"""Recognize the non-text parts of a page: running header, page number, figure caption."""
import re
from difflib import SequenceMatcher

CAPTION_RE = re.compile(r"^\s*(LEFT|RIGHT|ABOVE|BELOW|OPPOSITE|FACING)\s*:\s*([A-Z][A-Z .,'\-]+?)\s*$")
_NUM_LEAD = re.compile(r"^\s*(\d{1,3})\s{2,}(\S.*)$")
_NUM_TRAIL = re.compile(r"^(.*\S)\s{2,}(\d{1,3})\s*$")
_NUM_ONLY = re.compile(r"^\s*(\d{1,3})\s*$")


def norm(text):
    return re.sub(r"[^a-z]", "", text.lower())


def similar(a, b):
    return SequenceMatcher(None, norm(a), norm(b)).ratio()


def _mostly_upper(text):
    letters = [c for c in text if c.isalpha()]
    return (not letters) or sum(c.isupper() for c in letters) / len(letters) > 0.7


def header_zone(lines, title, extra_titles=()):
    """The running header at the top of a unit.
    Returns (first_line, last_line_exclusive, page_number or None) or None.
    The header starts at the first non-empty line and runs over the following lines while they are
    upper case (at most 3 lines in all), whether or not a blank line separates it from the text.
    It is a header if it carries a page number next to uppercase text, or reads as the book title or as
    the chapter title (extra_titles: the right-hand page repeats the chapter title, and its page number
    may be misread by the scan as a letter, so no number is found)."""
    i = 0
    while i < len(lines) and not lines[i].strip():
        i += 1
    if i >= len(lines):
        return None
    j = i + 1
    while j < len(lines) and lines[j].strip() and j - i < 3 and _mostly_upper(lines[j]):
        j += 1
    zone = lines[i:j]
    number, rest = None, []
    for ln in zone:
        m = _NUM_LEAD.match(ln) or None
        if m and number is None:
            number, rest = int(m.group(1)), rest + [m.group(2)]
            continue
        m = _NUM_TRAIL.match(ln)
        if m and number is None:
            number, rest = int(m.group(2)), rest + [m.group(1)]
            continue
        m = _NUM_ONLY.match(ln)
        if m and number is None:
            number = int(m.group(1))
            continue
        rest.append(ln)
    text = " ".join(rest)
    if number is not None and _mostly_upper(text) and len(zone) <= 3:
        return i, j, number
    for t in (title,) + tuple(extra_titles):
        if t and similar(text, t) >= 0.9:
            return i, j, None
    return None


def footer_number(lines):
    """A lone page number as the last non-empty line. Returns (line_index, number) or None."""
    k = len(lines) - 1
    while k >= 0 and not lines[k].strip():
        k -= 1
    if k >= 0 and _NUM_ONLY.match(lines[k]):
        return k, int(lines[k])
    return None