"""
Django LaTeX custom template tag for equations (using amsmath).

Creates a .png in the project media directory and returns an img tag that references it. 

Usage: 
    {% latex %}
    x=\frac{-b \pm \sqrt {b^2-4ac}}{2a}
    {% endlatex %}

Requirements:
    LaTeX (http://www.latex-project.org/ftp.html)
    dvipng (http://savannah.nongnu.org/projects/dvipng/)

Thanks to Kjell Magne Fauske (http://www.fauskes.net/nb/htmleqII/) for outlining the method.
"""

from django import template
register = template.Library()

class LaTeXNode(template.Node):
    # directory to append to MEDIA_ROOT and MEDIA_URL, terminate with /
    OUTPUT_DIR = 'image/latex/'

    # class name to assign to generated <img> tag
    IMG_CLASS_NAME = 'latex'

    LATEX_HEADER = '''
    \\documentclass{article}
    \\usepackage{amsmath}
    \\usepackage{amsthm}
    \\usepackage{amssymb}
    \\usepackage{bm}
    \\pagestyle{empty}
    \\begin{document}
    '''
    LATEX_FOOTER = '''
    \\end{document}
    '''

    def __init__(self, nodelist, label=None):
        self.nodelist = nodelist
        self.label = label

    def render(self, context):
        import subprocess
        import os
        from datetime import datetime
        from hashlib import md5
        from django.conf import settings
        
        latex = self.nodelist.render(context)
        filebase = md5(latex).hexdigest()
        workingdir = os.path.join(settings.MEDIA_ROOT, self.OUTPUT_DIR)

        if not os.path.exists(os.path.join(workingdir, filebase + '.png')):
            latexpath = r'%s%s.tex' % (workingdir, filebase)
            
            latexfile = open(latexpath, 'w')
            latexfile.write('\n'.join(('% generated at: ' + str(datetime.now()),
                                       self.LATEX_HEADER,
                                       '$%s$' % latex,  # delimit equation with $
                                       self.LATEX_FOOTER)))
            latexfile.close()
            
            old_workingdir = os.getcwd()
            os.chdir(workingdir)
            latex_exitcode = subprocess.call((r'latex',
                                              filebase + '.tex'))
            dvipng_exitcode = subprocess.call((r'dvipng',
                                               '-o', filebase + '.png',
                                               '-pp', '1',  # only page one
                                               '-T', 'tight',
                                               '-x', '1600',  # scale
                                               filebase + '.dvi'))
            if latex_exitcode or dvipng_exitcode:
                # TODO: There is room for better error reporting here,
                # but I'm happy to fail silently and leave the temp
                # files behind for debugging.
                return latex
            else:
                os.unlink(filebase + '.tex')
                os.unlink(filebase + '.log')
                os.unlink(filebase + '.aux')
                os.unlink(filebase + '.dvi')
                os.chdir(old_workingdir) 

        return '<img src="%s" alt="%s" %s/>' % (
            settings.MEDIA_URL + self.OUTPUT_DIR + filebase + '.png',
            latex,
            self.IMG_CLASS_NAME and 'class="%s" ' % self.IMG_CLASS_NAME or '')

def latex(parser, token):
    nodelist = parser.parse(('endlatex',))
    parser.delete_first_token()

    parts = token.contents.split()
    if len(parts) == 1:
        return LaTeXNode(nodelist)
    else:
        raise template.TemplateSyntaxError(
            "latex tag does not accept parameters, got '%s'" % (
                ' '.join(parts[1:])))

register.tag('latex', latex)