[gthumb] image viewer: use a proportional zoom increment

[Paolo Bacchilega <paobac src gnome org>]

commit 45dca1c895fd96cfd27733b036887206f4f2a829
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sun Nov 29 11:16:54 2020 +0100

    image viewer: use a proportional zoom increment
    
    Fixes https://gitlab.gnome.org/GNOME/gthumb/-/issues/104

 gthumb/gth-image-viewer.c | 29 +++++++----------------------
 1 file changed, 7 insertions(+), 22 deletions(-)
---
diff --git a/gthumb/gth-image-viewer.c b/gthumb/gth-image-viewer.c
index b0b556db..812fd669 100644
--- a/gthumb/gth-image-viewer.c
+++ b/gthumb/gth-image-viewer.c
@@ -183,39 +183,24 @@ gth_image_viewer_finalize (GObject *object)
 }
 
 
-static double zooms[] = {                  0.05, 0.07, 0.10,
-                        0.15, 0.20, 0.30, 0.50, 0.75, 1.0,
-                        1.5 , 2.0 , 3.0 , 5.0 , 7.5,  10.0,
-                        15.0, 20.0, 30.0, 50.0, 75.0, 100.0};
-
-static const int nzooms = sizeof (zooms) / sizeof (gdouble);
+#define MIN_ZOOM 0.05
+#define MAX_ZOOM 100.0
+#define ZOOM_STEP 0.1
 
 
 static double
 get_next_zoom (gdouble zoom)
 {
-       int i;
-
-       i = 0;
-       while ((i < nzooms) && (zooms[i] <= zoom))
-               i++;
-       i = CLAMP (i, 0, nzooms - 1);
-
-       return zooms[i];
+       zoom = zoom + (zoom * ZOOM_STEP);
+       return CLAMP (zoom, MIN_ZOOM, MAX_ZOOM);
 }
 
 
 static double
 get_prev_zoom (double zoom)
 {
-       int i;
-
-       i = nzooms - 1;
-       while ((i >= 0) && (zooms[i] >= zoom))
-               i--;
-       i = CLAMP (i, 0, nzooms - 1);
-
-       return zooms[i];
+       zoom = zoom - (zoom * ZOOM_STEP);
+       return CLAMP (zoom, MIN_ZOOM, MAX_ZOOM);
 }