dlc-manager 0.8.0

//! #PayoutFunction

use std::ops::Deref;

use crate::error::Error;
use bitcoin::Amount;
use dlc::{Payout, RangePayout};
#[cfg(feature = "use-serde")]
use serde::{Deserialize, Serialize};

/// Contains information to compute the set of payouts based on the outcomes.
#[derive(Clone, Debug, PartialEq)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub struct PayoutFunction {
    /// The pieces making up the function.
    pub(crate) payout_function_pieces: Vec<PayoutFunctionPiece>,
}

fn is_continuous(function_pieces: &[PayoutFunctionPiece]) -> bool {
    function_pieces
        .iter()
        .zip(function_pieces.iter().skip(1))
        .all(|(cur, next)| cur.get_last_point() == next.get_first_point())
}

impl PayoutFunction {
    /// Create a new payout function
    pub fn new(function_pieces: Vec<PayoutFunctionPiece>) -> Result<PayoutFunction, Error> {
        if !function_pieces.is_empty() && is_continuous(&function_pieces) {
            Ok(PayoutFunction {
                payout_function_pieces: function_pieces,
            })
        } else {
            Err(Error::InvalidParameters(
                "Function pieces are not continuous.".to_string(),
            ))
        }
    }

    /// Validate that the payout function is continuous and covers the interval [0, max_value]
    pub fn validate(&self, max_value: u64) -> Result<(), Error> {
        if !is_continuous(&self.payout_function_pieces) {
            return Err(Error::InvalidParameters(
                "Payout function is not continuous.".to_string(),
            ));
        }

        let covers = {
            let first = self
                .payout_function_pieces
                .first()
                .expect("to have at least one piece");
            let starts_at_zero = match first {
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(p) => {
                    p.payout_points
                        .first()
                        .expect("to have at least a point")
                        .event_outcome
                        == 0
                }
                PayoutFunctionPiece::HyperbolaPayoutCurvePiece(h) => {
                    h.left_end_point.event_outcome == 0
                }
            };

            let last = self
                .payout_function_pieces
                .last()
                .expect("to have at least one piece");
            let finishes_at_max = match last {
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(p) => {
                    p.payout_points
                        .last()
                        .expect("to have at least a point")
                        .event_outcome
                        == max_value
                }
                PayoutFunctionPiece::HyperbolaPayoutCurvePiece(h) => {
                    h.right_end_point.event_outcome == max_value
                }
            };

            starts_at_zero && finishes_at_max
        };

        if covers {
            Ok(())
        } else {
            Err(Error::InvalidParameters(
                "Payout function doesn't cover all the possible outcomes.".to_string(),
            ))
        }
    }

    /// Generate the range payouts from the function.
    pub fn to_range_payouts(
        &self,
        total_collateral: Amount,
        rounding_intervals: &RoundingIntervals,
    ) -> Result<Vec<RangePayout>, Error> {
        let mut range_payouts = Vec::new();
        for piece in &self.payout_function_pieces {
            piece.to_range_payouts(total_collateral, rounding_intervals, &mut range_payouts)?;
        }
        Ok(range_payouts)
    }
}

/// A piece of a payout function.
#[derive(Clone, Debug, PartialEq)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub enum PayoutFunctionPiece {
    /// A function piece represented by a polynomial.
    PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece),
    /// A function piece represented by an hyperbola.
    HyperbolaPayoutCurvePiece(HyperbolaPayoutCurvePiece),
}

impl PayoutFunctionPiece {
    /// Generate the range payouts for the function piece.
    pub fn to_range_payouts(
        &self,
        total_collateral: Amount,
        rounding_intervals: &RoundingIntervals,
        range_payouts: &mut Vec<RangePayout>,
    ) -> Result<(), Error> {
        match self {
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(p) => {
                p.to_range_payouts(rounding_intervals, total_collateral, range_payouts)
            }
            PayoutFunctionPiece::HyperbolaPayoutCurvePiece(h) => {
                h.to_range_payouts(rounding_intervals, total_collateral, range_payouts)
            }
        }
    }

