Add gzfilebuf::approxOffset() and sg_gzifstream::approxOffset()

gzfilebuf::approxOffset() is a wrapper for zlib's offset() function.
It can be useful to implement progress indicators and such.

simgear/misc/sgstream.cxx

@@ -25,6 +25,8 @@
 #include <ctype.h> // isspace()
 #include <cerrno>
 
+#include <zlib.h>
+
 #include "sgstream.hxx"
 
 #include <simgear/misc/sg_path.hxx>
@@ -95,6 +97,11 @@ sg_gzifstream::attach( int fd, ios_openmode io_mode )
     gzbuf.attach( fd, io_mode );
 }
 
+z_off_t
+sg_gzifstream::approxOffset() {
+  return gzbuf.approxOffset();
+}
+
 //
 // Manipulators
 //

simgear/misc/sgstream.hxx

@@ -39,6 +39,7 @@
 
 #include <string>
 
+#include <zlib.h>
 #include <simgear/misc/zfstream.hxx>
 
 class SGPath;
@@ -91,6 +92,15 @@ public:
     /** @return true if the file is successfully opened, false otherwise. */
     bool is_open() { return gzbuf.is_open(); }
 
+    /**
+     * @return the current offset in the file being read or written.
+     * The offset corresponds to compressed data if the file is compressed,
+     * and is influenced by buffering performed in zlib, hence the "approx"
+     * qualifier. It should be suitable for progress indicators and such,
+     * though.
+     */
+    z_off_t approxOffset();
+
 private:
     // Not defined!
     sg_gzifstream( const sg_gzifstream& );

simgear/misc/zfstream.cxx

@@ -30,7 +30,10 @@
 #include <fcntl.h>
 
 #include <simgear/misc/strutils.hxx>
+#include <simgear/debug/logstream.hxx>
+#include <simgear/structure/exception.hxx>
 
+#include <zlib.h>
 #include "zfstream.hxx"
 
 //
@@ -180,6 +183,27 @@ gzfilebuf::setcompressionstrategy( int comp_strategy )
     return gzsetparams(file, -2, comp_strategy);
 }
 
+z_off_t
+gzfilebuf::approxOffset() {
+    z_off_t res = gzoffset(file);
+
+    if (res == -1) {
+        int errnum;
+        std::string errMsg = "gzoffset() error: ";
+        const char *gzMsg = gzerror(file, &errnum);
+
+        if (errnum == Z_ERRNO) {
+            errMsg += simgear::strutils::error_string(errno);
+        } else {
+            errMsg += std::string(gzMsg);
+        }
+
+        SG_LOG( SG_GENERAL, SG_ALERT, errMsg );
+        throw sg_io_exception(errMsg);
+    }
+
+    return res;
+}
 
 std::streampos
 gzfilebuf::seekoff( std::streamoff, ios_seekdir, ios_openmode )

simgear/misc/zfstream.hxx

@@ -89,6 +89,15 @@ public:
     /** @return true if open, false otherwise */
     bool is_open() const { return (file != NULL); }
 
+    /**
+     * @return the current offset in the file being read or written.
+     * The offset corresponds to compressed data if the file is compressed,
+     * and is influenced by buffering performed in zlib, hence the "approx"
+     * qualifier. It should be suitable for progress indicators and such,
+     * though.
+     */
+    z_off_t approxOffset();
+
     /** @return stream position */
     virtual std::streampos seekoff( std::streamoff off, ios_seekdir way, ios_openmode which );