use hmac::{Hmac, Mac};
use hyper::body::Incoming;
use hyper::Request;
use sha2::{Digest, Sha256};
use std::collections::HashMap;

use crate::config::{Credential, S3Config};
use crate::s3_error::S3Error;

type HmacSha256 = Hmac<Sha256>;

/// The identity of an authenticated caller.
#[derive(Debug, Clone)]
pub struct AuthenticatedIdentity {
    pub access_key_id: String,
}

/// Parsed components of an AWS4-HMAC-SHA256 Authorization header.
struct SigV4Header {
    access_key_id: String,
    date_stamp: String,
    region: String,
    signed_headers: Vec<String>,
    signature: String,
}

/// Verify the request's SigV4 signature. Returns the caller identity on success.
pub fn verify_request(
    req: &Request<Incoming>,
    config: &S3Config,
) -> Result<AuthenticatedIdentity, S3Error> {
    let auth_header = req
        .headers()
        .get("authorization")
        .and_then(|v| v.to_str().ok())
        .unwrap_or("");

    // Reject SigV2
    if auth_header.starts_with("AWS ") {
        return Err(S3Error::authorization_header_malformed());
    }

    if !auth_header.starts_with("AWS4-HMAC-SHA256") {
        return Err(S3Error::authorization_header_malformed());
    }

    let parsed = parse_auth_header(auth_header)?;

    // Look up credential
    let credential = find_credential(&parsed.access_key_id, config)
        .ok_or_else(S3Error::invalid_access_key_id)?;

    // Get x-amz-date
    let amz_date = req
        .headers()
        .get("x-amz-date")
        .and_then(|v| v.to_str().ok())
        .or_else(|| {
            req.headers()
                .get("date")
                .and_then(|v| v.to_str().ok())
        })
        .ok_or_else(|| S3Error::missing_security_header("Missing x-amz-date header"))?;

    // Enforce 15-min clock skew
    check_clock_skew(amz_date)?;

    // Get payload hash
    let content_sha256 = req
        .headers()
        .get("x-amz-content-sha256")
        .and_then(|v| v.to_str().ok())
        .unwrap_or("UNSIGNED-PAYLOAD");

    // Build canonical request
    let canonical_request = build_canonical_request(req, &parsed.signed_headers, content_sha256);

    // Build string to sign
    let scope = format!(
        "{}/{}/s3/aws4_request",
        parsed.date_stamp, parsed.region
    );
    let canonical_hash = hex::encode(Sha256::digest(canonical_request.as_bytes()));
    let string_to_sign = format!(
        "AWS4-HMAC-SHA256\n{}\n{}\n{}",
        amz_date, scope, canonical_hash
    );

    // Derive signing key
    let signing_key = derive_signing_key(
        &credential.secret_access_key,
        &parsed.date_stamp,
        &parsed.region,
    );

    // Compute signature
    let computed = hmac_sha256(&signing_key, string_to_sign.as_bytes());
    let computed_hex = hex::encode(&computed);

    // Constant-time comparison
    if !constant_time_eq(computed_hex.as_bytes(), parsed.signature.as_bytes()) {
        return Err(S3Error::signature_does_not_match());
    }

    Ok(AuthenticatedIdentity {
        access_key_id: parsed.access_key_id,
    })
}

/// Parse the Authorization header into its components.
fn parse_auth_header(header: &str) -> Result<SigV4Header, S3Error> {
    // Format: AWS4-HMAC-SHA256 Credential=KEY/YYYYMMDD/region/s3/aws4_request, SignedHeaders=h1;h2, Signature=hex
    let after_algo = header
        .strip_prefix("AWS4-HMAC-SHA256")
        .ok_or_else(S3Error::authorization_header_malformed)?
        .trim();

    let mut credential_str = None;
    let mut signed_headers_str = None;
    let mut signature_str = None;

    for part in after_algo.split(',') {
        let part = part.trim();
        if let Some(val) = part.strip_prefix("Credential=") {
            credential_str = Some(val.trim());
        } else if let Some(val) = part.strip_prefix("SignedHeaders=") {
            signed_headers_str = Some(val.trim());
        } else if let Some(val) = part.strip_prefix("Signature=") {
            signature_str = Some(val.trim());
        }
    }

    let credential_str = credential_str
        .ok_or_else(S3Error::authorization_header_malformed)?;
    let signed_headers_str = signed_headers_str
        .ok_or_else(S3Error::authorization_header_malformed)?;
    let signature = signature_str
        .ok_or_else(S3Error::authorization_header_malformed)?
        .to_string();

    // Parse credential: KEY/YYYYMMDD/region/s3/aws4_request
    let cred_parts: Vec<&str> = credential_str.splitn(5, '/').collect();
    if cred_parts.len() < 5 {
        return Err(S3Error::authorization_header_malformed());
    }

    let access_key_id = cred_parts[0].to_string();
    let date_stamp = cred_parts[1].to_string();
    let region = cred_parts[2].to_string();

    let signed_headers: Vec<String> = signed_headers_str
        .split(';')
        .map(|s| s.trim().to_lowercase())
        .collect();

