Fix for bug #28121 "INSERT or UPDATE into DOUBLE(200,0) field being truncated to 31 digits"

When storing a large number to a FLOAT or DOUBLE field with fixed length, it could be incorrectly truncated if the field's length was greater than 31.

This patch also does some code cleanups to be able to reuse code which is common between Field_float::store() and Field_double::store().


include/m_string.h:
  Added declarations for log_10 and log_01 from strtod.c
mysql-test/r/type_float.result:
  Added the testcase for bug #28121 "INSERT or UPDATE into DOUBLE(200,0) field being truncated to 31 digits"
mysql-test/t/type_float.test:
  Added the testcase for bug #28121 "INSERT or UPDATE into DOUBLE(200,0) field being truncated to 31 digits"
sql/field.cc:
  Moved common code from Field_float::store() and Field_double:store() to Field_real::truncate()
  Fixed the algorithm to not truncate large input numbers if the field length is greater than 31.
  Fixed rounding to not depend on FLT_MAX/DBL_MAX constants.
sql/field.h:
  Moved not_fixed member from Field_double to Field_real to allow code reuse between Field_float::store() and Field_double::store()
  Added truncate() method to Field_real which is used by both Field_float and Field_double
sql/init.cc:
  log_10[] and log_01[] are now defined as statical arrays in strtod.c, no need to pre-computed them.
sql/item_cmpfunc.cc:
  log_01[] now starts from 1e0, not from 1e-1 for consistency.
sql/mysql_priv.h:
  Moved log_10[] and log_01[] from mysqld.cc to libmystrings.
sql/mysqld.cc:
  Moved log_10[] and log_01[] from mysqld.cc to libmystrings.
strings/strtod.c:
  Define and use log_10[] and log_01[] as static arrays of constants instead of values pre-computed at startup.

include/m_string.h

@@ -105,6 +105,10 @@ extern char *stpcpy(char *, const char *);	/* For AIX with gcc 2.95.3 */
 extern char NEAR _dig_vec_upper[];
 extern char NEAR _dig_vec_lower[];
 
+/* Defined in strtod.c */
+extern const double log_10[310];
+extern const double log_01[310];
+
 #ifdef BAD_STRING_COMPILER
 #define strmov(A,B)  (memccpy(A,B,0,INT_MAX)-1)
 #else

mysql-test/r/type_float.result

@@ -344,3 +344,36 @@ create table t1 (s1 float(0,2));
 ERROR 42000: For float(M,D), double(M,D) or decimal(M,D), M must be >= D (column 's1').
 create table t1 (s1 float(1,2));
 ERROR 42000: For float(M,D), double(M,D) or decimal(M,D), M must be >= D (column 's1').
+create table t1 (f1 double(200, 0));
+insert into t1 values (1e199), (-1e199);
+insert into t1 values (1e200), (-1e200);
+insert into t1 values (2e200), (-2e200);
+Warnings:
+Warning	1264	Out of range value adjusted for column 'f1' at row 1
+Warning	1264	Out of range value adjusted for column 'f1' at row 2
+select f1 + 0e0 from t1;
+f1 + 0e0
+1e+199
+-1e+199
+1e+200
+-1e+200
+1e+200
+-1e+200
+drop table t1;
+create table t1 (f1 float(30, 0));
+insert into t1 values (1e29), (-1e29);
+insert into t1 values (1e30), (-1e30);
+insert into t1 values (2e30), (-2e30);
+Warnings:
+Warning	1264	Out of range value adjusted for column 'f1' at row 1
+Warning	1264	Out of range value adjusted for column 'f1' at row 2
+select f1 + 0e0 from t1;
+f1 + 0e0
+1.0000000150475e+29
+-1.0000000150475e+29
+1.0000000150475e+30
+-1.0000000150475e+30
+1.0000000150475e+30
+-1.0000000150475e+30
+drop table t1;
+End of 5.0 tests

