/*
 * 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 "sud.h"

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#define REM_PROB_NUMER 2
#define REM_PROB_DENOM 3

#define MAX_FILL_ATTEMPTS 32

static void initposs(struct sudoku *sud)
{
	for (unsigned r = 0; r < NDIGITS; ++r) {
		for (unsigned c = 0; c < NDIGITS; ++c) {
			sud->cells[r][c].det = false;
			for (unsigned i = 0; i < NDIGITS; ++i)
				sud->cells[r][c].pvals[i] = true;
		}
	}
}

static void clear(struct sudoku *sud, unsigned r, unsigned c)
{
	unsigned tr, tc, val;

	/* Set undetermined with all possible values */
	sud->cells[r][c].det = false;
	for (unsigned i = 0; i < NDIGITS; ++i)
		sud->cells[r][c].pvals[i] = true;

	/* Update cell's possible values based off column. */
	for (tr = 0; tr < NDIGITS; ++tr) {
		if (tr == r || !sud->cells[tr][c].det)
			continue;
		val = sud->cells[tr][c].val;
		sud->cells[r][c].pvals[val] = false;
	}

	/* Update possible values of cells in same row. */
	for (tc = 0; tc < NDIGITS; ++tc) {
		if (tc == c || !sud->cells[r][tc].det)
			continue;
		val = sud->cells[r][tc].val;
		sud->cells[r][c].pvals[val] = false;
	}

	/* Update possible values of cells in same segment. */
	const unsigned segr0 = SEGLEN * (r / SEGLEN);
	const unsigned segc0 = SEGLEN * (c / SEGLEN);
	for (tr = segr0; tr < segr0 + SEGLEN; ++tr) {
		for (tc = segc0; tc < segc0 + SEGLEN; ++tc) {
			if ((tr == r && tc == c) || !sud->cells[tr][tc].det)
				continue;
			val = sud->cells[tr][tc].val;
			sud->cells[r][c].pvals[val] = false;
		}
	}
}

void gen(struct sudoku *sud)
{
	/* Generate a completed sudoku. */
retry:
	initposs(sud);
	for (unsigned r = 0; r < NDIGITS; ++r) {
		for (unsigned c = 0; c < NDIGITS; ++c) {
			unsigned val, n = 0;
			do {
				val = (unsigned)(rand() % NDIGITS);
				++n;
				if (n >= MAX_FILL_ATTEMPTS)
					goto retry;
			} while (update(sud, r, c, val) == NOT_ALLOWED);
		}
	}

	/* Remove cells with probability of `1 / REMOVE_PROB_RECIP`. */
	for (unsigned r = 0; r < NDIGITS; ++r) {
		for (unsigned c = 0; c < NDIGITS; ++c) {
			if (rand() % REM_PROB_DENOM < REM_PROB_NUMER)
				clear(sud, r, c);
		}
	}
}

enum update_res update(struct sudoku *sud, unsigned r, unsigned c, unsigned val)
{
	unsigned tr, tc;
	assert(val < NDIGITS);

	/* Check that the cell is undetermined and the value is allowed. */
	if (sud->cells[r][c].det)
		return ALREADY_DET;
	if (!sud->cells[r][c].pvals[val])
		return NOT_ALLOWED;

	/* Update possible values of cells in same column. */
	for (tr = 0; tr < NDIGITS; ++tr) {
		if (tr == r || sud->cells[tr][c].det)
			continue;
		sud->cells[tr][c].pvals[val] = false;
	}

	/* Update possible values of cells in same row. */
	for (tc = 0; tc < NDIGITS; ++tc) {
		if (tc == c || sud->cells[r][tc].det)
			continue;
		sud->cells[r][tc].pvals[val] = false;
	}

	/* Update possible values of cells in same segment. */
	const unsigned segr0 = SEGLEN * (r / SEGLEN);
	const unsigned segc0 = SEGLEN * (c / SEGLEN);
	for (tr = segr0; tr < segr0 + SEGLEN; ++tr) {
		for (tc = segc0; tc < segc0 + SEGLEN; ++tc) {
			if ((tr == r && tc == c) || sud->cells[tr][tc].det)
				continue;
			sud->cells[tr][tc].pvals[val] = false;
		}
	}

	/* Set the cell's value. */
	sud->cells[r][c].det = true;
	sud->cells[r][c].val = val;

	return OK;
}

void print(const struct sudoku *sud)
{
	for (unsigned r = 0; r < NDIGITS; ++r) {
		/*
		 * Print horizontal divider if on a segment boundary (but not
		 * at the start).
		 */
		if (r != 0 && r % SEGLEN == 0)
			puts("------+-------+------");

		for (unsigned c = 0; c < NDIGITS; ++c) {
			/*
			 * Print vertical divider if on a segment boundary (but
			 * not at the start).
			 */
			if (c != 0 && c % SEGLEN == 0)
				fputs("| ", stdout);

			if (sud->cells[r][c].det)
				printf("%u ", sud->cells[r][c].val + 1);
			else
				fputs("  ", stdout);
		}
		putchar('\n');
	}
}

static void zerocounts(unsigned counts[NDIGITS])
{
	for (unsigned i = 0; i < NDIGITS; ++i)
		counts[i] = 0;
}

static bool checkcounts(unsigned counts[NDIGITS])
{
	for (unsigned i = 0; i < NDIGITS; ++i) {
		if (counts[i] != 1)
			return false;
	}
	return true;
}

enum check_res check(const struct sudoku *sud)
{
	unsigned r, c, i, j, digitcounts[NDIGITS];

	/* Check each row. */
	for (r = 0; r < NDIGITS; ++r) {
		zerocounts(digitcounts);
		for (c = 0; c < NDIGITS; ++c) {
			if (!sud->cells[r][c].det)
				return INCOMPLETE;
			++digitcounts[sud->cells[r][c].val];
		}
		if (!checkcounts(digitcounts))
			return INCORRECT;
	}

	/* Check each column. */
	for (c = 0; c < NDIGITS; ++c) {
		zerocounts(digitcounts);
		for (r = 0; r < NDIGITS; ++r) {
			if (!sud->cells[r][c].det)
				return INCOMPLETE;
			++digitcounts[sud->cells[r][c].val];
		}
		if (!checkcounts(digitcounts))
			return INCORRECT;
	}

	/* Check each segment. */
	for (i = 0; i < NDIGITS; ++i) {
		zerocounts(digitcounts);
		for (j = 0; j < NDIGITS; ++j) {
			r = 3 * (i / 3) + j / 3;
			c = 3 * (i % 3) + j % 3;
			if (!sud->cells[r][c].det)
				return INCOMPLETE;
			++digitcounts[sud->cells[r][c].val];
		}
		if (!checkcounts(digitcounts))
			return INCORRECT;
	}

	/* If we've got this far, all is well. */
	return SOLVED;
}