#!/usr/bin/env awk -f


function log10(n) { return log(n)/log(10.0) }


function merge_meas(est_val, est_unc, meas_val, meas_unc) {

    # Kalman filtering compares variances,
    # convert uncertainties to square units
    est_unc = (0.5*est_unc)^2.0
    meas_unc = (0.5*meas_unc)^2.0

    K = (est_unc)/((est_unc) + (meas_unc))
    est_val = est_val + K*(meas_val - est_val)
    est_unc = (est_unc*meas_unc)/(est_unc + meas_unc)

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

    return sprintf(OFMT OFS OFMT ORS, est_val, est_unc)

}


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() {

    "uname" | getline uname
    close("uname")

    if (uname ~/OpenBSD/) {
        "sysctl kern.boottime" | getline t0_k
        close("sysctl kern.boottime")
        sub("^.*=", "", t0_k)
        split(t0_k, t0_k_arr)
        t0_k_arr[2] = (index("JanFebMarAprMayJunJulAugSepOctNovDec", t0_k_arr[2]) + 2)/3.0
        gsub(":", " ", t0_k_arr[4])
        t0_k = sprintf("%04d %02d %02d %s", t0_k_arr[5], t0_k_arr[2], t0_k_arr[3], t0_k_arr[4])
        t0_k = mktime(t0_k)
    }

    return t0_k

}


# 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] = ck_boottime()
        ARGV[2] ? t0_est[2] = ARGV[2] : t0_est[2] = systime()
    }
    #print("t0_est = ", t0_est[1])
    #print("t_boot_unc = ", t0_est[2])

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

}