Struct-based API / Hide raw types of decoding tables & decoding hash maps

[tats-u]

I want to switch this crate to struct-based API:

let encoder = get_cp_encoder(OEMCodePage::Cp437);
// let encoder = try_get_cp_encoder_from_number(437).unwrap(); // For those who want to pass a number directly
let cp437_bytes = encoder.encode_string_lossy("½ ± 1");

The current API looks ugly and exposes too much.

Especially [char; 128] and [Option<char>; 128] should not be exposed.

[mkroening]

I think we can base this on the char-based API and provide both in a nice API.

One question for clarification: Is it common to dynamically change the code page at runtime? Or would something like get_cp_encoder::<437>() be okay too?

[tats-u]

Is it common to dynamically change the code page at runtime?

Not just common but mandatory if you're going to develop apps for worldwide use.
Only for an app for a single country (e.g. one for internal use in your company), something like CP437_ENCODER is useful.

OEM code pages (= encoding for ZIP files and the terminal) depends on the language and region in Windows; CP437 in US English and Shift_JIS (CP932; not covered by this crate) in Japanese.

[tats-u]

Ah I got it. the enum-based is more preferred. There's no reason to prefer to use a raw code page number (like 437 instead of OEMCodePage::Cp437).
Enum can be easily completed and combined with match.

[mkroening]

Ah, makes sense. This is a draft of the static API I am imagining:

#[derive(Clone, Copy)]
pub struct Cp<const N: u16>(u8);

impl Cp<437> {
    pub const fn new(cp: u8) -> Option<Self>;
    pub const fn get(self) -> u8;
    pub const fn from_char(c: char) -> Option<Self>;
    pub const fn to_char(self) -> char;
}

pub type CpStr<N> = [Cp<N>];

impl CpStr<437> {
    pub const fn from_cp(bytes: &[u8]) -> Result<&Self, CpError>;
    pub const fn to_bytes(&self) -> &[u8];
}

pub struct CpString<const N: u16>(Box<CpStr<N>>);

impl CpString<437> {
    pub fn from_cp_lossy(bytes: &[u8]) -> Cow<'_, CpStr<N>>;
}

This API does not have structural differences between complete and incomplete tables: For complete tables, some operations just never fail.

If I understood you correctly, we also need to find a replacement for accessing DECODING_TABLE_CP_MAP with dynamic numbers, right? The dynamic API could look like this:

pub struct CpNumber(u16);

impl CpNumber {
    pub const fn new(n: u16) -> Option<Self>;
}

impl str {
    pub fn to_cp(&self, n: CpNumber) -> Result<&[u8], CpError>;
    pub fn to_cp_lossy(&self, n: CpNumber) -> Cow<'_, [u8]>;
}

impl [u8] {
    pub fn cp_to_str(&self, n: CpNumber) -> Result<&str, CpError>;
    pub fn cp_to_str_lossy(&self, n: CpNumber) -> Cow<'_, str>;
}

What do you think of this? Would this work for all use-cases? I can work on this and open a PR (or change the existing one) if you want.

[tats-u]

https://crates.io/crates/enum-map

This crate looks better than CpNumber(u16), which can't prevent the typo of the code page number by users who want to use a single code page.
Also I think it allows invalid objects like let foo = CpNumber(1252); (not CpNumber::new(1252)) by a user.

Your suggested API looks suitable for one-shot or casual/informal/simple usage because the library can't easily cache the reference of a encoding/decoding table. An additional static hash map (recent code page numbers → tables) object will be required.

[mkroening]

This crate looks better than CpNumber(u16)

I thought we need a way to go from u16 to code page at runtime, no?

If not, we can just use enums and don't need enum-map or what exactly were you thinking of?

Also I think it allows invalid objects like let foo = CpNumber(1252); (not CpNumber::new(1252)) by a user.

This is not possible, as the inner field is not public. Creation can only happen through new.

library can't easily cache the reference of a encoding/decoding table

How about putting the reference into the number to allow reuse? This makes the struct bigger, though. I am not sure if a phf lookup in exchange for smaller structs might be beneficial here, but we can do it if you want:

pub struct CpNumber {
    encode_table: &'static Map,
    decode_table: &'static Map,
}

impl CpNumber {
    pub fn new(n: u16) -> Self {
        /// look up the table
    }
}