Use a "format accumulator" for error strings

This makes it possible to internationalize strings while still using
I/O streams.  For example:

    std::cout << ACCUM(_("Hello to %1 and %2!") << "me" << "you")
              << std::endl;

1 files changed, 70 insertions, 0 deletions

diff --git a/src/accum.cc b/src/accum.cc
new file mode 100644
index 00000000..ac98c8cf
--- /dev/null
+++ b/src/accum.cc
@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2003-2009, John Wiegley.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ *
+ * - Neither the name of New Artisans LLC nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "utils.h"
+
+namespace ledger {
+
+std::streamsize straccbuf::xsputn(const char * s, std::streamsize num)
+{
+  if (index == 0) {
+    // The first item received is the format string
+    str = std::string(s, num);
+    index++;
+    return num;
+  }
+  else {
+    std::ostringstream buf;
+
+    // Every item thereafter is an argument that substitutes for %# in the
+    // format string
+    for (const char * p = str.c_str(); *p; p++) {
+      if (*p == '%') {
+	const char * q = p + 1;
+	if (*q && *q != '%' && std::isdigit(*q) &&
+	    std::size_t(*q - '0') == index) {
+	  p++;
+	  buf << std::string(s, num);
+	} else {
+	  buf << *p;
+	}
+      } else {
+	buf << *p;
+      }
+    }
+    str = buf.str();
+    index++;
+
+    return num;
+  }
+}
+
+} // namespace ledger