# 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
- `"nX"` indicates repeating the previous value `X` times
- each point is a delta of the previous point (which can be applied recursively)
- transformation of values (typically just to reduce their size, since the format is plain-text)

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 |