mysql-test/t/type_float.test

@@ -222,3 +222,23 @@ drop table t1;
 create table t1 (s1 float(0,2));
 --error 1427
 create table t1 (s1 float(1,2));
+
+#
+# Bug #28121 "INSERT or UPDATE into DOUBLE(200,0) field being truncated to 31 digits"
+#
+
+create table t1 (f1 double(200, 0));
+insert into t1 values (1e199), (-1e199);
+insert into t1 values (1e200), (-1e200);
+insert into t1 values (2e200), (-2e200);
+select f1 + 0e0 from t1;
+drop table t1;
+
+create table t1 (f1 float(30, 0));
+insert into t1 values (1e29), (-1e29);
+insert into t1 values (1e30), (-1e30);
+insert into t1 values (2e30), (-2e30);
+select f1 + 0e0 from t1;
+drop table t1;
+
+--echo End of 5.0 tests

sql/field.cc

@@ -3675,56 +3675,9 @@ int Field_float::store(const char *from,uint len,CHARSET_INFO *cs)
 
 int Field_float::store(double nr)
 {
-  float j;
-  int error= 0;
+  int error= truncate(&nr, FLT_MAX);
+  float j= nr;
 
-  if (isnan(nr))
-  {
-    j= 0;
-    set_null();
-    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-    error= 1;
-  }
-  else if (unsigned_flag && nr < 0)
-  {
-    j= 0;
-    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-    error= 1;
-  }
-  else
-  {
-    double max_value;
-    if (dec >= NOT_FIXED_DEC)
-    {
-      max_value= FLT_MAX;
-    }
-    else
-    {
-      uint tmp=min(field_length,array_elements(log_10)-1);
-      max_value= (log_10[tmp]-1)/log_10[dec];
-      /*
-	The following comparison is needed to not get an overflow if nr
-	is close to FLT_MAX
-      */
-      if (fabs(nr) < FLT_MAX/10.0e+32)
-	nr= floor(nr*log_10[dec]+0.5)/log_10[dec];
-    }
-    if (nr < -max_value)
-    {
-      j= (float)-max_value;
-      set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-      error= 1;
-    }
-    else if (nr > max_value)
-    {
-      j= (float)max_value;
-      set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-      error= 1;
-    }
-    else
-      j= (float) nr;
-  }
-  
 #ifdef WORDS_BIGENDIAN
   if (table->s->db_low_byte_first)
   {
@@ -3963,48 +3916,7 @@ int Field_double::store(const char *from,uint len,CHARSET_INFO *cs)
 
 int Field_double::store(double nr)
 {
-  int error= 0;
-
-  if (isnan(nr))
-  {
-    nr= 0;
-    set_null();
-    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-    error= 1;
-  }
-  else if (unsigned_flag && nr < 0)
-  {
-    nr= 0;
-    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-    error= 1;
-  }
-  else 
-  {
-    double max_value;
-    if (not_fixed)
-    {
-      max_value= DBL_MAX;
-    }
-    else
-    {
-      uint tmp=min(field_length,array_elements(log_10)-1);
-      max_value= (log_10[tmp]-1)/log_10[dec];
-      if (fabs(nr) < DBL_MAX/10.0e+32)
-	nr= floor(nr*log_10[dec]+0.5)/log_10[dec];
-    }
-    if (nr < -max_value)
-    {
-      nr= -max_value;
-      set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-      error= 1;
-    }
-    else if (nr > max_value)
-    {
-      nr= max_value;
-      set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
-      error= 1;
-    }
-  }
+  int error= truncate(&nr, DBL_MAX);
 
 #ifdef WORDS_BIGENDIAN
   if (table->s->db_low_byte_first)
@@ -4023,6 +3935,63 @@ int Field_double::store(longlong nr, bool unsigned_val)
   return store(unsigned_val ? ulonglong2double((ulonglong) nr) : (double) nr);
 }
 
