//! Resource loader: fetch URLs and decode their content.
//!
//! Brings together `net` (HTTP client), `encoding` (charset detection and decoding),
//! `url` (URL parsing and resolution), and `image` (image decoding) into a single
//! `ResourceLoader` that the browser uses to load web pages and subresources.

use std::fmt;

use we_encoding::sniff::sniff_encoding;
use we_encoding::Encoding;
use we_net::client::{ClientError, HttpClient};
use we_net::http::ContentType;
use we_url::Url;

// ---------------------------------------------------------------------------
// Error type
// ---------------------------------------------------------------------------

/// Errors that can occur during resource loading.
#[derive(Debug)]
pub enum LoadError {
    /// URL parsing failed.
    InvalidUrl(String),
    /// Network or HTTP error from the underlying client.
    Network(ClientError),
    /// HTTP response indicated an error status.
    HttpStatus { status: u16, reason: String },
    /// Encoding or decoding error.
    Encoding(String),
}

impl fmt::Display for LoadError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::InvalidUrl(s) => write!(f, "invalid URL: {s}"),
            Self::Network(e) => write!(f, "network error: {e}"),
            Self::HttpStatus { status, reason } => {
                write!(f, "HTTP {status} {reason}")
            }
            Self::Encoding(s) => write!(f, "encoding error: {s}"),
        }
    }
}

impl From<ClientError> for LoadError {
    fn from(e: ClientError) -> Self {
        Self::Network(e)
    }
}

// ---------------------------------------------------------------------------
// Resource types
// ---------------------------------------------------------------------------

/// A loaded resource with its decoded content and metadata.
#[derive(Debug)]
pub enum Resource {
    /// An HTML document.
    Html {
        text: String,
        base_url: Url,
        encoding: Encoding,
    },
    /// A CSS stylesheet.
    Css { text: String, url: Url },
    /// A decoded image.
    Image {
        data: Vec<u8>,
        mime_type: String,
        url: Url,
    },
    /// Any other resource type (binary).
    Other {
        data: Vec<u8>,
        mime_type: String,
        url: Url,
    },
}

// ---------------------------------------------------------------------------
// ResourceLoader
// ---------------------------------------------------------------------------

/// Loads resources over HTTP/HTTPS with encoding detection and content-type handling.
pub struct ResourceLoader {
    client: HttpClient,
}

impl ResourceLoader {
    /// Create a new resource loader with default settings.
    pub fn new() -> Self {
        Self {
            client: HttpClient::new(),
        }
    }

    /// Fetch a resource at the given URL.
    ///
    /// Determines the resource type from the HTTP Content-Type header, decodes
    /// text resources using the appropriate character encoding (per WHATWG spec),
    /// and returns the result as a typed `Resource`.
    pub fn fetch(&mut self, url: &Url) -> Result<Resource, LoadError> {
        let response = self.client.get(url)?;

        // Check for HTTP error status codes
        if response.status_code >= 400 {
            return Err(LoadError::HttpStatus {
                status: response.status_code,
                reason: response.reason.clone(),
            });
        }

        let content_type = response.content_type();
        let mime = content_type
            .as_ref()
            .map(|ct| ct.mime_type.as_str())
            .unwrap_or("application/octet-stream");

        match classify_mime(mime) {
            MimeClass::Html => {
                let (text, encoding) =
                    decode_text_resource(&response.body, content_type.as_ref(), true);
                Ok(Resource::Html {
                    text,
                    base_url: url.clone(),
                    encoding,
                })
            }
            MimeClass::Css => {
                let (text, _encoding) =
                    decode_text_resource(&response.body, content_type.as_ref(), false);
                Ok(Resource::Css {
                    text,
                    url: url.clone(),
                })
            }
            MimeClass::Image => Ok(Resource::Image {
                data: response.body,
                mime_type: mime.to_string(),
                url: url.clone(),
            }),
            MimeClass::Other => {
                // Check if it's a text type we should decode
                if mime.starts_with("text/") {
                    let (text, _encoding) =
                        decode_text_resource(&response.body, content_type.as_ref(), false);
                    Ok(Resource::Other {
                        data: text.into_bytes(),
                        mime_type: mime.to_string(),
                        url: url.clone(),
                    })
                } else {
                    Ok(Resource::Other {
                        data: response.body,
                        mime_type: mime.to_string(),
                        url: url.clone(),
                    })
                }
            }
        }
    }

