From James Harvey (via a period of collaborative polishing), a patch

to add two kinds of Penrose tiling to the grid types supported by
Loopy.

This has involved a certain amount of infrastructure work, because of
course the whole point of Penrose tilings is that they don't have to
be the same every time: so now grid.c has grown the capacity to
describe its grids as strings, and reconstitute them from those string
descriptions. Hence a Penrose Loopy game description consists of a
string identifying a particular piece of Penrose tiling, followed by
the normal Loopy clue encoding.

All the existing grid types decline to provide a grid description
string, so their Loopy game descriptions have not changed encoding.

[originally from svn r9159]

penrose.h

@@ -0,0 +1,59 @@
+/* penrose.h
+ *
+ * Penrose tiling functions.
+ *
+ * Provides an interface with which to generate Penrose tilings
+ * by recursive subdivision of an initial tile of choice (one of the
+ * four sets of two pairs kite/dart, or thin/thick rhombus).
+ *
+ * You supply a callback function and a context pointer, which is
+ * called with each tile in turn: you choose how many times to recurse.
+ */
+
+#ifndef _PENROSE_H
+#define _PENROSE_H
+
+#ifndef PHI
+#define PHI 1.6180339887
+#endif
+
+typedef struct vector vector;
+
+double v_x(vector *vs, int i);
+double v_y(vector *vs, int i);
+
+typedef struct penrose_state penrose_state;
+
+/* Return non-zero to clip the tree here (i.e. not recurse
+ * below this tile).
+ *
+ * Parameters are state, vector array, npoints, depth.
+ * ctx is inside state.
+ */
+typedef int (*tile_callback)(penrose_state *, vector *, int, int);
+
+struct penrose_state {
+    int start_size;  /* initial side length */
+    int max_depth;      /* Recursion depth */
+
+    tile_callback new_tile;
+    void *ctx;          /* for callback */
+};
+
+enum { PENROSE_P2, PENROSE_P3 };
+
+extern int penrose(penrose_state *state, int which);
+
+/* Returns the side-length of a penrose tile at recursion level
+ * gen, given a starting side length. */
+extern double penrose_side_length(double start_size, int depth);
+
+/* Returns the count of each type of tile at a given recursion depth. */
+extern void penrose_count_tiles(int gen, int *nlarge, int *nsmall);
+
+/* Calculate start size and recursion depth required to produce a
+ * width-by-height sized patch of penrose tiles with the given tilesize */
+extern void penrose_calculate_size(int which, int tilesize, int w, int h,
+                                   double *required_radius, int *start_size, int *depth);
+
+#endif