use keys::{Public, Signature};
use script::{script, Script, Num, VerificationFlags, Opcode, Error, Instruction};

#[derive(Debug, PartialEq, Clone, Copy)]
#[repr(u8)]
pub enum SignatureHash {
	All = 1,
	None = 2,
	Single = 3,
	AnyoneCanPay = 0x80,
}

#[derive(Debug, PartialEq, Clone, Copy)]
pub enum SignatureVersion {
	_Base,
	_WitnessV0,
}

pub trait SignatureChecker {
	fn check_signature(&self, script_signature: &[u8], public: &Public, script: &Script, version: SignatureVersion);

	fn check_lock_time(&self, lock_time: Num);

	fn check_sequence(&self, sequence: Num);
}

fn is_public_key(v: &[u8]) -> bool {
	match v.len() {
		33 if v[0] == 2 || v[0] == 3 => true,
		65 if v[0] == 4 => true,
		_ => false,
	}
}

/// A canonical signature exists of: <30> <total len> <02> <len R> <R> <02> <len S> <S> <hashtype>
/// Where R and S are not negative (their first byte has its highest bit not set), and not
/// excessively padded (do not start with a 0 byte, unless an otherwise negative number follows,
/// in which case a single 0 byte is necessary and even required).
///
/// See https://bitcointalk.org/index.php?topic=8392.msg127623#msg127623
///
/// This function is consensus-critical since BIP66.
fn is_valid_signature_encoding(sig: &[u8]) -> bool {
	// Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S] [sighash]
	// * total-length: 1-byte length descriptor of everything that follows,
	//   excluding the sighash byte.
	// * R-length: 1-byte length descriptor of the R value that follows.
	// * R: arbitrary-length big-endian encoded R value. It must use the shortest
	//   possible encoding for a positive integers (which means no null bytes at
	//   the start, except a single one when the next byte has its highest bit set).
	// * S-length: 1-byte length descriptor of the S value that follows.
	// * S: arbitrary-length big-endian encoded S value. The same rules apply.
	// * sighash: 1-byte value indicating what data is hashed (not part of the DER
	//   signature)

	// Minimum and maximum size constraints
	if sig.len() < 9 || sig.len() > 73 {
		return false;
	}

	// A signature is of type 0x30 (compound)
	if sig[0] != 0x30 {
		return false;
	}

	// Make sure the length covers the entire signature.
	if sig[1] as usize != sig.len() - 3 {
		return false;
	}

	// Extract the length of the R element.
	let len_r = sig[3] as usize;

	// Make sure the length of the S element is still inside the signature.
	if len_r + 5 >= sig.len() {
		return false;
	}

	// Extract the length of the S element.
	let len_s = sig[len_r + 5] as usize;

	// Verify that the length of the signature matches the sum of the length
	if len_r + len_s + 7 != sig.len() {
		return false;
	}

	// Check whether the R element is an integer.
	if sig[2] != 2 {
		return false;
	}

	// Zero-length integers are not allowed for R.
	if len_r == 0 {
		return false;
	}

	// Negative numbers are not allowed for R.
	if (sig[4] & 0x80) != 0 {
		return false;
	}

	// Null bytes at the start of R are not allowed, unless R would
	// otherwise be interpreted as a negative number.
	if len_r > 1 && sig[4] == 0 && (!(sig[5] & 0x80)) != 0 {
		return false;
	}

	// Check whether the S element is an integer.
	if sig[len_r + 4] != 2 {
		return false;
	}

	// Zero-length integers are not allowed for S.
	if len_s == 0 {
		return false;
	}

	// Negative numbers are not allowed for S.
	if (sig[len_r + 6] & 0x80) != 0 {
		return false;
	}

	// Null bytes at the start of S are not allowed, unless S would otherwise be
	// interpreted as a negative number.
	if len_s > 1 && (sig[len_r + 6] == 0) && (!(sig[len_r + 7] & 0x80)) != 0 {
		return false;
	}

	true
}

fn is_low_der_signature(sig: &[u8]) -> Result<bool, Error> {
	if !is_valid_signature_encoding(sig) {
		return Err(Error::SignatureDer);
	}

	let signature: Signature = sig.into();
	if !signature.check_low_s() {
		return Err(Error::SignatureHighS);
	}