    /// Fetch a URL string, resolving it against an optional base URL.
    pub fn fetch_url(&mut self, url_str: &str, base: Option<&Url>) -> Result<Resource, LoadError> {
        let url = match base {
            Some(base_url) => Url::parse_with_base(url_str, base_url)
                .or_else(|_| Url::parse(url_str))
                .map_err(|_| LoadError::InvalidUrl(url_str.to_string()))?,
            None => Url::parse(url_str).map_err(|_| LoadError::InvalidUrl(url_str.to_string()))?,
        };
        self.fetch(&url)
    }
}

impl Default for ResourceLoader {
    fn default() -> Self {
        Self::new()
    }
}

// ---------------------------------------------------------------------------
// MIME classification
// ---------------------------------------------------------------------------

enum MimeClass {
    Html,
    Css,
    Image,
    Other,
}

fn classify_mime(mime: &str) -> MimeClass {
    match mime {
        "text/html" | "application/xhtml+xml" => MimeClass::Html,
        "text/css" => MimeClass::Css,
        "image/png" | "image/jpeg" | "image/gif" | "image/webp" | "image/svg+xml" => {
            MimeClass::Image
        }
        _ => MimeClass::Other,
    }
}

// ---------------------------------------------------------------------------
// Text decoding
// ---------------------------------------------------------------------------

/// Decode a text resource's bytes to a String using WHATWG encoding sniffing.
///
/// For HTML resources, uses BOM > HTTP charset > meta prescan > default.
/// For non-HTML text resources, uses BOM > HTTP charset > default (UTF-8).
fn decode_text_resource(
    bytes: &[u8],
    content_type: Option<&ContentType>,
    is_html: bool,
) -> (String, Encoding) {
    let http_ct_value = content_type.map(|ct| {
        // Reconstruct a Content-Type header value for the sniffing function
        match &ct.charset {
            Some(charset) => format!("{}; charset={}", ct.mime_type, charset),
            None => ct.mime_type.clone(),
        }
    });

    if is_html {
        // Full WHATWG sniffing: BOM > HTTP > meta prescan > default (Windows-1252)
        let (encoding, _source) = sniff_encoding(bytes, http_ct_value.as_deref());
        let text = decode_with_bom_handling(bytes, encoding);
        (text, encoding)
    } else {
        // Non-HTML: BOM > HTTP charset > default (UTF-8)
        let (bom_enc, after_bom) = we_encoding::bom_sniff(bytes);
        if let Some(enc) = bom_enc {
            let text = we_encoding::decode(after_bom, enc);
            return (text, enc);
        }

        // Try HTTP charset
        if let Some(charset) = content_type.and_then(|ct| ct.charset.as_deref()) {
            if let Some(enc) = we_encoding::lookup(charset) {
                let text = we_encoding::decode(bytes, enc);
                return (text, enc);
            }
        }

        // Default to UTF-8 for non-HTML text
        let text = we_encoding::decode(bytes, Encoding::Utf8);
        (text, Encoding::Utf8)
    }
}

/// Decode bytes with BOM handling — strip BOM bytes before decoding.
fn decode_with_bom_handling(bytes: &[u8], encoding: Encoding) -> String {
    let (bom_enc, after_bom) = we_encoding::bom_sniff(bytes);
    if bom_enc.is_some() {
        // BOM was present — decode the bytes after the BOM
        we_encoding::decode(after_bom, encoding)
    } else {
        we_encoding::decode(bytes, encoding)
    }
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;

    // -----------------------------------------------------------------------
    // LoadError Display
    // -----------------------------------------------------------------------

