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bpo-43494: Make some minor changes to lnotab notes #24861

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Mar 19, 2021
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bpo-43494: Make some minor changes to lnotab notes #24861

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12 changes: 6 additions & 6 deletions Objects/lnotab_notes.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -3,16 +3,16 @@ Description of the internal format of the line number table
Conceptually, the line number table consists of a sequence of triples:
start-offset (inclusive), end-offset (exclusive), line-number.

Note that note all byte codes have a line number so we need handle `None` for the line-number.
Note that not all byte codes have a line number so we need handle `None` for the line-number.

However, storing the above sequence directly would be very inefficient as we would need 12 bytes per entry.

First of all, we can note that the end of one entry is the same as the start of the next, so we can overlap entries.
Secondly we also note that we don't really need arbitrary access to the sequence, so we can store deltas.
First, note that the end of one entry is the same as the start of the next, so we can overlap entries.
Second, we don't really need arbitrary access to the sequence, so we can store deltas.

We just need to store (end - start, line delta) pairs. The start offset of the first entry is always zero.

Thirdly, most deltas are small, so we can use a single byte for each value, as long we allow several entries for the same line.
Third, most deltas are small, so we can use a single byte for each value, as long we allow several entries for the same line.

Consider the following table
Start End Line
Expand All @@ -36,12 +36,12 @@ Stripping the redundant ends gives:

Note that the end - start value is always positive.

Finally in order, to fit into a single byte we need to convert start deltas to the range 0 <= delta <= 254,
Finally, in order to fit into a single byte we need to convert start deltas to the range 0 <= delta <= 254,
and line deltas to the range -127 <= delta <= 127.
A line delta of -128 is used to indicate no line number.
A start delta of 255 is used as a sentinel to mark the end of the table.
Also note that a delta of zero indicates that there are no bytecodes in the given range,
which means can use an invalidate line number for that range.
which means we can use an invalid line number for that range.

Final form:

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