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Constraint Programming Solver
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lib/solver/propagators/sum.ex
defmodule CPSolver.Propagator.Sum do
use CPSolver.Propagator
import CPSolver.Variable.View.Factory
@moduledoc """
The propagator for Sum constraint.
Sum(y, x) constrains y to be a sum of variables in the list x.
"""
@spec new(Common.variable_or_view(), [Common.variable_or_view()]) :: Propagator.t()
def new(y, x) do
new([minus(y) | x])
end
@impl true
def variables([y | x]) do
[
set_propagate_on(y, :domain_change)
| Enum.map(x, fn x_el -> set_propagate_on(x_el, :bound_change) end)
]
end
@impl true
def filter(all_vars) do
{sum_min, sum_max} = sum_min_max(all_vars)
filter_impl(all_vars, sum_min, sum_max)
end
defp filter_impl(_variables, sum_min, sum_max) when sum_min > 0 or sum_max < 0 do
:fail
end
defp filter_impl(variables, sum_min, sum_max) do
case Enum.reduce(variables, {0, 0}, fn v, {s_min, s_max} ->
## TODO: guard against operations on "failed" vars.
## If v is "failed", then we'd have
## ** (ArithmeticError) bad argument in arithmetic expression **
## if min(v) or max(v) are part of arithmetic experssions.
## This is not a big deal, but we don't want exceptions to show up.
##
cond do
removeAbove(v, -(sum_min - min(v))) == :fail -> throw({:fail, id(v)})
removeBelow(v, -(sum_max - max(v))) == :fail -> throw({:fail, id(v)})
true -> {s_min + min(v), s_max + max(v)}
end
end) do
:fail ->
:fail
## Enforce idempotence: we'll run filtering until there's no changes
{new_sum_min, new_sum_max} ->
((new_sum_min != sum_min ||
new_sum_max != sum_max) && filter_impl(variables, new_sum_min, new_sum_max)) ||
:ok
end
end
defp sum_min_max(variables) do
Enum.reduce(variables, {0, 0}, fn v, {s_min, s_max} = _acc ->
{s_min + min(v), s_max + max(v)}
end)
end
end