#!/usr/bin/env python
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"""
Package that implements a stream wrapper that has 'meters' as well as
regular output. A 'meter' is a single line of text that can be erased
and rewritten repeatedly, without producing multiple lines of output. It
can be used to produce effects like progress bars.

This package should only be called by the printing module in the layout_tests
package.
"""

class MeteredStream:
    """This class is a wrapper around a stream that allows you to implement
    meters (progress bars, etc.).

    It can be used directly as a stream, by calling write(), but also provides
    a method called update() that will overwite prior updates().
   """

    def __init__(self, stream):
        """
        Args:
          stream: output stream to write to
        """
        self._stream = stream
        self._dirty = False
        self._last_update = ""

    def write(self, txt):
        """Write to the stream, overwriting and resetting the meter."""

        # This routine is called by the logging infrastructure, and
        # must not call back into logging. It is not a public function.
        self._overwrite(txt)
        self._reset()

    def update(self, txt):
        """Write a message that will be overwritten by subsequent update() or write() calls."""
        self._overwrite(txt)

    def _overwrite(self, txt):
        # Print the necessary number of backspaces to erase the previous
        # message.
        if len(self._last_update):
            self._stream.write("\b" * len(self._last_update) +
                               " " * len(self._last_update) +
                               "\b" * len(self._last_update))
        self._stream.write(txt)
        last_newline = txt.rfind("\n")
        self._last_update = txt[(last_newline + 1):]
        self._dirty = True

    def _reset(self):
        self._dirty = False
        self._last_update = ''