Anders Englöf Ytterström
9a7a9c878b
Funny that original 2023 day 5 also was a PITA to figure out. Pt 1 was solved using BFS to flood-fill. After trying some different methods for pt 2, including: - wallcrawling, - side couting, - corner counting I never produced code to get past the test cases. - Wall crawling are hard due to overlapping regions. - corner couting and side counting are both hard, but will act as equally good solutions (since side count equals corner count). - Concave corners are hard, convex corners are easy. The final code is based on the posts on the solutions megathread. Changes: - Keep all areas in a set, defining a region. - find all convex and concave corners in each region. A new helper got introduced: Di, storing all diagonal neighbors for grid traversing. Convex corners: .. R. .R .. R. .. .. .R Concave corners: RR .R R. RR .R RR RR R.
73 lines
2.2 KiB
Python
73 lines
2.2 KiB
Python
from collections import deque
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from output import D, Di, cw, matrix
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def solve(data):
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grid, H, W = matrix(data)
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fence_cost = 0
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fence_cost_w_discount = 0
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seen = set()
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for r in range(H):
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for c in range(W):
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if (r, c) in seen:
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continue
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id = grid[r][c]
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regions = set()
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perimeters = 0
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q = deque([(r, c)])
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while q:
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rc = q.popleft()
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if rc in seen:
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continue
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seen.add(rc)
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regions.add(rc)
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y, x = rc
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for delta_y, delta_x in D:
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yn, xn = y + delta_y, x + delta_x
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if (0 <= yn < H and 0 <= xn < W) and grid[yn][xn] == id:
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q.append((yn, xn))
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else:
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perimeters += 1
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if len(regions) <= 2:
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corners = 4
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else:
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corners = count_corners(regions)
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fence_cost += len(regions) * perimeters
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fence_cost_w_discount += len(regions) * corners
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return fence_cost, fence_cost_w_discount
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def count_corners(region):
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corners = 0
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for y, x in region:
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for delta_y, delta_x in D:
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# convex corners: one horisontal and one vertical
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# neighbor are not members of region
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delta_ycw, delta_xcw = cw(delta_y, delta_x)
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if (y + delta_y, x + delta_x) not in region and (
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y + delta_ycw,
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x + delta_xcw,
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) not in region:
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corners += 1
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for delta_y, delta_x in Di:
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# concave corners: the diagonal neighbor are not
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# member of region, but the connected horisontal
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# and vertical neighbors are
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if (
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(y + delta_y, x + delta_x) not in region
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and (y + delta_y, x) in region
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and (y, x + delta_x) in region
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):
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corners += 1
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return corners
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if __name__ == "__main__":
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with open("./input/12.txt", "r") as f:
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inp = f.read().strip()
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p1, p2 = solve(inp)
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print(p1)
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print(p2)
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