1# Locations table
2
3For versions up to 3.10 see ./lnotab_notes.txt
4
5In version 3.11 the `co_linetable` bytes object of code objects contains a compact representation of the positions returned by the `co_positions()` iterator.
6
7The `co_linetable` consists of a sequence of location entries.
8Each entry starts with a byte with the most significant bit set, followed by zero or more bytes with most significant bit unset.
9
10Each entry contains the following information:
11* The number of code units covered by this entry (length)
12* The start line
13* The end line
14* The start column
15* The end column
16
17The first byte has the following format:
18
19Bit 7 | Bits 3-6 | Bits 0-2
20 ---- | ---- | ----
21 1 | Code | Length (in code units) - 1
22
23The codes are enumerated in the `_PyCodeLocationInfoKind` enum.
24
25## Variable length integer encodings
26
27Integers are often encoded using a variable length integer encoding
28
29### Unsigned integers (varint)
30
31Unsigned integers are encoded in 6 bit chunks, least significant first.
32Each chunk but the last has bit 6 set.
33For example:
34
35* 63 is encoded as `0x3f`
36* 200 is encoded as `0x48`, `0x03`
37
38### Signed integers (svarint)
39
40Signed integers are encoded by converting them to unsigned integers, using the following function:
41```Python
42def convert(s):
43    if s < 0:
44        return ((-s)<<1) | 1
45    else:
46        return (s<<1)
47```
48
49## Location entries
50
51The meaning of the codes and the following bytes are as follows:
52
53Code | Meaning | Start line | End line | Start column | End column
54 ---- | ---- | ---- | ---- | ---- | ----
55 0-9 | Short form | Δ 0 | Δ 0 | See below | See below
56 10-12 | One line form | Δ (code - 10) | Δ 0 | unsigned byte | unsigned byte
57 13 | No column info | Δ svarint | Δ 0 | None | None
58 14   | Long form | Δ svarint | Δ varint | varint | varint
59 15   | No location |  None | None | None | None
60
61The Δ means the value is encoded as a delta from another value:
62* Start line: Delta from the previous start line, or `co_firstlineno` for the first entry.
63* End line: Delta from the start line
64
65### The short forms
66
67Codes 0-9 are the short forms. The short form consists of two bytes, the second byte holding additional column information. The code is the start column divided by 8 (and rounded down).
68* Start column: `(code*8) + ((second_byte>>4)&7)`
69* End column: `start_column + (second_byte&15)`
70