--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/ch6ex2_4.py	Sun Jan 13 16:24:00 2013 -0600
@@ -0,0 +1,73 @@
+"""Chapter 6, exercise 2 in Allen Downey's Think Complexity book.
+
+4. Implement a Rule 30 CA on a ring with a few hundred cells, run it for as many
+   time steps as you can in a reasonable amount of time, and output the center
+   column as a sequence of bits. Test it using DieHarder.
+
+We'll take a slightly different tact. We will create a program that will
+continually output the center column as a bit stream. Once we reach the end of
+the time steps, we'll copy the last row to row 0 and start again. This way we
+can feed the data into dieharder over a pipe without it exhausting our stream.
+
+"""
+import sys
+
+from CircularCA import CircularCA
+
+
+def main(script, bytes_wanted):
+
+    bytes_wanted = int(bytes_wanted)
+
+    # Create a circular CA that is 257 columns; it is odd so that we have a true
+    # "center" column. Our CA will have 8192 steps. This will give us 8192
+    # / 8 == 1024 bytes of pseudo-random (we hope) data using rule 30. Once
+    # we've reached the end of the time steps, we'll copy the last row to the
+    # first and start over.
+
+    rows = 8193
+    cols = 257
+    mid = cols // 2
+    ca = CircularCA(30, rows, cols)
+    ca.start_single()
+
+    iters = 0
+    bytes_produced = 0
+    while True:
+        ca.loop(rows - 1)
+
+        iters += 1
+        if iters == 1:
+            # don't use the first iteration as the first few rows will have 0's
+            # until it fills up
+            ca.wrap()
+            continue
+
+        a = ca.get_array(mid, mid + 1)
+
+        # output the bit stream a byte at a time
+        x = 0
+        i = 0
+        for n, item in enumerate(a):
+            # don't use the first bit because it is a repeat from the previous
+            # iteration
+            if n == 0:
+                continue
+
+            x <<= 1
+            x |= item[0]
+            i += 1
+            if i == 8:
+                sys.stdout.write(chr(x))
+                x = 0
+                i = 0
+                bytes_produced += 1
+                if bytes_produced >= bytes_wanted:
+                    return  # we're done
+
+        # now start the CA over:
+        ca.wrap()
+
+
+if __name__ == '__main__':
+    main(*sys.argv)