(svn r11559) -Fix [FS#1505]: overflow when drawing graphics with high company values.

3 files changed, 53 insertions, 7 deletions

diff --git a/src/core/bitmath_func.cpp b/src/core/bitmath_func.cpp
index 9f36ed9a8..53139308a 100644
--- a/src/core/bitmath_func.cpp
+++ b/src/core/bitmath_func.cpp
@@ -25,7 +25,7 @@ const uint8 _ffb_64[64] = {
  * way further. If no bit is set return 0.
  *
  * @param x The value to search
- * @param The position of the first bit set
+ * @return The position of the first bit set
  */
 uint8 FindFirstBit(uint32 x)
 {
@@ -43,3 +43,29 @@ uint8 FindFirstBit(uint32 x)
 
 	return pos;
 }
+
+/**
+ * Search the last set bit in a 32 bit variable.
+ *
+ * This algorithm is a static implementation of a log
+ * conguence search algorithm. It checks the first half
+ * if there is a bit set search there further. And this
+ * way further. If no bit is set return 0.
+ *
+ * @param x The value to search
+ * @return The position of the last bit set
+ */
+uint8 FindLastBit(uint32 x)
+{
+	if (x == 0) return 0;
+
+	uint8 pos = 0;
+
+	if ((x & 0xffff0000) != 0) { x >>= 16; pos += 16; }
+	if ((x & 0x0000ff00) != 0) { x >>= 8;  pos += 8;  }
+	if ((x & 0x000000f0) != 0) { x >>= 4;  pos += 4;  }
+	if ((x & 0x0000000c) != 0) { x >>= 2;  pos += 2;  }
+	if ((x & 0x00000002) != 0) { pos += 1; }
+
+	return pos;
+}
diff --git a/src/core/bitmath_func.hpp b/src/core/bitmath_func.hpp
index 1877f003e..f4c6ff0bc 100644
--- a/src/core/bitmath_func.hpp
+++ b/src/core/bitmath_func.hpp
@@ -211,6 +211,7 @@ static inline uint8 FindFirstBit2x64(const int value)
 }
 
 uint8 FindFirstBit(uint32 x);
+uint8 FindLastBit(uint32 x);
 
 /**
  * Clear the first bit in an integer.
diff --git a/src/graph_gui.cpp b/src/graph_gui.cpp
index 7c2543011..ddabf882d 100644
--- a/src/graph_gui.cpp
+++ b/src/graph_gui.cpp
@@ -65,7 +65,7 @@ struct GraphDrawer {
 	uint height;    ///< The height of the graph in pixels.
 	StringID format_str_y_axis;
 	byte colors[GRAPH_MAX_DATASETS];
-	Money cost[GRAPH_MAX_DATASETS][24]; ///< last 2 years
+	OverflowSafeInt64 cost[GRAPH_MAX_DATASETS][24]; ///< last 2 years
 };
 
 static void DrawGraph(const GraphDrawer *gw)
@@ -133,7 +133,7 @@ static void DrawGraph(const GraphDrawer *gw)
 	for (int i = 0; i < gw->num_dataset; i++) {
 		if (!HasBit(gw->excluded_data, i)) {
 			for (int j = 0; j < gw->num_on_x_axis; j++) {
-				Money datapoint = gw->cost[i][j];
+				OverflowSafeInt64 datapoint = gw->cost[i][j];
 
 				if (datapoint != INVALID_DATAPOINT) {
 					/* For now, if the graph has negative values the scaling is
@@ -215,12 +215,31 @@ static void DrawGraph(const GraphDrawer *gw)
 			uint prev_y = INVALID_DATAPOINT_POS;
 
 			for (int j = 0; j < gw->num_on_x_axis; j++) {
-				Money datapoint = gw->cost[i][j];
+				OverflowSafeInt64 datapoint = gw->cost[i][j];
 
 				if (datapoint != INVALID_DATAPOINT) {
-					/* XXX: This can overflow if x_axis_offset * datapoint is
-					 * too big to fit in an int64. */
-					y = gw->top + x_axis_offset - (x_axis_offset * datapoint) / highest_value;
+					/*
+					 * Check whether we need to reduce the 'accuracy' of the
+					 * datapoint value and the highest value to splut overflows.
+					 * And when 'drawing' 'one million' or 'one million and one'
+					 * there is no significant difference, so the least
+					 * significant bits can just be removed.
+					 *
+					 * If there are more bits needed than would fit in a 32 bits
+					 * integer, so at about 31 bits because of the sign bit, the
+					 * least significant bits are removed.
+					 */
+					int mult_range = FindLastBit(x_axis_offset) + FindLastBit(abs(datapoint));
+					int reduce_range = max(mult_range - 31, 0);
+
+					/* Handle negative values differently (don't shift sign) */
+					if (datapoint < 0) {
+						datapoint = -(abs(datapoint) >> reduce_range);
+					} else {
+						datapoint >>= reduce_range;
+					}
+
+					y = gw->top + x_axis_offset - (x_axis_offset * datapoint) / (highest_value >> reduce_range);
 
 					/* Draw the point. */
 					GfxFillRect(x - 1, y - 1, x + 1, y + 1, color);