    fn get_first_point(&self) -> &PayoutPoint {
        match self {
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(p) => &p.payout_points[0],
            PayoutFunctionPiece::HyperbolaPayoutCurvePiece(h) => &h.left_end_point,
        }
    }

    fn get_last_point(&self) -> &PayoutPoint {
        match self {
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(p) => p.payout_points.last().unwrap(),
            PayoutFunctionPiece::HyperbolaPayoutCurvePiece(h) => &h.right_end_point,
        }
    }
}

trait Evaluable {
    fn evaluate(&self, outcome: u64) -> f64;

    fn get_rounded_payout(
        &self,
        outcome: u64,
        rounding_intervals: &RoundingIntervals,
        total_collateral: Amount,
    ) -> Result<Amount, Error> {
        let payout_double = self.evaluate(outcome);
        let total_collateral_sats = total_collateral.to_sat();
        if payout_double.is_sign_negative() || (payout_double != 0.0 && !payout_double.is_normal())
        {
            return Err(Error::InvalidParameters(format!(
                "Could not evaluate function for outcome {}, result was: {}",
                outcome, payout_double
            )));
        }

        if payout_double.round() > total_collateral_sats as f64 {
            return Err(Error::InvalidParameters(
                "Computed payout is greater than total collateral".to_string(),
            ));
        }

        // Ensure that we never round over the total collateral.
        Ok(Amount::from_sat(u64::min(
            rounding_intervals.round(outcome, payout_double),
            total_collateral_sats,
        )))
    }

    fn get_first_outcome(&self) -> u64;

    fn get_last_outcome(&self) -> u64;

    fn get_cur_range(
        &self,
        range_payouts: &mut Vec<RangePayout>,
        total_collateral: Amount,
        rounding_intervals: &RoundingIntervals,
    ) -> Result<RangePayout, Error> {
        let res = match range_payouts.pop() {
            Some(cur) => cur,
            None => {
                let first_outcome = self.get_first_outcome();
                let first_payout =
                    self.get_rounded_payout(first_outcome, rounding_intervals, total_collateral)?;
                RangePayout {
                    start: first_outcome as usize,
                    count: 1,
                    payout: Payout {
                        offer: first_payout,
                        accept: total_collateral - first_payout,
                    },
                }
            }
        };
        Ok(res)
    }

    fn to_range_payouts(
        &self,
        rounding_intervals: &RoundingIntervals,
        total_collateral: Amount,
        range_payouts: &mut Vec<RangePayout>,
    ) -> Result<(), Error> {
        compute_range_payouts(self, rounding_intervals, total_collateral, range_payouts)
    }
}

fn compute_range_payouts<E: Deref>(
    function: E,
    rounding_intervals: &RoundingIntervals,
    total_collateral: Amount,
    range_payouts: &mut Vec<RangePayout>,
) -> Result<(), Error>
where
    E::Target: Evaluable,
{
    let first_outcome = function.get_first_outcome();
    let mut cur_range =
        function.get_cur_range(range_payouts, total_collateral, rounding_intervals)?;

    let range_end = function.get_last_outcome().saturating_add(1);

    for outcome in (first_outcome + 1)..range_end {
        let payout = function.get_rounded_payout(outcome, rounding_intervals, total_collateral)?;
        if payout > total_collateral {
            return Err(Error::InvalidParameters(
                "Computed payout is greater than total collateral.".to_string(),
            ));
        }
        if cur_range.payout.offer == payout {
            cur_range.count += 1;
        } else {
            range_payouts.push(cur_range);
            cur_range = RangePayout {
                start: outcome as usize,
                count: 1,
                payout: Payout {
                    offer: payout,
                    accept: total_collateral - payout,
                },
            };
        }
    }

    range_payouts.push(cur_range);

