From 2ea6ca3ef8151d94d2a3f306a9035ae7ae001fcd Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 9 Jun 2014 19:37:51 -0700
Subject: * lib.c (match_str): Extended to suffix testing, with a negative
 start argument.

* txr.1: Documented.
---
 ChangeLog |  7 +++++++
 lib.c     | 31 +++++++++++++++++++++++--------
 txr.1     | 19 ++++++++++++++-----
 3 files changed, 44 insertions(+), 13 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index d60aac61..2fab8a4d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2014-06-09  Kaz Kylheku  <kaz@kylheku.com>
+
+	* lib.c (match_str): Extended to suffix testing, with a negative
+	start argument.
+
+	* txr.1: Documented.
+
 2014-06-09  Kaz Kylheku  <kaz@kylheku.com>
 
 	* Makefile: fix broken tests; numerous test cases output
diff --git a/lib.c b/lib.c
index d46ab2e6..b0118f45 100644
--- a/lib.c
+++ b/lib.c
@@ -2077,15 +2077,30 @@ val match_str(val bigstr, val str, val pos)
 
   pos = default_arg(pos, zero);
 
-  for (i = zero;
-       length_str_gt(bigstr, p = plus(pos, i)) && length_str_gt(str, i);
-       i = plus(i, one))
-  {
-    if (chr_str(bigstr, p) != chr_str(str, i))
-      return nil;
-  }
+  if (ge(pos, zero)) {
+    for (i = zero;
+         length_str_gt(bigstr, p = plus(pos, i)) && length_str_gt(str, i);
+         i = plus(i, one))
+    {
+      if (chr_str(bigstr, p) != chr_str(str, i))
+        return nil;
+    }
 
-  return length_str_le(str, i) ? t : nil;
+    return length_str_le(str, i) ? t : nil;
+  } else {
+    pos = plus(pos, length(bigstr));
+    pos = plus(minus(pos, length(str)), one);
+
+    for (i = minus(length(str), one);
+         ge(i, zero) && ge(p = plus(pos, i), zero);
+         i = minus(i, one))
+    {
+      if (chr_str(bigstr, p) != chr_str(str, i))
+        return nil;
+    }
+
+    return lt(i, zero) ? t : nil;
+  }
 }
 
 val match_str_tree(val bigstr, val tree, val pos)
diff --git a/txr.1 b/txr.1
index 9ac40b64..1d8a617b 100644
--- a/txr.1
+++ b/txr.1
@@ -8884,12 +8884,21 @@ Syntax:
 .TP
 Description:
 
-The match-str function determines how many characters of <littlestring> match a
-prefix of <bigstring>.
+Without the <start> argument, the match-str function determines whether
+<littlestring> is a prefix of <bigstring>, returning a t or nil
+indication.
 
-If the <start> argument is specified, then the function tests how many
-characters of <littlestring> match a prefix of that portion of <bigstring>
-which starts at the given position.
+If the <start> argument is specified, and is a non-negative integer, then the
+function tests whether <littlestring> matches a prefix of that portion of
+<bigstring> which starts at the given position.
+
+If the <start> argument is a negative integer, then match-str determines
+whether <littlestring> is a suffix of <bigstring>, ending on that position
+of bigstring, where -1 denotes the last character of <bigstring>,
+-2 the second last one and so on.
+
+If <start> is -1, then this corresponds to testing whether <littlestring>
+is a suffix of <bigstring>.
 
 .SS Function match-str-tree
 
-- 
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