# textdiff.py D. Parson August 27, 2023, simple differ for at home
__usage__ = 'USAGE: python3 textdiff.py YOURFILE REFFILE'

import sys

if __name__ == '__main__':  # Invoked from command line.
    if len(sys.argv) != 3:
        raise ValueError(__usage__)
    elif sys.argv[1] == sys.argv[2]:
        raise ValueError(sys.argv[1] + ' and ' 
            + sys.argv[2] + ' are the same file.')
    left = open(sys.argv[1], 'r')
    right = open(sys.argv[2], 'r')
    errors = 0
    leftlines = left.readlines()
    rightlines = right.readlines()
    left.close()
    right.close()
    # Reference file captured in Linux. Windows output tacks on a
    # <cr> that readlines() treats as an extra blank line. Filter
    # out any blank lines before processing.
    leftlines = [line.strip() for line in leftlines] # strip() removes spaces
    rightlines = [line.strip() for line in rightlines] # but not extra lines
    def filterFunc(line):
        return (line != '')
    leftlines = list(filter(filterFunc, leftlines))
    rightlines = list(filter(filterFunc, rightlines))
    if len(leftlines) != len(rightlines):
        sys.stderr.write('ERROR, ' + sys.argv[1] + ' has '
            + str(len(leftlines)) + ' lines, ' + sys.argv[2]
            + ' has ' + str(len(rightlines)) + ' lines.\n')
        errors = 1
    for lineix in range(0, len(rightlines)):
        if (lineix >= len(leftlines)) or (lineix >= len(rightlines)):
            # error was reported, do not compare them.
            break
        leftline = leftlines[lineix]
        rightline = rightlines[lineix]
        if leftline != rightline:
            sys.stderr.write('\nERROR, Diff at line ' + str(lineix+1) + '\n')
            sys.stderr.write('< ' + leftline + '\n')
            sys.stderr.write('> ' + rightline + '\n')
            errors += 1
            if errors > 4:
                break
    if errors:
        sys.exit(1)
    sys.exit(0)     # zero means no error, non-zero means error(s)