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Annotation of 41BSD/cmd/pxp/yytree.c, revision 1.1.1.1

1.1       root        1: /* Copyright (c) 1979 Regents of the University of California */
                      2: #
                      3: /*
                      4:  * pxp - Pascal execution profiler
                      5:  *
                      6:  * Bill Joy UCB
                      7:  * Version 1.2 January 1979
                      8:  */
                      9: 
                     10: #include "0.h"
                     11: #include "tree.h"
                     12: 
                     13: extern int *spacep;
                     14: 
                     15: /*
                     16:  * LIST MANIPULATION ROUTINES
                     17:  *
                     18:  * The grammar for Pascal is written left recursively.
                     19:  * Because of this, the portions of parse trees which are to resemble
                     20:  * lists are in the somewhat inconvenient position of having
                     21:  * the nodes built from left to right, while we want to eventually
                     22:  * have as semantic value the leftmost node.
                     23:  * We could carry the leftmost node as semantic value, but this
                     24:  * would be inefficient as to add a new node to the list we would
                     25:  * have to chase down to the end.  Other solutions involving a head
                     26:  * and tail pointer waste space.
                     27:  *
                     28:  * The simple solution to this apparent dilemma is to carry along
                     29:  * a pointer to the leftmost node in a list in the rightmost node
                     30:  * which is the current semantic value of the list.
                     31:  * When we have completed building the list, we can retrieve this
                     32:  * left end pointer very easily; neither adding new elements to the list
                     33:  * nor finding the left end is thus expensive.  As the bottommost node
                     34:  * has an unused next pointer in it, no space is wasted either.
                     35:  *
                     36:  * The nodes referred to here are of the T_LISTPP type and have
                     37:  * the form:
                     38:  *
                     39:  *     T_LISTPP        some_pointer            next_element
                     40:  *
                     41:  * Here some_pointer points to the things of interest in the list,
                     42:  * and next_element to the next thing in the list except for the
                     43:  * rightmost node, in which case it points to the leftmost node.
                     44:  * The next_element of the rightmost node is of course zapped to the
                     45:  * NIL pointer when the list is completed.
                     46:  *
                     47:  * Thinking of the lists as tree we heceforth refer to the leftmost
                     48:  * node as the "top", and the rightmost node as the "bottom" or as
                     49:  * the "virtual root".
                     50:  */
                     51: 
                     52: /*
                     53:  * Make a new list
                     54:  */
                     55: newlist(new)
                     56:        register int *new;
                     57: {
                     58: 
                     59:        if (new == NIL)
                     60:                return (NIL);
                     61:        return (tree3(T_LISTPP, new, spacep));
                     62: }
                     63: 
                     64: /*
                     65:  * Add a new element to an existing list
                     66:  */
                     67: addlist(vroot, new)
                     68:        register int *vroot;
                     69:        int *new;
                     70: {
                     71:        register int *top;
                     72: 
                     73:        if (new == NIL)
                     74:                return (vroot);
                     75:        if (vroot == NIL)
                     76:                return (newlist(new));
                     77:        top = vroot[2];
                     78:        vroot[2] = spacep;
                     79:        return (tree3(T_LISTPP, new, top));
                     80: }
                     81: 
                     82: /*
                     83:  * Fix up the list which has virtual root vroot.
                     84:  * We grab the top pointer and return it, zapping the spot
                     85:  * where it was so that the tree is not circular.
                     86:  */
                     87: fixlist(vroot)
                     88:        register int *vroot;
                     89: {
                     90:        register int *top;
                     91: 
                     92:        if (vroot == NIL)
                     93:                return (NIL);
                     94:        top = vroot[2];
                     95:        vroot[2] = NIL;
                     96:        return (top);
                     97: }
                     98: 
                     99: 
                    100: /*
                    101:  * Set up a T_VAR node for a qualified variable.
                    102:  * Init is the initial entry in the qualification,
                    103:  * or NIL if there is none.
                    104:  */
                    105: setupvar(var, init)
                    106:        char *var;
                    107:        register int *init;
                    108: {
                    109: 
                    110:        if (init != NIL)
                    111:                init = newlist(init);
                    112:        return (tree4(T_VAR, NOCON, var, init));
                    113: }

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