVER_SLOPE].flags, CFF_HAS_FLAT_SPRITE)) {
		image = GetCanalSprite(CF_RIVER_SLOPE, ti->tile);
		if (image == 0) {
			switch (ti->tileh) {
				case SLOPE_NW: image = SPR_WATER_SLOPE_Y_DOWN; break;
				case SLOPE_SW: image = SPR_WATER_SLOPE_X_UP;   break;
				case SLOPE_SE: image = SPR_WATER_SLOPE_Y_UP;   break;
				case SLOPE_NE: image = SPR_WATER_SLOPE_X_DOWN; break;
				default:       image = SPR_FLAT_WATER_TILE;    break;
			}
		} else {
			/* Flag bit 0 indicates that the first sprite is flat water. */
			offset = HasBit(_water_feature[CF_RIVER_SLOPE].flags, CFF_HAS_FLAT_SPRITE) ? 1 : 0;

			switch (ti->tileh) {
				case SLOPE_SE:              edges_offset += 12; break;
				case SLOPE_NE: offset += 1; edges_offset += 24; break;
				case SLOPE_SW: offset += 2; edges_offset += 36; break;
				case SLOPE_NW: offset += 3; edges_offset += 48; break;
				case SLOPE_E:  offset  = 0; edges_offset = 0; break;
				case SLOPE_S:  offset  = 0; edges_offset = 0; break;
				case SLOPE_W:  offset  = 0; edges_offset = 0; break;
				case SLOPE_N:  offset  = 0; edges_offset = 0; break;
				default:       offset  = 0; break;
			}

			offset = GetCanalSpriteOffset(CF_RIVER_SLOPE, ti->tile, offset);
		}
	}

	DrawGroundSprite(image + offset, PAL_NONE);

	/* Draw river edges if available. */
	DrawWaterEdges(false, edges_offset, ti->tile);
}