330 lines
15 KiB
Plaintext
330 lines
15 KiB
Plaintext
Advent of Code
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--- Day 15: Beverage Bandits ---
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Having perfected their hot chocolate, the Elves have a new problem: the
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Goblins that live in these caves will do anything to steal it. Looks like
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they're here for a fight.
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You scan the area, generating a map of the walls (#), open cavern (.), and
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starting position of every Goblin (G) and Elf (E) (your puzzle input).
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Combat proceeds in rounds; in each round, each unit that is still alive
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takes a turn, resolving all of its actions before the next unit's turn
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begins. On each unit's turn, it tries to move into range of an enemy (if it
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isn't already) and then attack (if it is in range).
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All units are very disciplined and always follow very strict combat rules.
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Units never move or attack diagonally, as doing so would be dishonorable.
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When multiple choices are equally valid, ties are broken in reading order:
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top-to-bottom, then left-to-right. For instance, the order in which units
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take their turns within a round is the reading order of their starting
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positions in that round, regardless of the type of unit or whether other
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units have moved after the round started. For example:
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would take their These units: turns in this order: #######
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####### #.G.E.# #.1.2.# #E.G.E# #3.4.5#
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#.G.E.# #.6.7.# ####### #######
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Each unit begins its turn by identifying all possible targets (enemy units).
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If no targets remain, combat ends.
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Then, the unit identifies all of the open squares (.) that are in range of
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each target; these are the squares which are adjacent (immediately up, down,
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left, or right) to any target and which aren't already occupied by a wall or
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another unit. Alternatively, the unit might already be in range of a target.
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If the unit is not already in range of a target, and there are no open
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squares which are in range of a target, the unit ends its turn.
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If the unit is already in range of a target, it does not move, but continues
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its turn with an attack. Otherwise, since it is not in range of a target, it
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moves.
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To move, the unit first considers the squares that are in range and
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determines which of those squares it could reach in the fewest steps. A step
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is a single movement to any adjacent (immediately up, down, left, or right)
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open (.) square. Units cannot move into walls or other units. The unit does
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this while considering the current positions of units and does not do any
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prediction about where units will be later. If the unit cannot reach (find
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an open path to) any of the squares that are in range, it ends its turn. If
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multiple squares are in range and tied for being reachable in the fewest
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steps, the square which is first in reading order is chosen. For example:
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Targets: In range: Reachable: Nearest: Chosen: #######
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####### ####### ####### ####### #E..G.# #E.?G?#
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#E.@G.# #E.!G.# #E.+G.# #...#.# --> #.?.#?# --> #.@.#.# -->
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#.!.#.# --> #...#.# #.G.#G# #?G?#G# #@G@#G# #!G.#G#
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#.G.#G# ####### ####### ####### ####### #######
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In the above scenario, the Elf has three targets (the three Goblins):
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• Each of the Goblins has open, adjacent squares which are in range
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(marked with a ? on the map). • Of those squares, four are reachable
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(marked @); the other two (on the right) would require moving through a
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wall or unit to reach. • Three of these reachable squares are nearest,
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requiring the fewest steps (only 2) to reach (marked !). • Of those, the
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square which is first in reading order is chosen (+).
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The unit then takes a single step toward the chosen square along the
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shortest path to that square. If multiple steps would put the unit equally
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closer to its destination, the unit chooses the step which is first in
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reading order. (This requires knowing when there is more than one shortest
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path so that you can consider the first step of each such path.) For
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example:
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In range: Nearest: Chosen: Distance: Step: #######
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####### ####### ####### ####### #.E...# #.E...#
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#.E...# #4E212# #..E..# #...?.# --> #...!.# --> #...+.# -->
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#32101# --> #.....# #..?G?# #..!G.# #...G.# #432G2#
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#...G.# ####### ####### ####### ####### #######
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The Elf sees three squares in range of a target (?), two of which are
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nearest (!), and so the first in reading order is chosen (+). Under
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"Distance", each open square is marked with its distance from the
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destination square; the two squares to which the Elf could move on this turn
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(down and to the right) are both equally good moves and would leave the Elf
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2 steps from being in range of the Goblin. Because the step which is first
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in reading order is chosen, the Elf moves right one square.
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Here's a larger example of movement:
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Initially: ######### #G..G..G# #.......# #.......# #G..E..G# #.......#
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#.......# #G..G..G# #########
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After 1 round: ######### #.G...G.# #...G...# #...E..G# #.G.....# #.......#
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#G..G..G# #.......# #########
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After 2 rounds: ######### #..G.G..# #...G...# #.G.E.G.# #.......# #G..G..G#
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#.......# #.......# #########
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After 3 rounds: ######### #.......# #..GGG..# #..GEG..# #G..G...# #......G#
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#.......# #.......# #########
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Once the Goblins and Elf reach the positions above, they all are either in
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range of a target or cannot find any square in range of a target, and so
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none of the units can move until a unit dies.
