Boyer Moore Majority Vote in Python

from bisect import bisect_left

# Boyer Moore Majority Vote in Python
#
# PURPOSE
# Majority vote cancels pairs of different values. If a value occurs more than half the
# time, it survives as the final candidate.
#
# METHOD
# Choose a new candidate whenever the balance is zero. Increment for matching values and
# decrement for different ones. Make a second pass to verify the candidate, since a majority
# need not exist.
#
# COMPLEXITY
# O(n) time and O(1) auxiliary space.
#
# CONTRACT
# Returns the INDEX of a verified majority value, or -1 if none exists. Returning an index
# avoids colliding with any valid integer value.
# 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 majority_index(a: list[int]) -> int:
    alg_require(isinstance(a, (list, tuple)) and len(a) <= 1000000)
    n: int = len(a)
    alg_require(n >= 0 and n <= 1000000)
    alg_require(len(a) == n)
    candidate: int = 0
    balance: int = 0
    i: int = 0
    count: int = 0
    while i < n:
        if balance == 0:
            candidate = i
        if a[i] == a[candidate]:
            balance += 1
        else:
            balance -= 1
        i += 1
    i = 0
    while i < n:
        if a[i] == a[candidate]:
            count += 1
        i += 1
    if count > n // 2:
        return candidate
    return -1

a = [2, 2, 1, 2]
result = majority_index(a)
print(result)