Library to load IDX file format into Common Lisp array
The definition comes from http://yann.lecun.com/exdb/mnist/
the IDX file format is a simple format for vectors and multidimensional matrices of various numerical types. The basic format is
magic number size in dimension 0 size in dimension 1 size in dimension 2 ..... size in dimension N data
The magic number is an integer (MSB first). The first 2 bytes are always 0.
The third byte codes the type of the data: 0x08: unsigned byte 0x09: signed byte 0x0B: short (2 bytes) 0x0C: int (4 bytes) 0x0D: float (4 bytes) 0x0E: double (8 bytes)
The 4-th byte codes the number of dimensions of the vector/matrix: 1 for vectors, 2 for matrices....
The sizes in each dimension are 4-byte integers (MSB first, high endian, like in most non-Intel processors).
The data is stored like in a C array, i.e. the index in the last dimension changes the fastest.