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After moving (or if the unit began its turn in range of a target), the unit
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attacks.
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To attack, the unit first determines all of the targets that are in range of
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it by being immediately adjacent to it. If there are no such targets, the
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unit ends its turn. Otherwise, the adjacent target with the fewest hit
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points is selected; in a tie, the adjacent target with the fewest hit points
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which is first in reading order is selected.
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The unit deals damage equal to its attack power to the selected target,
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reducing its hit points by that amount. If this reduces its hit points to 0
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or fewer, the selected target dies: its square becomes . and it takes no
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further turns.
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Each unit, either Goblin or Elf, has 3 attack power and starts with 200 hit
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points.
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For example, suppose the only Elf is about to attack:
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HP: HP: G.... 9 G.... 9 ..G.. 4 ..G.. 4
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..EG. 2 --> ..E.. ..G.. 2 ..G.. 2 ...G. 1 ...G. 1
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The "HP" column shows the hit points of the Goblin to the left in the
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corresponding row. The Elf is in range of three targets: the Goblin above it
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(with 4 hit points), the Goblin to its right (with 2 hit points), and the
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Goblin below it (also with 2 hit points). Because three targets are in
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range, the ones with the lowest hit points are selected: the two Goblins
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with 2 hit points each (one to the right of the Elf and one below the Elf).
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Of those, the Goblin first in reading order (the one to the right of the
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Elf) is selected. The selected Goblin's hit points (2) are reduced by the
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Elf's attack power (3), reducing its hit points to -1, killing it.
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After attacking, the unit's turn ends. Regardless of how the unit's turn
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ends, the next unit in the round takes its turn. If all units have taken
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turns in this round, the round ends, and a new round begins.
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The Elves look quite outnumbered. You need to determine the outcome of the
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battle: the number of full rounds that were completed (not counting the
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round in which combat ends) multiplied by the sum of the hit points of all
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remaining units at the moment combat ends. (Combat only ends when a unit
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finds no targets during its turn.)
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Below is an entire sample combat. Next to each map, each row's units' hit
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points are listed from left to right.
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Initially: ####### #.G...# G(200) #...EG# E(200), G(200) #.#.#G# G(200)
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#..G#E# G(200), E(200) #.....# #######
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After 1 round: ####### #..G..# G(200) #...EG# E(197), G(197) #.#G#G#
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G(200), G(197) #...#E# E(197) #.....# #######
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After 2 rounds: ####### #...G.# G(200) #..GEG# G(200), E(188), G(194)
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#.#.#G# G(194) #...#E# E(194) #.....# #######
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Combat ensues; eventually, the top Elf dies:
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After 23 rounds: ####### #...G.# G(200) #..G.G# G(200), G(131) #.#.#G#
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G(131) #...#E# E(131) #.....# #######
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After 24 rounds: ####### #..G..# G(200) #...G.# G(131) #.#G#G# G(200),
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G(128) #...#E# E(128) #.....# #######
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After 25 rounds: ####### #.G...# G(200) #..G..# G(131) #.#.#G# G(125)
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#..G#E# G(200), E(125) #.....# #######
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After 26 rounds: ####### #G....# G(200) #.G...# G(131) #.#.#G# G(122)
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#...#E# E(122) #..G..# G(200) #######
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After 27 rounds: ####### #G....# G(200) #.G...# G(131) #.#.#G# G(119)
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#...#E# E(119) #...G.# G(200) #######
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After 28 rounds: ####### #G....# G(200) #.G...# G(131) #.#.#G# G(116)
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#...#E# E(113) #....G# G(200) #######
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More combat ensues; eventually, the bottom Elf dies:
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After 47 rounds: ####### #G....# G(200) #.G...# G(131) #.#.#G# G(59)
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#...#.# #....G# G(200) #######
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Before the 48th round can finish, the top-left Goblin finds that there are
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no targets remaining, and so combat ends. So, the number of full rounds that
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were completed is 47, and the sum of the hit points of all remaining units
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is 200+131+59+200 = 590. From these, the outcome of the battle is 47 * 590 =
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27730.
