#!/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.
"""

import logging

_log = logging.getLogger("webkitpy.layout_tests.metered_stream")


class MeteredStream:
    """This class is a wrapper around a stream that allows you to implement
    meters.

    It can be used like a stream, but calling update() will print
    the string followed by only a carriage return (instead of a carriage
    return and a line feed). This can be used to implement progress bars and
    other sorts of meters. Note that anything written by update() will be
    erased by a subsequent update(), write(), or flush()."""

    def __init__(self, verbose, stream):
        """
        Args:
          verbose: whether update is a no-op
          stream: output stream to write to
        """
        self._dirty = False
        self._verbose = verbose
        self._stream = stream
        self._last_update = ""

    def write(self, txt):
        """Write to the stream, overwriting and resetting the meter."""
        if self._dirty:
            self.update("")
            self._dirty = False
        self._stream.write(txt)

    def flush(self):
        """Flush any buffered output."""
        self._stream.flush()

    def progress(self, str):
        """
        Write a message to the stream that will get overwritten.

        This is used for progress updates that don't need to be preserved in
        the log. If the MeteredStream was initialized with verbose==True,
        then this output is discarded. We have this in case we are logging
        lots of output and the update()s will get lost or won't work
        properly (typically because verbose streams are redirected to files).

        """
        if self._verbose:
            return
        self._write(str)

    def update(self, str):
        """
        Write a message that is also included when logging verbosely.

        This routine preserves the same console logging behavior as progress(),
        but will also log the message if verbose() was true.

        """
        # Note this is a separate routine that calls either into the logger
        # or the metering stream. We have to be careful to avoid a layering
        # inversion (stream calling back into the logger).
        if self._verbose:
            _log.info(str)
        else:
            self._write(str)

    def _write(self, str):
        """Actually write the message to the stream."""

        # FIXME: Figure out if there is a way to detect if we're writing
        # to a stream that handles CRs correctly (e.g., terminals). That might
        # be a cleaner way of handling this.

        # Print the necessary number of backspaces to erase the previous
        # message.
        self._stream.write("\b" * len(self._last_update))
        self._stream.write(str)
        num_remaining = len(self._last_update) - len(str)
        if num_remaining > 0:
            self._stream.write(" " * num_remaining + "\b" * num_remaining)
        self._last_update = str
        self._dirty = True