lib.rs

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Really Bad Markup Language (rbml) is an internal serialization format of rustc.
//! This is not intended to be used by users.
//!
//! Originally based on the Extensible Binary Markup Language
//! (ebml; http://www.matroska.org/technical/specs/rfc/index.html),
//! it is now a separate format tuned for the rust object metadata.
//!
//! # Encoding
//!
//! RBML document consists of the tag, length and data.
//! The encoded data can contain multiple RBML documents concatenated.
//!
//! **Tags** are a hint for the following data.
//! Tags are a number from 0x000 to 0xfff, where 0xf0 through 0xff is reserved.
//! Tags less than 0xf0 are encoded in one literal byte.
//! Tags greater than 0xff are encoded in two big-endian bytes,
//! where the tag number is ORed with 0xf000. (E.g. tag 0x123 = `f1 23`)
//!
//! **Lengths** encode the length of the following data.
//! It is a variable-length unsigned isize, and one of the following forms:
//!
//! - `80` through `fe` for lengths up to 0x7e;
//! - `40 ff` through `7f ff` for lengths up to 0x3fff;
//! - `20 40 00` through `3f ff ff` for lengths up to 0x1fffff;
//! - `10 20 00 00` through `1f ff ff ff` for lengths up to 0xfffffff.
//!
//! The "overlong" form is allowed so that the length can be encoded
//! without the prior knowledge of the encoded data.
//! For example, the length 0 can be represented either by `80`, `40 00`,
//! `20 00 00` or `10 00 00 00`.
//! The encoder tries to minimize the length if possible.
//! Also, some predefined tags listed below are so commonly used that
//! their lengths are omitted ("implicit length").
//!
//! **Data** can be either binary bytes or zero or more nested RBML documents.
//! Nested documents cannot overflow, and should be entirely contained
//! within a parent document.
//!
//! # Predefined Tags
//!
//! Most RBML tags are defined by the application.
//! (For the rust object metadata, see also `rustc::metadata::common`.)
//! RBML itself does define a set of predefined tags however,
//! intended for the auto-serialization implementation.
//!
//! Predefined tags with an implicit length:
//!
//! - `U8`  (`00`): 1-byte unsigned integer.
//! - `U16` (`01`): 2-byte big endian unsigned integer.
//! - `U32` (`02`): 4-byte big endian unsigned integer.
//! - `U64` (`03`): 8-byte big endian unsigned integer.
//!   Any of `U*` tags can be used to encode primitive unsigned integer types,
//!   as long as it is no greater than the actual size.
//!   For example, `u8` can only be represented via the `U8` tag.
//!
//! - `I8`  (`04`): 1-byte signed integer.
//! - `I16` (`05`): 2-byte big endian signed integer.
//! - `I32` (`06`): 4-byte big endian signed integer.
//! - `I64` (`07`): 8-byte big endian signed integer.
//!   Similar to `U*` tags. Always uses two's complement encoding.
//!
//! - `Bool` (`08`): 1-byte boolean value, `00` for false and `01` for true.
//!
//! - `Char` (`09`): 4-byte big endian Unicode scalar value.
//!   Surrogate pairs or out-of-bound values are invalid.
//!
//! - `F32` (`0a`): 4-byte big endian unsigned integer representing
//!   IEEE 754 binary32 floating-point format.
//! - `F64` (`0b`): 8-byte big endian unsigned integer representing
//!   IEEE 754 binary64 floating-point format.
//!
//! - `Sub8`  (`0c`): 1-byte unsigned integer for supplementary information.
//! - `Sub32` (`0d`): 4-byte unsigned integer for supplementary information.
//!   Those two tags normally occur as the first subdocument of certain tags,
//!   namely `Enum`, `Vec` and `Map`, to provide a variant or size information.
//!   They can be used interchangeably.
//!
//! Predefined tags with an explicit length:
//!
//! - `Str` (`10`): A UTF-8-encoded string.
//!
//! - `Enum` (`11`): An enum.
//!   The first subdocument should be `Sub*` tags with a variant ID.
//!   Subsequent subdocuments, if any, encode variant arguments.
//!
//! - `Vec` (`12`): A vector (sequence).
//! - `VecElt` (`13`): A vector element.
//!   The first subdocument should be `Sub*` tags with the number of elements.
//!   Subsequent subdocuments should be `VecElt` tag per each element.
//!
//! - `Map` (`14`): A map (associated array).
//! - `MapKey` (`15`): A key part of the map entry.
//! - `MapVal` (`16`): A value part of the map entry.
//!   The first subdocument should be `Sub*` tags with the number of entries.
//!   Subsequent subdocuments should be an alternating sequence of
//!   `MapKey` and `MapVal` tags per each entry.
//!
//! - `Opaque` (`17`): An opaque, custom-format tag.
//!   Used to wrap ordinary custom tags or data in the auto-serialized context.
//!   Rustc typically uses this to encode type informations.
//!
//! First 0x20 tags are reserved by RBML; custom tags start at 0x20.

#![crate_name = "rbml"]
#![unstable(feature = "rustc_private", issue = "27812")]
#![crate_type = "rlib"]
#![crate_type = "dylib"]
#![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
       html_root_url = "https://doc.rust-lang.org/nightly/",
       html_playground_url = "https://play.rust-lang.org/",
       test(attr(deny(warnings))))]
#![cfg_attr(not(stage0), deny(warnings))]

#![feature(rustc_private)]
#![feature(staged_api)]

#![cfg_attr(test, feature(test))]

extern crate serialize;

#[cfg(test)]
extern crate serialize as rustc_serialize; // Used by RustcEncodable

#[macro_use]
extern crate log;

#[cfg(test)]
extern crate test;

pub mod opaque;
pub mod leb128;

pub use self::EbmlEncoderTag::*;
pub use self::Error::*;

use std::str;
use std::fmt;

