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"""Comment copied from Python/compile.c:
All about a_lnotab.
c_lnotab is an array of unsigned bytes disguised as a Python string.
It is used to map bytecode offsets to source code line #s (when needed
for tracebacks).
The array is conceptually a list of
(bytecode offset increment, line number increment)
pairs. The details are important and delicate, best illustrated by example:
byte code offset source code line number
0 1
6 2
50 7
350 307
361 308
The first trick is that these numbers aren't stored, only the increments
from one row to the next (this doesn't really work, but it's a start):
0, 1, 6, 1, 44, 5, 300, 300, 11, 1
The second trick is that an unsigned byte can't hold negative values, or
values larger than 255, so (a) there's a deep assumption that byte code
offsets and their corresponding line #s both increase monotonically, and (b)
if at least one column jumps by more than 255 from one row to the next, more
than one pair is written to the table. In case #b, there's no way to know
from looking at the table later how many were written. That's the delicate
part. A user of c_lnotab desiring to find the source line number
corresponding to a bytecode address A should do something like this:
lineno = addr = 0
for addr_incr, line_incr in co_lnotab:
addr += addr_incr
if addr > A:
return lineno
if line_incr >= 0x80:
line_incr -= 0x100
lineno += line_incr
In order for this to work, when the addr field increments by more than 255,
the line # increment in each pair generated must be 0 until the remaining addr
increment is < 256. So, in the example above, assemble_lnotab (it used
to be called com_set_lineno) should not (as was actually done until 2.2)
expand 300, 300 to 255, 255, 45, 45,
but to 255, 0, 45, 255, 0, 45.
"""
def lnotab(pairs, first_lineno=0):
"""Yields byte integers representing the pairs of integers passed in."""
assert first_lineno <= pairs[0][1] # noqa: S101
cur_byte, cur_line = 0, first_lineno
for byte_off, line_off in pairs:
byte_delta = byte_off - cur_byte
line_delta = line_off - cur_line
assert byte_delta >= 0 # noqa: S101
while byte_delta > 255: # noqa: PLR2004
yield 255 # byte
yield 0 # line
byte_delta -= 255
yield byte_delta
while line_delta >= 0x80: # noqa: PLR2004
yield 0x7F # line
yield 0 # byte
line_delta -= 0x7F
while line_delta < -0x80: # noqa: PLR2004
yield 0x80 # line
yield 0 # byte
line_delta += 0x80
if line_delta < 0:
line_delta += 0x100
assert 0x80 <= line_delta <= 0xFF # noqa: S101, PLR2004
yield line_delta
cur_byte, cur_line = byte_off, line_off
def lnotab_string(pairs, first_lineno=0):
return bytes(lnotab(pairs, first_lineno))
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