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    // Copyright 2015 Brian Smith.
    //
    // Permission to use, copy, modify, and/or distribute this software for any
    // purpose with or without fee is hereby granted, provided that the above
    // copyright notice and this permission notice appear in all copies.
    //
    // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
    // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
    // SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
    // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
    // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

    //! Building blocks for parsing DER-encoded ASN.1 structures.
    //!
    //! This module contains the foundational parts of an ASN.1 DER parser.

    use untrusted;
    use error;

    pub const CONSTRUCTED: u8 = 1 << 5;
    pub const CONTEXT_SPECIFIC: u8 = 2 << 6;

    #[derive(Clone, Copy, PartialEq)]
    #[repr(u8)]
    pub enum Tag {
        Boolean = 0x01,
        Integer = 0x02,
        BitString = 0x03,
        OctetString = 0x04,
        Null = 0x05,
        OID = 0x06,
        Sequence = CONSTRUCTED | 0x10, // 0x30
        UTCTime = 0x17,
        GeneralizedTime = 0x18,

        ContextSpecificConstructed0 = CONTEXT_SPECIFIC | CONSTRUCTED | 0,
        ContextSpecificConstructed1 = CONTEXT_SPECIFIC | CONSTRUCTED | 1,
        ContextSpecificConstructed3 = CONTEXT_SPECIFIC | CONSTRUCTED | 3,
    }

    pub fn expect_tag_and_get_value<'a>(input: &mut untrusted::Reader<'a>,
                                        tag: Tag)
                                        -> Result<untrusted::Input<'a>,
                                                  error::Unspecified> {
        let (actual_tag, inner) = read_tag_and_get_value(input)?;
        if (tag as usize) != (actual_tag as usize) {
            return Err(error::Unspecified);
        }
        Ok(inner)
    }

    pub fn read_tag_and_get_value<'a>(input: &mut untrusted::Reader<'a>)
                                      -> Result<(u8, untrusted::Input<'a>),
                                                error::Unspecified> {
        let tag = input.read_byte()?;
        if (tag & 0x1F) == 0x1F {
            return Err(error::Unspecified); // High tag number form is not allowed.
        }

        // If the high order bit of the first byte is set to zero then the length
        // is encoded in the seven remaining bits of that byte. Otherwise, those
        // seven bits represent the number of bytes used to encode the length.
        let length = match input.read_byte()? {
            n if (n & 0x80) == 0 => n as usize,
            0x81 => {
                let second_byte = input.read_byte()?;
                if second_byte < 128 {
                    return Err(error::Unspecified); // Not the canonical encoding.
                }
                second_byte as usize
            },
            0x82 => {
                let second_byte = input.read_byte()? as usize;
                let third_byte = input.read_byte()? as usize;
                let combined = (second_byte << 8) | third_byte;
                if combined < 256 {
                    return Err(error::Unspecified); // Not the canonical encoding.
                }
                combined
            },
            _ => {
                return Err(error::Unspecified); // We don't support longer lengths.
            },
        };

        let inner = input.skip_and_get_input(length)?;
        Ok((tag, inner))
    }

    pub fn bit_string_with_no_unused_bits<'a>(input: &mut untrusted::Reader<'a>)
            -> Result<untrusted::Input<'a>, error::Unspecified> {
        nested(input, Tag::BitString, error::Unspecified, |value| {
            let unused_bits_at_end =
                value.read_byte().map_err(|_| error::Unspecified)?;
            if unused_bits_at_end != 0 {
                return Err(error::Unspecified);
            }
            Ok(value.skip_to_end())
        })
    }

    // TODO: investigate taking decoder as a reference to reduce generated code
    // size.
    pub fn nested<'a, F, R, E: Copy>(input: &mut untrusted::Reader<'a>, tag: Tag,
                                     error: E, decoder: F) -> Result<R, E>
                                     where F : FnOnce(&mut untrusted::Reader<'a>)
                                                      -> Result<R, E> {
        let inner = expect_tag_and_get_value(input, tag).map_err(|_| error)?;
        inner.read_all(error, decoder)
    }

    fn nonnegative_integer<'a>(input: &mut untrusted::Reader<'a>, min_value: u8)
                               -> Result<untrusted::Input<'a>, error::Unspecified> {
        // Verify that |input|, which has had any leading zero stripped off, is the
        // encoding of a value of at least |min_value|.
        fn check_minimum(input: untrusted::Input, min_value: u8)
                         -> Result<(), error::Unspecified> {
            input.read_all(error::Unspecified, |input| {
                let first_byte = input.read_byte()?;
                if input.at_end() && first_byte < min_value {
                    return Err(error::Unspecified);
                }
                let _ = input.skip_to_end();
                Ok(())
            })
        }

        let value = expect_tag_and_get_value(input, Tag::Integer)?;

        value.read_all(error::Unspecified, |input| {
            // Empty encodings are not allowed.
            let first_byte = input.read_byte()?;

            if first_byte == 0 {
                if input.at_end() {
                    // |value| is the legal encoding of zero.
                    if min_value > 0 {
                        return Err(error::Unspecified);
                    }
                    return Ok(value);
                }

                let r = input.skip_to_end();
                r.read_all(error::Unspecified, |input| {
                    let second_byte = input.read_byte()?;
                    if (second_byte & 0x80) == 0 {
                        // A leading zero is only allowed when the value's high bit
                        // is set.
                        return Err(error::Unspecified);
                    }
                    let _ = input.skip_to_end();
                    Ok(())
                })?;
                check_minimum(r, min_value)?;
                return Ok(r);
            }

