Function critically_low_hp

src/pray.c:115–156

critically low hit points if hp <= 5 or hp <= maxhp/N for some N */

/* critically low hit points if hp <= 5 or hp <= maxhp/N for some N */
boolean
critically_low_hp(
    boolean only_if_injured) /* determines whether maxhp <= 5 matters */
{
    int divisor, hplim,
        curhp = Upolyd ? u.mh : u.uhp,
        maxhp = Upolyd ? u.mhmax : u.uhpmax;

    if (only_if_injured && !(curhp < maxhp))
        return FALSE;
    /* if maxhp is extremely high, use lower threshold for the division test
       (golden glow cuts off at 11+5*lvl, nurse interaction at 25*lvl; this
       ought to use monster hit dice--and a smaller multiplier--rather than
       ulevel when polymorphed, but polyself doesn't maintain that) */
    hplim = 15 * u.ulevel;
    if (maxhp > hplim)
        maxhp = hplim;
    /* 7 used to be the unconditional divisor */
    switch (xlev_to_rank(u.ulevel)) { /* maps 1..30 into 0..8 */
    case 0:
    case 1:
        divisor = 5;
        break; /* explvl 1 to 5 */
    case 2:
    case 3:
        divisor = 6;
        break; /* explvl 6 to 13 */
    case 4:
    case 5:
        divisor = 7;
        break; /* explvl 14 to 21 */
    case 6:
    case 7:
        divisor = 8;
        break; /* explvl 22 to 29 */
    default:
        divisor = 9;
        break; /* explvl 30+ */
    }
    /* 5 is a magic number in TROUBLE_HIT handling below */
    return (boolean) (curhp <= 5 || curhp * divisor <= maxhp);
}

/* return True if surrounded by impassible rock, regardless of the state
   of your own location (for example, inside a doorless closet) */