// Copyright (c) 2023, Marvin Borner <dev@marvinborner.de>
// SPDX-License-Identifier: MIT

#include <stdio.h>

#include <term.h>
#include <map.h>
#include <reduce.h>
#include <log.h>
#include <assert.h>

// only substitutes (and therefore rehashes) -> no explicit beta reduction
static struct term *substitute(struct term *term, struct term *substitution,
			       size_t level)
{
	debug("substituting %lx with %lx\n", term->hash, substitution->hash);
	if (term->type == VAR) {
		if (term->u.var.index == level) {
			return substitution;
		} else {
			term_deref(substitution, 1);
			return term;
		}
	} else if (term->type == ABS) {
		struct term *previous = term->u.abs.term;
		struct term *new =
			substitute(term->u.abs.term, substitution, level + 1);
		if (previous->hash == new->hash)
			return new; // nothing changed
		struct term *rehashed = term_rehash_abs(term, new);
		term_rehash_parents(rehashed);
		if (rehashed->u.abs.term->hash == substitution->hash)
			term_deref_head(previous, 1);
		return rehashed;
	} else if (term->type == APP) {
		hash_t previous_lhs = term->u.app.lhs->hash;
		hash_t previous_rhs = term->u.app.rhs->hash;
		struct term *lhs =
			substitute(term->u.app.lhs, substitution, level);
		struct term *rhs =
			substitute(term->u.app.rhs, substitution, level);
		if (previous_lhs == lhs->hash && previous_rhs == rhs->hash)
			return term; // nothing changed
		struct term *rehashed = term_rehash_app(term, lhs, rhs);
		term_rehash_parents(rehashed);
		return rehashed;
	}
	fatal("invalid type %d\n", term->type);
}

// reduction of application
// ([X] Y) -> X/Y
struct term *reduce(struct term *term)
{
	if (!term_is_beta_redex(term))
		fatal("can't reduce non-beta-redex %d\n", term->type);

	debug("reducing %lx\n", term->hash);

	struct term *reduced = substitute(term->u.app.lhs, term->u.app.rhs, -1);
	return reduced;
}