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+/* cairo - a vector graphics library with display and print output
+ *
+ * Copyright © 2002 University of Southern California
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it either under the terms of the GNU Lesser General Public
+ * License version 2.1 as published by the Free Software Foundation
+ * (the "LGPL") or, at your option, under the terms of the Mozilla
+ * Public License Version 1.1 (the "MPL"). If you do not alter this
+ * notice, a recipient may use your version of this file under either
+ * the MPL or the LGPL.
+ *
+ * You should have received a copy of the LGPL along with this library
+ * in the file COPYING-LGPL-2.1; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335, USA
+ * You should have received a copy of the MPL along with this library
+ * in the file COPYING-MPL-1.1
+ *
+ * The contents of this file are subject to the Mozilla Public License
+ * Version 1.1 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
+ * OF ANY KIND, either express or implied. See the LGPL or the MPL for
+ * the specific language governing rights and limitations.
+ *
+ * The Original Code is the cairo graphics library.
+ *
+ * The Initial Developer of the Original Code is University of Southern
+ * California.
+ *
+ * Contributor(s):
+ * Carl D. Worth <cworth@cworth.org>
+ */
+
+#include "cairoint.h"
+
+#include "cairo-slope-private.h"
+
+/* Compare two slopes. Slope angles begin at 0 in the direction of the
+ positive X axis and increase in the direction of the positive Y
+ axis.
+
+ This function always compares the slope vectors based on the
+ smaller angular difference between them, (that is based on an
+ angular difference that is strictly less than pi). To break ties
+ when comparing slope vectors with an angular difference of exactly
+ pi, the vector with a positive dx (or positive dy if dx's are zero)
+ is considered to be more positive than the other.
+
+ Also, all slope vectors with both dx==0 and dy==0 are considered
+ equal and more positive than any non-zero vector.
+
+ < 0 => a less positive than b
+ == 0 => a equal to b
+ > 0 => a more positive than b
+*/
+int
+_cairo_slope_compare (const cairo_slope_t *a, const cairo_slope_t *b)
+{
+ cairo_int64_t ady_bdx = _cairo_int32x32_64_mul (a->dy, b->dx);
+ cairo_int64_t bdy_adx = _cairo_int32x32_64_mul (b->dy, a->dx);
+ int cmp;
+
+ cmp = _cairo_int64_cmp (ady_bdx, bdy_adx);
+ if (cmp)
+ return cmp;
+
+ /* special-case zero vectors. the intended logic here is:
+ * zero vectors all compare equal, and more positive than any
+ * non-zero vector.
+ */
+ if (a->dx == 0 && a->dy == 0 && b->dx == 0 && b->dy ==0)
+ return 0;
+ if (a->dx == 0 && a->dy == 0)
+ return 1;
+ if (b->dx == 0 && b->dy ==0)
+ return -1;
+
+ /* Finally, we're looking at two vectors that are either equal or
+ * that differ by exactly pi. We can identify the "differ by pi"
+ * case by looking for a change in sign in either dx or dy between
+ * a and b.
+ *
+ * And in these cases, we eliminate the ambiguity by reducing the angle
+ * of b by an infinitesimally small amount, (that is, 'a' will
+ * always be considered less than 'b').
+ */
+ if ((a->dx ^ b->dx) < 0 || (a->dy ^ b->dy) < 0) {
+ if (a->dx > 0 || (a->dx == 0 && a->dy > 0))
+ return -1;
+ else
+ return +1;
+ }
+
+ /* Finally, for identical slopes, we obviously return 0. */
+ return 0;
+}