Improve wxDataViewCtrl performance with wxDV_VARIABLE_LINE_HEIGHT

Store the line heights in a cache to make the (generic) wxDataViewCtrl
usable with this style.

include/wx/generic/private/rowheightcache.h

@@ -0,0 +1,142 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/rowheightcache.h
+// Purpose:     height cache of rows in a dataview
+// Author:      Jens Goepfert (mail@jensgoepfert.de)
+// Created:     2018-03-06
+// Copyright:   (c) wxWidgets team
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_ROWHEIGHTCACHE_H_
+#define _WX_PRIVATE_ROWHEIGHTCACHE_H_
+
+#include "wx/hashmap.h"
+#include "wx/vector.h"
+
+// struct describing a range of rows which contains rows <from> .. <to-1>
+struct RowRange
+{
+    unsigned int from;
+    unsigned int to;
+};
+
+/**
+@class RowRanges
+A helper class that manages a set of RowRange objects.
+It stores the indices that are members of a group in a memory
+efficiant way.
+*/
+class WXDLLIMPEXP_CORE RowRanges
+//class RowRanges
+{
+public:
+    /**
+    Adds a row index to this group by adding it to an existing RowRange
+    or by creating a new one.
+    */
+    void Add(unsigned int row);
+
+    /**
+    Removes a row index and all indices after idx from this group.
+    */
+    void Remove(unsigned int row);
+
+    /**
+    Checks whether a row index is contained in this group.
+    */
+    bool Has(unsigned int row) const;
+
+    /**
+    Returns the number of row indices that are contained in this group.
+    */
+    unsigned int CountAll() const;
+
+    /**
+    Returns the number of rows that are in this group before the given row index.
+    not including given row.
+    */
+    unsigned int CountTo(unsigned int row) const;
+    unsigned int GetSize() const; // for debugging statistics
+
+private:
+    wxVector<RowRange> m_ranges;
+    /**
+    If a new row index was inserted Cleanup checks if the neighbour ranges
+    of idx can includes the same row indices and discards
+    unnecessary RowRange objects.
+    */
+    void CleanUp(unsigned int idx);
+};
+
+WX_DECLARE_HASH_MAP(unsigned int, RowRanges*, wxIntegerHash, wxIntegerEqual,
+    HeightToRowRangesMap);
+
+/**
+@class HeightCache
+
+HeightCache implements a cache mechanism for the DataViewCtrl to give
+fast access to:
+* the height of one line (GetLineHeight)
+* the y-coordinate where a row starts (GetLineStart)
+* and vice versa (GetLineAt)
+
+The layout of the cache is a hashmap where the keys are all exisiting
+row heights in pixels. The values are RowRange objects that represents
+all having the specified height.
+
+{
+22: RowRange([0..10], [15..17], [20..2000]),
+42: RowRange([11..12], [18..18]),
+62: RowRange([13..14], [19..19])
+}
+
+Examples
+========
+
+GetLineStart
+------------
+To retrieve the y-coordinate of item 1000 it is neccessary to look into
+each key of the hashmap *m_heightToRowRange*. Get the row count of
+indices lower than 1000 (RowRange::CountTo) and multiplies it wich the
+according height.
+
+RowRange([0..10], [15..17], [20..2000]).CountTo(1000)
+--> 0..10 are 11 items, 15..17 are 3 items and 20..1000 are 980 items (1000-20)
+= 11 + 3 + 980 = 994 items
+
+GetLineStart(1000) --> (22 * 994) + (42 * 3) + (62 * 3) = 22180
+
+GetLineHeight
+-------------
+To retrieve the line height look into each key and check if row is
+contained in RowRange (RowRange::Has)
+
+GetLineAt
+---------
+To retrieve the row that starts at a specific y-coordinate.
+Look into each key and count all rows.
+Use bisect algorithm in combination with GetLineStart() to
+find the appropriate item
+*/
+class WXDLLIMPEXP_ADV HeightCache
+{
+public:
+    bool GetLineStart(unsigned int row, int &start);
+    bool GetLineHeight(unsigned int row, int &height);
+    bool GetLineAt(int y, unsigned int &row);
+
+    void Put(const unsigned int row, const int height);
+    /**
+    removes the stored height of the given row from the cache
+    and invalidates all cached rows (including row)
+    */
+    void Remove(const unsigned int row);
+    void Clear();
+
+private:
+    bool GetLineInfo(unsigned int row, int &start, int &height);
+    HeightToRowRangesMap  m_heightToRowRange;
+};
+
+
+#endif // _WX_PRIVATE_ROWHEIGHTCACHE_H_