    Ok(SigV4Header {
        access_key_id,
        date_stamp,
        region,
        signed_headers,
        signature,
    })
}

/// Find a credential by access key ID.
fn find_credential<'a>(access_key_id: &str, config: &'a S3Config) -> Option<&'a Credential> {
    config
        .auth
        .credentials
        .iter()
        .find(|c| c.access_key_id == access_key_id)
}

/// Check clock skew (15 minutes max).
fn check_clock_skew(amz_date: &str) -> Result<(), S3Error> {
    // Parse ISO 8601 basic format: YYYYMMDDTHHMMSSZ
    let parsed = chrono::NaiveDateTime::parse_from_str(amz_date, "%Y%m%dT%H%M%SZ")
        .map_err(|_| S3Error::authorization_header_malformed())?;

    let request_time = chrono::DateTime::<chrono::Utc>::from_naive_utc_and_offset(parsed, chrono::Utc);
    let now = chrono::Utc::now();
    let diff = (now - request_time).num_seconds().unsigned_abs();

    if diff > 15 * 60 {
        return Err(S3Error::request_time_too_skewed());
    }

    Ok(())
}

/// Build the canonical request string.
fn build_canonical_request(
    req: &Request<Incoming>,
    signed_headers: &[String],
    payload_hash: &str,
) -> String {
    let method = req.method().as_str();
    let uri_path = req.uri().path();

    // Canonical URI: the path, already percent-encoded by the client
    let canonical_uri = if uri_path.is_empty() { "/" } else { uri_path };

    // Canonical query string: sorted key=value pairs
    let canonical_query = build_canonical_query(req.uri().query().unwrap_or(""));

    // Canonical headers: sorted by lowercase header name
    let canonical_headers = build_canonical_headers(req, signed_headers);

    // Signed headers string
    let signed_headers_str = signed_headers.join(";");

    // Payload hash — accept UNSIGNED-PAYLOAD and STREAMING-AWS4-HMAC-SHA256-PAYLOAD as-is
    let effective_payload_hash = if payload_hash == "UNSIGNED-PAYLOAD"
        || payload_hash == "STREAMING-AWS4-HMAC-SHA256-PAYLOAD"
    {
        payload_hash.to_string()
    } else {
        payload_hash.to_string()
    };

    format!(
        "{}\n{}\n{}\n{}\n{}\n{}",
        method,
        canonical_uri,
        canonical_query,
        canonical_headers,
        signed_headers_str,
        effective_payload_hash
    )
}

/// Build canonical query string (sorted key=value pairs).
fn build_canonical_query(query: &str) -> String {
    if query.is_empty() {
        return String::new();
    }

    let mut pairs: Vec<(String, String)> = Vec::new();
    for pair in query.split('&') {
        let mut parts = pair.splitn(2, '=');
        let key = parts.next().unwrap_or("");
        let value = parts.next().unwrap_or("");
        pairs.push((key.to_string(), value.to_string()));
    }
    pairs.sort();

    pairs
        .iter()
        .map(|(k, v)| format!("{}={}", k, v))
        .collect::<Vec<_>>()
        .join("&")
}

/// Build canonical headers string.
fn build_canonical_headers(req: &Request<Incoming>, signed_headers: &[String]) -> String {
    let mut header_map: HashMap<String, Vec<String>> = HashMap::new();

    for (name, value) in req.headers() {
        let name_lower = name.as_str().to_lowercase();
        if signed_headers.contains(&name_lower) {
            if let Ok(val) = value.to_str() {
                header_map
                    .entry(name_lower)
                    .or_default()
                    .push(val.trim().to_string());
            }
        }
    }

    let mut result = String::new();
    for header_name in signed_headers {
        let values = header_map
            .get(header_name)
            .map(|v| v.join(","))
            .unwrap_or_default();
        result.push_str(header_name);
        result.push(':');
        result.push_str(&values);
        result.push('\n');
    }
    result
}

/// Derive the signing key via 4-step HMAC chain.
fn derive_signing_key(secret_key: &str, date_stamp: &str, region: &str) -> Vec<u8> {
    let k_secret = format!("AWS4{}", secret_key);
    let k_date = hmac_sha256(k_secret.as_bytes(), date_stamp.as_bytes());
    let k_region = hmac_sha256(&k_date, region.as_bytes());
    let k_service = hmac_sha256(&k_region, b"s3");
    hmac_sha256(&k_service, b"aws4_request")
}

/// Compute HMAC-SHA256.
fn hmac_sha256(key: &[u8], data: &[u8]) -> Vec<u8> {
    let mut mac = HmacSha256::new_from_slice(key).expect("HMAC key length is always valid");
    mac.update(data);
    mac.finalize().into_bytes().to_vec()
}

/// Constant-time byte comparison.
fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
    if a.len() != b.len() {
        return false;
    }
    let mut diff = 0u8;
    for (x, y) in a.iter().zip(b.iter()) {
        diff |= x ^ y;
    }
    diff == 0
}