/// Common data structures
#[derive(Clone, Copy)]
pub struct Doc<'a> {
    pub data: &'a [u8],
    pub start: usize,
    pub end: usize,
}

impl<'doc> Doc<'doc> {
    pub fn new(data: &'doc [u8]) -> Doc<'doc> {
        Doc {
            data: data,
            start: 0,
            end: data.len(),
        }
    }

    pub fn get<'a>(&'a self, tag: usize) -> Doc<'a> {
        reader::get_doc(*self, tag)
    }

    pub fn is_empty(&self) -> bool {
        self.start == self.end
    }

    pub fn as_str_slice<'a>(&'a self) -> &'a str {
        str::from_utf8(&self.data[self.start..self.end]).unwrap()
    }

    pub fn as_str(&self) -> String {
        self.as_str_slice().to_string()
    }
}

pub struct TaggedDoc<'a> {
    tag: usize,
    pub doc: Doc<'a>,
}

#[derive(Copy, Clone, Debug)]
pub enum EbmlEncoderTag {
    // tags 00..1f are reserved for auto-serialization.
    // first NUM_IMPLICIT_TAGS tags are implicitly sized and lengths are not encoded.
    EsU8 = 0x00, // + 1 byte
    EsU16 = 0x01, // + 2 bytes
    EsU32 = 0x02, // + 4 bytes
    EsU64 = 0x03, // + 8 bytes
    EsI8 = 0x04, // + 1 byte
    EsI16 = 0x05, // + 2 bytes
    EsI32 = 0x06, // + 4 bytes
    EsI64 = 0x07, // + 8 bytes
    EsBool = 0x08, // + 1 byte
    EsChar = 0x09, // + 4 bytes
    EsF32 = 0x0a, // + 4 bytes
    EsF64 = 0x0b, // + 8 bytes
    EsSub8 = 0x0c, // + 1 byte
    EsSub32 = 0x0d, // + 4 bytes
    // 0x0e and 0x0f are reserved
    EsStr = 0x10,
    EsEnum = 0x11, // encodes the variant id as the first EsSub*
    EsVec = 0x12, // encodes the # of elements as the first EsSub*
    EsVecElt = 0x13,
    EsMap = 0x14, // encodes the # of pairs as the first EsSub*
    EsMapKey = 0x15,
    EsMapVal = 0x16,
    EsOpaque = 0x17,
}

const NUM_TAGS: usize = 0x1000;
const NUM_IMPLICIT_TAGS: usize = 0x0e;

#[cfg_attr(rustfmt, rustfmt_skip)]
static TAG_IMPLICIT_LEN: [i8; NUM_IMPLICIT_TAGS] = [
    1, 2, 4, 8, // EsU*
    1, 2, 4, 8, // ESI*
    1, // EsBool
    4, // EsChar
    4, 8, // EsF*
    1, 4, // EsSub*
];

#[derive(Debug)]
pub enum Error {
    IntTooBig(usize),
    InvalidTag(usize),
    Expected(String),
    IoError(std::io::Error),
    ApplicationError(String),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // FIXME: this should be a more useful display form
        fmt::Debug::fmt(self, f)
    }
}
// --------------------------------------

pub mod reader {
    use std::char;

    use std::isize;
    use std::mem::transmute;

    use serialize;

    use super::opaque;
    use super::{ApplicationError, EsVec, EsMap, EsEnum, EsSub8, EsSub32, EsVecElt, EsMapKey,
                EsU64, EsU32, EsU16, EsU8, EsI64, EsI32, EsI16, EsI8, EsBool, EsF64, EsF32,
                EsChar, EsStr, EsMapVal, EsOpaque, EbmlEncoderTag, Doc, TaggedDoc, Error,
                IntTooBig, InvalidTag, Expected, NUM_IMPLICIT_TAGS, TAG_IMPLICIT_LEN};

    pub type DecodeResult<T> = Result<T, Error>;
    // rbml reading

    macro_rules! try_or {
        ($e:expr, $r:expr) => (
            match $e {
                Ok(e) => e,
                Err(e) => {
                    debug!("ignored error: {:?}", e);
                    return $r
                }
            }
        )
    }

    #[derive(Copy, Clone)]
    pub struct Res {
        pub val: usize,
        pub next: usize,
    }

    pub fn tag_at(data: &[u8], start: usize) -> DecodeResult<Res> {
        let v = data[start] as usize;
        if v < 0xf0 {
            Ok(Res {
                val: v,
                next: start + 1,
            })
        } else if v > 0xf0 {
            Ok(Res {
                val: ((v & 0xf) << 8) | data[start + 1] as usize,
                next: start + 2,
            })
        } else {
            // every tag starting with byte 0xf0 is an overlong form, which is prohibited.
            Err(InvalidTag(v))
        }
    }

    #[inline(never)]
    fn vuint_at_slow(data: &[u8], start: usize) -> DecodeResult<Res> {
        let a = data[start];
        if a & 0x80 != 0 {
            return Ok(Res {
                val: (a & 0x7f) as usize,
                next: start + 1,
            });
        }
        if a & 0x40 != 0 {
            return Ok(Res {
                val: ((a & 0x3f) as usize) << 8 | (data[start + 1] as usize),
                next: start + 2,
            });
        }
        if a & 0x20 != 0 {
            return Ok(Res {
                val: ((a & 0x1f) as usize) << 16 | (data[start + 1] as usize) << 8 |
                     (data[start + 2] as usize),
                next: start + 3,
            });
        }
        if a & 0x10 != 0 {
            return Ok(Res {
                val: ((a & 0x0f) as usize) << 24 | (data[start + 1] as usize) << 16 |
                     (data[start + 2] as usize) << 8 |
                     (data[start + 3] as usize),
                next: start + 4,
            });
        }
        Err(IntTooBig(a as usize))
    }

    pub fn vuint_at(data: &[u8], start: usize) -> DecodeResult<Res> {
        if data.len() - start < 4 {
            return vuint_at_slow(data, start);
        }

        // Lookup table for parsing EBML Element IDs as per
        // http://ebml.sourceforge.net/specs/ The Element IDs are parsed by
        // reading a big endian u32 positioned at data[start].  Using the four
        // most significant bits of the u32 we lookup in the table below how
        // the element ID should be derived from it.
        //
        // The table stores tuples (shift, mask) where shift is the number the
        // u32 should be right shifted with and mask is the value the right
        // shifted value should be masked with.  If for example the most
        // significant bit is set this means it's a class A ID and the u32
        // should be right shifted with 24 and masked with 0x7f. Therefore we
        // store (24, 0x7f) at index 0x8 - 0xF (four bit numbers where the most
        // significant bit is set).
        //
        // By storing the number of shifts and masks in a table instead of
        // checking in order if the most significant bit is set, the second
        // most significant bit is set etc. we can replace up to three
        // "and+branch" with a single table lookup which gives us a measured
        // speedup of around 2x on x86_64.
        static SHIFT_MASK_TABLE: [(usize, u32); 16] = [(0, 0x0),
                                                       (0, 0x0fffffff),
                                                       (8, 0x1fffff),
                                                       (8, 0x1fffff),
                                                       (16, 0x3fff),
                                                       (16, 0x3fff),
                                                       (16, 0x3fff),
                                                       (16, 0x3fff),
                                                       (24, 0x7f),
                                                       (24, 0x7f),
                                                       (24, 0x7f),
                                                       (24, 0x7f),
                                                       (24, 0x7f),
                                                       (24, 0x7f),
                                                       (24, 0x7f),
                                                       (24, 0x7f)];

