"""

Standard library functions for CEL that aren't available in cel-rust.

This module provides Python implementations of CEL standard library functions

that are missing from the upstream cel-rust implementation.

"""

from typing import Any

def substring(s: str, start: int, end: int | None = None) -> str:

"""

Extract a substring from a string.

This implements the CEL substring() function which is not yet available

in cel-rust upstream. See https://github.com/cel-rust/cel-rust/issues/200

Args:

s: The source string

start: Starting index (0-based, inclusive)

end: Optional ending index (0-based, exclusive). If not provided,

extracts to the end of the string.

Returns:

The extracted substring

Examples:

>>> substring("hello world", 0, 5)

'hello'

>>> substring("hello world", 6)

'world'

>>> substring("hello", 1, 4)

'ell'

"""

if end is None:

return s[start:]

return s[start:end]

# Dictionary mapping function names to their implementations

# This makes it easy to add all stdlib functions to a Context at once

STDLIB_FUNCTIONS = {

"substring": substring,

}

def add_stdlib_to_context(context: Any) -> None:

"""

Add all stdlib functions to a CEL Context.

Args:

context: A cel.Context object

Example:

>>> import cel

>>> from cel.stdlib import add_stdlib_to_context

>>> context = cel.Context()

>>> add_stdlib_to_context(context)

>>> cel.evaluate('substring("hello", 0, 2)', context)

'he'

"""

for name, func in STDLIB_FUNCTIONS.items():

context.add_function(name, func)