+/*
+  If a field has fixed length, truncate the double argument pointed to by 'nr'
+  appropriately.
+  Also ensure that the argument is within [-max_value; max_value] range.
+*/
+
+int Field_real::truncate(double *nr, double max_value)
+{
+  int error= 1;
+  double res= *nr;
+  
+  if (isnan(res))
+  {
+    res= 0;
+    set_null();
+    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
+    goto end;
+  }
+  else if (unsigned_flag && res < 0)
+  {
+    res= 0;
+    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
+    goto end;
+  }
+
+  if (!not_fixed)
+  {
+    uint order= field_length - dec;
+    uint step= array_elements(log_10) - 1;
+    max_value= 1.0;
+    for (; order > step; order-= step)
+      max_value*= log_10[step];
+    max_value*= log_10[order];
+    max_value-= 1.0 / log_10[dec];
+
+    double tmp= rint((res - floor(res)) * log_10[dec]) / log_10[dec];
+    res= floor(res) + tmp;
+  }
+  
+  if (res < -max_value)
+  {
+   res= -max_value;
+   set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
+  }
+  else if (res > max_value)
+  {
+    res= max_value;
+    set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, ER_WARN_DATA_OUT_OF_RANGE, 1);
+  }
+  else
+    error= 0;
+
+end:
+  *nr= res;
+  return error;
+}
+
 
 int Field_real::store_decimal(const my_decimal *dm)
 {

sql/field.h

@@ -453,6 +453,7 @@ public:
 /* base class for float and double and decimal (old one) */
 class Field_real :public Field_num {
 public:
+  my_bool not_fixed;
 
   Field_real(char *ptr_arg, uint32 len_arg, uchar *null_ptr_arg,
              uchar null_bit_arg, utype unireg_check_arg,
@@ -460,12 +461,14 @@ public:
              struct st_table *table_arg,
              uint8 dec_arg, bool zero_arg, bool unsigned_arg)
     :Field_num(ptr_arg, len_arg, null_ptr_arg, null_bit_arg, unireg_check_arg,
-               field_name_arg, table_arg, dec_arg, zero_arg, unsigned_arg)
+               field_name_arg, table_arg, dec_arg, zero_arg, unsigned_arg),
+    not_fixed(dec_arg >= NOT_FIXED_DEC)
     {}
 
 
   int store_decimal(const my_decimal *);
   my_decimal *val_decimal(my_decimal *);
+  int truncate(double *nr, double max_length);
   uint32 max_display_length() { return field_length; }
 };
 
@@ -758,7 +761,6 @@ public:
 
 class Field_double :public Field_real {
 public:
-  my_bool not_fixed;
   Field_double(char *ptr_arg, uint32 len_arg, uchar *null_ptr_arg,
 	       uchar null_bit_arg,
 	       enum utype unireg_check_arg, const char *field_name_arg,
@@ -766,21 +768,18 @@ public:
 	       uint8 dec_arg,bool zero_arg,bool unsigned_arg)
     :Field_real(ptr_arg, len_arg, null_ptr_arg, null_bit_arg,
                 unireg_check_arg, field_name_arg, table_arg,
-                dec_arg, zero_arg, unsigned_arg),
-     not_fixed(dec_arg >= NOT_FIXED_DEC)
+                dec_arg, zero_arg, unsigned_arg)
     {}
   Field_double(uint32 len_arg, bool maybe_null_arg, const char *field_name_arg,
 	       struct st_table *table_arg, uint8 dec_arg)
     :Field_real((char*) 0, len_arg, maybe_null_arg ? (uchar*) "" : 0, (uint) 0,
-                NONE, field_name_arg, table_arg, dec_arg, 0, 0),
-     not_fixed(dec_arg >= NOT_FIXED_DEC)
+                NONE, field_name_arg, table_arg, dec_arg, 0, 0)
     {}
   Field_double(uint32 len_arg, bool maybe_null_arg, const char *field_name_arg,
-	       struct st_table *table_arg, uint8 dec_arg, my_bool not_fixed_srg)
+	       struct st_table *table_arg, uint8 dec_arg, my_bool not_fixed_arg)
     :Field_real((char*) 0, len_arg, maybe_null_arg ? (uchar*) "" : 0, (uint) 0,
-	        NONE, field_name_arg, table_arg, dec_arg, 0, 0), 
-    not_fixed(not_fixed_srg)
-    {}
+                NONE, field_name_arg, table_arg, dec_arg, 0, 0)
+    {not_fixed= not_fixed_arg; }
   enum_field_types type() const { return FIELD_TYPE_DOUBLE;}
   enum ha_base_keytype key_type() const { return HA_KEYTYPE_DOUBLE; }
   int  store(const char *to,uint length,CHARSET_INFO *charset);

sql/init.cc

@@ -21,8 +21,6 @@
 
 void unireg_init(ulong options)
 {
-  uint i;
-  double nr;
   DBUG_ENTER("unireg_init");
 