        unsafe {
            let ptr = data.as_ptr().offset(start as isize) as *const u32;
            let val = u32::from_be(*ptr);

            let i = (val >> 28) as usize;
            let (shift, mask) = SHIFT_MASK_TABLE[i];
            Ok(Res {
                val: ((val >> shift) & mask) as usize,
                next: start + ((32 - shift) >> 3),
            })
        }
    }

    pub fn tag_len_at(data: &[u8], tag: Res) -> DecodeResult<Res> {
        if tag.val < NUM_IMPLICIT_TAGS && TAG_IMPLICIT_LEN[tag.val] >= 0 {
            Ok(Res {
                val: TAG_IMPLICIT_LEN[tag.val] as usize,
                next: tag.next,
            })
        } else {
            vuint_at(data, tag.next)
        }
    }

    pub fn doc_at<'a>(data: &'a [u8], start: usize) -> DecodeResult<TaggedDoc<'a>> {
        let elt_tag = try!(tag_at(data, start));
        let elt_size = try!(tag_len_at(data, elt_tag));
        let end = elt_size.next + elt_size.val;
        Ok(TaggedDoc {
            tag: elt_tag.val,
            doc: Doc {
                data: data,
                start: elt_size.next,
                end: end,
            },
        })
    }

    pub fn maybe_get_doc<'a>(d: Doc<'a>, tg: usize) -> Option<Doc<'a>> {
        let mut pos = d.start;
        while pos < d.end {
            let elt_tag = try_or!(tag_at(d.data, pos), None);
            let elt_size = try_or!(tag_len_at(d.data, elt_tag), None);
            pos = elt_size.next + elt_size.val;
            if elt_tag.val == tg {
                return Some(Doc {
                    data: d.data,
                    start: elt_size.next,
                    end: pos,
                });
            }
        }
        None
    }

    pub fn get_doc<'a>(d: Doc<'a>, tg: usize) -> Doc<'a> {
        match maybe_get_doc(d, tg) {
            Some(d) => d,
            None => {
                error!("failed to find block with tag {:?}", tg);
                panic!();
            }
        }
    }

    pub fn docs<'a>(d: Doc<'a>) -> DocsIterator<'a> {
        DocsIterator { d: d }
    }

    pub struct DocsIterator<'a> {
        d: Doc<'a>,
    }

    impl<'a> Iterator for DocsIterator<'a> {
        type Item = (usize, Doc<'a>);

        fn next(&mut self) -> Option<(usize, Doc<'a>)> {
            if self.d.start >= self.d.end {
                return None;
            }

            let elt_tag = try_or!(tag_at(self.d.data, self.d.start), {
                self.d.start = self.d.end;
                None
            });
            let elt_size = try_or!(tag_len_at(self.d.data, elt_tag), {
                self.d.start = self.d.end;
                None
            });

            let end = elt_size.next + elt_size.val;
            let doc = Doc {
                data: self.d.data,
                start: elt_size.next,
                end: end,
            };

            self.d.start = end;
            return Some((elt_tag.val, doc));
        }
    }

    pub fn tagged_docs<'a>(d: Doc<'a>, tag: usize) -> TaggedDocsIterator<'a> {
        TaggedDocsIterator {
            iter: docs(d),
            tag: tag,
        }
    }

    pub struct TaggedDocsIterator<'a> {
        iter: DocsIterator<'a>,
        tag: usize,
    }

    impl<'a> Iterator for TaggedDocsIterator<'a> {
        type Item = Doc<'a>;

        fn next(&mut self) -> Option<Doc<'a>> {
            while let Some((tag, doc)) = self.iter.next() {
                if tag == self.tag {
                    return Some(doc);
                }
            }
            None
        }
    }

    pub fn with_doc_data<T, F>(d: Doc, f: F) -> T
        where F: FnOnce(&[u8]) -> T
    {
        f(&d.data[d.start..d.end])
    }

    pub fn doc_as_u8(d: Doc) -> u8 {
        assert_eq!(d.end, d.start + 1);
        d.data[d.start]
    }

    pub fn doc_as_u64(d: Doc) -> u64 {
        if d.end >= 8 {
            // For performance, we read 8 big-endian bytes,
            // and mask off the junk if there is any. This
            // obviously won't work on the first 8 bytes
            // of a file - we will fall of the start
            // of the page and segfault.

            let mut b = [0; 8];
            b.clone_from_slice(&d.data[d.end - 8..d.end]);
            let data = unsafe { (*(b.as_ptr() as *const u64)).to_be() };
            let len = d.end - d.start;
            if len < 8 {
                data & ((1 << (len * 8)) - 1)
            } else {
                data
            }
        } else {
            let mut result = 0;
            for b in &d.data[d.start..d.end] {
                result = (result << 8) + (*b as u64);
            }
            result
        }
    }

    #[inline]
    pub fn doc_as_u16(d: Doc) -> u16 {
        doc_as_u64(d) as u16
    }
    #[inline]
    pub fn doc_as_u32(d: Doc) -> u32 {
        doc_as_u64(d) as u32
    }

    #[inline]
    pub fn doc_as_i8(d: Doc) -> i8 {
        doc_as_u8(d) as i8
    }
    #[inline]
    pub fn doc_as_i16(d: Doc) -> i16 {
        doc_as_u16(d) as i16
    }
    #[inline]
    pub fn doc_as_i32(d: Doc) -> i32 {
        doc_as_u32(d) as i32
    }
    #[inline]
    pub fn doc_as_i64(d: Doc) -> i64 {
        doc_as_u64(d) as i64
    }

    pub struct Decoder<'a> {
        parent: Doc<'a>,
        pos: usize,
    }

    impl<'doc> Decoder<'doc> {
        pub fn new(d: Doc<'doc>) -> Decoder<'doc> {
            Decoder {
                parent: d,
                pos: d.start,
            }
        }

