/*

 * Copyright 2010-2011 INRIA Saclay

 * Copyright 2012      Ecole Normale Superieure

 *

 * Use of this software is governed by the MIT license

 *

 * Written by Sven Verdoolaege, INRIA Saclay - Ile-de-France,

 * Parc Club Orsay Universite, ZAC des vignes, 4 rue Jacques Monod,

 * 91893 Orsay, France

 * and Ecole Normale Superieure, 45 rue d’Ulm, 75230 Paris, France

 */

#include <stdlib.h>

#include <isl/ctx.h>

#include <isl_tarjan.h>

void isl_tarjan_graph_free(struct isl_tarjan_graph *g)

{

	if (!g)

		return;

	free(g->node);

	free(g->stack);

	free(g->order);

	free(g);

}

static struct isl_tarjan_graph *isl_tarjan_graph_alloc(isl_ctx *ctx, int len)

{

	struct isl_tarjan_graph *g;

	int i;

	g = isl_calloc_type(ctx, struct isl_tarjan_graph);

	if (!g)

		return NULL;

	g->len = len;

	g->node = isl_alloc_array(ctx, struct isl_tarjan_node, len);

	if (len && !g->node)

		goto error;

	for (i = 0; i < len; ++i)

		g->node[i].index = -1;

	g->stack = isl_alloc_array(ctx, int, len);

	if (len && !g->stack)

		goto error;

	g->order = isl_alloc_array(ctx, int, 2 * len);

	if (len && !g->order)

		goto error;

	g->sp = 0;

	g->index = 0;

	g->op = 0;

	return g;

error:

	isl_tarjan_graph_free(g);

	return NULL;

}

/* Perform Tarjan's algorithm for computing the strongly connected components

 * in the graph with g->len nodes and with edges defined by "follows".

 */

static isl_stat isl_tarjan_components(struct isl_tarjan_graph *g, int i,

	isl_bool (*follows)(int i, int j, void *user), void *user)

{

	int j;

	g->node[i].index = g->index;

	g->node[i].min_index = g->index;

	g->node[i].on_stack = 1;

	g->index++;

	g->stack[g->sp++] = i;

	for (j = g->len - 1; j >= 0; --j) {

		isl_bool f;

		if (j == i)

			continue;

		if (g->node[j].index >= 0 &&

			(!g->node[j].on_stack ||

			 g->node[j].index > g->node[i].min_index))

			continue;

		f = follows(i, j, user);

		if (f < 0)

			return isl_stat_error;

		if (!f)

			continue;

		if (g->node[j].index < 0) {

			isl_tarjan_components(g, j, follows, user);

			if (g->node[j].min_index < g->node[i].min_index)

				g->node[i].min_index = g->node[j].min_index;

		} else if (g->node[j].index < g->node[i].min_index)

			g->node[i].min_index = g->node[j].index;

	}

	if (g->node[i].index != g->node[i].min_index)

		return isl_stat_ok;

	do {

		j = g->stack[--g->sp];

		g->node[j].on_stack = 0;

		g->order[g->op++] = j;

	} while (j != i);

	g->order[g->op++] = -1;

	return isl_stat_ok;

}

/* Decompose the graph with "len" nodes and edges defined by "follows"

 * into strongly connected components (SCCs).

 * follows(i, j, user) should return 1 if "i" follows "j" and 0 otherwise.

 * It should return -1 on error.

 *

 * If SCC a contains a node i that follows a node j in another SCC b

 * (i.e., follows(i, j, user) returns 1), then SCC a will appear after SCC b

 * in the result.

 */

struct isl_tarjan_graph *isl_tarjan_graph_init(isl_ctx *ctx, int len,

	isl_bool (*follows)(int i, int j, void *user), void *user)

{

	int i;

	struct isl_tarjan_graph *g = NULL;

	g = isl_tarjan_graph_alloc(ctx, len);

	if (!g)

		return NULL;

	for (i = len - 1; i >= 0; --i) {

		if (g->node[i].index >= 0)

			continue;

		if (isl_tarjan_components(g, i, follows, user) < 0)

			goto error;

	}

	return g;

error:

	isl_tarjan_graph_free(g);

	return NULL;

}