diff --git a/src/smallmap_gui.h b/src/smallmap_gui.h
--- a/src/smallmap_gui.h
+++ b/src/smallmap_gui.h
@@ -12,9 +12,150 @@
 #ifndef SMALLMAP_GUI_H
 #define SMALLMAP_GUI_H
 
+#include "industry_type.h"
+#include "window_gui.h"
+#include "strings_func.h"
+#include "blitter/factory.hpp"
+#include "widgets/smallmap_widget.h"
+
 void BuildIndustriesLegend();
 void ShowSmallMap();
 void BuildLandLegend();
 void BuildOwnerLegend();
 
+/** Structure for holding relevant data for legends in small map */
+struct LegendAndColour {
+	uint8 colour;              ///< Colour of the item on the map.
+	StringID legend;           ///< String corresponding to the coloured item.
+	IndustryType type;         ///< Type of industry. Only valid for industry entries.
+	uint8 height;              ///< Height in tiles. Only valid for height legend entries.
+	CompanyID company;         ///< Company to display. Only valid for company entries of the owner legend.
+	bool show_on_map;          ///< For filtering industries, if \c true, industry is shown on the map in colour.
+	bool end;                  ///< This is the end of the list.
+	bool col_break;            ///< Perform a column break and go further at the next column.
+};
+
+/** Class managing the smallmap window. */
+class SmallMapWindow : public Window {
+protected:
+	/** Types of legends in the #WID_SM_LEGEND widget. */
+	enum SmallMapType {
+		SMT_CONTOUR,
+		SMT_VEHICLES,
+		SMT_INDUSTRY,
+		SMT_ROUTES,
+		SMT_VEGETATION,
+		SMT_OWNER,
+	};
+
+	/** Available kinds of zoomlevel changes. */
+	enum ZoomLevelChange {
+		ZLC_INITIALIZE, ///< Initialize zoom level.
+		ZLC_ZOOM_OUT,   ///< Zoom out.
+		ZLC_ZOOM_IN,    ///< Zoom in.
+	};
+
+	static SmallMapType map_type; ///< Currently displayed legends.
+	static bool show_towns;       ///< Display town names in the smallmap.
+
+	static const uint LEGEND_BLOB_WIDTH = 8;              ///< Width of the coloured blob in front of a line text in the #WID_SM_LEGEND widget.
+	static const uint INDUSTRY_MIN_NUMBER_OF_COLUMNS = 2; ///< Minimal number of columns in the #WID_SM_LEGEND widget for the #SMT_INDUSTRY legend.
+	static const uint FORCE_REFRESH_PERIOD = 0x1F; ///< map is redrawn after that many ticks
+	static const uint BLINK_PERIOD         = 0x0F; ///< highlight blinking interval
+
+	uint min_number_of_fixed_rows; ///< Minimal number of rows in the legends for the fixed layouts only (all except #SMT_INDUSTRY).
+	uint column_width;             ///< Width of a column in the #WID_SM_LEGEND widget.
+
+	int32 scroll_x;  ///< Horizontal world coordinate of the base tile left of the top-left corner of the smallmap display.
+	int32 scroll_y;  ///< Vertical world coordinate of the base tile left of the top-left corner of the smallmap display.
+	int32 subscroll; ///< Number of pixels (0..3) between the right end of the base tile and the pixel at the top-left corner of the smallmap display.
+	int zoom;        ///< Zoom level. Bigger number means more zoom-out (further away).
+
+	uint8 refresh;   ///< Refresh counter, zeroed every FORCE_REFRESH_PERIOD ticks.
+
+	Point SmallmapRemapCoords(int x, int y) const;
+
+	/**
+	 * Draws vertical part of map indicator
+	 * @param x X coord of left/right border of main viewport
+	 * @param y Y coord of top border of main viewport
+	 * @param y2 Y coord of bottom border of main viewport
+	 */
+	static inline void DrawVertMapIndicator(int x, int y, int y2)
+	{
+		GfxFillRect(x, y,      x, y + 3, PC_VERY_LIGHT_YELLOW);
+		GfxFillRect(x, y2 - 3, x, y2,    PC_VERY_LIGHT_YELLOW);
+	}
+
+	/**
+	 * Draws horizontal part of map indicator
+	 * @param x X coord of left border of main viewport
+	 * @param x2 X coord of right border of main viewport
+	 * @param y Y coord of top/bottom border of main viewport
+	 */
+	static inline void DrawHorizMapIndicator(int x, int x2, int y)
+	{
+		GfxFillRect(x,      y, x + 3, y, PC_VERY_LIGHT_YELLOW);
+		GfxFillRect(x2 - 3, y, x2,    y, PC_VERY_LIGHT_YELLOW);
+	}
+
+	void DrawMapIndicators() const;
+	void DrawSmallMapColumn(void *dst, uint xc, uint yc, int pitch, int reps, int start_pos, int end_pos, Blitter *blitter) const;
+	void DrawVehicles(const DrawPixelInfo *dpi, Blitter *blitter) const;
+	void DrawTowns(const DrawPixelInfo *dpi) const;
+	void DrawSmallMap(DrawPixelInfo *dpi) const;
+
+	Point RemapTile(int tile_x, int tile_y) const;
+	Point PixelToTile(int px, int py, int *sub, bool add_sub = true) const;
+	Point ComputeScroll(int tx, int ty, int x, int y, int *sub);
+	void SetZoomLevel(ZoomLevelChange change, const Point *zoom_pt);
+	void SetupWidgetData();
+	uint32 GetTileColours(const TileArea &ta) const;
+
+	int GetPositionOnLegend(Point pt);
+
+
+public:
+
+	uint min_number_of_columns;    ///< Minimal number of columns in legends.
+
+	SmallMapWindow(const WindowDesc *desc, int window_number);
+
+	/**
+	 * Compute minimal required width of the legends.
+	 * @return Minimally needed width for displaying the smallmap legends in pixels.
+	 */
+	inline uint GetMinLegendWidth() const
+	{
+		return WD_FRAMERECT_LEFT + this->min_number_of_columns * this->column_width;
+	}
+
+	/**
+	 * Return number of columns that can be displayed in \a width pixels.
+	 * @return Number of columns to display.
+	 */
+	inline uint GetNumberColumnsLegend(uint width) const
+	{
+		return width / this->column_width;
+	}
+
+	uint GetLegendHeight(uint num_columns) const;
+
+	void SwitchMapType(SmallMapType map_type);
+	void SetNewScroll(int sx, int sy, int sub);
+	void SmallMapCenterOnCurrentPos();
+
+	virtual void SetStringParameters(int widget) const;
+	virtual void OnInit();
+	virtual void OnPaint();
+	virtual void DrawWidget(const Rect &r, int widget) const;
+	virtual void OnClick(Point pt, int widget, int click_count);
+	virtual void OnInvalidateData(int data = 0, bool gui_scope = true);
+	virtual bool OnRightClick(Point pt, int widget);
+	virtual void OnMouseWheel(int wheel);
+	virtual void OnTick();
+	virtual void OnScroll(Point delta);
+	virtual void OnMouseOver(Point pt, int widget);
+};
+
 #endif /* SMALLMAP_GUI_H */