        fn next_doc(&mut self, exp_tag: EbmlEncoderTag) -> DecodeResult<Doc<'doc>> {
            debug!(". next_doc(exp_tag={:?})", exp_tag);
            if self.pos >= self.parent.end {
                return Err(Expected(format!("no more documents in current node!")));
            }
            let TaggedDoc { tag: r_tag, doc: r_doc } = try!(doc_at(self.parent.data, self.pos));
            debug!("self.parent={:?}-{:?} self.pos={:?} r_tag={:?} r_doc={:?}-{:?}",
                   self.parent.start,
                   self.parent.end,
                   self.pos,
                   r_tag,
                   r_doc.start,
                   r_doc.end);
            if r_tag != (exp_tag as usize) {
                return Err(Expected(format!("expected EBML doc with tag {:?} but found tag {:?}",
                                            exp_tag,
                                            r_tag)));
            }
            if r_doc.end > self.parent.end {
                return Err(Expected(format!("invalid EBML, child extends to {:#x}, parent to \
                                             {:#x}",
                                            r_doc.end,
                                            self.parent.end)));
            }
            self.pos = r_doc.end;
            Ok(r_doc)
        }

        fn push_doc<T, F>(&mut self, exp_tag: EbmlEncoderTag, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            let d = try!(self.next_doc(exp_tag));
            let old_parent = self.parent;
            let old_pos = self.pos;
            self.parent = d;
            self.pos = d.start;
            let r = try!(f(self));
            self.parent = old_parent;
            self.pos = old_pos;
            Ok(r)
        }

        fn _next_sub(&mut self) -> DecodeResult<usize> {
            // empty vector/map optimization
            if self.parent.is_empty() {
                return Ok(0);
            }

            let TaggedDoc { tag: r_tag, doc: r_doc } = try!(doc_at(self.parent.data, self.pos));
            let r = if r_tag == (EsSub8 as usize) {
                doc_as_u8(r_doc) as usize
            } else if r_tag == (EsSub32 as usize) {
                doc_as_u32(r_doc) as usize
            } else {
                return Err(Expected(format!("expected EBML doc with tag {:?} or {:?} but found \
                                             tag {:?}",
                                            EsSub8,
                                            EsSub32,
                                            r_tag)));
            };
            if r_doc.end > self.parent.end {
                return Err(Expected(format!("invalid EBML, child extends to {:#x}, parent to \
                                             {:#x}",
                                            r_doc.end,
                                            self.parent.end)));
            }
            self.pos = r_doc.end;
            debug!("_next_sub result={:?}", r);
            Ok(r)
        }

        // variable-length unsigned integer with different tags.
        // `first_tag` should be a tag for u8 or i8.
        // `last_tag` should be the largest allowed integer tag with the matching signedness.
        // all tags between them should be valid, in the order of u8, u16, u32 and u64.
        fn _next_int(&mut self,
                     first_tag: EbmlEncoderTag,
                     last_tag: EbmlEncoderTag)
                     -> DecodeResult<u64> {
            if self.pos >= self.parent.end {
                return Err(Expected(format!("no more documents in current node!")));
            }

            let TaggedDoc { tag: r_tag, doc: r_doc } = try!(doc_at(self.parent.data, self.pos));
            let r = if first_tag as usize <= r_tag && r_tag <= last_tag as usize {
                match r_tag - first_tag as usize {
                    0 => doc_as_u8(r_doc) as u64,
                    1 => doc_as_u16(r_doc) as u64,
                    2 => doc_as_u32(r_doc) as u64,
                    3 => doc_as_u64(r_doc),
                    _ => unreachable!(),
                }
            } else {
                return Err(Expected(format!("expected EBML doc with tag {:?} through {:?} but \
                                             found tag {:?}",
                                            first_tag,
                                            last_tag,
                                            r_tag)));
            };
            if r_doc.end > self.parent.end {
                return Err(Expected(format!("invalid EBML, child extends to {:#x}, parent to \
                                             {:#x}",
                                            r_doc.end,
                                            self.parent.end)));
            }
            self.pos = r_doc.end;
            debug!("_next_int({:?}, {:?}) result={:?}", first_tag, last_tag, r);
            Ok(r)
        }

        pub fn read_opaque<R, F>(&mut self, op: F) -> DecodeResult<R>
            where F: FnOnce(&mut opaque::Decoder, Doc) -> DecodeResult<R>
        {
            let doc = try!(self.next_doc(EsOpaque));

            let result = {
                let mut opaque_decoder = opaque::Decoder::new(doc.data, doc.start);
                try!(op(&mut opaque_decoder, doc))
            };

            Ok(result)
        }

        pub fn position(&self) -> usize {
            self.pos
        }

        pub fn advance(&mut self, bytes: usize) {
            self.pos += bytes;
        }
    }

    impl<'doc> serialize::Decoder for Decoder<'doc> {
        type Error = Error;
        fn read_nil(&mut self) -> DecodeResult<()> {
            Ok(())
        }

        fn read_u64(&mut self) -> DecodeResult<u64> {
            self._next_int(EsU8, EsU64)
        }
        fn read_u32(&mut self) -> DecodeResult<u32> {
            Ok(try!(self._next_int(EsU8, EsU32)) as u32)
        }
        fn read_u16(&mut self) -> DecodeResult<u16> {
            Ok(try!(self._next_int(EsU8, EsU16)) as u16)
        }
        fn read_u8(&mut self) -> DecodeResult<u8> {
            Ok(doc_as_u8(try!(self.next_doc(EsU8))))
        }
        fn read_uint(&mut self) -> DecodeResult<usize> {
            let v = try!(self._next_int(EsU8, EsU64));
            if v > (::std::usize::MAX as u64) {
                Err(IntTooBig(v as usize))
            } else {
                Ok(v as usize)
            }
        }

        fn read_i64(&mut self) -> DecodeResult<i64> {
            Ok(try!(self._next_int(EsI8, EsI64)) as i64)
        }
        fn read_i32(&mut self) -> DecodeResult<i32> {
            Ok(try!(self._next_int(EsI8, EsI32)) as i32)
        }
        fn read_i16(&mut self) -> DecodeResult<i16> {
            Ok(try!(self._next_int(EsI8, EsI16)) as i16)
        }
        fn read_i8(&mut self) -> DecodeResult<i8> {
            Ok(doc_as_u8(try!(self.next_doc(EsI8))) as i8)
        }
        fn read_int(&mut self) -> DecodeResult<isize> {
            let v = try!(self._next_int(EsI8, EsI64)) as i64;
            if v > (isize::MAX as i64) || v < (isize::MIN as i64) {
                debug!("FIXME \\#6122: Removing this makes this function miscompile");
                Err(IntTooBig(v as usize))
            } else {
                Ok(v as isize)
            }
        }

        fn read_bool(&mut self) -> DecodeResult<bool> {
            Ok(doc_as_u8(try!(self.next_doc(EsBool))) != 0)
        }

        fn read_f64(&mut self) -> DecodeResult<f64> {
            let bits = doc_as_u64(try!(self.next_doc(EsF64)));
            Ok(unsafe { transmute(bits) })
        }
        fn read_f32(&mut self) -> DecodeResult<f32> {
            let bits = doc_as_u32(try!(self.next_doc(EsF32)));
            Ok(unsafe { transmute(bits) })
        }
        fn read_char(&mut self) -> DecodeResult<char> {
            Ok(char::from_u32(doc_as_u32(try!(self.next_doc(EsChar)))).unwrap())
        }
        fn read_str(&mut self) -> DecodeResult<String> {
            Ok(try!(self.next_doc(EsStr)).as_str())
        }