   MYSYS_PROGRAM_DONT_USE_CURSES();
@@ -39,16 +37,5 @@ void unireg_init(ulong options)
 
   VOID(strmov(reg_ext,".frm"));
   specialflag=SPECIAL_SAME_DB_NAME | options;  /* Set options from argv */
-  /* Make a tab of powers of 10 */
-  for (i=0,nr=1.0; i < array_elements(log_10) ; i++)
-  {					/* It's used by filesort... */
-    log_10[i]= nr ; nr*= 10.0;
-  }
-  /* Make a tab of powers of 0.1 */
-  for (i= 0, nr= 0.1; i < array_elements(log_01); i++)
-  {
-    log_01[i]= nr;
-    nr*= 0.1;
-  }
   DBUG_VOID_RETURN;
 }

sql/item_cmpfunc.cc

@@ -504,7 +504,7 @@ int Arg_comparator::set_compare_func(Item_bool_func2 *item, Item_result type)
   {
     if ((*a)->decimals < NOT_FIXED_DEC && (*b)->decimals < NOT_FIXED_DEC)
     {
-      precision= 5 * log_01[max((*a)->decimals, (*b)->decimals)];
+      precision= 5 * log_01[max((*a)->decimals, (*b)->decimals) + 1];
       if (func == &Arg_comparator::compare_real)
         func= &Arg_comparator::compare_real_fixed;
       else if (func == &Arg_comparator::compare_e_real)

sql/mysql_priv.h

@@ -1262,8 +1262,6 @@ extern char language[FN_REFLEN], reg_ext[FN_EXTLEN];
 extern char glob_hostname[FN_REFLEN], mysql_home[FN_REFLEN];
 extern char pidfile_name[FN_REFLEN], system_time_zone[30], *opt_init_file;
 extern char log_error_file[FN_REFLEN], *opt_tc_log_file;
-extern double log_10[32];
-extern double log_01[32];
 extern ulonglong log_10_int[20];
 extern ulonglong keybuff_size;
 extern ulonglong thd_startup_options;

sql/mysqld.cc

@@ -434,8 +434,6 @@ ulong slow_launch_threads = 0, sync_binlog_period;
 ulong expire_logs_days = 0;
 ulong rpl_recovery_rank=0;
 
-double log_10[32];			/* 10 potences */
-double log_01[32];
 time_t server_start_time;
 
 char mysql_home[FN_REFLEN], pidfile_name[FN_REFLEN], system_time_zone[30];

