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/* 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/. */

use std::cmp;

use encoding::label::encoding_from_whatwg_label;
use encoding::all::UTF_8;
use encoding::{EncodingRef, DecoderTrap, decode};


/// Determine the character encoding of a CSS stylesheet and decode it.
///
/// 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 a 2-tuple of a decoded Unicode string and the `Encoding` object that was used.
pub fn decode_stylesheet_bytes(css: &[u8], protocol_encoding_label: Option<&str>,
                               environment_encoding: Option<EncodingRef>)
                            -> (String, EncodingRef) {
    // https://drafts.csswg.org/css-syntax/#the-input-byte-stream
    match protocol_encoding_label {
        None => (),
        Some(label) => match encoding_from_whatwg_label(label) {
            None => (),
            Some(fallback) => return decode_replace(css, fallback)
        }
    }
    if css.starts_with("@charset \"".as_bytes()) {
        // 10 is "@charset \"".len()
        // 100 is arbitrary so that no encoding label is more than 100-10 bytes.
        match css[10..cmp::min(css.len(), 100)].iter().position(|&b| b == b'"') {
            None => (),
            Some(label_length)
            => if css[10 + label_length..].starts_with("\";".as_bytes()) {
                let label = &css[10..10 + label_length];
                let label = label.iter().map(|&b| b as char).collect::<String>();
                match encoding_from_whatwg_label(&*label) {
                    None => (),
                    Some(fallback) => match fallback.name() {
                        "utf-16be" | "utf-16le"
                        => return decode_replace(css, UTF_8 as EncodingRef),
                        _ => return decode_replace(css, fallback),
                    }
                }
            }
        }
    }
    match environment_encoding {
        None => (),
        Some(fallback) => return decode_replace(css, fallback)
    }
    return decode_replace(css, UTF_8 as EncodingRef)
}


#[inline]
fn decode_replace(input: &[u8], fallback_encoding: EncodingRef)-> (String, EncodingRef) {
    let (result, used_encoding) = decode(input, DecoderTrap::Replace, fallback_encoding);
    (result.unwrap(), used_encoding)
}