#!/bin/rc
rfork e

# get a sorted list of process in "ppid pid" format
fn procs {
  {cd /proc; for (i in */ppid) {cat $i; echo $i | sed -e 's/\/ppid//'}} | sort -n
}

# create variables $pid.<pidnum> that contains all child processes
fn collect {
  procs | while (pp=`{read}) {
    echo $pp(2) >> /env/pid.$pp(1)
  }
}

# print n spaces.
fn spaces {
   for (i in `{seq 1 $1}) echo -n ' '
}

# extract the proc name from /proc/$/status
fn status {
  if (test -e /proc/$1) {
    dd -bs 1 -count 20 -quiet 1 < /proc/$1/status | tr -d ' \t'
  }
  echo -n ([$1])
}

# recursively get the parent of the given pid.
fn parent {
  switch ($1) {
    case 0
    case *
      if (test -f /proc/$1/ppid) {
        parent `{cat /proc/$1/ppid}
        echo -n '->'
      }
  }
  status $1
}

# recursively get the children of the given pid.
# also passes in the number of levels of recursion so that
# we can use it to print indentation
fn children {
   spaces $2 ; echo -n '|-'; status $1 ; echo
   if (test -f /env/pid.^$1) {
     for (x in `{cat /env/pid.^$1}) {
       children $x `{echo $2 1 + p|dc}
     }
   }
}

fn usage {
  echo 'pstree -p pid'
  echo '   Show parents of the passed pid'
  echo 'pstree -c pid'
  echo '   Show children of the passed pid'
}

fn main {
  switch($1) {
   case -p
     collect
     parent $2
     echo
   case -c
     collect
     children $2 0 
   case *
     usage
  }
}

main $*