    Ok(())
}

/// A function piece represented by a polynomial.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub struct PolynomialPayoutCurvePiece {
    /// The set of points to be used to interpolate the polynomial.
    pub(crate) payout_points: Vec<PayoutPoint>,
}

impl PolynomialPayoutCurvePiece {
    /// Create a new PolynomialPayoutCurvePiece
    pub fn new(payout_points: Vec<PayoutPoint>) -> Result<Self, Error> {
        let is_ascending = payout_points.len() > 1
            && payout_points
                .iter()
                .zip(payout_points.iter().skip(1))
                .all(|(cur, next)| cur.event_outcome < next.event_outcome);
        if is_ascending {
            Ok(PolynomialPayoutCurvePiece { payout_points })
        } else {
            Err(Error::InvalidParameters(
                "Payout points must have ascending event outcome value.".to_string(),
            ))
        }
    }
}

impl Evaluable for PolynomialPayoutCurvePiece {
    fn evaluate(&self, outcome: u64) -> f64 {
        let nb_points = self.payout_points.len();

        // Optimizations for constant and linear cases.
        if nb_points == 2 {
            let (left_point, right_point) = (&self.payout_points[0], &self.payout_points[1]);
            let right_point_payout_sats = right_point.outcome_payout.to_sat() as f64;
            let left_point_payout_sats = left_point.outcome_payout.to_sat() as f64;
            return if left_point.outcome_payout == right_point.outcome_payout {
                right_point_payout_sats
            } else {
                let slope = (right_point_payout_sats - left_point_payout_sats)
                    / (right_point.event_outcome - left_point.event_outcome) as f64;
                (outcome - left_point.event_outcome) as f64 * slope + left_point_payout_sats
            };
        }

        let mut result = 0.0;
        let outcome = outcome as f64;

        for i in 0..nb_points {
            let mut l = self.payout_points[i].get_outcome_payout();
            for j in 0..nb_points {
                if i != j {
                    debug_assert!(
                        self.payout_points[i].event_outcome != self.payout_points[j].event_outcome
                    );
                    let i_outcome = self.payout_points[i].event_outcome as f64;
                    let j_outcome = self.payout_points[j].event_outcome as f64;
                    let denominator = i_outcome - j_outcome;
                    let numerator = outcome - j_outcome;
                    l *= numerator / denominator;
                }
            }
            result += l;
        }

        result
    }

    fn get_first_outcome(&self) -> u64 {
        self.payout_points[0].event_outcome
    }

    fn get_last_outcome(&self) -> u64 {
        self.payout_points.last().unwrap().event_outcome
    }

    fn to_range_payouts(
        &self,
        rounding_intervals: &RoundingIntervals,
        total_collateral: Amount,
        range_payouts: &mut Vec<RangePayout>,
    ) -> Result<(), Error> {
        if self.payout_points.len() == 2
            && self.payout_points[0].outcome_payout == self.payout_points[1].outcome_payout
        {
            let mut cur_range =
                self.get_cur_range(range_payouts, total_collateral, rounding_intervals)?;
            cur_range.count += (self.payout_points[1].event_outcome
                - self.payout_points[0].event_outcome) as usize;
            range_payouts.push(cur_range);
            return Ok(());
        }

        compute_range_payouts(self, rounding_intervals, total_collateral, range_payouts)
    }
}

/// A payout point representing a payout for a given outcome.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub struct PayoutPoint {
    /// The event outcome.
    pub event_outcome: u64,
    /// The payout for the outcome.
    pub outcome_payout: Amount,
    /// Extra precision to use when computing the payout.
    pub extra_precision: u16,
}

impl PayoutPoint {
    fn get_outcome_payout(&self) -> f64 {
        (self.outcome_payout.to_sat() as f64) + ((self.extra_precision as f64) / ((1 << 16) as f64))
    }
}

