//! Processing of `data:` URLs according to the Fetch Standard:
//! <https://fetch.spec.whatwg.org/#data-urls>
//! but starting from a string rather than a parsed URL to avoid extra copies.
//!
//! ```rust
//! use data_url::{DataUrl, mime};
//!
//! let url = DataUrl::process("data:,Hello%20World!").unwrap();
//! let (body, fragment) = url.decode_to_vec().unwrap();
//!
//! assert_eq!(url.mime_type().type_, "text");
//! assert_eq!(url.mime_type().subtype, "plain");
//! assert_eq!(url.mime_type().get_parameter("charset"), Some("US-ASCII"));
//! assert_eq!(body, b"Hello World!");
//! assert!(fragment.is_none());
//! ```
#![no_std]

// For forwards compatibility
#[cfg(feature = "std")]
extern crate std as _;

#[macro_use]
extern crate alloc;

#[cfg(not(feature = "alloc"))]
compile_error!("the `alloc` feature must be enabled");

use alloc::{string::String, vec::Vec};

macro_rules! require {
    ($condition: expr) => {
        if !$condition {
            return None;
        }
    };
}

pub mod forgiving_base64;
pub mod mime;

pub struct DataUrl<'a> {
    mime_type: mime::Mime,
    base64: bool,
    encoded_body_plus_fragment: &'a str,
}

#[derive(Debug)]
pub enum DataUrlError {
    NotADataUrl,
    NoComma,
}

impl<'a> DataUrl<'a> {
    /// <https://fetch.spec.whatwg.org/#data-url-processor>
    /// but starting from a string rather than a parsed `Url`, to avoid extra string copies.
    pub fn process(input: &'a str) -> Result<Self, DataUrlError> {
        use crate::DataUrlError::*;

        let after_colon = pretend_parse_data_url(input).ok_or(NotADataUrl)?;

        let (from_colon_to_comma, encoded_body_plus_fragment) =
            find_comma_before_fragment(after_colon).ok_or(NoComma)?;

        let (mime_type, base64) = parse_header(from_colon_to_comma);

        Ok(DataUrl {
            mime_type,
            base64,
            encoded_body_plus_fragment,
        })
    }

    pub fn mime_type(&self) -> &mime::Mime {
        &self.mime_type
    }

    /// Streaming-decode the data URL’s body to `write_body_bytes`,
    /// and return the URL’s fragment identifier if it has one.
    pub fn decode<F, E>(
        &self,
        write_body_bytes: F,
    ) -> Result<Option<FragmentIdentifier<'a>>, forgiving_base64::DecodeError<E>>
    where
        F: FnMut(&[u8]) -> Result<(), E>,
    {
        if self.base64 {
            decode_with_base64(self.encoded_body_plus_fragment, write_body_bytes)
        } else {
            decode_without_base64(self.encoded_body_plus_fragment, write_body_bytes)
                .map_err(forgiving_base64::DecodeError::WriteError)
        }
    }

    /// Return the decoded body, and the URL’s fragment identifier if it has one.
    pub fn decode_to_vec(
        &self,
    ) -> Result<(Vec<u8>, Option<FragmentIdentifier<'a>>), forgiving_base64::InvalidBase64> {
        let mut body = Vec::new();
        let fragment = self.decode(|bytes| {
            body.extend_from_slice(bytes);
            Ok(())
        })?;
        Ok((body, fragment))
    }
}

/// The URL’s fragment identifier (after `#`)
pub struct FragmentIdentifier<'a>(&'a str);

impl<'a> FragmentIdentifier<'a> {
    /// Like in a parsed URL
    pub fn to_percent_encoded(&self) -> String {
        let mut string = String::new();
        for byte in self.0.bytes() {
            match byte {
                // Ignore ASCII tabs or newlines like the URL parser would
                b'\t' | b'\n' | b'\r' => continue,
                // https://url.spec.whatwg.org/#fragment-percent-encode-set
                b'\0'..=b' ' | b'"' | b'<' | b'>' | b'`' | b'\x7F'..=b'\xFF' => {
                    percent_encode(byte, &mut string)
                }
                // Printable ASCII
                _ => string.push(byte as char),
            }
        }
        string
    }
}