    #[test]
    fn load_error_display_invalid_url() {
        let e = LoadError::InvalidUrl("bad://url".to_string());
        assert_eq!(e.to_string(), "invalid URL: bad://url");
    }

    #[test]
    fn load_error_display_http_status() {
        let e = LoadError::HttpStatus {
            status: 404,
            reason: "Not Found".to_string(),
        };
        assert_eq!(e.to_string(), "HTTP 404 Not Found");
    }

    #[test]
    fn load_error_display_encoding() {
        let e = LoadError::Encoding("bad charset".to_string());
        assert_eq!(e.to_string(), "encoding error: bad charset");
    }

    // -----------------------------------------------------------------------
    // MIME classification
    // -----------------------------------------------------------------------

    #[test]
    fn classify_text_html() {
        assert!(matches!(classify_mime("text/html"), MimeClass::Html));
    }

    #[test]
    fn classify_xhtml() {
        assert!(matches!(
            classify_mime("application/xhtml+xml"),
            MimeClass::Html
        ));
    }

    #[test]
    fn classify_text_css() {
        assert!(matches!(classify_mime("text/css"), MimeClass::Css));
    }

    #[test]
    fn classify_image_png() {
        assert!(matches!(classify_mime("image/png"), MimeClass::Image));
    }

    #[test]
    fn classify_image_jpeg() {
        assert!(matches!(classify_mime("image/jpeg"), MimeClass::Image));
    }

    #[test]
    fn classify_image_gif() {
        assert!(matches!(classify_mime("image/gif"), MimeClass::Image));
    }

    #[test]
    fn classify_application_json() {
        assert!(matches!(
            classify_mime("application/json"),
            MimeClass::Other
        ));
    }

    #[test]
    fn classify_text_plain() {
        assert!(matches!(classify_mime("text/plain"), MimeClass::Other));
    }

    #[test]
    fn classify_octet_stream() {
        assert!(matches!(
            classify_mime("application/octet-stream"),
            MimeClass::Other
        ));
    }

    // -----------------------------------------------------------------------
    // Text decoding — HTML
    // -----------------------------------------------------------------------

    #[test]
    fn decode_html_utf8_bom() {
        let bytes = b"\xEF\xBB\xBF<html>Hello</html>";
        let (text, enc) = decode_text_resource(bytes, None, true);
        assert_eq!(enc, Encoding::Utf8);
        assert_eq!(text, "<html>Hello</html>");
    }

    #[test]
    fn decode_html_utf8_from_http_charset() {
        let ct = ContentType {
            mime_type: "text/html".to_string(),
            charset: Some("utf-8".to_string()),
        };
        let bytes = b"<html>Hello</html>";
        let (text, enc) = decode_text_resource(bytes, Some(&ct), true);
        assert_eq!(enc, Encoding::Utf8);
        assert_eq!(text, "<html>Hello</html>");
    }

    #[test]
    fn decode_html_meta_charset() {
        let html = b"<meta charset=\"utf-8\"><html>Hello</html>";
        let (text, enc) = decode_text_resource(html, None, true);
        assert_eq!(enc, Encoding::Utf8);
        assert!(text.contains("Hello"));
    }

    #[test]
    fn decode_html_default_windows_1252() {
        let bytes = b"<html>Hello</html>";
        let (text, enc) = decode_text_resource(bytes, None, true);
        assert_eq!(enc, Encoding::Windows1252);
        assert!(text.contains("Hello"));
    }

    #[test]
    fn decode_html_windows_1252_special_chars() {
        // \x93 and \x94 are left/right double quotation marks in Windows-1252
        let bytes = b"<html>\x93Hello\x94</html>";
        let (text, enc) = decode_text_resource(bytes, None, true);
        assert_eq!(enc, Encoding::Windows1252);
        assert!(text.contains('\u{201C}')); // left double quote
        assert!(text.contains('\u{201D}')); // right double quote
    }

