Wfiris a C program performing classical FIR filter design using the window method. There is nothing unusual about this program, so almost any DSP book will give an adequate explanation of the algorithm as well as the meaning of its arguments. Briefly, the program permits the design of lowpass, highpass, bandpass, and bandstop filters using any of a number of windows. The method is to first compute the impulse response of an ideal (brick wall) filter, and then window it with the selected window to make the impulse response finite.
The window method can also be implemented directly using Ptolemy block diagrams. See "FIR filter design" on page 5-78.