# Efficient Integer Exponentiation in C

It's surprisingly difficult to find a good code snippet for this on Google, so here's an efficient computation of integer powers in C, using binary exponentiation:

# I confused the compiler

Clang is known for its great error messages, but I did manage to horribly confuse it:

# Standards-compliant* alloca()

The alloca() function in C is used to allocate a dynamic amount of memory on the stack. Despite its advantages in some situations, it is non-standard and will probably remain so forever.

# The Visitor Pattern in Python

The visitor pattern is tremendously useful when working with certain kinds of information like abstract syntax trees. It's basically a poor man's version of sum types for languages that don't natively support them. Unfortunately, they take advantage of function overloading, something which duck-typed languages like Python lack.

# Exact Bounding Boxes for Spheres/Ellipsoids

Finding the tightest axis-aligned bounding box for a sphere is trivial: the box extends from the center by the radius in all dimensions. But once the sphere is transformed, finding the minimal bounding box becomes trickier. Rotating a sphere, for example, shouldn't change its bounding box, but naïvely rotating the bounding box will expand it unnecessarily. Luckily there's a trick to computing minimal bounding boxes by representing the transformed sphere as a quadric surface.

# A Beautiful Ray/Mesh Intersection Algorithm

In my last post, I talked about a beautiful method for computing ray/triangle intersections. In this post, I will extend it to computing intersections with triangle fans. Since meshes are often stored in a corner table, which is simply an array of triangle fans, this gives an efficient algorithm for ray tracing triangle meshes.

# A Beautiful Ray/Triangle Intersection Method

3D ray/triangle intersections are obviously an important part of much of computer graphics. The Möller–Trumbore algorithm, for example, computes these intersections very quickly. But there is another method that I believe is more elegant, and in some cases allows you to compute the intersection for “free.”

# Never seen this one before

Never seen this one before:

malloc.c:2369: sysmalloc: Assertion (old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 *(sizeof(size_t))) - 1)) & ~((2 *(sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long) old_end & pagemask) == 0)' failed.

# Announcing Sangria

Sangria is a new project of mine to release various Guice extensions I've been working on recently. Right now the coolest thing it can do is context-sensitive injections, allowing (among other things) first-class Logger injection for more than just java.util.logging`.