From 2b249216eba72f0ea9700b7eede3711ff45a1327 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Fri, 7 Oct 2016 21:47:34 -0700
Subject: Check for self-assignment in replace_struct.

* struct.c (replace_struct): If target and source
are the same object, just do nothing and return
target.

* txr.1: Document self-assignment and return
value of replace-struct.
---
 struct.c | 28 ++++++++++++++++------------
 txr.1    | 11 +++++++++++
 2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/struct.c b/struct.c
index 066b6f44..f42c685b 100644
--- a/struct.c
+++ b/struct.c
@@ -605,20 +605,24 @@ val clear_struct(val strct, val value)
 val replace_struct(val target, val source)
 {
   const val self = lit("replace-struct");
-  struct struct_inst *tsi = struct_handle(target, self);
-  struct struct_inst *ssi = struct_handle(source, self);
-  struct struct_type *sst = ssi->type;
-  cnum nslots = sst->nslots;
-  size_t size = offsetof(struct struct_inst, slot) + sizeof (val) * nslots;
-  struct struct_inst *ssi_copy = coerce(struct struct_inst *, chk_malloc(size));
 
-  check_init_lazy_struct(source, ssi);
-  check_init_lazy_struct(target, tsi);
+  if (target != source) {
+    struct struct_inst *tsi = struct_handle(target, self);
+    struct struct_inst *ssi = struct_handle(source, self);
+    struct struct_type *sst = ssi->type;
+    cnum nslots = sst->nslots;
+    size_t size = offsetof(struct struct_inst, slot) + sizeof (val) * nslots;
+    struct struct_inst *ssi_copy = coerce(struct struct_inst *, chk_malloc(size));
+
+    check_init_lazy_struct(source, ssi);
+    check_init_lazy_struct(target, tsi);
+
+    memcpy(ssi_copy, ssi, size);
+    free(tsi);
+    target->co.handle = coerce(mem_t *, ssi_copy);
+    target->co.cls = source->co.cls;
+  }
 
-  memcpy(ssi_copy, ssi, size);
-  free(tsi);
-  target->co.handle = coerce(mem_t *, ssi_copy);
-  target->co.cls = source->co.cls;
   return target;
 }
 
diff --git a/txr.1 b/txr.1
index c1fc78ce..c7fde830 100644
--- a/txr.1
+++ b/txr.1
@@ -21521,6 +21521,17 @@ prior to using
 .codn replace-struct ,
 or to take other measures to handle the situation.
 
+If the
+.meta target-obj
+and
+.meta source-obj
+arguments are the same object,
+.code replace-struct
+has no effect.
+
+The return value is
+.metn target-obj .
+
 .coNP Function @ method
 .synb
 .mets (method < struct-obj << slot-name )
-- 
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