Conversion from MySQL code Note: decimal. c

Bytes ------------------------------------------------------------------------------------------------------------

*
Convert decimal to its binary fixed-length Representation
Two representations of the same length can be compared with memcmp
With the correct-1/0/+ 1 Result

Synopsis
Decimal2bin ()
From-value to convert
To-points to buffer Where string representation shoshould be stored
Precision/scale-see decimal_bin_size () below

Note
The buffer is assumed to be of the size decimal_bin_size (precision, scale)

Return Value
E_dec_ OK/e_dec_truncated/e_dec_overflow

Description
For storage decimal numbers are converted to the "binary" format.

This format has the following properties:
1. Length of the binary representation depends on the {precision, scale}
As provided by the caller and not on the intg/frac of the decimal
Convert.
2. Binary representations of the same {precision, scale} can be compared
With memcmp-with the same result as decimal_cmp () of the original
Decimals (not taking into account possible precision loss
Conversion ).

This binary format is as follows:
1. First the number is converted to have a requested precision and scale.
2. Every full dig_per_dec1 digits of intg part are stored in 4 bytes
As is
3. The first intg % dig_per_dec1 digits are stored in the specified CED
Number of bytes (enough bytes to store this number of digits-
See dig2bytes)
4. Same for frac-full decimal_digit_t's are stored as is,
The last frac % dig_per_dec1 digits-in the specified ced number of bytes.
5. If the number is negative-every byte is inversed.
5. The very first bit of the resulting byte array is inverted (because
Memcmp compares unsigned bytes, see property 2 above)

Example:

1234567890.1234

Internally is represented as 3 decimal_digit_t's

1 234567890 123400000

(Assuming we want a binary representation with precision = 14, scale = 4)
In hex it's

00-00-00-01 0d-fb-38-d2 07-5a-ef-40

Now, middle decimal_digit_t is full-It stores 9 decimal digits. It goes
Into binary representation as is:

...... 0d-fb-38-d2 ............

First decimal_digit_t has only one decimal digit. We can store one digit in
One byte, no need to waste four:

01 0d-fb-38-d2 ............

Now, last digit. It's 123400000. We can store 1234 in two bytes:

01 0d-fb-38-d2 04-d2

So, we 've packed 12 bytes number in 7 bytes.
And now we invert the highest bit to get the final result:

81 0d FB 38 D2 04 D2

And for-1234567890.1234 it wocould be

7E F2 04 37 2D FB 2D