[chaley]

The code calibre uses to shorten a path is under the spoiler. You will see that calibre shortens each component by a % of the amount needed, thus the longest components are shortened more.

Spoiler:

def shorten_component(s, by_what):
    l = len(s)
    if l < by_what:
        return s
    l = (l - by_what)//2
    if l <= 0:
        return s
    return s[:l] + s[-l:]

def shorten_components_to(length, components, more_to_take=0, last_has_extension=True):
    filepath = os.sep.join(components)
    extra = len(filepath) - (length - more_to_take)
    if extra < 1:
        return components
    deltas = []
    for x in components:
        pct = len(x)/float(len(filepath))
        deltas.append(int(ceil(pct*extra)))
    ans = []

    for i, x in enumerate(components):
        delta = deltas[i]
        if delta > len(x):
            r = x[0] if x is components[-1] else ''
        else:
            if last_has_extension and x is components[-1]:
                b, e = os.path.splitext(x)
                if e == '.':
                    e = ''
                r = shorten_component(b, delta)+e
                if r.startswith('.'):
                    r = x[0]+r
            else:
                r = shorten_component(x, delta)
            r = r.strip()
            if not r:
                r = x.strip()[0] if x.strip() else 'x'
        ans.append(r)
    if len(os.sep.join(ans)) > length:
        return shorten_components_to(length, components, more_to_take+2)
    return ans