[gnumeric] Column Fitting: don't go higher than 15 significant digits by default.

[Morten Welinder <mortenw src gnome org>]

commit 5a5ef23e153b8db04c4126b0257e4b2b5e8318f9
Author: Morten Welinder <terra gnome org>
Date:   Mon May 7 14:20:22 2018 -0400

    Column Fitting: don't go higher than 15 significant digits by default.

 ChangeLog            |    5 +++++
 NEWS                 |    1 +
 src/rendered-value.c |   44 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+), 0 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index d40e6a9..02a1358 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2018-05-07  Morten Welinder  <terra gnome org>
 
+       * src/rendered-value.c (gnm_rendered_value_new): When auto-fitting
+       a column, request no more room than for 15 significant digits.
+       Fixes #754391.  You can still get the maximum precision by
+       widening the column.
+
        * src/wbc-gtk.c (wbc_gtk_init): Use a resource for the default ui
        definition.
        (wbcg_auto_expr_value_changed): Limit the width of numbers
diff --git a/NEWS b/NEWS
index 20d2a7a..defb9a4 100644
--- a/NEWS
+++ b/NEWS
@@ -6,6 +6,7 @@ Morten:
        * Exit with non-zero with we fail to load file on command line.
        * Fix undo crash.  [#795861]
        * Introspection fixes.
+       * Don't offer more than 15 significant digits by default.  [#754391]
 
 --------------------------------------------------------------------------
 Gnumeric 1.12.40
diff --git a/src/rendered-value.c b/src/rendered-value.c
index 4b2c032..9387a14 100644
--- a/src/rendered-value.c
+++ b/src/rendered-value.c
@@ -62,6 +62,9 @@ static int rv_allocations;
 #define CHUNK_FREE(p,v) (rv_allocations--, g_slice_free1 (sizeof(*v),(v)))
 #endif
 
+// Number of decimal digits in a gnm_float rounded down to an integer.
+static int sane_digits;
+
 static gboolean
 debug_rv (void)
 {
@@ -218,6 +221,25 @@ rv_adjust_attributes (PangoAttrList *markup, double zoom, double scale, int rise
                                &raat);
 }
 
+static gboolean
+too_many_digits (const char *s)
+{
+       int count = 0;
+       const char *p;
+
+       // Count significant digits
+       for (p = s; *p; p = g_utf8_next_char (p)) {
+               gunichar uc = g_utf8_get_char (p);
+               if (uc == '0' && count == 0)
+                       continue;
+               if (g_unichar_isdigit (uc))
+                       count++;
+               if (uc == 'e' || uc == 'E')
+                       break;
+       }
+
+       return count > sane_digits;
+}
 
 /**
  * gnm_rendered_value_new: (skip)
@@ -483,6 +505,8 @@ gnm_rendered_value_new (GnmCell const *cell,
                                col_width_pixels = ci->size_pixels;
                        }
                        col_width_pixels -= (GNM_COL_MARGIN + GNM_COL_MARGIN + 1);
+                       if (col_width_pixels < 0)
+                               col_width_pixels = 0;
                        col_width = col_width_pixels * PANGO_SCALE;
                }
 
@@ -490,6 +514,24 @@ gnm_rendered_value_new (GnmCell const *cell,
                                         cell->value,
                                         col_width, date_conv, TRUE);
 
+               // If we are formatting a number as General without a limit
+               // on size [i.e., we autofitting a column] then avoid excess
+               // precision.  This is somewhat hacky.
+               if (col_width == -1 &&
+                   go_format_is_general (format) &&
+                   VALUE_IS_FLOAT (cell->value) &&
+                   font->go.metrics->min_digit_width > 0 &&
+                   too_many_digits (pango_layout_get_text (layout))) {
+                       int width;
+                       int delta = (font->go.metrics->min_digit_width + 1) / 2;
+                       pango_layout_get_size (layout, &width, NULL);
+                       col_width = width - delta;
+                       err = gnm_format_layout (layout,
+                                                font->go.metrics, format,
+                                                cell->value,
+                                                col_width, date_conv, TRUE);
+               }
+
                switch (err) {
                case GO_FORMAT_NUMBER_DATE_ERROR:
                        pango_layout_set_text (layout, "", -1);
@@ -686,6 +728,8 @@ gnm_rvc_remove (GnmRenderedValueCollection *rvc, GnmCell const *cell)
 void
 gnm_rendered_value_init (void)
 {
+       sane_digits = (int)gnm_floor (GNM_MANT_DIG * gnm_log10 (FLT_RADIX));
+
 #if USE_RV_POOLS
        rendered_value_pool =
                go_mem_chunk_new ("rendered value pool",