        // Compound types:
        fn read_enum<T, F>(&mut self, name: &str, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_enum({})", name);

            let doc = try!(self.next_doc(EsEnum));

            let (old_parent, old_pos) = (self.parent, self.pos);
            self.parent = doc;
            self.pos = self.parent.start;

            let result = try!(f(self));

            self.parent = old_parent;
            self.pos = old_pos;
            Ok(result)
        }

        fn read_enum_variant<T, F>(&mut self, _: &[&str], mut f: F) -> DecodeResult<T>
            where F: FnMut(&mut Decoder<'doc>, usize) -> DecodeResult<T>
        {
            debug!("read_enum_variant()");
            let idx = try!(self._next_sub());
            debug!("  idx={}", idx);

            f(self, idx)
        }

        fn read_enum_variant_arg<T, F>(&mut self, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_enum_variant_arg(idx={})", idx);
            f(self)
        }

        fn read_enum_struct_variant<T, F>(&mut self, _: &[&str], mut f: F) -> DecodeResult<T>
            where F: FnMut(&mut Decoder<'doc>, usize) -> DecodeResult<T>
        {
            debug!("read_enum_struct_variant()");
            let idx = try!(self._next_sub());
            debug!("  idx={}", idx);

            f(self, idx)
        }

        fn read_enum_struct_variant_field<T, F>(&mut self,
                                                name: &str,
                                                idx: usize,
                                                f: F)
                                                -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_enum_struct_variant_arg(name={}, idx={})", name, idx);
            f(self)
        }

        fn read_struct<T, F>(&mut self, name: &str, _: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_struct(name={})", name);
            f(self)
        }

        fn read_struct_field<T, F>(&mut self, name: &str, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_struct_field(name={}, idx={})", name, idx);
            f(self)
        }

        fn read_tuple<T, F>(&mut self, tuple_len: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_tuple()");
            self.read_seq(move |d, len| {
                if len == tuple_len {
                    f(d)
                } else {
                    Err(Expected(format!("Expected tuple of length `{}`, found tuple of length \
                                          `{}`",
                                         tuple_len,
                                         len)))
                }
            })
        }

        fn read_tuple_arg<T, F>(&mut self, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_tuple_arg(idx={})", idx);
            self.read_seq_elt(idx, f)
        }

        fn read_tuple_struct<T, F>(&mut self, name: &str, len: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_tuple_struct(name={})", name);
            self.read_tuple(len, f)
        }

        fn read_tuple_struct_arg<T, F>(&mut self, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_tuple_struct_arg(idx={})", idx);
            self.read_tuple_arg(idx, f)
        }

        fn read_option<T, F>(&mut self, mut f: F) -> DecodeResult<T>
            where F: FnMut(&mut Decoder<'doc>, bool) -> DecodeResult<T>
        {
            debug!("read_option()");
            self.read_enum("Option", move |this| {
                this.read_enum_variant(&["None", "Some"], move |this, idx| {
                    match idx {
                        0 => f(this, false),
                        1 => f(this, true),
                        _ => Err(Expected(format!("Expected None or Some"))),
                    }
                })
            })
        }

        fn read_seq<T, F>(&mut self, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>, usize) -> DecodeResult<T>
        {
            debug!("read_seq()");
            self.push_doc(EsVec, move |d| {
                let len = try!(d._next_sub());
                debug!("  len={}", len);
                f(d, len)
            })
        }

        fn read_seq_elt<T, F>(&mut self, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_seq_elt(idx={})", idx);
            self.push_doc(EsVecElt, f)
        }

        fn read_map<T, F>(&mut self, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>, usize) -> DecodeResult<T>
        {
            debug!("read_map()");
            self.push_doc(EsMap, move |d| {
                let len = try!(d._next_sub());
                debug!("  len={}", len);
                f(d, len)
            })
        }

        fn read_map_elt_key<T, F>(&mut self, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_map_elt_key(idx={})", idx);
            self.push_doc(EsMapKey, f)
        }

        fn read_map_elt_val<T, F>(&mut self, idx: usize, f: F) -> DecodeResult<T>
            where F: FnOnce(&mut Decoder<'doc>) -> DecodeResult<T>
        {
            debug!("read_map_elt_val(idx={})", idx);
            self.push_doc(EsMapVal, f)
        }

        fn error(&mut self, err: &str) -> Error {
            ApplicationError(err.to_string())
        }
    }
}

pub mod writer {
    use std::mem;
    use std::io::prelude::*;
    use std::io::{self, SeekFrom, Cursor};

    use super::opaque;
    use super::{EsVec, EsMap, EsEnum, EsSub8, EsSub32, EsVecElt, EsMapKey, EsU64, EsU32, EsU16,
                EsU8, EsI64, EsI32, EsI16, EsI8, EsBool, EsF64, EsF32, EsChar, EsStr, EsMapVal,
                EsOpaque, NUM_IMPLICIT_TAGS, NUM_TAGS};

    use serialize;


    pub type EncodeResult = io::Result<()>;

