A trivial Python C extension to map character positions to line numbers

example.py

import line_tuple

text = """\
line 1
line 2
line 3
"""

line_positions = line_tuple.build_line_tuple(text)
print(line_positions[0])  # 1
print(line_positions[5])  # 1
print(line_positions[10])  # 2
print(len(line_positions))  # Length of the text
print(line_positions)  # (1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3)

line_tuple.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>

static PyObject* build_line_tuple(PyObject* self, PyObject* args) {
    const char* text;
    Py_ssize_t text_length;

    if (!PyArg_ParseTuple(args, "s#", &text, &text_length)) {
        return NULL;
    }

    if (text_length == 0) {
        return PyTuple_New(0);
    }

    PyObject* line_tuple = PyTuple_New(text_length);
    if (!line_tuple) {
        return NULL;
    }

    int line_number = 1;
    PyObject* line_number_obj = PyLong_FromLong(line_number);
    if (!line_number_obj) {
        Py_DECREF(line_tuple);
        return NULL;
    }

    for (Py_ssize_t pos = 0; pos < text_length; ++pos) {
        Py_INCREF(line_number_obj);  // Increase reference count since PyTuple_SET_ITEM steals a reference
        PyTuple_SET_ITEM(line_tuple, pos, line_number_obj);
        if (text[pos] == '\n') {
            line_number++;
            Py_DECREF(line_number_obj);  // Decrease the old reference count
            line_number_obj = PyLong_FromLong(line_number);  // Create a new PyLong object
            if (!line_number_obj) {
                Py_DECREF(line_tuple);
                return NULL;
            }
        }
    }

    Py_DECREF(line_number_obj);  // Decrease reference count for the last line number object

    return line_tuple;
}

static PyMethodDef LineTupleMethods[] = {
    {"build_line_tuple", build_line_tuple, METH_VARARGS, "Build a line number tuple from text."},
    {NULL, NULL, 0, NULL}
};

static struct PyModuleDef linetuplemodule = {
    PyModuleDef_HEAD_INIT,
    "line_tuple",
    NULL,
    -1,
    LineTupleMethods
};

PyMODINIT_FUNC PyInit_line_tuple(void) {
    return PyModule_Create(&linetuplemodule);
}

python_version.py

def _build_line_tuple(text) -> tuple[int, ...]:
    """Build a tuple where each index corresponds to a character position and the 
    value at that index corresponds to a line number.
    """
    if not text:
        return ()
    line_list: list[int] = [0] * len(text)
    line_number = 1
    for pos, char in enumerate(text):
        line_list[pos] = line_number
        if char == '\n':
            line_number += 1
    line_tuple = tuple(line_list)
    return line_tuple

setup.py

from setuptools import setup, Extension

module = Extension("line_tuple", sources=["line_tuple.c"])

setup(
    name="line_tuple",
    version="0.0.1",
    description="C extension for line positions as a tuple",
    ext_modules=[module],
)