strings/strtod.c

@@ -31,13 +31,74 @@
 
 #define MAX_DBL_EXP	308
 #define MAX_RESULT_FOR_MAX_EXP 1.7976931348623157
-static double scaler10[] = {
-  1.0, 1e10, 1e20, 1e30, 1e40, 1e50, 1e60, 1e70, 1e80, 1e90
-};
-static double scaler1[] = {
-  1.0, 10.0, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9
+
+const double log_10[] = {
+  1e000, 1e001, 1e002, 1e003, 1e004, 1e005, 1e006, 1e007, 1e008, 1e009,
+  1e010, 1e011, 1e012, 1e013, 1e014, 1e015, 1e016, 1e017, 1e018, 1e019,
+  1e020, 1e021, 1e022, 1e023, 1e024, 1e025, 1e026, 1e027, 1e028, 1e029,
+  1e030, 1e031, 1e032, 1e033, 1e034, 1e035, 1e036, 1e037, 1e038, 1e039,
+  1e040, 1e041, 1e042, 1e043, 1e044, 1e045, 1e046, 1e047, 1e048, 1e049,
+  1e050, 1e051, 1e052, 1e053, 1e054, 1e055, 1e056, 1e057, 1e058, 1e059,
+  1e060, 1e061, 1e062, 1e063, 1e064, 1e065, 1e066, 1e067, 1e068, 1e069,
+  1e070, 1e071, 1e072, 1e073, 1e074, 1e075, 1e076, 1e077, 1e078, 1e079,
+  1e080, 1e081, 1e082, 1e083, 1e084, 1e085, 1e086, 1e087, 1e088, 1e089,
+  1e090, 1e091, 1e092, 1e093, 1e094, 1e095, 1e096, 1e097, 1e098, 1e099,
+  1e100, 1e101, 1e102, 1e103, 1e104, 1e105, 1e106, 1e107, 1e108, 1e109,
+  1e110, 1e111, 1e112, 1e113, 1e114, 1e115, 1e116, 1e117, 1e118, 1e119,
+  1e120, 1e121, 1e122, 1e123, 1e124, 1e125, 1e126, 1e127, 1e128, 1e129,
+  1e130, 1e131, 1e132, 1e133, 1e134, 1e135, 1e136, 1e137, 1e138, 1e139,
+  1e140, 1e141, 1e142, 1e143, 1e144, 1e145, 1e146, 1e147, 1e148, 1e149,
+  1e150, 1e151, 1e152, 1e153, 1e154, 1e155, 1e156, 1e157, 1e158, 1e159,
+  1e160, 1e161, 1e162, 1e163, 1e164, 1e165, 1e166, 1e167, 1e168, 1e169,
+  1e170, 1e171, 1e172, 1e173, 1e174, 1e175, 1e176, 1e177, 1e178, 1e179,
+  1e180, 1e181, 1e182, 1e183, 1e184, 1e185, 1e186, 1e187, 1e188, 1e189,
+  1e190, 1e191, 1e192, 1e193, 1e194, 1e195, 1e196, 1e197, 1e198, 1e199,
+  1e200, 1e201, 1e202, 1e203, 1e204, 1e205, 1e206, 1e207, 1e208, 1e209,
+  1e210, 1e211, 1e212, 1e213, 1e214, 1e215, 1e216, 1e217, 1e218, 1e219,
+  1e220, 1e221, 1e222, 1e223, 1e224, 1e225, 1e226, 1e227, 1e228, 1e229,
+  1e230, 1e231, 1e232, 1e233, 1e234, 1e235, 1e236, 1e237, 1e238, 1e239,
+  1e240, 1e241, 1e242, 1e243, 1e244, 1e245, 1e246, 1e247, 1e248, 1e249,
+  1e250, 1e251, 1e252, 1e253, 1e254, 1e255, 1e256, 1e257, 1e258, 1e259,
+  1e260, 1e261, 1e262, 1e263, 1e264, 1e265, 1e266, 1e267, 1e268, 1e269,
+  1e270, 1e271, 1e272, 1e273, 1e274, 1e275, 1e276, 1e277, 1e278, 1e279,
+  1e280, 1e281, 1e282, 1e283, 1e284, 1e285, 1e286, 1e287, 1e288, 1e289,
+  1e290, 1e291, 1e292, 1e293, 1e294, 1e295, 1e296, 1e297, 1e298, 1e299,
+  1e300, 1e301, 1e302, 1e303, 1e304, 1e305, 1e306, 1e307, 1e308, 1e309
 };
 
