From 41e5183c51e48cd2854099159a173da10eb30b8b Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sat, 9 Jan 2016 20:22:21 -0800
Subject: Rework lazy string optimization done in TXR 118.

This is also a bugfix.  Padding up the index to be at least
1024 characters longer than the existing prefix was dumb
and wrong; it changes the semantics of code which restores the
list from the lazy string, like the @(freeform) directive.
How much of the string is forced is visible to the caller!

* lib.c (lazy_str_force, lazy_str_force_upto): Don't
collect pieces from the lazy list and then catenate them
in one pass. Instead, use the existing function string_extend,
which grows the string exponentially.
---
 lib.c | 36 ++++++++++--------------------------
 1 file changed, 10 insertions(+), 26 deletions(-)

diff --git a/lib.c b/lib.c
index 1d034edf..cdc7ec1f 100644
--- a/lib.c
+++ b/lib.c
@@ -6146,18 +6146,18 @@ static val copy_lazy_str(val lstr)
 
 val lazy_str_force(val lstr)
 {
-  val lim, term;
-  list_collect_decl (strlist, ptail);
+  val lim, term, pfx;
   type_check(lstr, LSTR);
   lim = lstr->ls.props->limit;
   term = lstr->ls.props->term;
+  pfx = lstr->ls.prefix;
 
   while ((!lim || gt(lim, zero)) && lstr->ls.list) {
     val next = pop(&lstr->ls.list);
     if (!next)
       break;
-    ptail = list_collect(ptail, next);
-    ptail = list_collect(ptail, term);
+    string_extend(pfx, next);
+    string_extend(pfx, term);
     if (lim)
       lim = minus(lim, one);
   }
@@ -6165,11 +6165,6 @@ val lazy_str_force(val lstr)
   if (lim)
     set(mkloc(lstr->ls.props->limit, lstr), lim);
 
-  if (strlist) {
-    push(lstr->ls.prefix, &strlist);
-    set(mkloc(lstr->ls.prefix, lstr), cat_str(strlist, nil));
-  }
-
   return lstr->ls.prefix;
 }
 
@@ -6198,28 +6193,22 @@ val lazy_str_put(val lstr, val stream)
 val lazy_str_force_upto(val lstr, val index)
 {
   uses_or2;
-  val lim, term, ltrm, len, effidx = index;
-  list_collect_decl (strlist, ptail);
+  val lim, term, ltrm, pfx, len;
   type_check(lstr, LSTR);
   lim = lstr->ls.props->limit;
   term = lstr->ls.props->term;
   ltrm = length_str(term);
-  len = length_str(lstr->ls.prefix);
-
-  if (lt(effidx, len))
-    return t;
+  pfx = lstr->ls.prefix;
+  len = length_str(pfx);
 
-  if (minus(effidx, len) < num_fast(1024))
-    effidx = plus(len, num_fast(1024));
-
-  while (ge(effidx, len) && lstr->ls.list &&
+  while (ge(index, len) && lstr->ls.list &&
          or2(null(lim),gt(lim,zero)))
   {
     val next = pop(&lstr->ls.list);
     if (!next)
       break;
-    ptail = list_collect(ptail, next);
-    ptail = list_collect(ptail, term);
+    string_extend(pfx, next);
+    string_extend(pfx, term);
     if (lim)
       lim = minus(lim, one);
     len = plus(len, length_str(next));
@@ -6229,11 +6218,6 @@ val lazy_str_force_upto(val lstr, val index)
   if (lim)
     set(mkloc(lstr->ls.props->limit, lstr), lim);
 
-  if (strlist) {
-    push(lstr->ls.prefix, &strlist);
-    set(mkloc(lstr->ls.prefix, lstr), cat_str(strlist, nil));
-  }
-
   return lt(index, len);
 }
 
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