#!/bin/bash
# Emits {"rx":<bytes/s>,"tx":<bytes/s>,"iface":"<name>"} — network throughput
# since the previous invocation, cached in a state file, so this script must
# be polled periodically rather than run once.
set -uo pipefail

STATE_DIR="${XDG_CACHE_HOME:-$HOME/.cache}/omaspeedmeter"
STATE_FILE="$STATE_DIR/net"
mkdir -p -m 700 "$STATE_DIR"

IFACE="auto"
while (($# > 0)); do
    case "$1" in
    --iface)
        IFACE="${2:-auto}"
        shift 2
        ;;
    *) shift ;;
    esac
done

now=$(date +%s.%N)

prev_ts=""
prev_rx=0
prev_tx=0
if [[ -f "$STATE_FILE" ]]; then
    while IFS='=' read -r key value; do
        case "$key" in
        prev_ts) [[ "$value" =~ ^[0-9]+(\.[0-9]+)?$ ]] && prev_ts="$value" ;;
        prev_rx) [[ "$value" =~ ^[0-9]+$ ]] && prev_rx="$value" ;;
        prev_tx) [[ "$value" =~ ^[0-9]+$ ]] && prev_tx="$value" ;;
        esac
    done <"$STATE_FILE"
fi

iface="$IFACE"
if [[ "$iface" == "auto" || -z "$iface" ]]; then
    iface=$(ip route show default 2>/dev/null | awk '{for(i=1;i<=NF;i++) if ($i=="dev") {print $(i+1); exit}}')
fi

rx=0
tx=0
if [[ -n "$iface" && -r "/sys/class/net/$iface/statistics/rx_bytes" ]]; then
    rx=$(<"/sys/class/net/$iface/statistics/rx_bytes")
    tx=$(<"/sys/class/net/$iface/statistics/tx_bytes")
fi

rx_rate=0
tx_rate=0
if [[ -n "$prev_ts" ]]; then
    dt=$(awk -v a="$now" -v b="$prev_ts" 'BEGIN { d=a-b; if (d<=0) d=1; printf "%.3f", d }')
    if ((rx >= prev_rx)); then
        rx_rate=$(awk -v d="$((rx - prev_rx))" -v dt="$dt" 'BEGIN { printf "%.0f", d/dt }')
    fi
    if ((tx >= prev_tx)); then
        tx_rate=$(awk -v d="$((tx - prev_tx))" -v dt="$dt" 'BEGIN { printf "%.0f", d/dt }')
    fi
fi

tmp_file=$(mktemp "$STATE_DIR/net.XXXXXX")
{
    echo "prev_ts=$now"
    echo "prev_rx=$rx"
    echo "prev_tx=$tx"
} >"$tmp_file"
mv -f "$tmp_file" "$STATE_FILE"

printf '{"rx":%s,"tx":%s,"iface":"%s"}\n' "$rx_rate" "$tx_rate" "$iface"