/// Similar to <https://url.spec.whatwg.org/#concept-basic-url-parser>
/// followed by <https://url.spec.whatwg.org/#concept-url-serializer>
///
/// * `None`: not a data URL.
///
/// * `Some(s)`: sort of the result of serialization, except:
///
///   - `data:` prefix removed
///   - The fragment is included
///   - Other components are **not** UTF-8 percent-encoded
///   - ASCII tabs and newlines in the middle are **not** removed
fn pretend_parse_data_url(input: &str) -> Option<&str> {
    // Trim C0 control or space
    let left_trimmed = input.trim_start_matches(|ch| ch <= ' ');

    let mut bytes = left_trimmed.bytes();
    {
        // Ignore ASCII tabs or newlines like the URL parser would
        let mut iter = bytes
            .by_ref()
            .filter(|&byte| !matches!(byte, b'\t' | b'\n' | b'\r'));
        require!(iter.next()?.to_ascii_lowercase() == b'd');
        require!(iter.next()?.to_ascii_lowercase() == b'a');
        require!(iter.next()?.to_ascii_lowercase() == b't');
        require!(iter.next()?.to_ascii_lowercase() == b'a');
        require!(iter.next()? == b':');
    }
    let bytes_consumed = left_trimmed.len() - bytes.len();
    let after_colon = &left_trimmed[bytes_consumed..];

    // Trim C0 control or space
    Some(after_colon.trim_end_matches(|ch| ch <= ' '))
}

fn find_comma_before_fragment(after_colon: &str) -> Option<(&str, &str)> {
    for (i, byte) in after_colon.bytes().enumerate() {
        if byte == b',' {
            return Some((&after_colon[..i], &after_colon[i + 1..]));
        }
        if byte == b'#' {
            break;
        }
    }
    None
}

fn parse_header(from_colon_to_comma: &str) -> (mime::Mime, bool) {
    // "Strip leading and trailing ASCII whitespace"
    //     \t, \n, and \r would have been filtered by the URL parser
    //     \f percent-encoded by the URL parser
    //     space is the only remaining ASCII whitespace
    let trimmed = from_colon_to_comma.trim_matches(|c| matches!(c, ' ' | '\t' | '\n' | '\r'));

    let without_base64_suffix = remove_base64_suffix(trimmed);
    let base64 = without_base64_suffix.is_some();
    let mime_type = without_base64_suffix.unwrap_or(trimmed);

    let mut string = String::new();
    if mime_type.starts_with(';') {
        string.push_str("text/plain")
    }
    let mut in_query = false;
    for byte in mime_type.bytes() {
        match byte {
            // Ignore ASCII tabs or newlines like the URL parser would
            b'\t' | b'\n' | b'\r' => continue,

            // https://url.spec.whatwg.org/#c0-control-percent-encode-set
            b'\0'..=b'\x1F' | b'\x7F'..=b'\xFF' => percent_encode(byte, &mut string),

            // Bytes other than the C0 percent-encode set that are percent-encoded
            // by the URL parser in the query state.
            // '#' is also in that list but cannot occur here
            // since it indicates the start of the URL’s fragment.
            b' ' | b'"' | b'<' | b'>' if in_query => percent_encode(byte, &mut string),

            b'?' => {
                in_query = true;
                string.push('?')
            }

            // Printable ASCII
            _ => string.push(byte as char),
        }
    }

    // FIXME: does Mime::from_str match the MIME Sniffing Standard’s parsing algorithm?
    // <https://mimesniff.spec.whatwg.org/#parse-a-mime-type>
    let mime_type = string.parse().unwrap_or_else(|_| mime::Mime {
        type_: String::from("text"),
        subtype: String::from("plain"),
        parameters: vec![(String::from("charset"), String::from("US-ASCII"))],
    });