/// A function piece represented by a hyperbola.
#[derive(Clone, Debug, PartialEq)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub struct HyperbolaPayoutCurvePiece {
    /// The left end point of the piece.
    pub(crate) left_end_point: PayoutPoint,
    /// The right end point of the piece.
    pub(crate) right_end_point: PayoutPoint,
    /// Which piece to use in case of ambiguity.
    pub(crate) use_positive_piece: bool,
    /// X coordinate of the translation point.
    pub(crate) translate_outcome: f64,
    /// Y coordinate of the translation point.
    pub(crate) translate_payout: f64,
    /// a value of the transformation matrix.
    pub(crate) a: f64,
    /// b value of the transformation matrix.
    pub(crate) b: f64,
    /// c value of the transformation matrix.
    pub(crate) c: f64,
    /// d value of the transformation matrix.
    pub(crate) d: f64,
}

impl HyperbolaPayoutCurvePiece {
    /// Create a new HyperbolaPayoutCurvePiece
    pub fn new(
        left_end_point: PayoutPoint,
        right_end_point: PayoutPoint,
        use_positive_piece: bool,
        translate_outcome: f64,
        translate_payout: f64,
        a: f64,
        b: f64,
        c: f64,
        d: f64,
    ) -> Result<Self, Error> {
        if a * d == b * c {
            Err(Error::InvalidParameters(
                "a * c cannot equal d * c".to_string(),
            ))
        } else if left_end_point.event_outcome >= right_end_point.event_outcome {
            Err(Error::InvalidParameters(
                "Left end point outcome must be strictly less than right end point outcome"
                    .to_string(),
            ))
        } else {
            Ok(HyperbolaPayoutCurvePiece {
                left_end_point,
                right_end_point,
                use_positive_piece,
                translate_outcome,
                translate_payout,
                a,
                b,
                c,
                d,
            })
        }
    }
}

impl Evaluable for HyperbolaPayoutCurvePiece {
    fn evaluate(&self, outcome: u64) -> f64 {
        let outcome = outcome as f64;
        let translated_outcome = outcome - self.translate_outcome;
        let sqrt_term_abs_val = (translated_outcome.powi(2) - 4.0 * self.a * self.b).sqrt();
        let sqrt_term = if self.use_positive_piece {
            sqrt_term_abs_val
        } else {
            -sqrt_term_abs_val
        };

        let first_term = self.c * (translated_outcome + sqrt_term) / (2.0 * self.a);
        let second_term = 2.0 * self.a * self.d / (translated_outcome + sqrt_term);
        first_term + second_term + self.translate_payout
    }

    fn get_first_outcome(&self) -> u64 {
        self.left_end_point.event_outcome
    }
    fn get_last_outcome(&self) -> u64 {
        self.right_end_point.event_outcome
    }
}

/// Provides information on if and how to round the payouts of a payout function
/// to reduce the number of adaptor signatures required. A `rounding_mod` value
/// of 1 indicates that no rounding is performed.
#[derive(Clone, Debug)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub struct RoundingInterval {
    /// The start of the rounding interval.
    pub begin_interval: u64,
    /// The rounding modulus value.
    pub rounding_mod: u64,
}

/// A set of rounding intervals.
#[derive(Clone, Debug)]
#[cfg_attr(
    feature = "use-serde",
    derive(Serialize, Deserialize),
    serde(rename_all = "camelCase")
)]
pub struct RoundingIntervals {
    /// Contains the rounding intervals.
    pub intervals: Vec<RoundingInterval>,
}

impl RoundingIntervals {
    /// Round the given payout based on the rounding modulus matching the given
    /// outcome.
    pub fn round(&self, outcome: u64, payout: f64) -> u64 {
        let rounding_mod = match self
            .intervals
            .binary_search_by(|x| x.begin_interval.cmp(&outcome))
        {
            Ok(index) => self.intervals[index].rounding_mod,
            Err(index) if index != 0 => self.intervals[index - 1].rounding_mod,
            _ => unreachable!(),
        } as f64;

        let m = if payout >= 0.0 {
            payout % rounding_mod
        } else {
            payout % rounding_mod + rounding_mod
        };

        if m >= rounding_mod / 2.0 {
            (payout + rounding_mod - m).round() as u64
        } else {
            (payout - m).round() as u64
        }
    }

