/* This Source Code Form is subject to the terms of the Mozilla Public

 * License, v. 2.0. If a copy of the MPL was not distributed with this

 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/// Abstraction for avoiding a dependency from cssparser to an encoding library

pub trait EncodingSupport {

    /// One character encoding

    type Encoding;

    /// https://encoding.spec.whatwg.org/#concept-encoding-get

    fn from_label(ascii_label: &[u8]) -> Option<Self::Encoding>;

    /// Return the UTF-8 encoding

    fn utf8() -> Self::Encoding;

    /// Whether the given encoding is UTF-16BE or UTF-16LE

    fn is_utf16_be_or_le(encoding: &Self::Encoding) -> bool;

}

/// Determine the character encoding of a CSS stylesheet.

///

/// This is based on the presence of a BOM (Byte Order Mark), an `@charset` rule, and

/// encoding meta-information.

///

/// * `css_bytes`: A byte string.

/// * `protocol_encoding`: The encoding label, if any, defined by HTTP or equivalent protocol.

///   (e.g. via the `charset` parameter of the `Content-Type` header.)

/// * `environment_encoding`: An optional `Encoding` object for the [environment encoding]

///   (https://drafts.csswg.org/css-syntax/#environment-encoding), if any.

///

/// Returns the encoding to use.

pub fn stylesheet_encoding<E>(

    css: &[u8],

    protocol_encoding_label: Option<&[u8]>,

    environment_encoding: Option<E::Encoding>,

) -> E::Encoding

where

    E: EncodingSupport,

{

    // https://drafts.csswg.org/css-syntax/#the-input-byte-stream

    if let Some(label) = protocol_encoding_label {

        if let Some(protocol_encoding) = E::from_label(label) {

            return protocol_encoding;

        };

    };

    let prefix = b"@charset \"";

    if css.starts_with(prefix) {

        let rest = &css[prefix.len()..];

        if let Some(label_length) = rest.iter().position(|&b| b == b'"') {

            if rest[label_length..].starts_with(b"\";") {

                let label = &rest[..label_length];

                if let Some(charset_encoding) = E::from_label(label) {

                    if E::is_utf16_be_or_le(&charset_encoding) {

                        return E::utf8();

                    } else {

                        return charset_encoding;

                    }

                }

            }

        }

    }

    environment_encoding.unwrap_or_else(E::utf8)

}