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

#include <stdio.h>

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

static struct term *substitute(struct term *term, struct term *substitution,
			       size_t level)
{
	if (term->type == VAR && term->u.var.index == level) {
		return substitution;
	} else if (term->type == ABS) {
		struct term *new =
			substitute(term->u.abs.term, substitution, level + 1);
		if (term->u.abs.term->hash == substitution->hash)
			return term;
		/* term_deref(term->u.abs.term); */
		struct term *rehashed = term_rehash_abs(term, new);
		/* term_deref_head(term); */
		return rehashed;
	} else if (term->type == APP) {
		struct term *lhs =
			substitute(term->u.app.lhs, substitution, level);
		struct term *rhs =
			substitute(term->u.app.rhs, substitution, level);
		if (term->u.app.lhs->hash == substitution->hash &&
		    term->u.app.rhs->hash == substitution->hash)
			return term;
		/* if (term->u.app.lhs->hash != substitution->hash) */
		/* 	term_deref(term->u.app.lhs); */
		/* if (term->u.app.rhs->hash != substitution->hash) */
		/* 	term_deref(term->u.app.rhs); */
		struct term *rehashed = term_rehash_app(term, lhs, rhs);
		/* term_deref_head(term); */
		return rehashed;
	}
	fatal("invalid type %d\n", term->type);
}

// reduction of application
// ([X],Y) -> X/Y
void reduce(struct term *term)
{
	assert(term->type == APP);
	assert(term->u.app.lhs->type == ABS);

	substitute(term->u.app.lhs->u.abs.term, term->u.app.rhs, 0);
}