[gegl] buffer: add gegl_memeq_zero()

[Ell <ell src gnome org>]

commit b0c68ce4da747c609ecfac0d6637c6b3e8a5603c
Author: Ell <ell_se yahoo com>
Date:   Fri Nov 30 05:25:02 2018 -0500

    buffer: add gegl_memeq_zero()
    
    ... which checks if a block of memory is fully zeroed.

 gegl/buffer/gegl-buffer.h    | 13 ++++++++
 gegl/buffer/gegl-rectangle.c | 72 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)
---
diff --git a/gegl/buffer/gegl-buffer.h b/gegl/buffer/gegl-buffer.h
index 3553ccaef..042d7a286 100644
--- a/gegl/buffer/gegl-buffer.h
+++ b/gegl/buffer/gegl-buffer.h
@@ -755,6 +755,19 @@ void     gegl_free                    (gpointer mem);
  */
 gpointer gegl_calloc (gsize size, int n_memb) G_GNUC_MALLOC;
 
+/**
+ * gegl_memeq_zero: (skip)
+ * @ptr: pointer to the memory block
+ * @size: block size
+ *
+ * Checks if all the bytes of the memory block @ptr, of size @size,
+ * are equal to zero.
+ *
+ * Returns: TRUE if all the bytes are equal to zero.
+ */
+gboolean gegl_memeq_zero (gconstpointer ptr,
+                          gsize         size);
+
 /**
  * gegl_memset_pattern: (skip)
  * @dst_ptr: pointer to copy to
diff --git a/gegl/buffer/gegl-rectangle.c b/gegl/buffer/gegl-rectangle.c
index 391e2c129..4c8b31afb 100644
--- a/gegl/buffer/gegl-rectangle.c
+++ b/gegl/buffer/gegl-rectangle.c
@@ -304,6 +304,78 @@ gegl_free (gpointer buf)
   g_free ((gchar*)buf - *((guint8*)buf -1));
 }
 
+gboolean
+gegl_memeq_zero (gconstpointer ptr,
+                 gsize         size)
+{
+  const guint8 *p = ptr;
+
+  if (size >= 1 && (guintptr) p & 0x1)
+    {
+      if (*(const guint8 *) p)
+        return FALSE;
+
+      p    += 1;
+      size -= 1;
+    }
+
+  if (size >= 2 && (guintptr) p & 0x2)
+    {
+      if (*(const guint16 *) p)
+        return FALSE;
+
+      p    += 2;
+      size -= 2;
+    }
+
+  if (size >= 4 && (guintptr) p & 0x4)
+    {
+      if (*(const guint32 *) p)
+        return FALSE;
+
+      p    += 4;
+      size -= 4;
+    }
+
+  while (size >= 8)
+    {
+      if (*(const guint64 *) p)
+        return FALSE;
+
+      p    += 8;
+      size -= 8;
+    }
+
+  if (size >= 4)
+    {
+      if (*(const guint32 *) p)
+        return FALSE;
+
+      p    += 4;
+      size -= 4;
+    }
+
+  if (size >= 2)
+    {
+      if (*(const guint16 *) p)
+        return FALSE;
+
+      p    += 2;
+      size -= 2;
+    }
+
+  if (size >= 1)
+    {
+      if (*(const guint8 *) p)
+        return FALSE;
+
+      p    += 1;
+      size -= 1;
+    }
+
+  return TRUE;
+}
+
 void
 gegl_memset_pattern (void * restrict       dst_ptr,
                             const void * restrict src_ptr,