From c17054aa390ce42e867d9ce7b5fd821e0c666d45 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 5 Oct 2015 06:17:31 -0700
Subject: Copy lazy strings without forcing.

* lib.c (copy_lazy_str): New static function.
(copy_str): Use copy_lazy_str to copy lazy strings.

* txr.1: Documentation added under copy-str.
---
 lib.c | 17 ++++++++++++++++-
 txr.1 | 12 ++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/lib.c b/lib.c
index c54924fe..87abf0b9 100644
--- a/lib.c
+++ b/lib.c
@@ -2543,9 +2543,13 @@ val init_str(val str, const wchar_t *data)
   return str;
 }
 
+static val copy_lazy_str(val lstr);
+
 val copy_str(val str)
 {
-  return string(c_str(str));
+  return if3(lazy_stringp(str),
+             copy_lazy_str(str),
+             string(c_str(str)));
 }
 
 val upcase_str(val str)
@@ -5683,6 +5687,17 @@ val lazy_str(val lst, val term, val limit)
   return obj;
 }
 
+static val copy_lazy_str(val lstr)
+{
+  val obj = make_obj();
+  obj->ls.type = LSTR;
+  obj->ls.opts = obj->ls.list = obj->ls.prefix = nil;
+  obj->ls.prefix = copy_str(lstr->ls.prefix);
+  obj->ls.list = lstr->ls.list;
+  obj->ls.opts = lstr->ls.opts;
+  return obj;
+}
+
 val lazy_str_force(val lstr)
 {
   val lim, term;
diff --git a/txr.1 b/txr.1
index 309b5d8b..f8d2dd89 100644
--- a/txr.1
+++ b/txr.1
@@ -16133,6 +16133,18 @@ function constructs a new string whose contents are identical
 to
 .metn string .
 
+If
+.meta string
+is a lazy string, then a lazy string is constructed with the
+same attributes as
+.metn string .
+The new lazy string has its own copy of the prefix portion of
+.meta string
+which has been forced so far. The unforced list and separator
+string are shared between
+.meta string
+and the newly constructed lazy string.
+
 .coNP Function @ upcase-str
 .synb
 .mets (upcase-str << string )
-- 
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