	Ok(true)
}

fn is_defined_hashtype_signature(sig: &[u8]) -> bool {
	if sig.is_empty() {
		return false;
	}

	let n_hashtype = sig[sig.len() -1] & !(SignatureHash::AnyoneCanPay as u8);
	if n_hashtype < SignatureHash::All as u8 && n_hashtype > SignatureHash::Single as u8 {
		return false
	}
	true
}

fn check_signature_encoding(sig: &[u8], flags: &VerificationFlags) -> Result<bool, Error> {
	// Empty signature. Not strictly DER encoded, but allowed to provide a
	// compact way to provide an invalid signature for use with CHECK(MULTI)SIG

	if sig.is_empty() {
		return Ok(true);
	}

	if (flags.verify_dersig || flags.verify_low_s || flags.verify_strictenc) && !is_valid_signature_encoding(sig) {
		Err(Error::SignatureDer)
	} else if flags.verify_low_s && !try!(is_low_der_signature(sig)) {
		Ok(false)
	} else if flags.verify_strictenc && !is_defined_hashtype_signature(sig) {
		Err(Error::SignatureHashtype)
	} else {
		Ok(true)
	}
}

fn check_pubkey_encoding(v: &[u8], flags: &VerificationFlags) -> Result<bool, Error> {
	if flags.verify_strictenc && !is_public_key(v) {
		return Err(Error::PubkeyType);
	}

	Ok(true)
}

fn check_minimal_push(data: &[u8], opcode: Opcode) -> bool {
	if data.is_empty() {
		// Could have used OP_0.
		opcode == Opcode::OP_0
	} else if data.len() == 1 && data[0] >= 1 && data[0] <= 16 {
		// Could have used OP_1 .. OP_16.
		opcode as u8 == Opcode::OP_1 as u8 + (data[0] - 1)
	} else if data.len() == 1 && data[0] == 0x81 {
		// Could have used OP_1NEGATE
		opcode == Opcode::OP_1NEGATE
	} else if data.len() <= 75 {
		// Could have used a direct push (opcode indicating number of bytes pushed + those bytes).
		opcode as usize == data.len()
	} else if data.len() <= 255 {
		// Could have used OP_PUSHDATA.
		opcode == Opcode::OP_PUSHDATA1
	} else if data.len() <= 65535 {
		// Could have used OP_PUSHDATA2.
		opcode == Opcode::OP_PUSHDATA2
	} else {
		true
	}
}

pub fn eval_script(
	stack: &mut Vec<Vec<u8>>,
	script: &Script,
	flags: &VerificationFlags,
	_checker: &SignatureChecker,
	_version: SignatureVersion
) -> Result<bool, Error> {
	if script.len() > script::MAX_SCRIPT_SIZE {
		return Err(Error::ScriptSize);
	}

	for i in script.into_iter() {
		match try!(i) {
			Instruction::PushValue(_opcode, num) => {
				stack.push(num.to_vec());
			},
			Instruction::PushBytes(opcode, bytes) => {
				// TODO: if fExec
				if flags.verify_minimaldata && !check_minimal_push(bytes, opcode) {
					return Err(Error::Minimaldata);
				}
				stack.push(bytes.to_vec());
			},
			Instruction::Normal(_opcode) => {
			},
		}
	}

	Ok(false)
}

#[cfg(test)]
mod tests {
	use hex::FromHex;
	use super::is_public_key;

	#[test]
	fn tests_is_public_key() {
		assert!(!is_public_key(&[]));
		assert!(!is_public_key(&[1]));
		assert!(is_public_key(&"0495dfb90f202c7d016ef42c65bc010cd26bb8237b06253cc4d12175097bef767ed6b1fcb3caf1ed57c98d92e6cb70278721b952e29a335134857acd4c199b9d2f".from_hex().unwrap()));
		assert!(is_public_key(&[2; 33]));
		assert!(is_public_key(&[3; 33]));
		assert!(!is_public_key(&[4; 33]));
	}
}