from typing import Optional, Sequence

from .monitor_series_window import MetricSample
from .monitor_counter_increase_types import CounterIncrease


def counter_increase(samples: Sequence[MetricSample]) -> CounterIncrease:
    """How much a counter grew over the samples, Prometheus-style: a drop means
    the counter restarted from zero, so the value after it is all increase. No
    extrapolation to window edges, so the answer is a whole number."""
    increase = 0
    resets = 0
    for i, s in enumerate(samples):
        if i > 0 and s.at <= samples[i - 1].at:
            raise ValueError("samples must be in strictly ascending time order: %d follows %d" % (s.at, samples[i - 1].at))
        if s.value < 0:
            raise ValueError("counter value must not be negative: %d at %d" % (s.value, s.at))
        if i == 0:
            continue
        prev = samples[i - 1].value
        if s.value >= prev:
            increase += s.value - prev
        else:
            resets += 1
            increase += s.value
    if not samples:
        return CounterIncrease(increase=0, resets=0, rate_per_second_milli=None, first_at=None, last_at=None)
    first_at = samples[0].at
    last_at = samples[-1].at
    rate: Optional[int] = None
    if len(samples) >= 2:
        n = increase * 1000
        d = last_at - first_at
        rate = (2 * n + d) // (2 * d)
    return CounterIncrease(increase=increase, resets=resets, rate_per_second_milli=rate, first_at=first_at, last_at=last_at)