aqiguess.sh

#bash
linear() {
  local y0=$1 y1=$2 x0=$3 x1=$4 x=$5
  printf '%d\n' $(((x-x1) / (x1-x0) * (y1-y0) + y1)) # Already int
}

guess_aqi() {
  local pm_25=$1
  if ((pm_25 < 0))
    return 1
  elif ((pm25 <= 12))
    linear 50 0 12 0 pm_25
  elif ((pm25 <= 35))
    linear 100 51 35 12 pm_25
  elif ((pm25 <= 55))
    linear 150 101 55 35 pm_25
  elif ((pm25 <= 150))
    linear 200 151 150 55 pm_25
  elif ((pm25 <= 250))
    linear 300 201 250 150 pm_25
  elif ((pm25 <= 350))
    linear 400 301 350 250 pm_25
  elif ((pm25 <= 500))
    linear 500 401 500 350 pm_25
  else
    echo 555   
  fi
}