XDelta Compression Format

The XINA Delta (xd) compression format is a simple, high performance format for compression and decompression of a list of numeric values into a string. The compression is most efficient for data sets with frequently repeated values, or predictable value iterations.

The format consists of a few main features:

• a comma separated list of values
• blank values indicating either repeating the previous value, or a static value
• "n#" indicates repeating the previous value # times
• each point is a delta of the previous point (which can be applied recursively)
• transformation of values (typically just to reduce their character length, since the format is plain-text)

The format grammar is as follows:

string

• parameters ':' values

parameters

• parameter
• parameter ',' parameters

parameter

• 'v' integer
• 'd' integer
• 'd'
• 'dd'
• 'br'
• 'b'
• 'b' number
• '+' number
• '-' number
• '*' number
• '/' number

values

• value
• value ',' values

value

• ""
• number
• number 'n' integer
• 'n' integer

number

• integer fraction exponent

integer

• digit
• onenine digits
• '-' digit
• '-' onenine digits

digits

• digit
• digit digits

digit

• '0'
• onenine

onenine

• '1' . '9'

fraction

• ""
• '.' digits

exponent

• ""
• 'E' sign digits
• 'e' sign digits

sign

• ""
• '+'
• '-'

The configuration of the encoding of a particular string is denoted by a comma delimited set of steps. These are:

• b<number> - blanks are provided static value
• br - blanks repeat previous value
• d - single iteration delta
• dd - double iteration delta
• +<number> - add static value
• -<number> - subtract static value
• *<number> - multiply by static value
• /<number> - divide by static value

This is followed by a colon, and the encoded data.

For example, given the array of numbers:

1, 1, 1, 1, 2, 3, 4, 4, 4, 3, 2, 1, 0, 0, 0

One possible encoding would be:

"d,b0:1,,n2,1,n2,,,-1,n3,,"

Breaking this down:

Dec	Enc
1	1
1	blank = d0 = 1+0 = 1
1	n2 = previous value twice = d0 = 1+0 = 1
1	"
2	1 = d1 = 1+1 = 2
3	n2 = previous value twice = d1 = 2+1 = 3
4	" = d1 = 3+1 = 4
4	blank = d0 = 4+0 = 4
4	blank = d0 = 4+0 = 4
3	-1 = d-1 = 4-1 = 3
2	n3 = previous value thrice = d-1 = 3-1 = 2
1	" = d-1 = 2-1 = 1
0	" = d-1 = 1-1 = 0
0	blank = d0 = 0+0 = 0
0	blank = d0 = 0+0 = 0