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

#include <stdio.h>

#include <term.h>
#include <reduce.h>
#include <log.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);
		return term_rehash_abs(term, new);
	} 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);
		return term_rehash_app(term, lhs, rhs);
	}
	fatal("invalid type %d\n", term->type);
}

static struct term *reduce_app(struct term *term)
{
	if (term->u.app.lhs->type == ABS) {
		return substitute(term->u.app.lhs->u.abs.term, term->u.app.rhs,
				  0);
	} else {
		struct term *lhs = reduce(term->u.app.lhs);
		struct term *rhs = reduce(term->u.app.rhs);
		return term_rehash_app(term, lhs, rhs);
	}
}

static struct term *reduce_abs(struct term *term)
{
	return reduce(term->u.abs.term);
}

struct term *reduce(struct term *term)
{
	if (term->type == APP) {
		return reduce_app(term);
	} else if (term->type == ABS) {
		return reduce_abs(term);
	} else if (term->type == VAR) {
		return term;
	}
	fatal("invalid type %d\n", term->type);
}