use aes_gcm::{
    aead::{Aead, AeadCore, KeyInit, OsRng},
    Aes256Gcm, Key, Nonce,
};
use anyhow::{anyhow, Context, Result};
use base64::{engine::general_purpose::STANDARD as BASE64, Engine as _};
use std::env;
use std::sync::OnceLock;

static RAW_KEY: OnceLock<[u8; 32]> = OnceLock::new();

/// Initialize the encryption key from a hex or base64 encoded string.
/// Must be called before any encryption/decryption operations.
/// Returns error if the key is invalid or already initialized with a different key.
pub fn init_with_key(key_str: &str) -> Result<()> {
    let key_bytes = hex::decode(key_str)
        .or_else(|_| BASE64.decode(key_str))
        .context("Encryption key must be hex or base64 encoded")?;
    if key_bytes.len() != 32 {
        anyhow::bail!(
            "Encryption key must be 32 bytes (256 bits), got {} bytes",
            key_bytes.len()
        );
    }
    let key_array: [u8; 32] = key_bytes.try_into().unwrap(); // safe due to length check
                                                             // Check if already initialized with same key
    if let Some(existing) = RAW_KEY.get() {
        if existing == &key_array {
            // Same key already initialized, okay
            return Ok(());
        } else {
            anyhow::bail!("Encryption key already initialized with a different key");
        }
    }
    // Store raw key bytes
    RAW_KEY
        .set(key_array)
        .map_err(|_| anyhow::anyhow!("Encryption key already initialized"))?;
    Ok(())
}

/// Initialize the encryption key from the environment variable `LLM_PROXY__ENCRYPTION_KEY`.
/// Must be called before any encryption/decryption operations.
/// Panics if the environment variable is missing or invalid.
pub fn init_from_env() -> Result<()> {
    let key_str =
        env::var("LLM_PROXY__ENCRYPTION_KEY").context("LLM_PROXY__ENCRYPTION_KEY environment variable not set")?;
    init_with_key(&key_str)
}

/// Get the encryption key bytes, panicking if not initialized.
fn get_key() -> &'static [u8; 32] {
    RAW_KEY
        .get()
        .expect("Encryption key not initialized. Call crypto::init_with_key() or crypto::init_from_env() first.")
}

/// Encrypt a plaintext string and return a base64-encoded ciphertext (nonce || ciphertext || tag).
pub fn encrypt(plaintext: &str) -> Result<String> {
    let key = Key::<Aes256Gcm>::from_slice(get_key());
    let cipher = Aes256Gcm::new(key);
    let nonce = Aes256Gcm::generate_nonce(&mut OsRng); // 12 bytes
    let ciphertext = cipher
        .encrypt(&nonce, plaintext.as_bytes())
        .map_err(|e| anyhow!("Encryption failed: {}", e))?;
    // Combine nonce and ciphertext (ciphertext already includes tag)
    let mut combined = Vec::with_capacity(nonce.len() + ciphertext.len());
    combined.extend_from_slice(&nonce);
    combined.extend_from_slice(&ciphertext);
    Ok(BASE64.encode(combined))
}

/// Decrypt a base64-encoded ciphertext (nonce || ciphertext || tag) to a plaintext string.
pub fn decrypt(ciphertext_b64: &str) -> Result<String> {
    let key = Key::<Aes256Gcm>::from_slice(get_key());
    let cipher = Aes256Gcm::new(key);
    let combined = BASE64.decode(ciphertext_b64).context("Invalid base64 ciphertext")?;
    if combined.len() < 12 {
        anyhow::bail!("Ciphertext too short");
    }
    let (nonce_bytes, ciphertext_and_tag) = combined.split_at(12);
    let nonce = Nonce::from_slice(nonce_bytes);
    let plaintext_bytes = cipher
        .decrypt(nonce, ciphertext_and_tag)
        .map_err(|e| anyhow!("Decryption failed (invalid key or corrupted ciphertext): {}", e))?;
    String::from_utf8(plaintext_bytes).context("Decrypted bytes are not valid UTF-8")
}

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

    const TEST_KEY_HEX: &str = "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f";

    #[test]
    fn test_encrypt_decrypt() {
        init_with_key(TEST_KEY_HEX).unwrap();
        let plaintext = "super secret api key";
        let ciphertext = encrypt(plaintext).unwrap();
        assert_ne!(ciphertext, plaintext);
        let decrypted = decrypt(&ciphertext).unwrap();
        assert_eq!(decrypted, plaintext);
    }

    #[test]
    fn test_different_inputs_produce_different_ciphertexts() {
        init_with_key(TEST_KEY_HEX).unwrap();
        let plaintext = "same";
        let cipher1 = encrypt(plaintext).unwrap();
        let cipher2 = encrypt(plaintext).unwrap();
        assert_ne!(cipher1, cipher2, "Nonce should make ciphertexts differ");
        assert_eq!(decrypt(&cipher1).unwrap(), plaintext);
        assert_eq!(decrypt(&cipher2).unwrap(), plaintext);
    }

    #[test]
    fn test_invalid_key_length() {
        let result = init_with_key("tooshort");
        assert!(result.is_err());
    }

    #[test]
    fn test_init_from_env() {
        unsafe { std::env::set_var("LLM_PROXY__ENCRYPTION_KEY", TEST_KEY_HEX) };
        let result = init_from_env();
        assert!(result.is_ok());
        // Ensure encryption works
        let ciphertext = encrypt("test").unwrap();
        let decrypted = decrypt(&ciphertext).unwrap();
        assert_eq!(decrypted, "test");
    }

    #[test]
    fn test_missing_env_key() {
        unsafe { std::env::remove_var("LLM_PROXY__ENCRYPTION_KEY") };
        let result = init_from_env();
        assert!(result.is_err());
    }

    #[test]
    fn test_key_hex_and_base64() {
        // Hex key works
        init_with_key(TEST_KEY_HEX).unwrap();
        // Base64 key (same bytes encoded as base64)
        let base64_key = BASE64.encode(hex::decode(TEST_KEY_HEX).unwrap());
        // Re-initialization with same key (different encoding) is allowed
        let result = init_with_key(&base64_key);
        assert!(result.is_ok());
        // Encryption should still work
        let ciphertext = encrypt("test").unwrap();
        let decrypted = decrypt(&ciphertext).unwrap();
        assert_eq!(decrypted, "test");
    }

    #[test]
    #[ignore] // conflicts with global state from other tests
    fn test_already_initialized() {
        init_with_key(TEST_KEY_HEX).unwrap();
        let result = init_with_key(TEST_KEY_HEX);
        assert!(result.is_ok()); // same key allowed
    }

    #[test]
    #[ignore] // conflicts with global state from other tests
    fn test_already_initialized_different_key() {
        init_with_key(TEST_KEY_HEX).unwrap();
        let different_key = "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e20";
        let result = init_with_key(different_key);
        assert!(result.is_err());
    }
}