Deltas and per-second rates from successive SNMP samples, with Counter32 and Counter64 wraparound handled.

Works on the enriched varbind maps every SnmpKit.SNMP call returns, on {type, value} tuples, or on bare integers with an explicit type.

# two polls of the same object
{:ok, %{value: 4_294_967_000, type: :counter32} = t0} = SnmpKit.SNMP.get(host, "ifInOctets.1")
# ... 10 seconds later, the counter wrapped ...
{:ok, %{value: 1_000} = t1} = SnmpKit.SNMP.get(host, "ifInOctets.1")

SnmpKit.SNMP.Rate.delta(t0, t1)          #=> {:ok, 1296}
SnmpKit.SNMP.Rate.rate(t0, t1, 10_000)   #=> {:ok, 129.6}  (per second)

Whole walks are paired by OID with rates/3, which reads the interval from the sysUpTime.0 varbinds when both samples carry them and reports a device restart instead of inventing rates from reset counters.

Restarts versus wraps

A Counter32 that wrapped and a counter that reset on reboot look the same from one object alone. delta/2 assumes a wrap, which is right for a busy 32-bit counter. rates/3 compares sysUpTime.0 when present and returns {:error, :device_restarted} if it went backwards; pass sysUpTime.0 along with the objects you poll to get that protection.

sysUpTime is itself a 32-bit TimeTicks value and wraps after about 497 days. A wrap looks exactly like a restart, so rates/3 reports {:error, :device_restarted} for it too. When a poll may span that boundary pass interval_ms: from your own clock; it always takes priority over the uptime varbinds.

Multiple wraps

A counter that wrapped more than once between two samples cannot be told from one that wrapped once, so its delta is under-reported. Poll fast enough that no counter can wrap twice (a 32-bit octet counter wraps in under 35 seconds at 1 Gbps), or give max_rate:, the highest per-second rate the object can plausibly show, and a rate that only a multiple wrap explains becomes {:error, :implausible_rate}.

Options

• :type - the SNMP type of bare numeric samples.
• :max_rate - per-second bound; a rate whose magnitude exceeds it is reported as :implausible_rate.
• :interval_ms (rates/3) - elapsed time between the two samples, takes priority over sysUpTime.0.
• :unrateable (rates/3) - :skip (default) leaves out entries that could not be rated; :error returns {:error, {:unrateable, oid, reason}} for the first one.

Sample values

Integer values are what the decoder produces. Hand-built samples may carry floats for :gauge32, :integer and :unsigned32; a counter is an integer by definition, so a float counter is {:error, {:not_a_sample, _}}.