"""SAFETY-97: what a battery's power reading shows while it gives nothing.

B's reading (0x0518) shows -95 to -118 W at idle (standby draw, live
2026-10-03); A's shows -2 to -4 W. "The battery gives nothing" is graded
against this idle reading, not against 0 W, or a refusing B never latches.
DECISIONS/SAFETY-97-idle-offset.md.

Learned per unit, never configured:
- only while control runs, the unit is not refused, stale or tripped, and
  its command has been within IDLE_ASK_W for IDLE_QUIET_S (a refusing battery
  is asked more than that, so it cannot teach its refusal as idle);
- a reading beyond IDLE_SAMPLE_W is not a sample;
- first value: the mean of IDLE_FIRST_S of samples; then a slow follow
  (IDLE_TAU_S);
- persisted per unit and restored, clamped to IDLE_SAMPLE_W, at start.
The band is centred on it clamped to IDLE_MAX_W (owner, 2026-10-04). Until
learned the idle reading is 0 W, the rule as it was.
"""

import math

IDLE_MAX_W = 75.0       # |idle| used: under the 100 W band, so it keeps 0 W +-25 W
IDLE_SAMPLE_W = 250.0   # a reading beyond this is not a sample
IDLE_ASK_W = 50.0       # a command this small counts as "asked ~0"
IDLE_QUIET_S = 120.0    # 6 lags of 20 s: a 2500 W step has decayed to 6 W
IDLE_FIRST_S = 120.0    # seconds of samples averaged for the first value
IDLE_TAU_S = 1800.0     # then follow drift this slowly
IDLE_SAVE_W = 5.0       # persist when the value moved this far
IDLE_GAP_S = 10.0       # a longer gap between reads counts as this


def _num(v):
    try:
        v = float(v)
    except (TypeError, ValueError):
        return None
    return v if math.isfinite(v) else None


class IdleReading:
    """One unit's idle reading. `w` is what "gives nothing" is centred on."""

    def __init__(self, saved=None):
        v = _num(saved)
        self.value = (None if v is None
                      else max(-IDLE_SAMPLE_W, min(IDLE_SAMPLE_W, v)))
        self.saved = self.value
        self.quiet_since = None
        self.last = None
        self.acc_w = 0.0
        self.acc_s = 0.0

    @property
    def w(self) -> float:
        """The centre used: the learned value clamped to IDLE_MAX_W."""
        if self.value is None:
            return 0.0
        return max(-IDLE_MAX_W, min(IDLE_MAX_W, self.value))

    def observe(self, reading, standing_w, now_mono, may_learn) -> bool:
        """One read. Returns True when the value should be persisted."""
        now = float(now_mono)
        dt = (0.0 if self.last is None
              else max(0.0, min(IDLE_GAP_S, now - self.last)))
        self.last = now
        standing = _num(standing_w)
        if not may_learn or standing is None or abs(standing) > IDLE_ASK_W:
            self.quiet_since = None
            return False
        if self.quiet_since is None:
            self.quiet_since = now
        b = _num(reading)
        if (now - self.quiet_since < IDLE_QUIET_S or b is None
                or abs(b) > IDLE_SAMPLE_W or dt <= 0.0):
            return False
        if self.value is None:
            self.acc_w += b * dt
            self.acc_s += dt
            if self.acc_s < IDLE_FIRST_S:
                return False
            self.value = self.acc_w / self.acc_s
        else:
            self.value += (b - self.value) * min(1.0, dt / IDLE_TAU_S)
        if self.saved is None or abs(self.value - self.saved) >= IDLE_SAVE_W:
            self.saved = self.value
            return True
        return False