            // Negative values are not allowed.
            if (first_byte & 0x80) != 0 {
                return Err(error::Unspecified);
            }

            let _ = input.skip_to_end();
            check_minimum(value, min_value)?;
            Ok(value)
        })
    }

    /// Parse as integer with a value in the in the range [0, 255], returning its
    /// numeric value. This is typically used for parsing version numbers.
    #[inline]
    pub fn small_nonnegative_integer(input: &mut untrusted::Reader)
                                     -> Result<u8, error::Unspecified> {
        let value = nonnegative_integer(input, 0)?;
        value.read_all(error::Unspecified, |input| {
            let r = input.read_byte()?;
            Ok(r)
        })
    }

    /// Parses a positive DER integer, returning the big-endian-encoded value, sans
    /// any leading zero byte.
    #[inline]
    pub fn positive_integer<'a>(input: &mut untrusted::Reader<'a>)
                                -> Result<untrusted::Input<'a>, error::Unspecified> {
        nonnegative_integer(input, 1)
    }

    #[cfg(test)]
    mod tests {
        use error;
        use super::*;
        use untrusted;

        fn with_good_i<F, R>(value: &[u8], f: F)
                             where F: FnOnce(&mut untrusted::Reader)
                                             -> Result<R, error::Unspecified> {
            let r = untrusted::Input::from(value).read_all(error::Unspecified, f);
            assert!(r.is_ok());
        }

        fn with_bad_i<F, R>(value: &[u8], f: F)
                            where F: FnOnce(&mut untrusted::Reader)
                                            -> Result<R, error::Unspecified> {
            let r = untrusted::Input::from(value).read_all(error::Unspecified, f);
            assert!(r.is_err());
        }

        static ZERO_INTEGER: &'static [u8] = &[0x02, 0x01, 0x00];

        static GOOD_POSITIVE_INTEGERS: &'static [(&'static [u8], u8)] =
            &[(&[0x02, 0x01, 0x01], 0x01),
              (&[0x02, 0x01, 0x02], 0x02),
              (&[0x02, 0x01, 0x7e], 0x7e),
              (&[0x02, 0x01, 0x7f], 0x7f),

              // Values that need to have an 0x00 prefix to disambiguate them from
              // them from negative values.
              (&[0x02, 0x02, 0x00, 0x80], 0x80),
              (&[0x02, 0x02, 0x00, 0x81], 0x81),
              (&[0x02, 0x02, 0x00, 0xfe], 0xfe),
              (&[0x02, 0x02, 0x00, 0xff], 0xff)];

        static BAD_NONNEGATIVE_INTEGERS: &'static [&'static [u8]] =
            &[&[], // At end of input
              &[0x02], // Tag only
              &[0x02, 0x00], // Empty value

              // Length mismatch
              &[0x02, 0x00, 0x01],
              &[0x02, 0x01],
              &[0x02, 0x01, 0x00, 0x01],
              &[0x02, 0x01, 0x01, 0x00], // Would be valid if last byte is ignored.
              &[0x02, 0x02, 0x01],

              // Negative values
              &[0x02, 0x01, 0x80],
              &[0x02, 0x01, 0xfe],
              &[0x02, 0x01, 0xff],

              // Values that have an unnecessary leading 0x00
              &[0x02, 0x02, 0x00, 0x00],
              &[0x02, 0x02, 0x00, 0x01],
              &[0x02, 0x02, 0x00, 0x02],
              &[0x02, 0x02, 0x00, 0x7e],
              &[0x02, 0x02, 0x00, 0x7f]];

        #[test]
        fn test_small_nonnegative_integer() {
            with_good_i(ZERO_INTEGER, |input| {
                assert_eq!(small_nonnegative_integer(input)?, 0x00);
                Ok(())
            });
            for &(ref test_in, test_out) in GOOD_POSITIVE_INTEGERS.iter() {
                with_good_i(test_in, |input| {
                    assert_eq!(small_nonnegative_integer(input)?, test_out);
                    Ok(())
                });
            }
            for &test_in in BAD_NONNEGATIVE_INTEGERS.iter() {
                with_bad_i(test_in, |input| {
                    let _ = small_nonnegative_integer(input)?;
                    Ok(())
                });
            }
        }

        #[test]
        fn test_positive_integer() {
            with_bad_i(ZERO_INTEGER, |input| {
                let _ = positive_integer(input)?;
                Ok(())
            });
            for &(ref test_in, test_out) in GOOD_POSITIVE_INTEGERS.iter() {
                with_good_i(test_in, |input| {
                    let test_out = [test_out];
                    assert_eq!(positive_integer(input)?,
                               untrusted::Input::from(&test_out[..]));
                    Ok(())
                });
            }
            for &test_in in BAD_NONNEGATIVE_INTEGERS.iter() {
                with_bad_i(test_in, |input| {
                    let _ = positive_integer(input)?;
                    Ok(())
                });
            }
        }
    }

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