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Constraint Programming Solver
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lib/examples/bin_packing/upper_bound.ex
defmodule CPSolver.Examples.BinPacking.UpperBound do
@moduledoc """
Precompute upper bounds for bin packing instance
based on heuristics.
'First fit decreasing' is the one that is implemented
"""
alias InPlace.Array
def first_fit_decreasing(weights, capacity) do
first_fit(Enum.sort(weights, :desc), capacity)
end
def first_fit(weights, capacity) do
if hd(weights) > capacity do
throw(:item_weight_over_capacity)
end
remaining_bin_space = Array.new(length(weights), capacity)
Enum.reduce(weights, 1, fn w, count_acc ->
if place_item(remaining_bin_space, w, count_acc) do
count_acc
else
(count_acc + 1)
|> tap(fn new_bin_idx ->
Array.update(remaining_bin_space, new_bin_idx, fn val -> val - w end)
end)
end
end)
end
defp place_item(bin_space, weight, bin_count) do
Enum.reduce_while(1..bin_count, false, fn bin_idx, _acc ->
if Array.get(bin_space, bin_idx) >= weight do
## Can place item to this bin
Array.update(bin_space, bin_idx, fn val -> val - weight end)
{:halt, true}
else
{:cont, false}
end
end)
end
end