* lib.c (search_str): Support negative starting index.

Hoist uselessly repeated c_str operation out of loop.

* txr.1: Document negative starting index for search-str.

3 files changed, 27 insertions, 6 deletions

diff --git a/ChangeLog b/ChangeLog
index d2a40266..906b02b2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2014-07-19  Kaz Kylheku  <kaz@kylheku.com>
 
+	* lib.c (search_str): Support negative starting index.
+	Hoist uselessly repeated c_str operation out of loop.
+
+	* txr.1: Document negative starting index for search-str.
+
+2014-07-19  Kaz Kylheku  <kaz@kylheku.com>
+
 	* hash.c (hash_construct): Nullify the pairs argument so that
 	it works correctly with an empty vector.
 
diff --git a/lib.c b/lib.c
index 0f705211..3f38ab97 100644
--- a/lib.c
+++ b/lib.c
@@ -2145,11 +2145,14 @@ val search_str(val haystack, val needle, val start_num, val from_end)
     const wchar_t *n = c_str(needle), *h;
 
     if (!h_is_lazy) {
-      do {
-        const wchar_t *f;
-        h = c_str(haystack);
+      h = c_str(haystack);
+
+      if (start < 0)
+        start += wcslen(h);
 
-        f = wcsstr(h + start, n);
+    nonlazy:
+      do {
+        const wchar_t *f = wcsstr(h + start, n);
 
         if (f)
           pos = f - h;
@@ -2159,6 +2162,13 @@ val search_str(val haystack, val needle, val start_num, val from_end)
     } else {
       size_t ln = c_num(length_str(needle));
 
+      if (start < 0) {
+        lazy_str_force(haystack);
+        h = c_str(haystack->ls.prefix);
+        start += wcslen(h);
+        goto nonlazy;
+      }
+
       do {
         lazy_str_force_upto(haystack, plus(num(start + 1), length_str(needle)));
         h = c_str(haystack->ls.prefix);
diff --git a/txr.1 b/txr.1
index 666d238c..b3f4b3b8 100644
--- a/txr.1
+++ b/txr.1
@@ -9337,8 +9337,12 @@ The search-str function finds an occurrence of the string <needle> inside
 the <haystack> string and returns its position. If no such occurrence exists,
 it returns nil.
 
-If a <start> argument is specified, it gives the starting index for the
-search. If the <from-end> argument is specified and is not nil, it means
+If a <start> argument is not specified, it defaults to zero. If it is
+a non-negative integer, it specifies the starting character position for
+the search.  Negative values of <start> indicate positions from the end of the
+string, such that -1 is the last character of the string.
+
+If the <from-end> argument is specified and is not nil, it means
 that the search is conducted right-to-left. If multiple matches are possible,
 it will find the rightmost one rather than the leftmost one.