"""codecontext - display the block context above the edit window

Once code has scrolled off the top of a window, it can be difficult to
determine which block you are in.  This extension implements a pane at the top
of each IDLE edit window which provides block structure hints.  These hints are
the lines which contain the block opening keywords, e.g. 'if', for the
enclosing block.  The number of hint lines is determined by the maxlines
variable in the codecontext section of config-extensions.def. Lines which do
not open blocks are not shown in the context hints pane.

"""
import re
from sys import maxsize as INFINITY

import tkinter
from tkinter.constants import NSEW, SUNKEN

from idlelib.config import idleConf

BLOCKOPENERS = {"class", "def", "elif", "else", "except", "finally", "for",
                "if", "try", "while", "with", "async"}


def get_spaces_firstword(codeline, c=re.compile(r"^(\s*)(\w*)")):
    "Extract the beginning whitespace and first word from codeline."
    return c.match(codeline).groups()


def get_line_info(codeline):
    """Return tuple of (line indent value, codeline, block start keyword).

    The indentation of empty lines (or comment lines) is INFINITY.
    If the line does not start a block, the keyword value is False.
    """
    spaces, firstword = get_spaces_firstword(codeline)
    indent = len(spaces)
    if len(codeline) == indent or codeline[indent] == '#':
        indent = INFINITY
    opener = firstword in BLOCKOPENERS and firstword
    return indent, codeline, opener


class CodeContext:
    "Display block context above the edit window."
    UPDATEINTERVAL = 100  # millisec

    def __init__(self, editwin):
        """Initialize settings for context block.

        editwin is the Editor window for the context block.
        self.text is the editor window text widget.

        self.context displays the code context text above the editor text.
          Initially None, it is toggled via <<toggle-code-context>>.
        self.topvisible is the number of the top text line displayed.
        self.info is a list of (line number, indent level, line text,
          block keyword) tuples for the block structure above topvisible.
          self.info[0] is initialized with a 'dummy' line which
          starts the toplevel 'block' of the module.

        self.t1 and self.t2 are two timer events on the editor text widget to