use crate::core::Method;
use crate::core::{Error, PeriodType, ValueType, Window};
use crate::helpers::Peekable;

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

/// Moving [Standard Deviation](https://en.wikipedia.org/wiki/Standard_deviation) over the window of size `length` for timeseries of type [`ValueType`]
///
/// # Parameters
///
/// Has a single parameter `length`: [`PeriodType`]
///
/// `length` should be > `1`
///
/// # Input type
///
/// Input type is [`ValueType`]
///
/// # Output type
///
/// Output type is [`ValueType`]
///
/// # Examples
///
/// ```
/// use yata::prelude::*;
/// use yata::methods::StDev;
///
/// // StDev over the window with length=3
/// let mut stdev = StDev::new(3, &1.0).unwrap();
///
/// stdev.next(&1.0);
/// stdev.next(&2.0);
///
/// assert_eq!(stdev.next(&3.0), 1.0);
/// assert_eq!(stdev.next(&4.0), 1.0);
/// ```
///
/// # Performance
///
/// O(1)
///
/// [`ValueType`]: crate::core::ValueType
/// [`PeriodType`]: crate::core::PeriodType
#[derive(Debug, Clone)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct StDev {
	mean: ValueType,
	val_sum: ValueType,
	sq_val_sum: ValueType,
	divider: ValueType,
	k: ValueType,
	window: Window<ValueType>,
}

impl Method for StDev {
	type Params = PeriodType;
	type Input = ValueType;
	type Output = Self::Input;

	fn new(length: Self::Params, &value: &Self::Input) -> Result<Self, Error> {
		match length {
			0 | 1 => Err(Error::WrongMethodParameters),
			length => {
				let k = ((length - 1) as ValueType).recip();

				let float_length = length as ValueType;
				let mean = -value;
				let divider = -float_length.recip();

				Ok(Self {
					mean,
					val_sum: value * float_length,
					sq_val_sum: value * value * float_length,
					divider,
					k,
					window: Window::new(length, value),
				})
			}
		}
	}

	#[inline]
	fn next(&mut self, &value: &Self::Input) -> Self::Output {
		let prev_value = self.window.push(value);
		let diff = value - prev_value;

		// same as `value * value - prev_value * prev_value`
		self.sq_val_sum += diff * (value + prev_value);

		self.val_sum += diff;
		self.mean += diff * self.divider;

		self.peek()
	}
}

impl Peekable<<Self as Method>::Output> for StDev {
	fn peek(&self) -> <Self as Method>::Output {
		// self.sq_val_sum - self.val_sum * self.mean;
		let sum = self.val_sum.mul_add(self.mean, self.sq_val_sum);

		(sum * self.k)
			.abs() // sometimes float values may produce negative values, when sum is really near to zero value
			.sqrt()
	}
}

#[cfg(test)]
#[allow(clippy::suboptimal_flops)]
mod tests {
	use super::{Method, StDev as TestingMethod};
	use crate::core::ValueType;
	use crate::helpers::{assert_eq_float, RandomCandles};
	use crate::methods::tests::test_const_float;

	#[test]
	fn test_st_dev_const() {
		for i in 2..255 {
			let input = (i as ValueType + 56.0) / 16.3251;
			let mut method = TestingMethod::new(i, &input).unwrap();

			test_const_float(&mut method, &input, 0.0);
		}
	}

	#[test]
	fn test_st_dev() {
		let candles = RandomCandles::default();

		let src: Vec<ValueType> = candles
			.take(300)
			.enumerate()
			.map(|(i, x)| x.close * if i % 2 == 0 { 1.0 } else { -1.0 })
			.collect();

		(2..255).for_each(|ma_length| {
			let mut ma = TestingMethod::new(ma_length, &src[0]).unwrap();
			let ma_length = ma_length as usize;

			src.iter().enumerate().for_each(|(i, x)| {
				let mut avg = 0.;
				for j in 0..ma_length {
					avg += src[i.saturating_sub(j)] / ma_length as ValueType;
				}

				let mut diff_sq_sum = 0.;
				for j in 0..ma_length {
					diff_sq_sum +=
						(src[i.saturating_sub(j)] - avg).powi(2) / (ma_length - 1) as ValueType;
				}

				let value = ma.next(x);
				let value2 = diff_sq_sum.sqrt();

				println!("{value2} <=> {value} at {i} with length {ma_length}");
				assert_eq_float(value2, value);
			});
		});
	}
}