#! /usr/bin/python

import cgi, cgitb
cgitb.enable()

HTML_Header = 'Content-type: text/html\n\n'
HTML_file = 'LittleCalc.htm'

# constants needed to recognize where the TextArea is, and replace it
text_comment_start='<!--start-textarea-->'
text_comment_end='<!--end-textarea-->'
text_area_start='<textarea rows="15" name="the_commands" cols="30">'
text_area_end='</textarea>'
answer_start='<!--answer_start-->'
answer_end='<!--answer_end-->'

# ------------------------------- Main ------------------------
def Main():
    form = cgi.FieldStorage()
    f=open(HTML_file,'r')
    html=f.read()
    f.close()

    # get commands and calculate answer(s)
    if 'the_commands' in form:
        the_commands = form['the_commands'].value
        the_commands = the_commands.replace('\r','') # remove extraneous \r
        if the_commands.strip() == '':
            answer='No commands'
        else:
            answer = CalcAnswer(the_commands)
    else:
        the_commands=''
        answer='No commands'

    # modify the html and print
    new_commands = text_area_start+the_commands+text_area_end
    html = StringReplace(html,text_comment_start,text_comment_end,new_commands)
    html = StringReplace(html,answer_start,answer_end,answer)

    print HTML_Header+html

# ------------------------------- StringReplace -----------------
def StringReplace(the_string, from_after, until_before, with_this):
    pos1 = the_string.find(from_after) + len(from_after)
    pos2 = the_string.find(until_before)
    new_string = the_string[:pos1] + with_this + the_string[pos2:]
    return new_string

# ------------------------------- IsNumber ----------------------
def IsNumber(s):
    try:
        f=float(s)
        return True
    except:
        return False
    
# ------------------------------ CalcAnswer ---------------------
def CalcAnswer(commands):
    lines=commands.split('\n')
    #--------------------------------
    #  This is where your language interpreter code should go...
    # -------------------------------


Main()