    // rbml writing
    pub struct Encoder<'a> {
        pub writer: &'a mut Cursor<Vec<u8>>,
        size_positions: Vec<u64>,
        relax_limit: u64, // do not move encoded bytes before this position
    }

    fn write_tag<W: Write>(w: &mut W, n: usize) -> EncodeResult {
        if n < 0xf0 {
            w.write_all(&[n as u8])
        } else if 0x100 <= n && n < NUM_TAGS {
            w.write_all(&[0xf0 | (n >> 8) as u8, n as u8])
        } else {
            Err(io::Error::new(io::ErrorKind::Other, &format!("invalid tag: {}", n)[..]))
        }
    }

    fn write_sized_vuint<W: Write>(w: &mut W, n: usize, size: usize) -> EncodeResult {
        match size {
            1 => w.write_all(&[0x80 | (n as u8)]),
            2 => w.write_all(&[0x40 | ((n >> 8) as u8), n as u8]),
            3 => w.write_all(&[0x20 | ((n >> 16) as u8), (n >> 8) as u8, n as u8]),
            4 => w.write_all(&[0x10 | ((n >> 24) as u8), (n >> 16) as u8, (n >> 8) as u8, n as u8]),
            _ => Err(io::Error::new(io::ErrorKind::Other, &format!("isize too big: {}", n)[..])),
        }
    }

    pub fn write_vuint<W: Write>(w: &mut W, n: usize) -> EncodeResult {
        if n < 0x7f {
            return write_sized_vuint(w, n, 1);
        }
        if n < 0x4000 {
            return write_sized_vuint(w, n, 2);
        }
        if n < 0x200000 {
            return write_sized_vuint(w, n, 3);
        }
        if n < 0x10000000 {
            return write_sized_vuint(w, n, 4);
        }
        Err(io::Error::new(io::ErrorKind::Other, &format!("isize too big: {}", n)[..]))
    }

    impl<'a> Encoder<'a> {
        pub fn new(w: &'a mut Cursor<Vec<u8>>) -> Encoder<'a> {
            Encoder {
                writer: w,
                size_positions: vec![],
                relax_limit: 0,
            }
        }

        pub fn start_tag(&mut self, tag_id: usize) -> EncodeResult {
            debug!("Start tag {:?}", tag_id);
            assert!(tag_id >= NUM_IMPLICIT_TAGS);

            // Write the enum ID:
            try!(write_tag(self.writer, tag_id));

            // Write a placeholder four-byte size.
            let cur_pos = try!(self.writer.seek(SeekFrom::Current(0)));
            self.size_positions.push(cur_pos);
            let zeroes: &[u8] = &[0, 0, 0, 0];
            self.writer.write_all(zeroes)
        }

        pub fn end_tag(&mut self) -> EncodeResult {
            let last_size_pos = self.size_positions.pop().unwrap();
            let cur_pos = try!(self.writer.seek(SeekFrom::Current(0)));
            try!(self.writer.seek(SeekFrom::Start(last_size_pos)));
            let size = (cur_pos - last_size_pos - 4) as usize;

            // relax the size encoding for small tags (bigger tags are costly to move).
            // we should never try to move the stable positions, however.
            const RELAX_MAX_SIZE: usize = 0x100;
            if size <= RELAX_MAX_SIZE && last_size_pos >= self.relax_limit {
                // we can't alter the buffer in place, so have a temporary buffer
                let mut buf = [0u8; RELAX_MAX_SIZE];
                {
                    let last_size_pos = last_size_pos as usize;
                    let data = &self.writer.get_ref()[last_size_pos + 4..cur_pos as usize];
                    buf[..size].clone_from_slice(data);
                }

                // overwrite the size and data and continue
                try!(write_vuint(self.writer, size));
                try!(self.writer.write_all(&buf[..size]));
            } else {
                // overwrite the size with an overlong encoding and skip past the data
                try!(write_sized_vuint(self.writer, size, 4));
                try!(self.writer.seek(SeekFrom::Start(cur_pos)));
            }

            debug!("End tag (size = {:?})", size);
            Ok(())
        }

        pub fn wr_tag<F>(&mut self, tag_id: usize, blk: F) -> EncodeResult
            where F: FnOnce() -> EncodeResult
        {
            try!(self.start_tag(tag_id));
            try!(blk());
            self.end_tag()
        }

        pub fn wr_tagged_bytes(&mut self, tag_id: usize, b: &[u8]) -> EncodeResult {
            assert!(tag_id >= NUM_IMPLICIT_TAGS);
            try!(write_tag(self.writer, tag_id));
            try!(write_vuint(self.writer, b.len()));
            self.writer.write_all(b)
        }

        pub fn wr_tagged_u64(&mut self, tag_id: usize, v: u64) -> EncodeResult {
            let bytes: [u8; 8] = unsafe { mem::transmute(v.to_be()) };
            // tagged integers are emitted in big-endian, with no
            // leading zeros.
            let leading_zero_bytes = v.leading_zeros() / 8;
            self.wr_tagged_bytes(tag_id, &bytes[leading_zero_bytes as usize..])
        }

        #[inline]
        pub fn wr_tagged_u32(&mut self, tag_id: usize, v: u32) -> EncodeResult {
            self.wr_tagged_u64(tag_id, v as u64)
        }

        #[inline]
        pub fn wr_tagged_u16(&mut self, tag_id: usize, v: u16) -> EncodeResult {
            self.wr_tagged_u64(tag_id, v as u64)
        }

        #[inline]
        pub fn wr_tagged_u8(&mut self, tag_id: usize, v: u8) -> EncodeResult {
            self.wr_tagged_bytes(tag_id, &[v])
        }

        #[inline]
        pub fn wr_tagged_i64(&mut self, tag_id: usize, v: i64) -> EncodeResult {
            self.wr_tagged_u64(tag_id, v as u64)
        }

        #[inline]
        pub fn wr_tagged_i32(&mut self, tag_id: usize, v: i32) -> EncodeResult {
            self.wr_tagged_u32(tag_id, v as u32)
        }

        #[inline]
        pub fn wr_tagged_i16(&mut self, tag_id: usize, v: i16) -> EncodeResult {
            self.wr_tagged_u16(tag_id, v as u16)
        }

        #[inline]
        pub fn wr_tagged_i8(&mut self, tag_id: usize, v: i8) -> EncodeResult {
            self.wr_tagged_bytes(tag_id, &[v as u8])
        }

        pub fn wr_tagged_str(&mut self, tag_id: usize, v: &str) -> EncodeResult {
            self.wr_tagged_bytes(tag_id, v.as_bytes())
        }

        // for auto-serialization
        fn wr_tagged_raw_bytes(&mut self, tag_id: usize, b: &[u8]) -> EncodeResult {
            try!(write_tag(self.writer, tag_id));
            self.writer.write_all(b)
        }

        fn wr_tagged_raw_u64(&mut self, tag_id: usize, v: u64) -> EncodeResult {
            let bytes: [u8; 8] = unsafe { mem::transmute(v.to_be()) };
            self.wr_tagged_raw_bytes(tag_id, &bytes)
        }

        fn wr_tagged_raw_u32(&mut self, tag_id: usize, v: u32) -> EncodeResult {
            let bytes: [u8; 4] = unsafe { mem::transmute(v.to_be()) };
            self.wr_tagged_raw_bytes(tag_id, &bytes)
        }

        fn wr_tagged_raw_u16(&mut self, tag_id: usize, v: u16) -> EncodeResult {
            let bytes: [u8; 2] = unsafe { mem::transmute(v.to_be()) };
            self.wr_tagged_raw_bytes(tag_id, &bytes)
        }

        fn wr_tagged_raw_u8(&mut self, tag_id: usize, v: u8) -> EncodeResult {
            self.wr_tagged_raw_bytes(tag_id, &[v])
        }

        fn wr_tagged_raw_i64(&mut self, tag_id: usize, v: i64) -> EncodeResult {
            self.wr_tagged_raw_u64(tag_id, v as u64)
        }

        fn wr_tagged_raw_i32(&mut self, tag_id: usize, v: i32) -> EncodeResult {
            self.wr_tagged_raw_u32(tag_id, v as u32)
        }

        fn wr_tagged_raw_i16(&mut self, tag_id: usize, v: i16) -> EncodeResult {
            self.wr_tagged_raw_u16(tag_id, v as u16)
        }

        fn wr_tagged_raw_i8(&mut self, tag_id: usize, v: i8) -> EncodeResult {
            self.wr_tagged_raw_bytes(tag_id, &[v as u8])
        }

        pub fn wr_bytes(&mut self, b: &[u8]) -> EncodeResult {
            debug!("Write {:?} bytes", b.len());
            self.writer.write_all(b)
        }

        pub fn wr_str(&mut self, s: &str) -> EncodeResult {
            debug!("Write str: {:?}", s);
            self.writer.write_all(s.as_bytes())
        }

        /// Returns the current position while marking it stable, i.e.
        /// generated bytes so far wouldn't be affected by relaxation.
        pub fn mark_stable_position(&mut self) -> u64 {
            let pos = self.writer.seek(SeekFrom::Current(0)).unwrap();
            if self.relax_limit < pos {
                self.relax_limit = pos;
            }
            pos
        }
    }

    impl<'a> Encoder<'a> {
        // used internally to emit things like the vector length and so on
        fn _emit_tagged_sub(&mut self, v: usize) -> EncodeResult {
            if v as u8 as usize == v {
                self.wr_tagged_raw_u8(EsSub8 as usize, v as u8)
            } else if v as u32 as usize == v {
                self.wr_tagged_raw_u32(EsSub32 as usize, v as u32)
            } else {
                Err(io::Error::new(io::ErrorKind::Other,
                                   &format!("length or variant id too big: {}", v)[..]))
            }
        }

        pub fn emit_opaque<F>(&mut self, f: F) -> EncodeResult
            where F: FnOnce(&mut opaque::Encoder) -> EncodeResult
        {
            try!(self.start_tag(EsOpaque as usize));

            {
                let mut opaque_encoder = opaque::Encoder::new(self.writer);
                try!(f(&mut opaque_encoder));
            }

            self.mark_stable_position();
            self.end_tag()
        }
    }

    impl<'a> serialize::Encoder for Encoder<'a> {
        type Error = io::Error;

        fn emit_nil(&mut self) -> EncodeResult {
            Ok(())
        }

        fn emit_uint(&mut self, v: usize) -> EncodeResult {
            self.emit_u64(v as u64)
        }
        fn emit_u64(&mut self, v: u64) -> EncodeResult {
            if v as u32 as u64 == v {
                self.emit_u32(v as u32)
            } else {
                self.wr_tagged_raw_u64(EsU64 as usize, v)
            }
        }
        fn emit_u32(&mut self, v: u32) -> EncodeResult {
            if v as u16 as u32 == v {
                self.emit_u16(v as u16)
            } else {
                self.wr_tagged_raw_u32(EsU32 as usize, v)
            }
        }
        fn emit_u16(&mut self, v: u16) -> EncodeResult {
            if v as u8 as u16 == v {
                self.emit_u8(v as u8)
            } else {
                self.wr_tagged_raw_u16(EsU16 as usize, v)
            }
        }
        fn emit_u8(&mut self, v: u8) -> EncodeResult {
            self.wr_tagged_raw_u8(EsU8 as usize, v)
        }

        fn emit_int(&mut self, v: isize) -> EncodeResult {
            self.emit_i64(v as i64)
        }
        fn emit_i64(&mut self, v: i64) -> EncodeResult {
            if v as i32 as i64 == v {
                self.emit_i32(v as i32)
            } else {
                self.wr_tagged_raw_i64(EsI64 as usize, v)
            }
        }
        fn emit_i32(&mut self, v: i32) -> EncodeResult {
            if v as i16 as i32 == v {
                self.emit_i16(v as i16)
            } else {
                self.wr_tagged_raw_i32(EsI32 as usize, v)
            }
        }
        fn emit_i16(&mut self, v: i16) -> EncodeResult {
            if v as i8 as i16 == v {
                self.emit_i8(v as i8)
            } else {
                self.wr_tagged_raw_i16(EsI16 as usize, v)
            }
        }
        fn emit_i8(&mut self, v: i8) -> EncodeResult {
            self.wr_tagged_raw_i8(EsI8 as usize, v)
        }

        fn emit_bool(&mut self, v: bool) -> EncodeResult {
            self.wr_tagged_raw_u8(EsBool as usize, v as u8)
        }

        fn emit_f64(&mut self, v: f64) -> EncodeResult {
            let bits = unsafe { mem::transmute(v) };
            self.wr_tagged_raw_u64(EsF64 as usize, bits)
        }
        fn emit_f32(&mut self, v: f32) -> EncodeResult {
            let bits = unsafe { mem::transmute(v) };
            self.wr_tagged_raw_u32(EsF32 as usize, bits)
        }
        fn emit_char(&mut self, v: char) -> EncodeResult {
            self.wr_tagged_raw_u32(EsChar as usize, v as u32)
        }

        fn emit_str(&mut self, v: &str) -> EncodeResult {
            self.wr_tagged_str(EsStr as usize, v)
        }

        fn emit_enum<F>(&mut self, _name: &str, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            try!(self.start_tag(EsEnum as usize));
            try!(f(self));
            self.end_tag()
        }

        fn emit_enum_variant<F>(&mut self, _: &str, v_id: usize, _: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            try!(self._emit_tagged_sub(v_id));
            f(self)
        }

        fn emit_enum_variant_arg<F>(&mut self, _: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            f(self)
        }

        fn emit_enum_struct_variant<F>(&mut self,
                                       v_name: &str,
                                       v_id: usize,
                                       cnt: usize,
                                       f: F)
                                       -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_enum_variant(v_name, v_id, cnt, f)
        }

