1 files changed, 64 insertions, 12 deletions

diff --git a/lib/sitecing_enflesher.ll b/lib/sitecing_enflesher.ll
index 5f631d7..46489c7 100644
--- a/lib/sitecing_enflesher.ll
+++ b/lib/sitecing_enflesher.ll
@@ -22,12 +22,7 @@ ID	[A-Za-z_][A-Za-z0-9_]*
     string line = yytext;
     line.erase(0,3);
     line.erase(line.length()-1);
-    outs.flush();
-    outs.close();
-    outs.clear();
-    outs.open((parser.output_basename+line).c_str(),ios::trunc);
-    if(!outs.good())
-        throw preprocessor_error(CODEPOINT,"failed to write preprocessor output");
+    outs_open(parser.output_basename+line);
     anchor();
     anchoraged = true;
 }
@@ -35,12 +30,7 @@ ID	[A-Za-z_][A-Za-z0-9_]*
     string line = yytext;
     line.erase(0,2);
     line.erase(line.length()-1);
-    outs.flush();
-    outs.close();
-    outs.clear();
-    outs.open((parser.output_basename+line).c_str(),ios::trunc);
-    if(!outs.good())
-        throw preprocessor_error(CODEPOINT,"failed to write preprocessor output");
+    outs_open(parser.output_basename+line);
     anchoraged = false;
 }
 
@@ -197,6 +187,68 @@ void sitecing_enflesher::anchor() {
     outs << "\n#line " << lineno() << " \"" << parser.skeleton << "\"\n";
     anchor_time = false;
 }
+
+void sitecing_enflesher::outs_open(const string& nfile) {
+    if(!outs_filename.empty()) {
+        outs.flush();
+        outs.close();
+        outs.clear();
+        /*
+         * compare source and destination files.
+         *
+         * one can also keep a hash for the old one and compute one for the
+         * output while writing, but I'm not sure if it's any better. Surely a
+         * bit less accurate, unless we're going to compare it in case of
+         * difference, anyway.
+         */
+        bool overwrite = false;
+        struct stat st_s, st_d;
+        string fn_s = outs_filename+".new";
+        string fn_d = outs_filename;
+        if(stat(fn_d.c_str(),&st_d)) {
+            overwrite = true;
+        }else{
+            if(stat(fn_s.c_str(),&st_s))
+                throw preprocessor_error(CODEPOINT,"failed to stat() supposedly created file");
+            if(st_s.st_size!=st_d.st_size) {
+                overwrite = true;
+            }else{
+                ifstream i_s(fn_s.c_str(),ios::in);
+                if(!i_s)
+                    throw preprocessor_error(CODEPOINT,"failed to open supposedly created file");
+                ifstream i_d(fn_d.c_str(),ios::in);
+                if(!i_d)
+                    throw preprocessor_error(CODEPOINT,"failed to open the old preprocessed source");
+                off_t remaining = st_s.st_size;
+                char t1[2048];
+                char t2[sizeof(t1)];
+                while(remaining) {
+                    int rb = remaining;
+                    if(rb>sizeof(t1))
+                        rb = sizeof(t1);
+                    if(i_s.read(t1,rb).gcount()!=rb)
+                        throw preprocessor_error(CODEPOINT,"error reading just created file");
+                    if(i_d.read(t2,rb).gcount()!=rb)
+                        throw preprocessor_error(CODEPOINT,"error reading the old preprocessed source");
+                    if(memcmp(t1,t2,rb)) {
+                        overwrite = true;
+                        break;
+                    }
+                    remaining -= rb;
+                }
+            }
+        }
+        if(overwrite) {
+            cerr << "renaming '" << fn_s << "'" << endl;
+            if(rename(fn_s.c_str(),fn_d.c_str()))
+                throw preprocessor_error(CODEPOINT,"failed to rename() generated output");
+        }
+    }
+    outs_filename = nfile;
+    outs.open((nfile+".new").c_str(),ios::trunc);
+    if(!outs.good())
+        throw preprocessor_error(CODEPOINT,"failed to write preprocessor output");
+}
 /*
  * vim:set ft=lex:
  */