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diff --git a/libdimension/dimension/geometry.h b/libdimension/dimension/geometry.h
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+/*************************************************************************
+ * Copyright (C) 2008 Tavian Barnes <tavianator@gmail.com>               *
+ *                                                                       *
+ * This file is part of Dimension.                                       *
+ *                                                                       *
+ * Dimension is free software; you can redistribute it and/or modify it  *
+ * under the terms of the GNU Lesser General Public License as published *
+ * by the Free Software Foundation; either version 3 of the License, or  *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * Dimension 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     *
+ * Lesser General Public License for more details.                       *
+ *                                                                       *
+ * You should have received a copy of the GNU Lesser General Public      *
+ * License along with this program.  If not, see                         *
+ * <http://www.gnu.org/licenses/>.                                       *
+ *************************************************************************/
+
+/*
+ * Core geometric types like scalars, vectors, and rays.
+ */
+
+#ifndef DIMENSION_GEOMETRY_H
+#define DIMENSION_GEOMETRY_H
+
+/* Scalar and vector types. */
+typedef double                           dmnsn_scalar;
+typedef struct { dmnsn_scalar x, y, z; } dmnsn_vector;
+
+/* Vector arithmetic */
+
+inline dmnsn_vector
+dmnsn_vector_construct(dmnsn_scalar x, dmnsn_scalar y, dmnsn_scalar z)
+{
+  dmnsn_vector v = { .x = x, .y = y, .z = z };
+  return v;
+}
+
+inline dmnsn_vector
+dmnsn_vector_add(dmnsn_vector lhs, dmnsn_vector rhs)
+{
+  return dmnsn_vector_construct(lhs.x + rhs.x, lhs.y + rhs.y, lhs.z + rhs.z);
+}
+
+inline dmnsn_vector
+dmnsn_vector_sub(dmnsn_vector lhs, dmnsn_vector rhs)
+{
+  return dmnsn_vector_construct(lhs.x - rhs.x, lhs.y - rhs.y, lhs.z - rhs.z);
+}
+
+inline dmnsn_vector
+dmnsn_vector_mul(dmnsn_scalar lhs, dmnsn_vector rhs)
+{
+  return dmnsn_vector_construct(lhs*rhs.x, lhs*rhs.y, lhs*rhs.z);
+}
+
+inline dmnsn_vector
+dmnsn_vector_div(dmnsn_vector lhs, dmnsn_scalar rhs)
+{
+  return dmnsn_vector_construct(lhs.x/rhs, lhs.y/rhs, lhs.z/rhs);
+}
+
+inline dmnsn_scalar
+dmnsn_vector_dot(dmnsn_vector lhs, dmnsn_vector rhs)
+{
+  return lhs.x*rhs.x + lhs.y*rhs.y + lhs.z*rhs.z;
+}
+
+inline dmnsn_vector
+dmnsn_vector_cross(dmnsn_vector lhs, dmnsn_vector rhs)
+{
+  return dmnsn_vector_construct(lhs.y*rhs.z - lhs.z*rhs.y,
+                                lhs.z*rhs.x - lhs.x*rhs.z,
+                                lhs.x*rhs.y - lhs.y*rhs.x);
+}
+
+/* A line, or ray. */
+typedef struct {
+  dmnsn_vector x0; /* A point on the line */
+  dmnsn_vector n;  /* A normal vector; the direction of the line */
+} dmnsn_line;
+
+/* A point on a line, defined by x0 + t*n */
+inline dmnsn_vector
+dmnsn_line_point(dmnsn_line l, dmnsn_scalar t)
+{
+  return dmnsn_vector_add(l.x0, dmnsn_vector_mul(t, l.n));
+}
+
+#endif /* DIMENSION_GEOMETRY_H */