#!/usr/bin/env awk -f


function log10(n) {

    return log(n)/log(10.0)

}


function merge_meas(val_est, unc_est, val_meas, unc_meas) {

    # Kalman filtering compares variances,
    # convert uncerainties to square units
    unc_est = (0.5*unc_est)^2.0
    unc_meas = (0.5*unc_meas)^2.0

    G_K = (unc_est)/((unc_est) + (unc_meas))
    val_est = val_est + G_K*(val_meas - val_est)
    unc_est = (unc_est*unc_meas)/(unc_est + unc_meas)

    # convert unc_est back to original units
    unc_est = 2.0*sqrt(unc_est)

    return sprintf(OFMT OFS OFMT ORS, val_est, unc_est)

}


function sigfig(val, unc) {

    ordmag_val = log10(val)
    ordmag_unc = log10(unc)

    #sigfig_unc = sprintf("%.f", 6)
    sigfig_unc = sprintf("%.f", sqrt((ordmag_val - ordmag_unc)^2.0) + 1.0)
    sigfig_val = sprintf("%.f", (ordmag_val - ordmag_unc + sigfig_unc))

    ofmt_val = "%." sigfig_val "g"
    ofmt_unc = "%." sigfig_unc "g"

    return sprintf(ofmt_val OFS ofmt_unc ORS, val, unc)

}


# function ck_boottime() {
# 
#     "sysctl kern.boottime" | getline t_kboot
#     close("sysctl kern.boottime")
#     sub("^.*\{", "", t_kboot)
#     sub("\}.*$", "", t_kboot)
#     split(t_kboot, t_kboot_arr, ",")
# 
#     for (i in t_kboot_arr) {
#         sub("^.*= ", "", t_kboot_arr[i])
#     }
# 
#     t_kboot = sprintf(t_kboot_arr[1] "." t_kboot_arr[2])
#     return t_kboot
# 
# }


function ck_uptime(t0_est, t0_unc) {

    t_sys_meas[1] = systime()
    t_sys_meas[2] = 1.0

    "uptime" | getline t_up_cmd
    close("uptime")

    split(merge_meas(t_sys_meas[1], t_sys_meas[2], systime(), 1.0), t_sys_meas)

    # estimate (predict) uptime
    t_up_est[1] = t_sys_meas[1] - t0_est
    t_up_est[2] = t_sys_meas[2] + t0_unc

    # evaluate measured uptime
    t_up_meas[1] = 0.0
    t_up_meas[2] = systime()

    sub("^.*up ", "", t_up_cmd)
    sub(", load.*$", "", t_up_cmd)
    split(t_up_cmd, t_up_cmd_arr, ",")

    for (i in t_up_cmd_arr) {

        # TODO: add cases for days, months, years, etc.
        if (t_up_cmd_arr[i] ~ /day/) {
            split(t_up_cmd_arr[i], days)
            t_up_meas[1] += 86400.0*(days[1] + 0.0)
            (t_up_meas[2] > 86400.0) ? t_up_meas[2] = 86400.0 : t_up_meas[2] += 0.0
        }

        if (t_up_cmd_arr[i] ~ /hr/) {
            split(t_up_cmd_arr[i], hrs)
            t_up_meas[1] += 3600.0*(hrs[1] + 0.0)
            (t_up_meas[2] > 3600.0) ? t_up_meas[2] = 3600.0 : t_up_meas[2] += 0.0
        }

        if (t_up_cmd_arr[i] ~ /min/) {
            split(t_up_cmd_arr[i], mins)
            t_up_meas[1] += (60.0*(mins[1] + 0.0))
            (t_up_meas[2] > 60.0) ? t_up_meas[2] = 60.0 : t_up_meas[2] += 0.0
        }

        if (t_up_cmd_arr[i] ~ /:/) {
            split(t_up_cmd_arr[i], hrs_min, ":")
            t_up_meas[1] += 3600.0*(hrs_min[1] + 0.0)
            (t_up_meas[2] > 3600.0) ? t_up_meas[2] = 3600.0 : t_up_meas[2] += 0.0
            t_up_meas[1] += 60.0*(hrs_min[2] + 0.0)
            (t_up_meas[2] > 60.0) ? t_up_meas[2] = 60.0 : t_up_meas[2] += 0.0
        }

    }

    # merge predicted and measured uptimes
    split(merge_meas(t_up_est[1], t_up_est[2], t_up_meas[1], t_up_meas[2]), t_up_est)

    # evaluate measured boot time, t0
    t_boot_meas[1] = t_sys_meas[1] - t_up_meas[1]
    t_boot_meas[2] = t_sys_meas[2] + t_up_meas[2]

    # merge previous and updated boot time, t0
    split(merge_meas(t0_est, t0_unc, t_boot_meas[1], t_boot_meas[2]), t_boot_est)

    return sigfig(t_boot_est[1], t_boot_est[2])

}


BEGIN {

    OFMT="%.21g"
    pi = 4.0*atan2(1.0, 1.0)
    c0 = 299792458  # m/sec

    #print("_systime_", systime())
    #print("_boottime_", ck_boottime())

    # check ARGV for previous estimate
    if (ARGC > 0) {
        ARGV[1] ? t0_est[1] = ARGV[1] : t0_est[1] = 0.0
        ARGV[2] ? t0_est[2] = ARGV[2] : t0_est[2] = systime()
    }
    #print("t0_est = ", t0_est[1])
    #print("t_boot_unc = ", t0_est[2])

    # wait (sleep) based on uncertainty
    # print(60.0/t0_est[2])

    # check uptime, update estimate
    print(ck_uptime(t0_est[1], t0_est[2]))

}