    #[test]
    fn decode_html_bom_beats_http_charset() {
        let ct = ContentType {
            mime_type: "text/html".to_string(),
            charset: Some("windows-1252".to_string()),
        };
        let mut bytes = vec![0xEF, 0xBB, 0xBF];
        bytes.extend_from_slice(b"<html>Hello</html>");
        let (text, enc) = decode_text_resource(&bytes, Some(&ct), true);
        assert_eq!(enc, Encoding::Utf8);
        assert_eq!(text, "<html>Hello</html>");
    }

    // -----------------------------------------------------------------------
    // Text decoding — non-HTML (CSS, etc.)
    // -----------------------------------------------------------------------

    #[test]
    fn decode_css_utf8_default() {
        let bytes = b"body { color: red; }";
        let (text, enc) = decode_text_resource(bytes, None, false);
        assert_eq!(enc, Encoding::Utf8);
        assert_eq!(text, "body { color: red; }");
    }

    #[test]
    fn decode_css_bom_utf8() {
        let bytes = b"\xEF\xBB\xBFbody { color: red; }";
        let (text, enc) = decode_text_resource(bytes, None, false);
        assert_eq!(enc, Encoding::Utf8);
        assert_eq!(text, "body { color: red; }");
    }

    #[test]
    fn decode_css_http_charset() {
        let ct = ContentType {
            mime_type: "text/css".to_string(),
            charset: Some("utf-8".to_string()),
        };
        let bytes = b"body { color: red; }";
        let (text, enc) = decode_text_resource(bytes, Some(&ct), false);
        assert_eq!(enc, Encoding::Utf8);
        assert_eq!(text, "body { color: red; }");
    }

    // -----------------------------------------------------------------------
    // BOM handling
    // -----------------------------------------------------------------------

    #[test]
    fn decode_with_bom_strips_utf8_bom() {
        let bytes = b"\xEF\xBB\xBFHello";
        let text = decode_with_bom_handling(bytes, Encoding::Utf8);
        assert_eq!(text, "Hello");
    }

    #[test]
    fn decode_without_bom_passes_through() {
        let bytes = b"Hello";
        let text = decode_with_bom_handling(bytes, Encoding::Utf8);
        assert_eq!(text, "Hello");
    }

    #[test]
    fn decode_with_utf16le_bom() {
        let bytes = b"\xFF\xFEH\x00e\x00l\x00l\x00o\x00";
        let text = decode_with_bom_handling(bytes, Encoding::Utf16Le);
        assert_eq!(text, "Hello");
    }

    #[test]
    fn decode_with_utf16be_bom() {
        let bytes = b"\xFE\xFF\x00H\x00e\x00l\x00l\x00o";
        let text = decode_with_bom_handling(bytes, Encoding::Utf16Be);
        assert_eq!(text, "Hello");
    }

    // -----------------------------------------------------------------------
    // ResourceLoader construction
    // -----------------------------------------------------------------------

    #[test]
    fn resource_loader_new() {
        let _loader = ResourceLoader::new();
    }

    #[test]
    fn resource_loader_default() {
        let _loader = ResourceLoader::default();
    }

    // -----------------------------------------------------------------------
    // URL resolution
    // -----------------------------------------------------------------------

    #[test]
    fn fetch_url_invalid_url_error() {
        let mut loader = ResourceLoader::new();
        let result = loader.fetch_url("not a url at all", None);
        assert!(matches!(result, Err(LoadError::InvalidUrl(_))));
    }

    #[test]
    fn fetch_url_relative_without_base_errors() {
        let mut loader = ResourceLoader::new();
        let result = loader.fetch_url("/relative/path", None);
        assert!(matches!(result, Err(LoadError::InvalidUrl(_))));
    }

    #[test]
    fn fetch_url_relative_with_base_resolves() {
        let mut loader = ResourceLoader::new();
        let base = Url::parse("http://example.com/page").unwrap();
        // This will fail since we can't actually connect in tests,
        // but the URL resolution itself should work (it won't be InvalidUrl).
        let result = loader.fetch_url("/style.css", Some(&base));
        assert!(result.is_err());
        // The error should NOT be InvalidUrl — the URL resolved successfully.
        assert!(!matches!(result, Err(LoadError::InvalidUrl(_))));
    }
}