    (mime_type, base64)
}

/// None: no base64 suffix
#[allow(clippy::skip_while_next)]
fn remove_base64_suffix(s: &str) -> Option<&str> {
    let mut bytes = s.bytes();
    {
        // Ignore ASCII tabs or newlines like the URL parser would
        let iter = bytes
            .by_ref()
            .filter(|&byte| !matches!(byte, b'\t' | b'\n' | b'\r'));

        // Search from the end
        let mut iter = iter.rev();

        require!(iter.next()? == b'4');
        require!(iter.next()? == b'6');
        require!(iter.next()?.to_ascii_lowercase() == b'e');
        require!(iter.next()?.to_ascii_lowercase() == b's');
        require!(iter.next()?.to_ascii_lowercase() == b'a');
        require!(iter.next()?.to_ascii_lowercase() == b'b');
        require!(iter.skip_while(|&byte| byte == b' ').next()? == b';');
    }
    Some(&s[..bytes.len()])
}

fn percent_encode(byte: u8, string: &mut String) {
    const HEX_UPPER: [u8; 16] = *b"0123456789ABCDEF";
    string.push('%');
    string.push(HEX_UPPER[(byte >> 4) as usize] as char);
    string.push(HEX_UPPER[(byte & 0x0f) as usize] as char);
}

/// This is <https://url.spec.whatwg.org/#string-percent-decode> while also:
///
/// * Ignoring ASCII tab or newlines
/// * Stopping at the first '#' (which indicates the start of the fragment)
///
/// Anything that would have been UTF-8 percent-encoded by the URL parser
/// would be percent-decoded here.
/// We skip that round-trip and pass it through unchanged.
fn decode_without_base64<F, E>(
    encoded_body_plus_fragment: &str,
    mut write_bytes: F,
) -> Result<Option<FragmentIdentifier<'_>>, E>
where
    F: FnMut(&[u8]) -> Result<(), E>,
{
    let bytes = encoded_body_plus_fragment.as_bytes();
    let mut slice_start = 0;
    for (i, &byte) in bytes.iter().enumerate() {
        // We only need to look for 5 different "special" byte values.
        // For everything else we make slices as large as possible, borrowing the input,
        // in order to make fewer write_all() calls.
        if matches!(byte, b'%' | b'#' | b'\t' | b'\n' | b'\r') {
            // Write everything (if anything) "non-special" we’ve accumulated
            // before this special byte
            if i > slice_start {
                write_bytes(&bytes[slice_start..i])?;
            }
            // Then deal with the special byte.
            match byte {
                b'%' => {
                    let l = bytes.get(i + 2).and_then(|&b| (b as char).to_digit(16));
                    let h = bytes.get(i + 1).and_then(|&b| (b as char).to_digit(16));
                    if let (Some(h), Some(l)) = (h, l) {
                        // '%' followed by two ASCII hex digits
                        let one_byte = h as u8 * 0x10 + l as u8;
                        write_bytes(&[one_byte])?;
                        slice_start = i + 3;
                    } else {
                        // Do nothing. Leave slice_start unchanged.
                        // The % sign will be part of the next slice.
                    }
                }

                b'#' => {
                    let fragment_start = i + 1;
                    let fragment = &encoded_body_plus_fragment[fragment_start..];
                    return Ok(Some(FragmentIdentifier(fragment)));
                }

                // Ignore over '\t' | '\n' | '\r'
                _ => slice_start = i + 1,
            }
        }
    }
    write_bytes(&bytes[slice_start..])?;
    Ok(None)
}

/// `decode_without_base64()` composed with
/// <https://infra.spec.whatwg.org/#isomorphic-decode> composed with
/// <https://infra.spec.whatwg.org/#forgiving-base64-decode>.
fn decode_with_base64<F, E>(
    encoded_body_plus_fragment: &str,
    write_bytes: F,
) -> Result<Option<FragmentIdentifier<'_>>, forgiving_base64::DecodeError<E>>
where
    F: FnMut(&[u8]) -> Result<(), E>,
{
    let mut decoder = forgiving_base64::Decoder::new(write_bytes);
    let fragment = decode_without_base64(encoded_body_plus_fragment, |bytes| decoder.feed(bytes))?;
    decoder.finish()?;
    Ok(fragment)
}