    /// Validate that the instance is well formed, meaning non empty and with the
    /// first interval starting at zero.
    pub fn validate(&self) -> Result<(), Error> {
        if self.intervals.is_empty() {
            return Err(Error::InvalidParameters(
                "Empty rounding intervals.".to_string(),
            ));
        }

        if self.intervals[0].begin_interval != 0 {
            return Err(Error::InvalidParameters(
                "Rounding interval doesn't start at 0.".to_string(),
            ));
        }

        let mut cur = self.intervals[0].begin_interval;

        if self.intervals.iter().skip(1).any(|x| {
            let res = x.begin_interval <= cur;
            cur = x.begin_interval;
            res
        }) {
            return Err(Error::InvalidParameters(
                "Rounding intervals being value are not strictly increasing".to_string(),
            ));
        }

        Ok(())
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use secp256k1_zkp::rand::{thread_rng, RngCore};

    #[test]
    fn lagrange_interpolate_test() {
        let polynomial = PolynomialPayoutCurvePiece {
            payout_points: vec![
                PayoutPoint {
                    event_outcome: 0,
                    outcome_payout: Amount::from_sat(1),
                    extra_precision: 0,
                },
                PayoutPoint {
                    event_outcome: 2,
                    outcome_payout: Amount::from_sat(5),
                    extra_precision: 0,
                },
                PayoutPoint {
                    event_outcome: 4,
                    outcome_payout: Amount::from_sat(17),
                    extra_precision: 0,
                },
            ],
        };

        assert_eq!(101_f64, polynomial.evaluate(10));
        assert_eq!(10001_f64, polynomial.evaluate(100));
    }

    #[test]
    fn polynomial_to_range_outcome_test() {
        struct TestCase {
            payout_points: Vec<PayoutPoint>,
            expected_len: usize,
            expected_first_start: usize,
            expected_first_payout: Amount,
            expected_last_start: usize,
            expected_last_payout: Amount,
            total_collateral: Amount,
        }
        let test_cases: Vec<TestCase> = vec![
            TestCase {
                payout_points: vec![
                    PayoutPoint {
                        event_outcome: 0,
                        outcome_payout: Amount::ZERO,
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 20,
                        outcome_payout: Amount::from_sat(20),
                        extra_precision: 0,
                    },
                ],
                expected_len: 21,
                expected_first_start: 0,
                expected_first_payout: Amount::ZERO,
                expected_last_start: 20,
                expected_last_payout: Amount::from_sat(20),
                total_collateral: Amount::from_sat(20),
            },
            TestCase {
                payout_points: vec![
                    PayoutPoint {
                        event_outcome: 10,
                        outcome_payout: Amount::from_sat(10),
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 20,
                        outcome_payout: Amount::from_sat(10),
                        extra_precision: 0,
                    },
                ],
                expected_len: 1,
                expected_first_start: 10,
                expected_first_payout: Amount::from_sat(10),
                expected_last_start: 10,
                expected_last_payout: Amount::from_sat(10),
                total_collateral: Amount::from_sat(10),
            },
            TestCase {
                payout_points: vec![
                    PayoutPoint {
                        event_outcome: 50000,
                        outcome_payout: Amount::ZERO,
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 1048575,
                        outcome_payout: Amount::ZERO,
                        extra_precision: 0,
                    },
                ],
                expected_len: 1,
                expected_first_start: 50000,
                expected_first_payout: Amount::ZERO,
                expected_last_start: 50000,
                expected_last_payout: Amount::ZERO,
                total_collateral: Amount::from_sat(200000000),
            },
        ];

