[goffice] dtoa: implement some approximation of shortest-string versions.

[Morten Welinder <mortenw src gnome org>]

commit 02df4e11989d375bd3f16b50337b97292bd38b1c
Author: Morten Welinder <terra gnome org>
Date:   Tue Apr 22 21:00:39 2014 -0400

    dtoa: implement some approximation of shortest-string versions.

 ChangeLog              |    3 ++
 goffice/math/go-dtoa.c |   53 ++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 52 insertions(+), 4 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 1e24497..c10d1e5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2014-04-22  Morten Welinder  <terra gnome org>
 
+       * goffice/math/go-dtoa.c (go_dtoa): Mostly implement the
+       shortest-version flag.
+
        * goffice/graph/gog-object-xml.c (gog_object_write_property_sax):
        Properly handle float/double properties.
 
diff --git a/goffice/math/go-dtoa.c b/goffice/math/go-dtoa.c
index 72db547..e2e42a7 100644
--- a/goffice/math/go-dtoa.c
+++ b/goffice/math/go-dtoa.c
@@ -452,6 +452,26 @@ parse_fmt (const char *fmt, va_list args, gboolean *is_long,
                *d = va_arg (args, double);
 }
 
+static long double
+strto (const char *str, gboolean is_long, gboolean is_ascii)
+{
+       if (is_long) {
+#ifdef GOFFICE_WITH_LONG_DOUBLE
+               /*
+                * FIXME: ignore ascii flag for now.  Given the limited
+                * and checked usage of this function this should still
+                * be safe, if suboptimal.
+                */
+               return go_strtold (str, NULL);
+#else
+               return go_nan;
+#endif
+       } else {
+               return is_ascii
+                       ? g_ascii_strtod (str, NULL)
+                       : go_strtod (str, NULL);
+       }
+}
 
 
 void
@@ -462,18 +482,43 @@ go_dtoa (GString *dst, const char *fmt, ...)
        long double d;
        gboolean is_long;
        gboolean debug = FALSE;
+       size_t oldlen;
 
        va_start (args, fmt);
        parse_fmt (fmt, args, &is_long, &w, &p, &fl, &t, &d);
        va_end (args);
 
-       /*
-        * FLAG_SHORTEST isn't fully implemented yet.  For now we just
-        * ensure a roundtrip.
-        */
        if (fl & FLAG_SHORTEST) p = is_long ? 20 : 17;
+       oldlen = (fl & FLAG_TRUNCATE) ? 0 : dst->len;
 
        if (debug) g_printerr ("%Lg [%s] t=%c  p=%d\n", d, fmt, t, p);
        fmt_fp (dst, d, w, p, fl, t);
        if (debug) g_printerr ("  --> %s\n", dst->str);
+
+       if (fl & FLAG_SHORTEST) {
+               const char *dec = (fl & FLAG_ASCII)
+                       ? "."
+                       : go_locale_get_decimal()->str;
+               const char *dot = strstr (dst->str + oldlen, dec);
+               if (dot) {
+                       /*
+                        * This is crude.  We have a dot, so try to render
+                        * the same number with less precision and check
+                        * that the result round-trips.
+                        */
+                       GString *alt = g_string_new (NULL);
+                       long double dalt;
+
+                       fmt_fp (alt, d, w, p - 1, fl, t);
+
+                       dalt = strto (alt->str, is_long, (fl & FLAG_ASCII));
+
+                       if (dalt == d) {
+                               g_string_truncate (dst, oldlen);
+                               go_string_append_gstring (dst, alt);
+                               if (debug) g_printerr ("  --> %s\n", dst->str);
+                       }
+                       g_string_free (alt, TRUE);
+               }
+       }
 }