2019 day 7, part 1

2019/day07/README.org

@@ -93,10 +93,73 @@ Here are some example programs:
 Try every combination of phase settings on the amplifiers. /What is the
 highest signal that can be sent to the thrusters?/
 
-To begin, [[file:7/input][get your puzzle input]].
+Your puzzle answer was =65464=.
 
-Answer:
+The first half of this puzzle is complete! It provides one gold star: *
 
-You can also [Shareon
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-[[javascript:void(0);][Mastodon]]] this puzzle.
+** --- Part Two ---
+It's no good - in this configuration, the amplifiers can't generate a
+large enough output signal to produce the thrust you'll need. The Elves
+quickly talk you through rewiring the amplifiers into a /feedback loop/:
+
+#+BEGIN_EXAMPLE
+        O-------O  O-------O  O-------O  O-------O  O-------O
+  0 -+->| Amp A |->| Amp B |->| Amp C |->| Amp D |->| Amp E |-.
+     |  O-------O  O-------O  O-------O  O-------O  O-------O |
+     |                                                        |
+     '--------------------------------------------------------+
+                                                              |
+                                                              v
+                                                       (to thrusters)
+#+END_EXAMPLE
+
+Most of the amplifiers are connected as they were before; amplifier
+=A='s output is connected to amplifier =B='s input, and so on.
+/However,/ the output from amplifier =E= is now connected into amplifier
+=A='s input. This creates the feedback loop: the signal will be sent
+through the amplifiers /many times/.
+
+In feedback loop mode, the amplifiers need /totally different phase
+settings/: integers from =5= to =9=, again each used exactly once. These
+settings will cause the Amplifier Controller Software to repeatedly take
+input and produce output many times before halting. Provide each
+amplifier its phase setting at its first input instruction; all further
+input/output instructions are for signals.
+
+Don't restart the Amplifier Controller Software on any amplifier during
+this process. Each one should continue receiving and sending signals
+until it halts.
+
+All signals sent or received in this process will be between pairs of
+amplifiers except the very first signal and the very last signal. To
+start the process, a =0= signal is sent to amplifier =A='s input
+/exactly once/.
+
+Eventually, the software on the amplifiers will halt after they have
+processed the final loop. When this happens, the last output signal from
+amplifier =E= is sent to the thrusters. Your job is to /find the largest
+output signal that can be sent to the thrusters/ using the new phase
+settings and feedback loop arrangement.
+
+Here are some example programs:
+
+- Max thruster signal /=139629729=/ (from phase setting sequence
+  =9,8,7,6,5=):
+
+  #+BEGIN_EXAMPLE
+    3,26,1001,26,-4,26,3,27,1002,27,2,27,1,27,26,
+    27,4,27,1001,28,-1,28,1005,28,6,99,0,0,5
+  #+END_EXAMPLE
+
+- Max thruster signal /=18216=/ (from phase setting sequence
+  =9,7,8,5,6=):
+
+  #+BEGIN_EXAMPLE
+    3,52,1001,52,-5,52,3,53,1,52,56,54,1007,54,5,55,1005,55,26,1001,54,
+    -5,54,1105,1,12,1,53,54,53,1008,54,0,55,1001,55,1,55,2,53,55,53,4,
+    53,1001,56,-1,56,1005,56,6,99,0,0,0,0,10
+  #+END_EXAMPLE
+
+Try every combination of the new phase settings on the amplifier
+feedback loop. /What is the highest signal that can be sent to the
+thrusters?/