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+# Internal representation of the cube
+
+The plan (TODO) is to have multiple implementations: some that
+take advantage of advanced CPU instructions (SIMD) and a fallback
+"array" representation that works on any architecture.
+
+# Array representation (fallback)
+
+In this implementation of the cube.h interface, the cube is represented
+by two arrays of 8-bit unsigned integers, one for centers and one for
+corners.  The 4 leas-significant digits of each bit determine the piece,
+the other 4 are used for orientation or kept to 0.
+
+Edges:
+        xxxopppp (x = unused, o = orientation, p = piece)
+
+Corners:
+        xooxpppp (x = unused, o = orientation, p = piece)
+
+The two bits for CO are shifted to make it possible to perform mod 3
+operations (sum, inverse) using only addition and bitwise operators.
+See below for details.
+
+The third bit is needed because x+y+1 can exceed 4.