Documentation to explain the behaviour of map().

Subject: Re: Map is still slow
Message-Id: <200007152305.QAA26887@molotok.activestate.com>

p4raw-id: //depot/perl@6428

diff --git a/pp_ctl.c b/pp_ctl.c
index a924d2e..75673dc 100644 (file)
--- a/pp_ctl.c
+++ b/pp_ctl.c
@@ -725,36 +725,49 @@ PP(pp_mapstart)
 PP(pp_mapwhile)
 {
     djSP;
-    I32 diff = (SP - PL_stack_base) - *PL_markstack_ptr;
+    I32 items = (SP - PL_stack_base) - *PL_markstack_ptr; /* how many new items */
     I32 count;
     I32 shift;
     SV** src;
     SV** dst; 
 
+    /* first, move source pointer to the next item in the source list */
     ++PL_markstack_ptr[-1];
-    if (diff) {
-       if (diff > PL_markstack_ptr[-1] - PL_markstack_ptr[-2]) {
-           shift = diff - (PL_markstack_ptr[-1] - PL_markstack_ptr[-2]);
-           count = (SP - PL_stack_base) - PL_markstack_ptr[-1] + 2;
+
+    /* if there are new items, push them into the destination list */
+    if (items) {
+       /* might need to make room back there first */
+       if (items > PL_markstack_ptr[-1] - PL_markstack_ptr[-2]) {
+           /* XXX this implementation is very pessimal because the stack
+            * is repeatedly extended for every set of items.  Is possible
+            * to do this without any stack extension or copying at all
+            * by maintaining a separate list over which the map iterates
+            * (like foreach does). */
+
+           /* everything in the stack after the destination list moves
+            * towards the end the stack by the amount of room needed */
+           shift = items - (PL_markstack_ptr[-1] - PL_markstack_ptr[-2]);
+
+           /* items to shift up (accounting for the moved source pointer) */
+           count = (SP - PL_stack_base) - (PL_markstack_ptr[-1] - 1);
            
            EXTEND(SP,shift);
            src = SP;
            dst = (SP += shift);
            PL_markstack_ptr[-1] += shift;
            *PL_markstack_ptr += shift;
-           while (--count)
+           while (count--)
                *dst-- = *src--;
        }
-       dst = PL_stack_base + (PL_markstack_ptr[-2] += diff) - 1; 
-       ++diff;
-       while (--diff)
+       /* copy the new items down to the destination list */
+       dst = PL_stack_base + (PL_markstack_ptr[-2] += items) - 1; 
+       while (items--)
            *dst-- = SvTEMP(TOPs) ? POPs : sv_mortalcopy(POPs); 
     }
     LEAVE;                                     /* exit inner scope */
 
     /* All done yet? */
     if (PL_markstack_ptr[-1] > *PL_markstack_ptr) {
-       I32 items;
        I32 gimme = GIMME_V;
 
        (void)POPMARK;                          /* pop top */
@@ -777,6 +790,7 @@ PP(pp_mapwhile)
        ENTER;                                  /* enter inner scope */
        SAVEVPTR(PL_curpm);
 
+       /* set $_ to the new source item */
        src = PL_stack_base[PL_markstack_ptr[-1]];
        SvTEMP_off(src);
        DEFSV = src;