+const double log_01[] = {
+  1e-000,1e-001,1e-002,1e-003,1e-004,1e-005,1e-006,1e-007,1e-008,1e-009,
+  1e-010,1e-011,1e-012,1e-013,1e-014,1e-015,1e-016,1e-017,1e-018,1e-019,
+  1e-020,1e-021,1e-022,1e-023,1e-024,1e-025,1e-026,1e-027,1e-028,1e-029,
+  1e-030,1e-031,1e-032,1e-033,1e-034,1e-035,1e-036,1e-037,1e-038,1e-039,
+  1e-040,1e-041,1e-042,1e-043,1e-044,1e-045,1e-046,1e-047,1e-048,1e-049,
+  1e-050,1e-051,1e-052,1e-053,1e-054,1e-055,1e-056,1e-057,1e-058,1e-059,
+  1e-060,1e-061,1e-062,1e-063,1e-064,1e-065,1e-066,1e-067,1e-068,1e-069,
+  1e-070,1e-071,1e-072,1e-073,1e-074,1e-075,1e-076,1e-077,1e-078,1e-079,
+  1e-080,1e-081,1e-082,1e-083,1e-084,1e-085,1e-086,1e-087,1e-088,1e-089,
+  1e-090,1e-091,1e-092,1e-093,1e-094,1e-095,1e-096,1e-097,1e-098,1e-099,
+  1e-100,1e-101,1e-102,1e-103,1e-104,1e-105,1e-106,1e-107,1e-108,1e-109,
+  1e-110,1e-111,1e-112,1e-113,1e-114,1e-115,1e-116,1e-117,1e-118,1e-119,
+  1e-120,1e-121,1e-122,1e-123,1e-124,1e-125,1e-126,1e-127,1e-128,1e-129,
+  1e-130,1e-131,1e-132,1e-133,1e-134,1e-135,1e-136,1e-137,1e-138,1e-139,
+  1e-140,1e-141,1e-142,1e-143,1e-144,1e-145,1e-146,1e-147,1e-148,1e-149,
+  1e-150,1e-151,1e-152,1e-153,1e-154,1e-155,1e-156,1e-157,1e-158,1e-159,
+  1e-160,1e-161,1e-162,1e-163,1e-164,1e-165,1e-166,1e-167,1e-168,1e-169,
+  1e-170,1e-171,1e-172,1e-173,1e-174,1e-175,1e-176,1e-177,1e-178,1e-179,
+  1e-180,1e-181,1e-182,1e-183,1e-184,1e-185,1e-186,1e-187,1e-188,1e-189,
+  1e-190,1e-191,1e-192,1e-193,1e-194,1e-195,1e-196,1e-197,1e-198,1e-199,
+  1e-200,1e-201,1e-202,1e-203,1e-204,1e-205,1e-206,1e-207,1e-208,1e-209,
+  1e-210,1e-211,1e-212,1e-213,1e-214,1e-215,1e-216,1e-217,1e-218,1e-219,
+  1e-220,1e-221,1e-222,1e-223,1e-224,1e-225,1e-226,1e-227,1e-228,1e-229,
+  1e-230,1e-231,1e-232,1e-233,1e-234,1e-235,1e-236,1e-237,1e-238,1e-239,
+  1e-240,1e-241,1e-242,1e-243,1e-244,1e-245,1e-246,1e-247,1e-248,1e-249,
+  1e-250,1e-251,1e-252,1e-253,1e-254,1e-255,1e-256,1e-257,1e-258,1e-259,
+  1e-260,1e-261,1e-262,1e-263,1e-264,1e-265,1e-266,1e-267,1e-268,1e-269,
+  1e-270,1e-271,1e-272,1e-273,1e-274,1e-275,1e-276,1e-277,1e-278,1e-279,
+  1e-280,1e-281,1e-282,1e-283,1e-284,1e-285,1e-286,1e-287,1e-288,1e-289,
+  1e-290,1e-291,1e-292,1e-293,1e-294,1e-295,1e-296,1e-297,1e-298,1e-299,
+  1e-300,1e-301,1e-302,1e-303,1e-304,1e-305,1e-306,1e-307,1e-308,1e-309
+};
 
 /*
   Convert string to double (string doesn't have to be null terminated)
@@ -57,7 +118,7 @@ double my_strtod(const char *str, char **end_ptr, int *error)
 {
   double result= 0.0;
   uint negative= 0, ndigits, dec_digits= 0, neg_exp= 0;
-  int exponent= 0, digits_after_dec_point= 0, tmp_exp;
+  int exponent= 0, digits_after_dec_point= 0, tmp_exp, step;
   const char *old_str, *end= *end_ptr, *start_of_number;
   char next_char;
   my_bool overflow=0;
@@ -178,17 +239,13 @@ double my_strtod(const char *str, char **end_ptr, int *error)
     {
       exponent= -exponent;
       neg_exp= 1;                               /* neg_exp was 0 before */
+      step= array_elements(log_01) - 1;
     }
-    while (exponent >= 100)
-    {
-      result= neg_exp ? result/1.0e100 : result*1.0e100;
-      exponent-= 100;
-    }
-    scaler= scaler10[exponent/10]*scaler1[exponent%10];
-    if (neg_exp)
-      result/= scaler;
     else
-      result*= scaler;
+      step= array_elements(log_10) - 1;
+    for (; exponent > step; exponent-= step)
+      result*= neg_exp ? log_01[step] : log_10[step];
+    result*= neg_exp ? log_01[exponent] : log_10[exponent];
   }
 
 done: