// -*- mode: rust; bidi-display-reordering: nil -*-

/*! # Sixbit - a crate for small packed strings.

This crate provides densely-packed 8-bit, 16-bit, 32-bit, 64-bit, and
128-bit "small strings", in a variety of custom script-specific
6-bit-per-character encodings. It does not deal with any scripts that have
repertoires that significantly exceed reasonable 6-bit-per-character codes
(eg. Chinese); such scripts are probably better off using UTF-16 or BOCU-1.

(Also its treatment of Latin leaves something to be desired, since you
cannot encode mixed-case strings; this is a deliberate choice to let us do
alphanumeric-with-symbols strings in at least one case -- upper case.)

This sort of datatype is a low-level optimization to use in a system
processing a lot of small strings (eg. single words): you can avoid
allocating them on the heap, store them in string-structure headers, NaN
boxes or other sorts of small-literal value types.

Perhaps most enticingly: you can pack _more than one of them_ into a SIMD
vector, and operate on (eg. search) multiple such strings at once, in
parallel, as a single register. Vector registers just keep growing these
days.

Of course not every string is short enough to fit, and not every
short-enough string has codepoints that fall inside one of the "code pages"
that this crate provides. The encoding functions are therefore all
partial. But they should handle a significant enough quantity of strings to
make it worthwhile.

## Usage Summary

Encoding is done via the `EncodeSixbit` trait attached to `Iterator<char>`,
so you can just do: `let enc = "hello".chars().encode_sixbit::<u64>()`. If
there is a failure (say, the string spans pages or doesn't fit) you'll get
back an `Err(EncodeError)` with details, otherwise `Ok(n)` where `n` is a
`u64`.

Decoding is a `DecodeSixbitIter` iterator implementing `Iterator<char>`,
attached to the various packed types, allowing you to write `let s:String =
someu64.decode_sixbit().collect()`, or any other pattern that takes an
`Iterator<char>`.

In several cases you will need to normalize or decompose "standard" unicode text
before pushing it through these interfaces. For example, the Hangul page only
has compatibility jamo, so you have to decompose standard Korean text to that
form before encoding. Similarly the Halfwidth Kana are unlikely to be the
characters standard Japanese text arrives in, and Devanagari strings with nuktas
will need to be decomposed before mapping. This crate does none of these tasks:
it's a building block, not a complete solution.

## Code Pages

Every packed string produced by this crate begins with a small tag
indicating the "code page" of the rest of the string. A code page here is a
set of 64 unicode character values that the 6-bit codes of the rest of the
string are interpreted as. Strings that mix characters from multiple code
pages are not supported. Again, think "single words".

I have chosen the contents of the code pages to the best of my abilities in
script knowledge, guesswork, consulting with friends and experts, scouring
wikipedia and so forth, and subject to some constraints outlined below. I'm
happy to take PRs to improve the contents of them to better capture "many
words" in specific scripts; code page contents will therefore be slightly in
flux until we get to a 1.0 release, so if by some bizarre chance you are
reading this and choose to use the crate and are going to store values from
this crate in stable storage, you should lock your client to a specific
point-revision of the crate until 1.0.

### Constraints

There is only enough room in the tag to reference a handful of code pages;
not every script will make it, but luckily only a few scripts account for
much of the text in the world.

We want to avoid wasting bits, and the number of spare bits in a packed
value of N bits (modulo 6) varies, depending on its size: 2 spare bits for
8, 32 and 128-bit values; 4 spare bits for 16 and 64-bit values.

We want to be able to sort these strings using machine-provided integer
comparison, and have that order correspond to unicode code-point
lexicographical string order (including "short strings sort before
long"). This dictates a fair amount about the tag values, code repertoires,
and normal form of the encoded strings.

### Design

Code pages are taken from (or in some cases, across) unicode blocks, and
tags are ordered by (initial) unicode block. Codes within each code page are
further ordered by unicode codepoint: each code page is essentially a
"likely-useful subsequence" of the contents of 1 or more corresponding
unicode block(s). This unfortunately means that common punctuation
characters and digits are only available for strings using the
uppercase-latin page. I've tried to include some additional punctuation
where it's available in blocks. Since mixing pages is also not possible,
"supplementary" blocks have been mostly avoided unless they happen to follow
an unused sequence-prefix that can be combined to make a useful
self-contained page (eg. the second "lower case and extended latin" page).

A script can only work if there's a "likely-useful subsequence" that fits
inside 63 code points. The zero code in each page is reserved as the string
_terminator_ and padding value. Strings that encode shorter than their
packed value container are left-shifted to always begin at the
most-significant coding bit, and the trailing bits are all set to zero (this
does not mean you can encode nulls -- the zeroes here mean "past end of
string").

The high order / 2-bit tags select among 4 "primary" (most-likely) scripts
spread across the unicode range (in code block order). These are available
in all packed values:

  | tag | page contents                              |
  |-----|--------------------------------------------|
  |  00 | Latin upper case, digits and punctuation   |
  |  01 | Cyrillic                                   |
  |  10 | Arabic                                     |
  |  11 | Devanagari                                 |

When packing 64-bit and 16-bit values we get _4_ spare bits to use for a
tag, not just 2. In these cases we therefore have 12 additional scripts
available, which for simplicity sake casting up and down between value sizes
we keep the high order bits the same and add 2 bits below, picking
additional scripts _from the block ranges between_ those of the primary
scripts (again, in unicode block order):

  | tag   | page contents                                 |
  |-------|-----------------------------------------------|
  | 00 00 | Latin upper case, digits and punctuation      |
  | 00 01 | Latin lower case and extended lowercase forms |
  | 00 10 | *reserved*                                    |
  | 00 11 | Greek                                         |
  |       |                                               |
  | 01 00 | Cyrillic                                      |
  | 01 01 | *reserved*                                    |
  | 01 10 | Hebrew                                        |
  | 01 11 | *reserved*                                    |
  |       |                                               |
  | 10 00 | Arabic                                        |
  | 10 01 | *reserved*                                    |
  | 10 10 | *reserved*                                    |
  | 10 11 | *reserved*                                    |
  |       |                                               |
  | 11 00 | Devanagari                                    |
  | 11 01 | *reserved*                                    |
  | 11 10 | Hangul Compatibility Jamo                     |
  | 11 11 | Halfwidth Kana                                |

The *reserved* cases are where I either didn't know enough about the scripts
available in that range of unicode, or ran out of good candidates, or both.
I might assign them to something in the future, or "compact out" the gaps /
reassign the 4-bit codes so their high bits are _not_ the same as the 2-bit
cases, but I've already exceeded my realistic level of armchair-linguist
knowledge and I figured simplifying design choices would be better than
pretending I could do any better. Patches welcome!

The overall assignment of bits is summarized as follows:

| packed type | tag bits | coding bits | max string length  |
|-------------|----------|-------------|--------------------|
| u128        | 2        | 126         | 21                 |
|  u64        | 4        |  60         | 10                 |
|  u32        | 2        |  30         |  5                 |
|  u16        | 4        |  12         |  2                 |
|   u8        | 2        |   6         |  1                 |

*/

use std::ops::{BitOrAssign, ShlAssign};
use std::mem::size_of;

// Page 00 00: U+0000, then U+0020-U+005F excepting the backslash (U+005C).
const LATIN_UPPER_ETC : [char; 64] = [
    '\0', ' ', '!', '"', '#', '$', '%', '&', '\'', '(', ')', '*', '+', ',', '-', '.',
    '/',  '0', '1', '2', '3', '4', '5', '6', '7',  '8', '9', ':', ';', '<', '=', '>',
    '?',  '@', 'A', 'B', 'C', 'D', 'E', 'F', 'G',  'H', 'I', 'J', 'K', 'L', 'M', 'N',
    'O',  'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W',  'X', 'Y', 'Z', '[', ']', '^', '_'
];

// Page 00 01: U+0000, then the end of basic Latin (U+0061-U+007E) excepting
// the backtick character, then some punctuators from Latin-1 (U+00A7, U+00AB,
// U+00AC, U+00B2, U+00B3, U+00B9 and U+00BB), then precomposed forms from
// Latin-1 (U+00E0-U+00FF excepting a bunch as noted below) and Latin-extended-A
// (U+011F, U+0131, U+015F) to handle the largest few language groups.
const LATIN_LOWER_EXT : [char; 64] = [
    '\0',
    // 30 lower-case and punct chars from the end of of basic Latin, minus backtick.
    'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',
    'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '{', '|', '}', '~',
    // 7 punctuators and superscripts from Latin-1 supplement
    '§', '«', '¬', '²', '³', '¹', '»',
    // 23 Latin-1 supplement characters chosen by need of larger language groups:
    // (F)rench, (G)erman, (P)ortuguese, (S)panish, (T)urkish
    'à', // a-with-grave:        FGP
    'á', // a-with-acute:          PS
    'â', // a-with-circumflex:   F P T
    'ã', // a-with-tilde:          P
    'ä', // a-with-diaeresis:     G
    // omit: U+00E5 a-with-ring-above
    // omit: U+00E6 small ae
    'ç', // c-with-cedilla:      F P T
    'è', // e-with-grave:        FG
    'é', // e-with-acute:        F PS
    'ê', // e-with-circumflex:   F P
    'ë', // e-with-diaeresis:    FG
    // omit: U+00EC i-with-grave
    'í', // i-with-acute:          PS
    'î', // i-with-circumflex:   F   T
    'ï', // i-with-diaeresis:    F
    // omit: U+00F0 eth
    'ñ', // n-with-tilde:           S
    // omit: o-with-grave
    'ó', // o-with-acute:           S
    'ô', // o-with-circumflex:   F P
    'õ', // o-with-tilde:          P
    'ö', // o-with-diaeresis:     G  T
    // omit: U+00F7 solidus
    // omit: U+00F8 o-with-stroke
    'ù', // u-with-grave:        F
    'ú', // u-with-acute:           S
    'û', // u-with-circumflex:   F   T
    'ü', // u-with-diaeresis:     G ST
    // omit: U+00FD y-with-acute
    // omit: U+00FE thorn
    'ÿ', // y-with-diaeresis:    F
    // 3 Latin-extended-A characters to finish off Turkish
    'ğ', // g-with-breve:            T
    'ı', // i-dotless:               T
    'ş'  // s-with-cedilla           T
];

// Page 00 11: U+0000, then upper and lowercase characters in order from
// U+0386-U+03CE, including stressed forms but omitting diaeresis forms.
const GREEK : [char; 64] = [
    '\0',
    // 7 stressed uppercase characters
    'Ά', 'Έ', 'Ή', 'Ί', 'Ό', 'Ύ', 'Ώ',
    // 24 uppercase characters
    'Α', 'Β', 'Γ', 'Δ', 'Ε', 'Ζ', 'Η', 'Θ', 'Ι', 'Κ', 'Λ', 'Μ', 'Ν', 'Ξ', 'Ο', 'Π',
    'Ρ', 'Σ', 'Τ', 'Υ', 'Φ', 'Χ', 'Ψ', 'Ω',
    // 4 stressed lowercase characters
    'ά', 'έ', 'ή', 'ί',
    // 25 lowercase characters (two variants of sigma)
    'α', 'β', 'γ', 'δ', 'ε', 'ζ', 'η', 'θ', 'ι', 'κ', 'λ', 'μ', 'ν', 'ξ', 'ο', 'π',
    'ρ', 'ς', 'σ', 'τ', 'υ', 'φ', 'χ', 'ψ', 'ω',
    // 3 stressed lowercase characters
    'ό', 'ύ', 'ώ'
];

// Page 01 00: U+0000, then U+0410-U+044F excepting the lowercase hard-sign U+044A
const CYRILLIC : [char; 64] = [
    '\0', 'А', 'Б', 'В', 'Г', 'Д', 'Е', 'Ж', 'З', 'И', 'Й', 'К', 'Л', 'М', 'Н', 'О',
    'П', 'Р', 'С', 'Т', 'У', 'Ф', 'Х', 'Ц', 'Ч', 'Ш', 'Щ', 'Ъ', 'Ы', 'Ь', 'Э', 'Ю',
    'Я', 'а', 'б', 'в', 'г', 'д', 'е', 'ж', 'з', 'и', 'й', 'к', 'л', 'м', 'н', 'о',
    'п', 'р', 'с', 'т', 'у', 'ф', 'х', 'ц', 'ч', 'ш', 'щ', 'ы', 'ь', 'э', 'ю', 'я'
];

// Page 01 01: U+0000, then U+05B0-U+05F4
const HEBREW : [char; 64] = [
    '\0', 'ְ', 'ֱ', 'ֲ', 'ֳ', 'ִ', 'ֵ', 'ֶ', 'ַ', 'ָ', 'ֹ', 'ֺ', 'ֻ', 'ּ', 'ֽ', '־',
    'ֿ', '׀', 'ׁ', 'ׂ', '׃', 'ׄ', 'ׅ', '׆', 'ׇ', 'א', 'ב', 'ג', 'ד', 'ה', 'ו', 'ז',
    'ח', 'ט', 'י', 'ך', 'כ', 'ל', 'ם', 'מ', 'ן', 'נ', 'ס', 'ע', 'ף', 'פ', 'ץ', 'צ',
    'ק', 'ר', 'ש', 'ת', 'װ', 'ױ', 'ײ', '׳', '״',
    // Space for 7 more, not sure which to include: expert help wanted!
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}',
];

// Page 10 00: U+0000, then a selection (leaning Perso-Arabic) from the Arabic
// block U+0600–U+06FF, detailed below. Characters selected on the advice of
// @Manishearth who, unlike me, knows something about Arabic script.
const ARABIC : [char; 64] = [
    '\0',

    // 3 punctuators
    // U+060C comma
    '،',
    // U+061B semicolon
    '؛',
    // U+061F question mark
    '؟',

    // 1 hamza
    'ء',

    // 29 main characters in range U+0627-U+0649
    'ا', 'ب', 'ة', 'ت', 'ث', 'ج', 'ح', 'خ',
    'د', 'ذ', 'ر', 'ز', 'س', 'ش', 'ص', 'ض',
    'ط', 'ظ', 'ع', 'غ',
    // omit: U+063B-U+063F "early Persian and Azerbaijani"
    // omit: U+0640 kashida
    'ف', 'ق',
    // omit: U+0643 isolated kaf
    'ل', 'م', 'ن', 'ه', 'و', 'ى', 'ي',

    // 3 short vowels and 1 shadda
    // (sorry my editor balked at displaying some literals here)
    // fatha     damma       kasra       shadda
    '\u{064e}', '\u{064f}', '\u{0650}', '\u{0651}',
    // 2 combining forms of maddah and hamza
    // maddah    hamza
    '\u{0653}', '\u{0654}',
    // 2 vowels used only in Urdu
    // subscript alef
    '\u{0656}', 
    // inverted damma / ulta pesh
    '\u{0657}',

    // 1 superscript alef
    '\u{0670}',

    // 11 extended characters for Persian or Urdu
    // U+0679 tteh (Urdu)
    'ٹ',
    // U+067E peh (Persian, Urdu)
    'پ',
    // U+0686 tcheh (Persian, Urdu)
    'چ',
    // U+0688 ddal (Urdu)
    'ڈ',
    // U+0691 rreh (Urdu)
    'ڑ',
    // U+0698 jeh (Persian, Urdu)
    'ژ',
    // U+06A9 keheh / kaf (Persian, Urdu)
    'ک',
    // U+06AF gaf (Persian, Urdu)
    'گ',
    // U+06BA noon ghunna (Urdu)
    'ں',
    // U+06BE heh doachashmee (Urdu)
    'ھ',
    // U+06D2 yeh barree (Urdu)
    'ے',

    // U+06D4 full stop
    '۔',

    // Space for 9 more, not sure which to include; expert help wanted!
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}',
];

// Page 11 00: U+0000, then a selection detailed below from U+0901-U+0965;
// characters selected on the advice of @Manishearth who, unlike me, knows
// something about Devanagari script.
const DEVANAGARI : [char; 64] = [
    '\0',
    // 3 diacritics U+901 candrabindu, U+902 anusvara, U+903 visarga
    'ँ', 'ं', 'ः',
    // omit: U+0904 short A (Awadhi)
    // 11 standalone vowels (U+0905-U+0914)
    'अ', 'आ', 'इ', 'ई', 'उ', 'ऊ', 'ऋ',
    // omit: U+090C vocalic L (Sanskrit)
    // omit: U+090D candra E (transcription)
    // omit: U+090E short E (Kashmiri, Bihari, transcription)
    'ए', 'ऐ',
    // omit: U+0911 candra o (transcription)
    // omit: U+0912 short o (Kashmiri, Bihari)
    'ओ', 'औ',
    // 34 consonants (U+0915-U+0939)
    'क', 'ख', 'ग', 'घ', 'ङ', 'च', 'छ', 'ज', 'झ', 'ञ', 'ट', 'ठ', 'ड', 'ढ', 'ण', 'त',
    'थ', 'द', 'ध', 'न', 'प', 'फ', 'ब', 'भ', 'म', 'य', 'र', 'ल', 'ळ', 'व', 'श', 'ष',
    'स','ह',
    // omit: U+0934 letter LLLA (transcription)
    // 1 diacritic nukta
    '़',
    // 10 combining vowels (U+093E-U+094C)
    'ा', 'ि', 'ी', 'ु', 'ू', 'ृ',
    // omit: U+0944 vocalic rr (Sanskrit)
    // omit: U+0945 candra e (transcription)
    // omit: U+0946 short e (Kashmiri, Bihari, transcription)
    'े', 'ै',
    // omit: U+0949 candra o (transcription)
    // omit: U+094A short o (Kashmiri, Bihari, transcription)
    'ो', 'ौ',
    // omit: multiple diacritics for Kashmiri, Sanskrit, obsolete orthographies,
    // as well as old vocalics and Hindi nukta consonants (users should decompose them)
    // 1 diacritic virama
    '्',
    // 2 punctuators danda and double danda
    '।', '॥',
    // Space for 1 more, not sure which to include: expert help wanted!
    '\u{ffff}',
];

// Page 11 10: U+0000, then U+3131-U+3163 (initial part of KS X 1001 - 0x24 / 0xA4)
const HANGUL_COMPATIBILITY_JAMO : [char; 64] = [
    '\0', 'ㄱ', 'ㄲ', 'ㄳ', 'ㄴ', 'ㄵ', 'ㄶ', 'ㄷ', 'ㄸ', 'ㄹ', 'ㄺ', 'ㄻ', 'ㄼ', 'ㄽ', 'ㄾ', 'ㄿ',
    'ㅀ', 'ㅁ', 'ㅂ', 'ㅃ', 'ㅄ', 'ㅅ', 'ㅆ', 'ㅇ', 'ㅈ', 'ㅉ', 'ㅊ', 'ㅋ', 'ㅌ', 'ㅍ', 'ㅎ', 'ㅏ',
    'ㅐ', 'ㅑ', 'ㅒ', 'ㅓ', 'ㅔ', 'ㅕ', 'ㅖ', 'ㅗ', 'ㅘ', 'ㅙ', 'ㅚ', 'ㅛ', 'ㅜ', 'ㅝ', 'ㅞ', 'ㅟ',
    'ㅠ', 'ㅡ', 'ㅢ', 'ㅣ',
    // Space for 12 more, not sure which to include: expert help wanted!
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
];

// Page 11 11: U+0000, then U+FF61-U+FF9F (latter part of JIS-X-0201)
const HALFWIDTH_KANA : [char; 64] = [
    '\0', '｡', '｢', '｣', '､', '･', 'ｦ', 'ｧ', 'ｨ', 'ｩ', 'ｪ', 'ｫ', 'ｬ', 'ｭ', 'ｮ', 'ｯ',
    'ｰ', 'ｱ', 'ｲ', 'ｳ', 'ｴ', 'ｵ', 'ｶ', 'ｷ', 'ｸ', 'ｹ', 'ｺ', 'ｻ', 'ｼ', 'ｽ', 'ｾ', 'ｿ',
    'ﾀ', 'ﾁ', 'ﾂ', 'ﾃ', 'ﾄ', 'ﾅ', 'ﾆ', 'ﾇ', 'ﾈ', 'ﾉ', 'ﾊ', 'ﾋ', 'ﾌ', 'ﾍ', 'ﾎ', 'ﾏ',
    'ﾐ', 'ﾑ', 'ﾒ', 'ﾓ', 'ﾔ', 'ﾕ', 'ﾖ', 'ﾗ', 'ﾘ', 'ﾙ', 'ﾚ', 'ﾛ', 'ﾜ', 'ﾝ', 'ﾞ', 'ﾟ'
];

const RESERVED : [char; 64] = [
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
    '\u{ffff}', '\u{ffff}', '\u{ffff}', '\u{ffff}',
];

const PAGES : [[char; 64]; 16] = [
    LATIN_UPPER_ETC,
    LATIN_LOWER_EXT,
    RESERVED,
    GREEK,

    CYRILLIC,
    RESERVED,
    HEBREW,
    RESERVED,

    ARABIC,
    RESERVED,
    RESERVED,
    RESERVED,

    DEVANAGARI,
    RESERVED,
    HANGUL_COMPATIBILITY_JAMO,
    HALFWIDTH_KANA
];

pub trait PackedValue
where
    Self: Copy,
    Self: ShlAssign<usize>,
    Self: BitOrAssign<Self>,
    Self: ::std::cmp::PartialOrd,
    Self: ::std::fmt::Debug,
    Self: ::std::fmt::LowerHex
{
    const NBITS: usize = size_of::<Self>() * 8;
    const NCHARS: usize = Self::NBITS / 6;
    const NTAGBITS: usize = Self::NBITS - (Self::NCHARS * 6);
    // This is a bit ridiculous; I literally tried 4 different crates and every
    // trait I could find in the stdlib and it seems like there is some sort of
    // community-wide conspiracy to ensure the absence of generic truncating
    // casts.
    fn truncating_cast_from(i: usize) -> Self;

    // This also seems somewhat contorted to express via existing traits.
    fn most_significant_byte(self) -> u8;
}

impl PackedValue for u8 {
    fn truncating_cast_from(i: usize) -> u8 { i as u8 }
    fn most_significant_byte(self) -> u8 { self }
}

impl PackedValue for u16 {
    fn truncating_cast_from(i: usize) -> u16 { i as u16 }
    fn most_significant_byte(self) -> u8 { (self >> 8) as u8 }
}

impl PackedValue for u32 {
    fn truncating_cast_from(i: usize) -> u32 { i as u32 }
    fn most_significant_byte(self) -> u8 { (self >> 24) as u8 }
}

impl PackedValue for u64 {
    fn truncating_cast_from(i: usize) -> u64 { i as u64 }
    fn most_significant_byte(self) -> u8 { (self >> 56) as u8 }
}

impl PackedValue for u128 {
    fn truncating_cast_from(i: usize) -> u128 { i as u128 }
    fn most_significant_byte(self) -> u8 { (self >> 120) as u8 }
}

#[derive(PartialEq, Debug)]
pub enum EncodeError {
    TooLong,
    NoCodePageFor(char),
    PageUnavailable(usize),
    MissingFromPage(char)
}

pub fn encode<N, IT>(i: IT) -> Result<N, EncodeError>
where
    N: PackedValue,
    IT: Iterator<Item = char>
{
    let mut pi = i.peekable();
    let mut out : N = N::truncating_cast_from(0);
    match pi.peek() {
        // Zero-length strings map to page 0, code 0.
        | None => Ok(out),
        | Some(&init) => {
            // Pick page: just try each one, there are only 16.
            match PAGES.iter().position(|&p| p.binary_search(&init).is_ok()) {
                // No page means this string won't work.
                | None => Err(EncodeError::NoCodePageFor(init)),
                | Some(p) => {
                    let mut tag = p;
                    let mut rem : usize = N::NCHARS;
                    // Check and adjust tag by size.
                    if N::NTAGBITS == 2 {
                        // Tried a "secondary tag" when only
                        // using 2 tag bits, sorry!
                        if tag & 0b11 != 0 {
                            return Err(EncodeError::PageUnavailable(tag))
                        }
                        tag >>= 2;
                    }
                    // Set tag.
                    out |= N::truncating_cast_from(tag);
                    // Encode chars.
                    for c in pi {
                        if rem == 0 {
                            // String is too long.
                            return Err(EncodeError::TooLong)
                        }
                        match PAGES[p].binary_search(&c) {
                            // No code for c in page.
                            | Err(_) => return Err(EncodeError::MissingFromPage(c)),
                            // Got a code, use it!
                            | Ok(i) => {
                                out <<= 6;
                                out |= N::truncating_cast_from(i);
                                rem -= 1;
                            }
                        }
                    }
                    // Pad remainder.
                    out <<= 6 * rem;
                    Ok(out)
                }
            }
        }
    }
}

pub trait EncodeSixbit: Sized + Iterator<Item = char>
{
    fn encode_sixbit<N: PackedValue>(self) -> Result<N, EncodeError>;
}

impl<T> EncodeSixbit for T
where
    T: Sized,
    T: Iterator<Item = char>
{
    fn encode_sixbit<N: PackedValue>(self) -> Result<N, EncodeError> {
        encode::<N, Self>(self)
    }
}

pub struct DecodeSixbitIter<N: PackedValue> {
    tag: usize,
    tmp: N
}

impl<N> Iterator for DecodeSixbitIter<N>
where
    N: PackedValue
{
    type Item = char;
    fn next(self: &mut Self) -> Option<char> {
        let mut ch = self.tmp.most_significant_byte();
        ch >>= 2;
        match ch {
            | 0 => None,
            | i => {
                self.tmp <<= 6;
                Some(PAGES[self.tag][i as usize])
            }
        }
    }
}

pub trait DecodeSixbit
where Self: PackedValue
{
    fn decode_sixbit(self) -> DecodeSixbitIter<Self>;
}

impl<N> DecodeSixbit for N
where N: PackedValue
{
    fn decode_sixbit(self) -> DecodeSixbitIter<Self> {
        let mut tmp = self;
        let mut tag = self.most_significant_byte();
        tag >>= 8 - N::NTAGBITS;
        if N::NTAGBITS == 2 {
            tag <<= 2;
        }
        tmp <<= N::NTAGBITS;
        DecodeSixbitIter {
            tag: tag as usize,
            tmp,
        }
    }
}


#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn misc_invariants() {
        // Check that pages are ordered by unicode ranges.
        for pair in PAGES.windows(2) {
            if pair[0][1] != '\u{ffff}' && pair[1][1] != '\u{ffff}' {
                if pair[0][1] >= pair[1][1] {
                    println!("mis-ordered page pair: {:?} >= {:?}", pair[0][1], pair[1][1]);
                }
                assert!(pair[0][1] < pair[1][1]);
            }
        }
        for p in PAGES.iter() {
            // Check that every page has a zero code, or is invalid.
            assert!(p[0] == '\0' || p[0] == '\u{ffff}');
            // Check that every page is sorted.
            for pair in p.windows(2) {
                if pair[0] != '\0' && pair[1] != '\0' &&
                    pair[0] != '\u{ffff}' && pair[1] != '\u{ffff}' {
                        if pair[0] >= pair[1] {
                            println!("mis-ordered char pair: {:?} >= {:?}", pair[0], pair[1]);
                        }
                        assert!(pair[0] < pair[1]);
                    }
            }
        }
    }

    fn round_trip<N:PackedValue>(s: &str) -> Result<N, EncodeError> {
        match s.chars().encode_sixbit::<N>() {
            Ok(enc) => {
                let dec:String = enc.decode_sixbit().collect();
                println!("roundtrip: {:?} => {:x} => {:?}", s, enc, dec);
                assert!(dec == s);
                Ok(enc)
            }
            err => err
        }
    }

    // For Latin Upper we try a full-width, a not-full-width, and each of the
    // error conditions.
    #[test]
    fn test_latin_upper() {
        // Full width.
        assert!(round_trip::<u128>("PRINTER IS ON FIRE!!").is_ok());
        assert!(round_trip::<u64>("NO_CARRIER").is_ok());
        assert!(round_trip::<u32>("[CAT]").is_ok());
        assert!(round_trip::<u16>("OK").is_ok());
        assert!(round_trip::<u8>("+").is_ok());

        // Non-full-width.
        assert!(round_trip::<u128>("PRINTER WORKING").is_ok());
        assert!(round_trip::<u64>("ATDT 123").is_ok());
        assert!(round_trip::<u32>("-=-").is_ok());
        assert!(round_trip::<u16>("?").is_ok());
        assert!(round_trip::<u8>("").is_ok());

        // Error conditions: TooLong.
        assert!(round_trip::<u128>("PRINTER FULLY OPERATIONAL") == Err(EncodeError::TooLong));
        assert!(round_trip::<u64>("ATDT 123-4567") == Err(EncodeError::TooLong));
        assert!(round_trip::<u32>("-/-=-/-") == Err(EncodeError::TooLong));
        assert!(round_trip::<u16>("?!?") == Err(EncodeError::TooLong));
        assert!(round_trip::<u8>("OOH") == Err(EncodeError::TooLong));

        // Error conditions: NoCodePageFor.
        assert!(round_trip::<u128>("©2018") == Err(EncodeError::NoCodePageFor('©')));

        // Error conditions: PageUnavailable.
        assert!(round_trip::<u128>("shark") == Err(EncodeError::PageUnavailable(1)));

        // Error conditions: MissingFromPage.
        assert!(round_trip::<u64>("sh@rk") == Err(EncodeError::MissingFromPage('@')));
    }

    fn check_order<N:PackedValue>(a: &str, b: &str) {
        assert!(a < b);
        assert!(a.chars().encode_sixbit::<N>().unwrap() < b.chars().encode_sixbit::<N>().unwrap());
    }

    #[test]
    fn sorting() {
        // Check encoding order preservation within pages.
        check_order::<u32>("", "AB");
        check_order::<u64>("abcd", "abcde");
        check_order::<u64>("abcde", "abcdf");
        check_order::<u64>("α", "αβγ");
        check_order::<u64>("αβ", "αβγ");
        check_order::<u64>("αβγ", "αβδ");
        // Check encoding order preservation across pages.
        check_order::<u64>("abc", "αβγ");
        check_order::<u64>("αβγ", "абв");
        check_order::<u64>("абв", "אבג");
        check_order::<u64>("אבג", "ابة");
        check_order::<u64>("ابة", "कखग");
        check_order::<u64>("कखग", "ㄱㄲㄳ");
        check_order::<u64>("ㄱㄲㄳ", "ｦｧｨ");
    }

    // For Latin-lower and non-Latin scripts we just check a word at each width
    // to make sure they work.
    #[test]
    fn test_latin_lower() {
        // Non-primary tag: only available in u64 and u16 forms.
        assert!(round_trip::<u64>("«öğrenmek»").is_ok());
        assert!(round_trip::<u16>("où").is_ok());
    }

    #[test]
    fn test_greek() {
        // Non-primary tag: only available in u64 and u16 forms.
        assert!(round_trip::<u64>("αλήθεια").is_ok());
        assert!(round_trip::<u16>("γη").is_ok());
    }

    #[test]
    fn test_cyrillic() {
        // Primary tag: available in all forms.
        assert!(round_trip::<u128>("скоропреходящий").is_ok());
        assert!(round_trip::<u64>("содержать").is_ok());
        assert!(round_trip::<u32>("форма").is_ok());
        assert!(round_trip::<u16>("же").is_ok());
        assert!(round_trip::<u8>("Я").is_ok());
    }

    #[test]
    fn test_hebrew() {
        // Non-primary tag: only available in u64 and u16 forms.
        assert!(round_trip::<u64>("לעשות").is_ok());
        assert!(round_trip::<u16>("כל").is_ok());
    }

    #[test]
    fn test_arabic() {
        // Primary tag: available in all forms.
        assert!(round_trip::<u128>("محافظت").is_ok());
        assert!(round_trip::<u64>("العاصمة").is_ok());
        assert!(round_trip::<u32>("البعض").is_ok());
        assert!(round_trip::<u16>("از").is_ok());
        assert!(round_trip::<u8>("و").is_ok());
    }

    #[test]
    fn test_devanagari() {
        // Primary tag: available in all forms.
        assert!(round_trip::<u128>("किंकर्तव्यविमूढ़").is_ok());
        assert!(round_trip::<u64>("आवश्यकता").is_ok());
        assert!(round_trip::<u32>("सपना").is_ok());
        assert!(round_trip::<u16>("पल").is_ok());
        assert!(round_trip::<u8>("आ").is_ok());
    }

    #[test]
    fn test_compatibility_hangul_jamo() {
        // Non-primary tag: only available in u64 and u16 forms.
        assert!(round_trip::<u64>("ㅇㅜㅁㅈㅣㄱㅇㅣㅁ").is_ok());
        assert!(round_trip::<u16>("ㅅㅜ").is_ok());
    }

    #[test]
    fn test_halfwidth_kana() {
        // Non-primary tag: only available in u64 and u16 forms.
        assert!(round_trip::<u64>("ｲｸﾂｶﾉ").is_ok());
        assert!(round_trip::<u16>("ﾔﾙ").is_ok());
    }
}