"""Utils about histogram metrics"""


def quantile(tuples: list[tuple[float, float]], q_value: float):
    """Calculate the quantile of the histogram.
    refer to https://github.com/prometheus/prometheus/blob/main/promql/quantile.go
    """
    if len(tuples) == 0:
        return 0

    tuples.sort(key=lambda x: x[0])
    observations = tuples[len(tuples) - 1][1]
    if observations == 0:
        return 0

    rank = q_value * observations

    for idx, bucket in enumerate(tuples):
        if rank > bucket[1]:
            continue

        bucket_start = 0
        bucket_end = bucket[0]
        count = bucket[1]

        if idx == 0 and bucket_end <= 0:
            return bucket_end

        if idx > 0:
            bucket_start = tuples[idx - 1][0]
            count -= tuples[idx - 1][1]
            rank -= tuples[idx - 1][1]
        return bucket_start + (bucket_end - bucket_start) * (rank / count)
    return tuples[len(tuples) - 2][0]