import os
from os.path import (isabs, islink)

def realpath(path):
    if isinstance(path, bytes):
        sep = b'/'
        dot = b'.'
        dotdot = b'..'
        getcwd = os.getcwdb
    else:
        sep = '/'
        dot = '.'
        dotdot = '..'
        getcwd = os.getcwd
    root = sep
    
    # The path is broken into components. Each component may resolve to a
    # sub-path, and its components may require further resolution.
    
    # Stack of component iterators for each sub-path being resolved
    unres = [iter(path.split(sep))]
    if not isabs(path):
        unres.append(iter(getcwd().split(sep)))
    
    links = []  # Stack of links names of sub-paths being resolved
    resolved = []  # Fully resolved path components
    while unres:
        while True:
            try:
                component = next(unres[-1])
            except StopIteration:
                break
            if not component or component == dot:
                continue
            if component == dotdot:
                if resolved:
                    resolved.pop()
                continue
            
            resolved.append(component)
            dir = root + sep.join(resolved)
            if islink(dir):
                if dir in links:
                    while unres:
                        resolved.extend(unres.pop())
                    return root + sep.join(resolved)
                resolved.pop()
                links.append(dir)
                target = os.readlink(dir)
                unres.append(iter(target.split(sep)))
                if isabs(target):
                    resolved = []
        
        unres.pop()
        if links:
            links.pop()
    return root + sep.join(resolved)