1 files changed, 66 insertions, 4 deletions

diff --git a/src/marker.c b/src/marker.c
index 866dd6ac9c0..dcdadd705ad 100644
--- a/src/marker.c
+++ b/src/marker.c
@@ -32,6 +32,10 @@ static int cached_bytepos;
 static struct buffer *cached_buffer;
 static int cached_modiff;
 
+/* Nonzero means enable debugging checks on byte/char correspondences.  */
+
+static int byte_debug_flag;
+
 clear_charpos_cache (b)
      struct buffer *b;
 {
@@ -58,7 +62,12 @@ clear_charpos_cache (b)
   int changed = 0;                                                      \
                                                                         \
   if (this_charpos == charpos)                                          \
-    return (BYTEPOS);                                                   \
+    {                                                                   \
+      int value = (BYTEPOS);                                            \
+      if (byte_debug_flag)                                              \
+        byte_char_debug_check (b, charpos, value);                      \
+      return value;                                                     \
+    }                                                                   \
   else if (this_charpos > charpos)                                      \
     {                                                                   \
       if (this_charpos < best_above)                                    \
@@ -78,11 +87,37 @@ clear_charpos_cache (b)
   if (changed)                                                          \
     {                                                                   \
       if (best_above - best_below == best_above_byte - best_below_byte) \
-        return best_below_byte + (charpos - best_below);                \
+        {                                                               \
+          int value = best_below_byte + (charpos - best_below);         \
+          if (byte_debug_flag)                                          \
+            byte_char_debug_check (b, charpos, value);                  \
+          return value;                                                 \
+        }                                                               \
     }                                                                   \
 }
 
 int
+byte_char_debug_check (b, charpos, bytepos)
+     struct buffer *b;
+     int charpos, bytepos;
+{
+  int nchars = 0;
+
+  if (bytepos > BUF_GPT_BYTE (b))
+    {
+      nchars = chars_in_text (BUF_BEG_ADDR (b),
+                              BUF_GPT_BYTE (b) - BUF_BEG_BYTE (b));
+      nchars += chars_in_text (BUF_GAP_END_ADDR (b),
+                               bytepos - BUF_GPT_BYTE (b));
+    }
+  else
+    nchars = chars_in_text (BUF_BEG_ADDR (b), bytepos - BUF_BEG_BYTE (b));
+
+  if (charpos - 1 != nchars)
+    abort ();
+}
+
+int
 charpos_to_bytepos (charpos)
      int charpos;
 {
@@ -169,6 +204,9 @@ buf_charpos_to_bytepos (b, charpos)
           set_marker_both (marker, Qnil, best_below, best_below_byte);
         }
 
+      if (byte_debug_flag)
+        byte_char_debug_check (b, charpos, best_below_byte);
+
       cached_buffer = b;
       cached_modiff = BUF_MODIFF (b);
       cached_charpos = best_below;
@@ -196,6 +234,9 @@ buf_charpos_to_bytepos (b, charpos)
           set_marker_both (marker, Qnil, best_above, best_above_byte);
         }
 
+      if (byte_debug_flag)
+        byte_char_debug_check (b, charpos, best_above_byte);
+
       cached_buffer = b;
       cached_modiff = BUF_MODIFF (b);
       cached_charpos = best_above;
@@ -218,7 +259,12 @@ buf_charpos_to_bytepos (b, charpos)
   int changed = 0;                                                      \
                                                                         \
   if (this_bytepos == bytepos)                                          \
-    return (CHARPOS);                                                   \
+    {                                                                   \
+      int value = (CHARPOS);                                            \
+      if (byte_debug_flag)                                              \
+        byte_char_debug_check (b, value, bytepos);                      \
+      return value;                                                     \
+    }                                                                   \
   else if (this_bytepos > bytepos)                                      \
     {                                                                   \
       if (this_bytepos < best_above_byte)                               \
@@ -238,7 +284,12 @@ buf_charpos_to_bytepos (b, charpos)
   if (changed)                                                          \
     {                                                                   \
       if (best_above - best_below == best_above_byte - best_below_byte) \
-        return best_below + (bytepos - best_below_byte);                \
+        {                                                               \
+          int value = best_below + (bytepos - best_below_byte);         \
+          if (byte_debug_flag)                                          \
+            byte_char_debug_check (b, value, bytepos);                  \
+          return value;                                                 \
+        }                                                               \
     }                                                                   \
 }
 
@@ -319,6 +370,9 @@ buf_bytepos_to_charpos (b, bytepos)
           set_marker_both (marker, Qnil, best_below, best_below_byte);
         }
 
+      if (byte_debug_flag)
+        byte_char_debug_check (b, best_below, bytepos);
+
       cached_buffer = b;
       cached_modiff = BUF_MODIFF (b);
       cached_charpos = best_below;
@@ -346,6 +400,9 @@ buf_bytepos_to_charpos (b, bytepos)
           set_marker_both (marker, Qnil, best_above, best_above_byte);
         }
 
+      if (byte_debug_flag)
+        byte_char_debug_check (b, best_above, bytepos);
+
       cached_buffer = b;
       cached_modiff = BUF_MODIFF (b);
       cached_charpos = best_above;
@@ -838,4 +895,9 @@ syms_of_marker ()
   defsubr (&Smarker_insertion_type);
   defsubr (&Sset_marker_insertion_type);
   defsubr (&Sbuffer_has_markers_at);
+
+  DEFVAR_BOOL ("byte-debug-flag", &byte_debug_flag,
+   "Non-nil enables debugging checks in byte/char position conversions.");
+  byte_debug_flag = 0;
+
 }