The derives list

The next task for bison is to generate a list of rules assosciated with Non-terminals. By simply walking thru all the rules, and some complex linked list programming, an array DERIVES[0..NVARS] is computed in the file derives.c (derives_compute()). I am able to document the structure of the derives array for now. Why is this array required and how it will be used will be filled in later.

A look at the print_derives() funtion reveals the structure of DERIVES:

Derives[i] = list of rules with i th nonterminal as LHS.

The list of rules is flat list of rule numbers followed by NULL.

e.g, S->NUM X | Y
X -> NUM
Y -> 'a'

derives[0] = 0 NULL (Non terminal $accept is the LHS of rule 0 $accept -> S $end)
derives[1] = 1 2 NULL (S is the LHS of rules 1 and 2)
derives[2] = 3 NULL
derives[3] = 4 NULL