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    //! Compact representation of `U256`

    use bigint::{U256, Uint};

    /// Compact representation of `U256`
    #[derive(Debug, PartialEq, Clone, Copy)]
    pub struct Compact(u32);

    impl From<u32> for Compact {
    	fn from(u: u32) -> Self {
    		Compact(u)
    	}
    }

    impl From<Compact> for u32 {
    	fn from(c: Compact) -> Self {
    		c.0
    	}
    }

    impl From<U256> for Compact {
    	fn from(u: U256) -> Self {
    		Compact::from_u256(u)
    	}
    }

    impl From<Compact> for U256 {
    	fn from(c: Compact) -> Self {
    		// ignore overflows and negative values
    		c.to_u256().unwrap_or_else(|x| x)
    	}
    }

    impl Compact {
    	pub fn new(u: u32) -> Self {
    		Compact(u)
    	}

    	pub fn max_value() -> Self {
    		U256::max_value().into()
    	}

    	/// Computes the target [0, T] that a blockhash must land in to be valid
    	/// Returns value in error, if there is an overflow or its negative value
    	pub fn to_u256(&self) -> Result<U256, U256> {
    		let size = self.0 >> 24;
    		let mut word = self.0 & 0x007fffff;

    		let result = if size <= 3 {
    			word >>= 8 * (3 - size as usize);
    			word.into()
    		} else {
    			U256::from(word) << (8 * (size as usize - 3))
    		};

    		let is_negative = word != 0 && (self.0 & 0x00800000) != 0;
    		let is_overflow = (word != 0 && size > 34) ||
    				(word > 0xff && size > 33) ||
    				(word > 0xffff && size > 32);

    		if is_negative || is_overflow {
    			Err(result)
    		} else {
    			Ok(result)
    		}
    	}

    	pub fn from_u256(val: U256) -> Self {
    		let mut size = (val.bits() + 7) / 8;
    		let mut compact = if size <= 3 {
    			(val.low_u64() << (8 * (3 - size))) as u32
    		} else {
    			let bn = val >> (8 * (size - 3));
    			bn.low_u32()
    		};

    		if (compact & 0x00800000) != 0 {
    			compact >>= 8;
    			size += 1;
    		}

    		assert!((compact & !0x007fffff) == 0);
    		assert!(size < 256);
    		Compact(compact | (size << 24) as u32)
    	}

    	pub fn to_f64(&self) -> f64 {
    		let mut shift = (self.0 >> 24) & 0xff;
    		let mut diff = f64::from(0x0000ffffu32) / f64::from(self.0 & 0x00ffffffu32);
    		while shift < 29 {
    			diff *= f64::from(256);
    			shift += 1;
    		}
    		while shift > 29 {
    			diff /= f64::from(256.0);
    			shift -= 1;
    		}
    		diff
    	}
    }

    #[cfg(test)]
    mod tests {
    	use bigint::{U256, Uint};
    	use super::Compact;

    	#[test]
    	fn test_compact_to_u256() {
    		assert_eq!(Compact::new(0x01003456).to_u256(), Ok(0.into()));
    		assert_eq!(Compact::new(0x01123456).to_u256(), Ok(0x12.into()));
    		assert_eq!(Compact::new(0x02008000).to_u256(), Ok(0x80.into()));
    		assert_eq!(Compact::new(0x05009234).to_u256(), Ok(0x92340000u64.into()));
    		// negative -0x12345600
    		assert!(Compact::new(0x04923456).to_u256().is_err());
    		assert_eq!(Compact::new(0x04123456).to_u256(), Ok(0x12345600u64.into()));
    	}

    	#[test]
    	fn test_from_u256() {
    		let test1 = U256::from(1000u64);
    		assert_eq!(Compact::new(0x0203e800), Compact::from_u256(test1));

    		let test2 = U256::from(2).pow(U256::from(256-32)) - U256::from(1);
    		assert_eq!(Compact::new(0x1d00ffff), Compact::from_u256(test2));
    	}

    	#[test]
    	fn test_compact_to_from_u256() {
    		// TODO: it does not work both ways for small values... check why
    		let compact = Compact::new(0x1d00ffff);
    		let compact2 = Compact::from_u256(compact.to_u256().unwrap());
    		assert_eq!(compact, compact2);

    		let compact = Compact::new(0x05009234);
    		let compact2 = Compact::from_u256(compact.to_u256().unwrap());
    		assert_eq!(compact, compact2);
    	}

    	#[test]
    	fn difficulty() {
    		fn compare_f64(v1: f64, v2: f64) -> bool {
    			(v1 - v2).abs() < 0.00001
    		}

    		assert!(compare_f64(Compact::new(0x1b0404cb).to_f64(), 16307.42094));

    		// tests from original bitcoin client:
    		// https://github.com/bitcoin/bitcoin/blob/1e8f88e071019907785b260477bd359bef6f9a8f/src/test/blockchain_tests.cpp

    		assert!(compare_f64(Compact::new(0x1f111111).to_f64(), 0.000001));
    		assert!(compare_f64(Compact::new(0x1ef88f6f).to_f64(), 0.000016));
    		assert!(compare_f64(Compact::new(0x1df88f6f).to_f64(), 0.004023));
    		assert!(compare_f64(Compact::new(0x1cf88f6f).to_f64(), 1.029916));
    		assert!(compare_f64(Compact::new(0x12345678).to_f64(), 5913134931067755359633408.0));
    	}
    }

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