Why you should use Fill Pointers in LISP

Fill pointers are a powerful tool in LISP that can help improve the performance of your code. By using fill pointers, you can ensure that your code only allocates the necessary amount of memory for each data structure. This can help reduce memory usage and improve the speed of your code.

A fill pointer is a special kind of pointer used in some Lisp implementations to indicate the end of the accessible part of a vector. It is not necessarily the end of the vector itself; the total size of the vector may be larger. The fill pointer can be moved with certain functions to make more or less of the vector accessible, effectively changing its size. Vectors are a data structure that can be used to store an ordered collection of elements. In many programming languages, including Lisp, vectors are implemented as arrays. An array is a contiguous block of memory that holds a fixed number of elements of the same type, each element in the array is accessed by its index, which is an integer offset from the beginning of the array.

The size of an array is determined when it is created and cannot be changed without creating a new array. This can be inefficient if the size of the data set stored in the array changes frequently. Vectors address this problem by allowing their size to be changed dynamically. A fill pointer is used to keep track of which parts of a vector have been initialized with data. It points to the next available slot in the vector, i.e., the place where new data will be inserted when it is added. When data is removed from a vector, the fill pointer moves accordingly so that it always points to the next available slot. Functions that add or remove data from vectors automatically update the fill pointer as needed.