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Constraint Programming Solver
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lib/solver/constraints/propagators/element_var.ex
defmodule CPSolver.Propagator.ElementVar do
use CPSolver.Propagator
@moduledoc """
The propagator for Element constraint.
array[index] = value,
where array is an array of variables.
"""
def new(var_array, var_index, var_value) do
new([var_array, var_index, var_value])
end
@impl true
def bind(%{args: [var_array, var_index, var_value] = _args} = propagator, source, var_field) do
bound_args =
[
Enum.map(var_array, fn var -> Propagator.bind_to_variable(var, source, var_field) end),
Propagator.bind_to_variable(var_index, source, var_field),
Propagator.bind_to_variable(var_value, source, var_field)
]
Map.put(propagator, :args, bound_args)
end
@impl true
def variables([var_array, var_index, var_value]) do
Enum.map(var_array, fn var ->
set_propagate_on(var, :domain_change)
end) ++
[
set_propagate_on(var_index, :domain_change),
set_propagate_on(var_value, :domain_change)
]
end
defp initial_reduction([], _var_index, _var_value) do
throw(:fail)
end
defp initial_reduction(var_array, var_index, _var_value) do
# var_index is an index in array2d,
# so we trim D(var_index) to the size of array (0-based).
removeBelow(var_index, 0)
removeAbove(var_index, length(var_array) - 1)
end
@impl true
def filter([var_array, var_index, var_value] = _args, state, _changes) do
state || initial_reduction(var_array, var_index, var_value)
filter_impl(var_array, var_index, var_value)
{:state, %{}}
end
defp filter_impl(var_array, var_index, var_value) do
# Step 1
## For all variables in var_array, if no values in D(var_value)
## present in their domains, then the corresponding index has to be removed.
value_domain = domain(var_value) |> Domain.to_list()
index_domain = domain(var_index) |> Domain.to_list()
total_value_intersection =
Enum.reduce(index_domain, MapSet.new(), fn idx, intersection_acc ->
case Enum.at(var_array, idx) do
nil ->
IO.inspect("Unexpected: no element at #{idx}")
throw(:unexpected_no_element)
elem_var ->
elem_var_domain = domain(elem_var) |> Domain.to_list()
intersection = MapSet.intersection(value_domain, elem_var_domain)
(MapSet.size(intersection) == 0 && remove(var_index, idx) && intersection_acc) ||
MapSet.union(intersection, intersection_acc)
end
end)
## Step 2
## `total_value_intersection` has domain values from D(var_value)
## such that each of them is present in at least one domain of variables
## of `var_array`
## Hence, we can remove values that are not in `total_value_intersection` from
## D(var_value)
Enum.each(value_domain, fn val ->
!MapSet.member?(total_value_intersection, val) && remove(var_value, val)
end)
end
end