        fn emit_enum_struct_variant_field<F>(&mut self, _: &str, idx: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_enum_variant_arg(idx, f)
        }

        fn emit_struct<F>(&mut self, _: &str, _len: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            f(self)
        }

        fn emit_struct_field<F>(&mut self, _name: &str, _: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            f(self)
        }

        fn emit_tuple<F>(&mut self, len: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_seq(len, f)
        }
        fn emit_tuple_arg<F>(&mut self, idx: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_seq_elt(idx, f)
        }

        fn emit_tuple_struct<F>(&mut self, _: &str, len: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_seq(len, f)
        }
        fn emit_tuple_struct_arg<F>(&mut self, idx: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_seq_elt(idx, f)
        }

        fn emit_option<F>(&mut self, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            self.emit_enum("Option", f)
        }
        fn emit_option_none(&mut self) -> EncodeResult {
            self.emit_enum_variant("None", 0, 0, |_| Ok(()))
        }
        fn emit_option_some<F>(&mut self, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {

            self.emit_enum_variant("Some", 1, 1, f)
        }

        fn emit_seq<F>(&mut self, len: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            if len == 0 {
                // empty vector optimization
                return self.wr_tagged_bytes(EsVec as usize, &[]);
            }

            try!(self.start_tag(EsVec as usize));
            try!(self._emit_tagged_sub(len));
            try!(f(self));
            self.end_tag()
        }

        fn emit_seq_elt<F>(&mut self, _idx: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {

            try!(self.start_tag(EsVecElt as usize));
            try!(f(self));
            self.end_tag()
        }

        fn emit_map<F>(&mut self, len: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            if len == 0 {
                // empty map optimization
                return self.wr_tagged_bytes(EsMap as usize, &[]);
            }

            try!(self.start_tag(EsMap as usize));
            try!(self._emit_tagged_sub(len));
            try!(f(self));
            self.end_tag()
        }

        fn emit_map_elt_key<F>(&mut self, _idx: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {

            try!(self.start_tag(EsMapKey as usize));
            try!(f(self));
            self.end_tag()
        }

        fn emit_map_elt_val<F>(&mut self, _idx: usize, f: F) -> EncodeResult
            where F: FnOnce(&mut Encoder<'a>) -> EncodeResult
        {
            try!(self.start_tag(EsMapVal as usize));
            try!(f(self));
            self.end_tag()
        }
    }
}

// ___________________________________________________________________________
// Testing

#[cfg(test)]
mod tests {
    use super::{Doc, reader, writer};

    use serialize::{Encodable, Decodable};

    use std::io::Cursor;

    #[test]
    fn test_vuint_at() {
        let data = &[
            0x80,
            0xff,
            0x40, 0x00,
            0x7f, 0xff,
            0x20, 0x00, 0x00,
            0x3f, 0xff, 0xff,
            0x10, 0x00, 0x00, 0x00,
            0x1f, 0xff, 0xff, 0xff
        ];

        let mut res: reader::Res;

        // Class A
        res = reader::vuint_at(data, 0).unwrap();
        assert_eq!(res.val, 0);
        assert_eq!(res.next, 1);
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, (1 << 7) - 1);
        assert_eq!(res.next, 2);

        // Class B
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, 0);
        assert_eq!(res.next, 4);
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, (1 << 14) - 1);
        assert_eq!(res.next, 6);

        // Class C
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, 0);
        assert_eq!(res.next, 9);
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, (1 << 21) - 1);
        assert_eq!(res.next, 12);

        // Class D
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, 0);
        assert_eq!(res.next, 16);
        res = reader::vuint_at(data, res.next).unwrap();
        assert_eq!(res.val, (1 << 28) - 1);
        assert_eq!(res.next, 20);
    }

    #[test]
    fn test_option_int() {
        fn test_v(v: Option<isize>) {
            debug!("v == {:?}", v);
            let mut wr = Cursor::new(Vec::new());
            {
                let mut rbml_w = writer::Encoder::new(&mut wr);
                let _ = v.encode(&mut rbml_w);
            }
            let rbml_doc = Doc::new(wr.get_ref());
            let mut deser = reader::Decoder::new(rbml_doc);
            let v1 = Decodable::decode(&mut deser).unwrap();
            debug!("v1 == {:?}", v1);
            assert_eq!(v, v1);
        }

        test_v(Some(22));
        test_v(None);
        test_v(Some(3));
    }
}

#[cfg(test)]
mod bench {
    #![allow(non_snake_case)]
    use test::Bencher;
    use super::reader;

    #[bench]
    pub fn vuint_at_A_aligned(b: &mut Bencher) {
        let data = (0..4 * 100)
                       .map(|i| {
                           match i % 2 {
                               0 => 0x80,
                               _ => i as u8,
                           }
                       })
                       .collect::<Vec<_>>();
        let mut sum = 0;
        b.iter(|| {
            let mut i = 0;
            while i < data.len() {
                sum += reader::vuint_at(&data, i).unwrap().val;
                i += 4;
            }
        });
    }

    #[bench]
    pub fn vuint_at_A_unaligned(b: &mut Bencher) {
        let data = (0..4 * 100 + 1)
                       .map(|i| {
                           match i % 2 {
                               1 => 0x80,
                               _ => i as u8,
                           }
                       })
                       .collect::<Vec<_>>();
        let mut sum = 0;
        b.iter(|| {
            let mut i = 1;
            while i < data.len() {
                sum += reader::vuint_at(&data, i).unwrap().val;
                i += 4;
            }
        });
    }

    #[bench]
    pub fn vuint_at_D_aligned(b: &mut Bencher) {
        let data = (0..4 * 100)
                       .map(|i| {
                           match i % 4 {
                               0 => 0x10,
                               3 => i as u8,
                               _ => 0,
                           }
                       })
                       .collect::<Vec<_>>();
        let mut sum = 0;
        b.iter(|| {
            let mut i = 0;
            while i < data.len() {
                sum += reader::vuint_at(&data, i).unwrap().val;
                i += 4;
            }
        });
    }

    #[bench]
    pub fn vuint_at_D_unaligned(b: &mut Bencher) {
        let data = (0..4 * 100 + 1)
                       .map(|i| {
                           match i % 4 {
                               1 => 0x10,
                               0 => i as u8,
                               _ => 0,
                           }
                       })
                       .collect::<Vec<_>>();
        let mut sum = 0;
        b.iter(|| {
            let mut i = 1;
            while i < data.len() {
                sum += reader::vuint_at(&data, i).unwrap().val;
                i += 4;
            }
        });
    }
}