nilmdb/nilmdb/server/pyrocket.py

# Python implementation of the "rocket" data parsing interface.
# This interface translates between the binary format on disk
# and the ASCII format used when communicating with clients.

# This is slow!  Use the C version instead.

import struct
import layout as _layout

class Rocket(object):
    def __init__(self, layout):
        self.layout = layout

        # For packing/unpacking into a binary file.
        # This will change in the C version
        try:
            (self.ltype, lcount) = layout.split('_', 2)
            self.lcount = int(lcount)
        except:
            raise ValueError("no such layout: badly formatted string")
        if self.lcount < 1:
            raise ValueError("no such layout: bad count")
        try:
            struct_fmt = '<d'  # Little endian, double timestamp
            struct_mapping = {
                "int8": 'b',
                "uint8": 'B',
                "int16": 'h',
                "uint16": 'H',
                "int32": 'i',
                "uint32": 'I',
                "int64": 'q',
                "uint64": 'Q',
                "float32": 'f',
                "float64": 'd',
                }
            struct_fmt += struct_mapping[self.ltype] * self.lcount
        except KeyError:
            raise ValueError("no such layout: bad data type")
        self.packer = struct.Struct(struct_fmt)

        # For packing/unpacking from strings.
        self.formatter = _layout.Formatter(self.layout)

    @property
    def binary_size(self):
        """Return size of one row of data in the binary file, in bytes"""
        return self.packer.size

    def append_list(self, file, data):
        """Append the list data to the file"""
        # We assume the file is opened in append mode,
        # so all writes go to the end.
        for row in data:
            file.write(self.packer.pack(*row))
        file.flush()

    def extract_list(self, file, offset, count):
        """Extract count rows of data from the file at offset offset.
        Return a list of lists [[row],[row],...]"""
        ret = []
        file.seek(offset)
        for i in xrange(count):
            data = file.read(self.binary_size)
            ret.append(list(self.packer.unpack(data)))
        return ret

    def extract_string(self, file, offset, count):
        """Extract count rows of data from the file at offset offset.
        Return an ascii formatted string according to the layout"""
        return self.formatter.format(self.extract_list(file, offset, count))