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/*
* esodata - data structures and other things, of varying utility
* Copyright 2022, Ben Culkin
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
package bjc.functypes;
import java.util.*;
import java.util.function.*;
import bjc.data.*;
/**
* Function combinators to form 'fixpoints' or recursive functions.
*
* @author Ben Culkin
*
*/
public interface Fixpoints {
/**
* Convert a two-argument function to a single argument recursive one.
*
* @param <InputType> The input type for the function.
* @param <ReturnType> The output type for the function.
*
* @param func The function to make recursive.
*
* @return The newly recursive function.
*/
static <InputType, ReturnType> Function<InputType, ReturnType> fix(
BiFunction<InputType, Function<InputType, ReturnType>, ReturnType> func) {
Holder<Function<InputType, ReturnType>> inner = Holder.of(null);
inner.replace((arg) -> {
return func.apply(arg, inner.getValue());
});
return inner.getValue();
}
/**
* Convert a two-argument function to a single argument recursive memoized one.
*
* @param <InputType> The input type for the function.
* @param <ReturnType> The output type for the function.
*
* @param func The function to make recursive.
*
* @return The newly recursive memoized function.
*/
static <InputType, ReturnType> Function<InputType, ReturnType> memoFix(
BiFunction<InputType, Function<InputType, ReturnType>, ReturnType> func) {
Map<InputType, ReturnType> memoMap = new HashMap<>();
Holder<Function<InputType, ReturnType>> inner = Holder.of(null);
inner.replace((arg) ->
memoMap.computeIfAbsent(
arg,
(newArg) -> func.apply(newArg, inner.getValue())
)
);
return inner.getValue();
}
}
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