2017 Day 13 Complete!
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Advent of Code
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--- Day 13: Packet Scanners ---
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You need to cross a vast firewall. The firewall consists of several layers, each with a security
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scanner that moves back and forth across the layer. To succeed, you must not be detected by a
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scanner.
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By studying the firewall briefly, you are able to record (in your puzzle input) the depth of each
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layer and the range of the scanning area for the scanner within it, written as depth: range. Each
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layer has a thickness of exactly 1. A layer at depth 0 begins immediately inside the firewall; a
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layer at depth 1 would start immediately after that.
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For example, suppose you've recorded the following:
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0: 3
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1: 2
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4: 4
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6: 4
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This means that there is a layer immediately inside the firewall (with range 3), a second layer
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immediately after that (with range 2), a third layer which begins at depth 4 (with range 4), and a
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fourth layer which begins at depth 6 (also with range 4). Visually, it might look like this:
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[ ] [ ] [ ]
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[ ] [ ]
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Within each layer, a security scanner moves back and forth within its range. Each security scanner
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starts at the top and moves down until it reaches the bottom, then moves up until it reaches the
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top, and repeats. A security scanner takes one picosecond to move one step. Drawing scanners as S,
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the first few picoseconds look like this:
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Picosecond 0:
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0 1 2 3 4 5 6
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[S] [S] ... ... [S] ... [S]
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[ ] [ ] [ ] [ ]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 1:
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 2:
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0 1 2 3 4 5 6
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[ ] [S] ... ... [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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Picosecond 3:
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] ... [ ]
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[S] [S] [ ] [ ]
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[ ] [ ] [ ]
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[S] [S]
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Your plan is to hitch a ride on a packet about to move through the firewall. The packet will travel
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along the top of each layer, and it moves at one layer per picosecond. Each picosecond, the packet
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moves one layer forward (its first move takes it into layer 0), and then the scanners move one step.
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If there is a scanner at the top of the layer as your packet enters it, you are caught. (If a
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scanner moves into the top of its layer while you are there, you are not caught: it doesn't have
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time to notice you before you leave.) If you were to do this in the configuration above, marking
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your current position with parentheses, your passage through the firewall would look like this:
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Initial state:
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0 1 2 3 4 5 6
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[S] [S] ... ... [S] ... [S]
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[ ] [ ] [ ] [ ]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 0:
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0 1 2 3 4 5 6
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(S) [S] ... ... [S] ... [S]
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[ ] [ ] [ ] [ ]
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[ ] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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( ) [ ] ... ... [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 1:
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0 1 2 3 4 5 6
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[ ] ( ) ... ... [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] (S) ... ... [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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Picosecond 2:
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0 1 2 3 4 5 6
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[ ] [S] (.) ... [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [ ] (.) ... [ ] ... [ ]
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[S] [S] [ ] [ ]
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[ ] [ ] [ ]
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[S] [S]
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Picosecond 3:
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0 1 2 3 4 5 6
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[ ] [ ] ... (.) [ ] ... [ ]
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[S] [S] [ ] [ ]
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[ ] [ ] [ ]
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[S] [S]
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0 1 2 3 4 5 6
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[S] [S] ... (.) [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[ ] [S] [S]
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[ ] [ ]
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Picosecond 4:
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0 1 2 3 4 5 6
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[S] [S] ... ... ( ) ... [ ]
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[ ] [ ] [ ] [ ]
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[ ] [S] [S]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [ ] ... ... ( ) ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 5:
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] (.) [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [S] ... ... [S] (.) [S]
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[ ] [ ] [ ] [ ]
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[S] [ ] [ ]
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[ ] [ ]
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Picosecond 6:
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0 1 2 3 4 5 6
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[ ] [S] ... ... [S] ... (S)
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[ ] [ ] [ ] [ ]
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[S] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] ... ( )
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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In this situation, you are caught in layers 0 and 6, because your packet entered the layer when its
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scanner was at the top when you entered it. You are not caught in layer 1, since the scanner moved
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into the top of the layer once you were already there.
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The severity of getting caught on a layer is equal to its depth multiplied by its range. (Ignore
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layers in which you do not get caught.) The severity of the whole trip is the sum of these values.
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In the example above, the trip severity is 0*3 + 6*4 = 24.
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Given the details of the firewall you've recorded, if you leave immediately, what is the severity of
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your whole trip?
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Your puzzle answer was _______.
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The first half of this puzzle is complete! It provides one gold star: *
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--- Part Two ---
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Now, you need to pass through the firewall without being caught - easier said than done.
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You can't control the speed of the packet, but you can delay it any number of picoseconds. For each
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picosecond you delay the packet before beginning your trip, all security scanners move one step.
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You're not in the firewall during this time; you don't enter layer 0 until you stop delaying the
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packet.
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In the example above, if you delay 10 picoseconds (picoseconds 0 - 9), you won't get caught:
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State after delaying:
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0 1 2 3 4 5 6
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[ ] [S] ... ... [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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Picosecond 10:
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0 1 2 3 4 5 6
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( ) [S] ... ... [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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0 1 2 3 4 5 6
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( ) [ ] ... ... [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 11:
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0 1 2 3 4 5 6
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[ ] ( ) ... ... [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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[S] (S) ... ... [S] ... [S]
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[ ] [ ] [ ] [ ]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 12:
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0 1 2 3 4 5 6
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[S] [S] (.) ... [S] ... [S]
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[ ] [ ] [ ] [ ]
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[ ] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [ ] (.) ... [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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Picosecond 13:
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0 1 2 3 4 5 6
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[ ] [ ] ... (.) [ ] ... [ ]
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [S] ... (.) [ ] ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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Picosecond 14:
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0 1 2 3 4 5 6
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[ ] [S] ... ... ( ) ... [ ]
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[ ] [ ] [ ] [ ]
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[S] [S] [S]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [ ] ... ... ( ) ... [ ]
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[S] [S] [ ] [ ]
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[ ] [ ] [ ]
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[S] [S]
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Picosecond 15:
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] (.) [ ]
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[S] [S] [ ] [ ]
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[ ] [ ] [ ]
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[S] [S]
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0 1 2 3 4 5 6
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[S] [S] ... ... [ ] (.) [ ]
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[ ] [ ] [ ] [ ]
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[ ] [S] [S]
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[ ] [ ]
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Picosecond 16:
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0 1 2 3 4 5 6
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[S] [S] ... ... [ ] ... ( )
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[ ] [ ] [ ] [ ]
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[ ] [S] [S]
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[ ] [ ]
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0 1 2 3 4 5 6
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[ ] [ ] ... ... [ ] ... ( )
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[S] [S] [S] [S]
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[ ] [ ] [ ]
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[ ] [ ]
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Because all smaller delays would get you caught, the fewest number of picoseconds you would need to
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delay to get through safely is 10.
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What is the fewest number of picoseconds that you need to delay the packet to pass through the
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firewall without being caught?
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Although it hasn't changed, you can still get your puzzle input.
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Answer: _____________________ [ [Submit] ]
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2017/about
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. http://adventofcode.com/2017/support
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. http://adventofcode.com/2017/events
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. http://adventofcode.com/2017/settings
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. http://adventofcode.com/2017/auth/logout
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017/leaderboard
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. http://adventofcode.com/2017/stats
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. http://adventofcode.com/2017/sponsors
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. http://adventofcode.com/2017/sponsors
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. http://adventofcode.com/2017/day/13/input
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