#!/usr/bin/env python3
#
# Script to read a GDS file, and find the given cellname if it has the
# standard random two-character prefix generated by magic when writing
# out a vendor GDS, and report the prefix.  No changes are made to any
# file.

import os
import sys

def usage():
    print('find_gds_prefix.py <cell_name> <path_to_cell_gds>')

if __name__ == '__main__':
    debug = False

    if len(sys.argv) == 1:
        print("No options given to find_gds_prefix.py.")
        usage()
        sys.exit(0)

    optionlist = []
    arguments = []

    for option in sys.argv[1:]:
        if option.find('-', 0) == 0:
            optionlist.append(option)
        else:
            arguments.append(option)

    if len(arguments) != 2:
        print("Wrong number of arguments given to find_gds_prefix.py.")
        usage()
        sys.exit(0)

    for option in optionlist:
        if option == '-debug':
            debug = True

    cellname = arguments[0]
    cellsource = arguments[1]

    cellsrcdir = os.path.split(cellsource)[0]
    cellinfile = os.path.split(cellsource)[1]

    print('Reading GDS file looking for prefixed cell ' + cellname)
    with open(cellsource, 'rb') as ifile:
        celldata = ifile.read()

    #----------------------------------------------------------------------
    # Assume that celldata contains the cell in question.
    # Find the extend of the data from 'beginstr' to 'endstr'
    #----------------------------------------------------------------------
    
    datalen = len(celldata)
    dataptr = 0
    found = False
    while dataptr < datalen:
        # Read stream records up to 'beginstr'
        bheader = celldata[dataptr:dataptr + 2]
        reclen = int.from_bytes(bheader, 'big')
        if reclen == 0:
            print('Error: found zero-length record at position ' + str(dataptr))
            break

        rectype = celldata[dataptr + 2]
        datatype = celldata[dataptr + 3]

        brectype = rectype.to_bytes(1, byteorder='big')
        bdatatype = datatype.to_bytes(1, byteorder='big')

        if rectype == 5:            # beginstr
            saveptr = dataptr

        elif rectype == 6:            # strname
            if datatype != 6:
                print('Error: Structure name record is not a string!')
                sys.exit(1)

            bstring = celldata[dataptr + 4: dataptr + reclen]
            # Odd length strings end in null byte which needs to be removed
            if bstring[-1] == 0:
                bstring = bstring[:-1]
            strname = bstring.decode('ascii')
            if strname[3:] == cellname:
                print('Cell ' + strname + ' found at position ' + str(saveptr))
                print('Prefix: ' + strname[0:3])
                found = True
                break
            elif strname == cellname:
                print('Unprefixed cell ' + strname + ' found at position ' + str(saveptr))
            elif debug:
                print('Cell ' + strname + ' position ' + str(dataptr) + ' (ignored)')

        # Advance the pointer past the data
        dataptr += reclen

    if not found:
        print('Failed to find a prefixed cell ' + cellname)
        sys.exit(1)

    exit(0)