Iterative Fibonacci in Python

from bisect import bisect_left

# Iterative Fibonacci in Python
#
# PURPOSE
# The Fibonacci recurrence needs only the previous two terms. Keeping those terms avoids
# both exponential recursion and a full table.
#
# METHOD
# Maintain F(i) and F(i+1), then advance the pair. Return F(n) after n advances.
#
# COMPLEXITY
# O(n) time and O(1) auxiliary space, under fixed-width arithmetic.
#
# CONTRACT
# n >= 0, with F(0)=0 and F(1)=1. Use n <= 45 for every intermediate value to fit signed
# 32-bit integers in this version.
# Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
# counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
# Sorted order, correct tree topology, and caller-owned data-structure invariants are
# preconditions; checking them on every query would change query complexity.
# Scalar and shape checks are included. Value-range checks are included where needed to
# prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
# workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
# uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
# partly updated after arithmetic failure. These are single-threaded, in-memory
# implementations.
#
# LANGUAGE DETAILS
# Python 3.9+: automatic lengths, native lists, arbitrary-precision arithmetic, and
# bisect_left for LIS. Invalid scalar/shape inputs raise ValueError. Integer element types
# and sorted/tree invariants remain caller preconditions.
#
# EXAMPLE
# Expected return: 0.
# Run: python example.py
# Tests for this exact source artifact run before publication. The accompanying source
# archive includes unit cases, runtime results, and source hashes.

def alg_require(condition):
    if not condition:
        raise ValueError("invalid algorithm input; see contract")
def alg_size(n):
    alg_require(0 <= n <= 4000000)
    return n

def fibonacci(n: int) -> int:
    alg_require(type(n) is int and -2147483648 <= n <= 2147483647)
    alg_require(n >= 0 and n <= 1000000)
    alg_require(n >= 0 and n <= 45)
    a: int = 0
    b: int = 1
    next_value: int = 0
    i: int = 0
    while i < n:
        next_value = a + b
        a = b
        b = next_value
        i += 1
    return a

n = 0
result = fibonacci(n)
print(result)