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Here are a few example summarized combats:
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####### ####### #G..#E# #...#E# E(200) #E#E.E# #E#...#
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E(197) #G.##.# --> #.E##.# E(185) #...#E# #E..#E# E(200), E(200)
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#...E.# #.....# ####### #######
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Combat ends after 37 full rounds Elves win with 982 total hit points left
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Outcome: 37 * 982 = 36334
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####### ####### #E..EG# #.E.E.# E(164), E(197) #.#G.E#
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#.#E..# E(200) #E.##E# --> #E.##.# E(98) #G..#.# #.E.#.# E(200)
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#..E#.# #...#.# ####### #######
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Combat ends after 46 full rounds Elves win with 859 total hit points left
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Outcome: 46 * 859 = 39514
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####### ####### #E.G#.# #G.G#.# G(200), G(98) #.#G..#
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#.#G..# G(200) #G.#.G# --> #..#..# #G..#.# #...#G# G(95) #...E.#
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#...G.# G(200) ####### #######
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Combat ends after 35 full rounds Goblins win with 793 total hit points left
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Outcome: 35 * 793 = 27755
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####### ####### #.E...# #.....# #.#..G# #.#G..# G(200)
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#.###.# --> #.###.# #E#G#G# #.#.#.# #...#G# #G.G#G# G(98),
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G(38), G(200) ####### #######
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Combat ends after 54 full rounds Goblins win with 536 total hit points left
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Outcome: 54 * 536 = 28944
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######### ######### #G......# #.G.....# G(137) #.E.#...#
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#G.G#...# G(200), G(200) #..##..G# #.G##...# G(200) #...##..# -->
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#...##..# #...#...# #.G.#...# G(200) #.G...G.# #.......#
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#.....G.# #.......# ######### #########
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Combat ends after 20 full rounds Goblins win with 937 total hit points left
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Outcome: 20 * 937 = 18740
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What is the outcome of the combat described in your puzzle input?
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Your puzzle answer was 225096.
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--- Part Two ---
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According to your calculations, the Elves are going to lose badly. Surely,
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you won't mess up the timeline too much if you give them just a little
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advanced technology, right?
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You need to make sure the Elves not only win, but also suffer no losses:
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even the death of a single Elf is unacceptable.
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However, you can't go too far: larger changes will be more likely to
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permanently alter spacetime.
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So, you need to find the outcome of the battle in which the Elves have the
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lowest integer attack power (at least 4) that allows them to win without a
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single death. The Goblins always have an attack power of 3.
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In the first summarized example above, the lowest attack power the Elves
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need to win without losses is 15:
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####### ####### #.G...# #..E..# E(158) #...EG# #...E.#
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E(14) #.#.#G# --> #.#.#.# #..G#E# #...#.# #.....# #.....#
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####### #######
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Combat ends after 29 full rounds Elves win with 172 total hit points left
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Outcome: 29 * 172 = 4988
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In the second example above, the Elves need only 4 attack power:
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####### ####### #E..EG# #.E.E.# E(200), E(23) #.#G.E#
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#.#E..# E(200) #E.##E# --> #E.##E# E(125), E(200) #G..#.# #.E.#.#
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E(200) #..E#.# #...#.# ####### #######
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Combat ends after 33 full rounds Elves win with 948 total hit points left
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Outcome: 33 * 948 = 31284
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In the third example above, the Elves need 15 attack power:
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####### ####### #E.G#.# #.E.#.# E(8) #.#G..# #.#E..#
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E(86) #G.#.G# --> #..#..# #G..#.# #...#.# #...E.# #.....#
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####### #######
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Combat ends after 37 full rounds Elves win with 94 total hit points left
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Outcome: 37 * 94 = 3478
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In the fourth example above, the Elves need 12 attack power:
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####### ####### #.E...# #...E.# E(14) #.#..G# #.#..E#
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E(152) #.###.# --> #.###.# #E#G#G# #.#.#.# #...#G# #...#.#
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####### #######
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Combat ends after 39 full rounds Elves win with 166 total hit points left
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Outcome: 39 * 166 = 6474
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In the last example above, the lone Elf needs 34 attack power:
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######### ######### #G......# #.......# #.E.#...# #.E.#...#
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E(38) #..##..G# #..##...# #...##..# --> #...##..# #...#...#
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#...#...# #.G...G.# #.......# #.....G.# #.......# #########
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#########
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Combat ends after 30 full rounds Elves win with 38 total hit points left
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Outcome: 30 * 38 = 1140
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After increasing the Elves' attack power until it is just barely enough for
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them to win without any Elves dying, what is the outcome of the combat
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described in your puzzle input?
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Your puzzle answer was 35354.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should return to your Advent calendar and try another
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puzzle.
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If you still want to see it, you can get your puzzle input.
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References
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Visible links
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. https://adventofcode.com/
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. https://adventofcode.com/2018/about
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. https://adventofcode.com/2018/events
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. https://adventofcode.com/2018/settings
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. https://adventofcode.com/2018/auth/logout
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. Advent of Code Supporter
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https://adventofcode.com/2018/support
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. https://adventofcode.com/2018
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. https://adventofcode.com/2018
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. https://adventofcode.com/2018/support
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. https://adventofcode.com/2018/sponsors
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. https://adventofcode.com/2018/leaderboard
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. https://adventofcode.com/2018/stats
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. https://adventofcode.com/2018/sponsors
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. https://en.wikipedia.org/wiki/Goblin
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. https://adventofcode.com/2018
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. https://adventofcode.com/2018/day/15/input
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