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day15 p2: proper Bellman-Ford

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Bryce Allen
2021-12-17 16:34:24 -05:00
parent 3a843c6b3e
commit ceff063ed0
1 changed files with 37 additions and 78 deletions
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113
day15/part2.jl
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@@ -171,10 +171,35 @@ function neighbor_idx(nrows, ncols, y, x)
return v
end
function relax(nrows, ncols, R, costs, row, col; first=false)
idx = neighbor_idx(nrows, ncols, row, col)
c = minimum(costs[i] for i in idx) + R[row, col]
if first || c < costs[row, col]
costs[row, col] = c
return true
end
return false
end
function relax_all(nrows, ncols, R, costs)
relaxed = false
for row in 1:nrows
for col in 1:ncols
v_relaxed = relax(nrows, ncols, R, costs, row, col)
relaxed = relaxed || v_relaxed
end
end
return relaxed
end
function get_costs(R)
costs = zeros(Int, size(R))
dirs = zeros(Int, size(R))
nrows, ncols = size(R)
# first pass, unconditional update looking only right/down. Avoids
# having to add an infinity value
costs[1,1] = 0
for col in 2:ncols
costs[1, col] = costs[1, col-1] + R[1, col]
end
@@ -190,90 +215,24 @@ function get_costs(R)
end
end
if false
# more passes
for i in 1:max(nrows, ncols)
for col in ncols-1:-1:1
rcost = costs[end, col+1] + R[end, col]
if rcost < costs[end, col]
costs[end, col] = rcost
end
end
for row in nrows-1:-1:1
dcost = costs[row+1, end] + R[row, end]
if dcost < costs[row, end]
costs[row, end] = dcost
end
for col in ncols-1:-1:1
rcost = costs[row, col+1] + R[row, col]
if rcost < costs[row, col]
costs[row, col] = rcost
end
dcost = costs[row+1, col] + R[row, col]
if dcost < costs[row, col]
costs[row, col] = dcost
end
end
end
end
end
edges = ( (nrows-2)*(ncols-2)*4
+(nrows+ncols-4)*3
+4*2 )
max_passes = (nrows*ncols - 1) * edges
for i in 1:max(nrows, ncols)
for row in nrows:-1:1
for col in ncols:-1:1
idx = neighbor_idx(nrows, ncols, row, col)
c = minimum(costs[i] for i in idx) + R[row, col]
if c < costs[row, col]
costs[row, col] = c
i = 1
while i <= max_passes
relaxed = relax_all(nrows, ncols, R, costs)
if !relaxed
break
end
i += 1
end
end
for row in nrows:-1:1
for col in 1:ncols
idx = neighbor_idx(nrows, ncols, row, col)
c = minimum(costs[i] for i in idx) + R[row, col]
if c < costs[row, col]
costs[row, col] = c
end
end
end
for row in 1:nrows
for col in ncols:-1:1
idx = neighbor_idx(nrows, ncols, row, col)
c = minimum(costs[i] for i in idx) + R[row, col]
if c < costs[row, col]
costs[row, col] = c
end
end
end
for row in 1:nrows
for col in 1:ncols
idx = neighbor_idx(nrows, ncols, row, col)
c = minimum(costs[i] for i in idx) + R[row, col]
if c < costs[row, col]
costs[row, col] = c
end
end
end
end
println("passes ", i)
return costs, dirs
end
#=
cost, path = least_risk(risk_map)
display(path)
println()
println("cost = ", cost)
=#
costs, dirs = get_costs(risk_map)
if length(costs) < 1000