/*
 * Copyright (C) 2022 Camden Dixie O'Brien
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero 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
 * Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public
 * License along with this program.  If not, see
 * <https://www.gnu.org/licenses/>.
 */

#include "solve.h"

#include <stdio.h>

struct match {
	unsigned row;
	unsigned col;
	unsigned val;
};

struct cellref {
	unsigned row, col;
	struct cellstate *state;
};

enum apply_res { ERROR, NO_MATCH, MATCH };

typedef struct cellref cellgroup[NDIGITS];
typedef bool (*rulefn)(cellgroup *group, struct match *match_out);

static bool one_place_for_digit(cellgroup *group, struct match *match_out)
{
	for (unsigned val = 0; val < NDIGITS; ++val) {
		unsigned n = 0, row = NDIGITS, col = NDIGITS;
		for (unsigned i = 0; i < NDIGITS; ++i) {
			if (!(*group)[i].state->det && (*group)[i].state->pvals[val]) {
				row = (*group)[i].row;
				col = (*group)[i].col;
				++n;
				if (n > 1)
					break;
			}
		}

		if (n == 1) {
			match_out->row = row;
			match_out->col = col;
			match_out->val = val;
			return true;
		}
	}

	return false;
}

static const rulefn rules[] = {
	&one_place_for_digit,
};
static const unsigned nrules = sizeof(rules) / sizeof(rules[0]);

static enum apply_res apply_rules(struct sudoku *sud, cellgroup *group)
{
	struct match m;
	bool matched = false;
	for (unsigned i = 0; i < nrules; ++i) {
		if (!rules[i](group, &m))
			continue;
		if (!update(sud, m.row, m.col, m.val))
			return ERROR;
		matched = true;
	}
	return matched ? MATCH : NO_MATCH;
}

bool solve(struct sudoku *sud)
{
	cellgroup group = { 0 };
	unsigned r, c;
	bool match;
	enum apply_res res;

	for (unsigned i = 0;; ++i) {
		match = false;

		/* Apply rules to each row. */
		for (r = 0; r < NDIGITS; ++r) {
			for (c = 0; c < NDIGITS; ++c) {
				group[c].row = r;
				group[c].col = c;
				group[c].state = &sud->cells[r][c];
			}

			res = apply_rules(sud, &group);
			if (res == ERROR)
				return false;
			else if (res == MATCH)
				match = true;
		}

		/* Apply rules to each column. */
		for (c = 0; c < NDIGITS; ++c) {
			for (r = 0; r < NDIGITS; ++r) {
				group[r].row = r;
				group[r].col = c;
				group[r].state = &sud->cells[r][c];
			}

			res = apply_rules(sud, &group);
			if (res == ERROR)
				return false;
			else if (res == MATCH)
				match = true;
		}

		/* TODO: Apply rules to each segment. */

		/* Exit if no matches. */
		if (!match)
			return true;
	}
}