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from __future__ import annotations
from collections.abc import Iterable, Iterator
from typing import TypeVar
X = TypeVar("X")
Y = TypeVar("Y")
def ffill(x: Iterable[X], xp: Iterable[X], fp: Iterable[Y], left: Y) -> Iterator[Y]:
"""Forward-fill interpolation
Parameters
----------
x:
Output x series. Must be monotonic ascending.
xp:
Input x series. Must be strictly monotonic ascending.
fp:
Input y series. If it contains more or less elements than xp, the two series
will be clipped to the shortest one (like in :func:`zip`).
left:
Value to yield for x < xp[0]
Yields
------
Forward-fill interpolated elements from fp matching x
Examples
--------
>>> list(ffill([0.5, 2.2, 2.3, 4.5], [1, 2, 3], "abc", "-"))
["-", "b", "b", "c"]
"""
it = zip(xp, fp)
xp_done = False
xp1, fp1 = None, left
for xi in x:
while not xp_done and (xp1 is None or xi >= xp1): # type: ignore[unreachable]
fp0 = fp1
try:
xp1, fp1 = next(it)
except StopIteration:
xp_done = True
yield fp0
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