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//! Gemini Response Headers
//!
//! This module contains types to represent Gemini response headers, construct
//! them from their component parts, and for examining the type and content of
//! the Gemini <META> field, which represents, among other things, the MIME type
//! of successful Gemini responses. See `MetaKind` for more.
//!
//! # Examples
//!
//! ```
//! use gemini::{Header, MetaKind, Status};
//!
//! assert_eq!(Header::new(Status::INPUT, "Hello!".to_string()).unwrap().meta_kind(), MetaKind::Prompt);
//! assert_eq!(Header::new(Status::SUCCESS, "".to_string()).unwrap().mime_type().unwrap(), "text/gemini; charset=utf-8");
//! ```
//!
//! some item comments are taken verbatim from
//! [the Gemini spec](https://gemini.circumlunar.space/docs/specification.html)

use crate::status::{Category, Status};

// I'd love for this struct to derive(Copy), but a naive implementation would
// always incur 1kb of overhead per header, which seems unnecessary when most
// will probably be merely a few bytes. How to weigh those concerns?
/// Gemini response headers.
///
/// Consist of a valid `Status` along with a <META> field, which has a maximum
/// length of 1024 bytes and must be valid utf-8 text.
#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)]
pub struct Header {
    /// Status associated with the response header.
    pub status: Status,
    meta: String,
}

/// Type that represents the semantics of the <META> field for different sorts
/// of response statuses.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
pub enum MetaKind {
    /// The <META> line is a prompt which should be displayed to the user.
    Prompt,
    /// <META> is a new URL for the requested resource. The URL may be absolute or relative.
    MimeType,
    /// <META> is a new URL for the requested resource.
    RedirectTarget,
    /// The contents of <META> may provide additional information on the response, and should be displayed to human users.
    Message,
}

impl Header {
    /// Maximum length, in bytes, of the <META> field.
    pub const MAX_META_LEN: usize = 1024;
    /// Default MIME type for successful Gemini responses.
    pub const DEFAULT_MIME_TYPE: &'static str = "text/gemini; charset=utf-8";

    /// Retrieve the <META> field as a utf-8 encoded string slice.
    pub fn meta(&self) -> &str {
        self.meta.as_str()
    }

    /// Return which `MetaKind` is assoicated with the response.
    ///
    /// Clients should use this to decide how to interpret response bodies.
    pub fn meta_kind(&self) -> MetaKind {
        match self.status.category() {
            Category::Input => MetaKind::Prompt,
            Category::Success => MetaKind::MimeType,
            Category::Redirect => MetaKind::RedirectTarget,
            Category::TemporaryFailure
            | Category::PermanentFailure
            | Category::ClientCertificateRequired => MetaKind::Message,
        }
    }

    /// Return the MIME media type for a header which has it.
    pub fn mime_type(&self) -> Option<&str> {
        match self.meta_kind() {
            MetaKind::MimeType => Some(self.meta()),
            _ => None,
        }
    }

    /// Construct a new `Header` from a valid `Status` and a utf-8 string.
    ///
    /// Will return an `Err` if the length of the provided meta exceeds
    /// `Self::MAX_META_LEN`.
    pub fn new(status: Status, meta: String) -> Option<Self> {
        if meta.len() < Self::MAX_META_LEN {
            let meta = match status {
                Status::SUCCESS if meta.trim().is_empty() => Self::DEFAULT_MIME_TYPE.to_string(),
                _ => meta,
            };
            Some(Header { status, meta })
        } else {
            None
        }
    }

    /// Construct a `Status::SUCCESS` header with the given mime-type.
    pub fn success(mime_type: String) -> Option<Self> {
        Self::new(Status::SUCCESS, mime_type)
    }

    /// Construct a `Status::SUCCESS` header with the text/gemini mime-type.
    pub fn gemtext() -> Self {
        let status = Status::SUCCESS;
        let meta = Self::DEFAULT_MIME_TYPE.to_string();
        Header { status, meta }
    }
}