From 2c1c44c4fe7609cdfa3f04c2f156a2da773a5b0a Mon Sep 17 00:00:00 2001
From: Andrew Bettison <andrewb@zip.com.au>
Date: Tue, 10 Jul 2012 19:36:25 +0930
Subject: [PATCH] Add strbuf_trunc() primitive, fix bug in strbuf_substr()

---
 strbuf.c | 18 +++++++++++++++++-
 strbuf.h | 28 ++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/strbuf.c b/strbuf.c
index d2d0da1b..b89a75c5 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -98,7 +98,7 @@ char *strbuf_substr(const_strbuf sb, int offset)
 {
   char *s;
   if (offset < 0) {
-    s = sb->end + offset;
+    s = (sb->current < sb->end ? sb->current : sb->end) + offset;
     if (s < sb->start)
       s = sb->start;
   } else {
@@ -108,3 +108,19 @@ char *strbuf_substr(const_strbuf sb, int offset)
   }
   return s;
 }
+
+strbuf strbuf_trunc(strbuf sb, int offset)
+{
+  char *s;
+  if (offset < 0) {
+    s = (sb->current < sb->end ? sb->current : sb->end) + offset;
+    if (s < sb->start)
+      s = sb->start;
+  } else {
+    s = sb->start + offset;
+    if (s > sb->end)
+      s = sb->end;
+  }
+  sb->current = s;
+  return sb;
+}
diff --git a/strbuf.h b/strbuf.h
index caf0ecb3..c018255c 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -311,6 +311,34 @@ __STRBUF_INLINE char *strbuf_str(const_strbuf sb) {
 char *strbuf_substr(const_strbuf sb, int offset);
 
 
+/** Truncate the string in the strbuf to a given offset.  If the offset is
+ * negative, then it is taken from the end of the string, ie, the length of the
+ * string is added to it.  If the string is shorter than the given offset, then
+ * it is unchanged.  Otherwise, a terminating nul char is written at the offset
+ * and the string's length truncated accordingly.  Return a pointer to the
+ * strbuf so that operations can be chained in a single line.
+ *
+ * After the operation:
+ *      len = strbuf_len(sb);
+ *      strbuf_trunc(sb, off);
+ * the following invariants hold:
+ * where len <= off, sb is unchanged;
+ * where 0 <= off < len:
+ *      strbuf_count(sb) == off
+ *      strbuf_len(sb) == off
+ * where -len <= off < 0:
+ *      strbuf_count(sb) == len + off
+ *      strbuf_len(sb) == len + off
+ * where off < -len:
+ *      strbuf_count(sb) == 0
+ *      strbuf_len(sb) == 0
+ *      strbuf_str(sb)[0] == '\0'
+ *
+ * @author Andrew Bettison <andrew@servalproject.com>
+ */
+strbuf strbuf_trunc(strbuf sb, int offset);
+
+
 /** Return true if the given strbuf is "empty", ie, not modified since being
  * initialised to STRUCT_STRBUF_EMPTY or with strbuf_init(sb, NULL, 0);
  *