import sys
import traceback


BROWSER_ERROR_TEMPLATE = """\
body:before {{
    content: {0};

    display: block;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;

    font-size: 14px;
    margin: 1em;
    padding: 1em;
    border: 3px double red;

    white-space: pre;
    font-family: monospace;
    background: #fcebeb;
    color: black;
}}
"""

def add_error_marker(text, position, start_line=1):
    """Add a caret marking a given position in a string of input.

    Returns (new_text, caret_line).
    """
    indent = "    "
    lines = []
    caret_line = start_line
    for line in text.split("\n"):
        lines.append(indent + line)

        if 0 <= position <= len(line):
            lines.append(indent + (" " * position) + "^")
            caret_line = start_line

        position -= len(line)
        position -= 1  # for the newline
        start_line += 1

    return "\n".join(lines), caret_line


class SassError(Exception):
    """Error class that wraps another exception and attempts to bolt on some
    useful context.
    """
    def __init__(self, exc, rule=None, expression=None, expression_pos=None):
        self.exc = exc

        self.rule_stack = []
        if rule:
            self.rule_stack.append(rule)

        self.expression = expression
        self.expression_pos = expression_pos

        _, _, self.original_traceback = sys.exc_info()

    def add_rule(self, rule):
        """Add a new rule to the "stack" of rules -- this is used to track,
        e.g., how a file was ultimately imported.
        """
        self.rule_stack.append(rule)

    def format_prefix(self):
        """Return the general name of the error and the contents of the rule or
        property that caused the failure.  This is the initial part of the
        error message and should be error-specific.
        """
        # TODO this contains NULs and line numbers; could be much prettier
        if self.rule_stack:
            return (
                "Error parsing block:\n" +
                "    " + self.rule_stack[0].unparsed_contents + "\n"
            )
        else:
            return "Unknown error\n"

    def format_sass_stack(self):
        """Return a "traceback" of Sass imports."""
        if not self.rule_stack:
            return ""

        ret = ["From ", self.rule_stack[0].file_and_line, "\n"]
        last_file = self.rule_stack[0].source_file

        # TODO this could go away if rules knew their import chains...
        for rule in self.rule_stack[1:]:
            if rule.source_file is not last_file:
                ret.extend(("...imported from ", rule.file_and_line, "\n"))
            last_file = rule.source_file

        return "".join(ret)

    def format_python_stack(self):
        """Return a traceback of Python frames, from where the error occurred
        to where it was first caught and wrapped.
        """
        ret = ["Traceback:\n"]
        ret.extend(traceback.format_tb(self.original_traceback))
        return "".join(ret)

    def format_original_error(self):
        """Return the typical "TypeError: blah blah" for the original wrapped
        error.
        """
        # TODO eventually we'll have sass-specific errors that will want nicer
        # "names" in browser display and stderr
        return "".join((type(self.exc).__name__, ": ", str(self.exc), "\n"))

    def __str__(self):
        try:
            prefix = self.format_prefix()
            sass_stack = self.format_sass_stack()
            python_stack = self.format_python_stack()
            original_error = self.format_original_error()

            # TODO not very well-specified whether these parts should already
            # end in newlines, or how many
            return prefix + "\n" + sass_stack + python_stack + original_error
        except Exception:
            # "unprintable error" is not helpful
            return str(self.exc)

    def to_css(self):
        """Return a stylesheet that will show the wrapped error at the top of
        the browser window.
        """
        # TODO should this include the traceback?  any security concerns?
        prefix = self.format_prefix()
        original_error = self.format_original_error()
        sass_stack = self.format_sass_stack()

        message = prefix + "\n" + sass_stack + original_error

        # Super simple escaping: only quotes and newlines are illegal in css
        # strings
        message = message.replace('\\', '\\\\')
        message = message.replace('"', '\\"')
        # use the maximum six digits here so it doesn't eat any following
        # characters that happen to look like hex
        message = message.replace('\n', '\\00000A')

        return BROWSER_ERROR_TEMPLATE.format('"' + message + '"')


class SassParseError(SassError):
    """Error raised when parsing a Sass expression fails."""

    def format_prefix(self):
        decorated_expr, line = add_error_marker(self.expression, self.expression_pos or -1)
        return """Error parsing expression:\n{0}\n""".format(decorated_expr)


class SassEvaluationError(SassError):
    """Error raised when evaluating a parsed expression fails."""

    def format_prefix(self):
        # TODO would be nice for the AST to have position information
        # TODO might be nice to print the AST and indicate where the failure
        # was?
        decorated_expr, line = add_error_marker(self.expression, self.expression_pos or -1)
        return """Error evaluating expression:\n{0}\n""".format(decorated_expr)