        for test_case in test_cases {
            let polynomial = PolynomialPayoutCurvePiece {
                payout_points: test_case.payout_points,
            };

            let rounding_intervals = RoundingIntervals {
                intervals: vec![RoundingInterval {
                    begin_interval: 0,
                    rounding_mod: 1,
                }],
            };

            let mut range_payouts = Vec::new();
            polynomial
                .to_range_payouts(
                    &rounding_intervals,
                    test_case.total_collateral,
                    &mut range_payouts,
                )
                .expect("to be able to compute the range payouts");
            let first = range_payouts.first().unwrap();
            let last = range_payouts.last().unwrap();

            assert_eq!(test_case.expected_len, range_payouts.len());
            assert_eq!(test_case.expected_first_start, first.start);
            assert_eq!(test_case.expected_first_payout, first.payout.offer);
            assert_eq!(test_case.expected_last_start, last.start);
            assert_eq!(test_case.expected_last_payout, last.payout.offer);
        }
    }

    #[test]
    fn hyperbola_evaluate_test() {
        let d = (thread_rng().next_u64() as f64) + (thread_rng().next_u64() as f64 / 100.0);
        let translate_payout =
            (thread_rng().next_u64() as f64) + (thread_rng().next_u64() as f64 / 100.0);
        let outcomes: Vec<_> = (0..100).map(|_| thread_rng().next_u64()).collect();
        let expected_payout = |outcome: u64| -> f64 { d / outcome as f64 + translate_payout };

        let hyperbola = HyperbolaPayoutCurvePiece {
            left_end_point: PayoutPoint {
                event_outcome: 1,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            right_end_point: PayoutPoint {
                event_outcome: u64::MAX,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            use_positive_piece: true,
            translate_outcome: 0.0,
            translate_payout,
            a: 1.0,
            b: 0.0,
            c: 0.0,
            d,
        };

        for outcome in outcomes {
            assert_eq!(expected_payout(outcome), hyperbola.evaluate(outcome));
        }
    }

    #[test]
    fn hyperbola_to_range_outcome_invalid_curve_test() {
        let hyperbola = HyperbolaPayoutCurvePiece {
            left_end_point: PayoutPoint {
                event_outcome: 1,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            right_end_point: PayoutPoint {
                event_outcome: 1000,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            use_positive_piece: true,
            translate_outcome: 2.5,
            translate_payout: 0.0,
            a: 1.0,
            b: -1.4,
            c: -0.1,
            d: 10.0,
        };

        hyperbola
            .to_range_payouts(
                &RoundingIntervals {
                    intervals: vec![RoundingInterval {
                        begin_interval: 0,
                        rounding_mod: 1,
                    }],
                },
                Amount::from_sat(200000000),
                &mut Vec::new(),
            )
            .expect_err("Should not tolerate negative payout");
    }

    #[test]
    fn hyperbola_to_range_outcome_test() {
        let hyperbola = HyperbolaPayoutCurvePiece {
            left_end_point: PayoutPoint {
                event_outcome: 1,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            right_end_point: PayoutPoint {
                event_outcome: 1000,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            use_positive_piece: true,
            translate_outcome: 2.5,
            translate_payout: 0.0,
            a: 1.0,
            b: -1.4,
            c: 0.0,
            d: 10.0,
        };

        hyperbola
            .to_range_payouts(
                &RoundingIntervals {
                    intervals: vec![RoundingInterval {
                        begin_interval: 0,
                        rounding_mod: 1,
                    }],
                },
                Amount::from_sat(200000000),
                &mut Vec::new(),
            )
            .expect("to be able to compute the range payouts");
    }

    #[test]
    fn hyperbola_invalid_parameters_tests() {
        HyperbolaPayoutCurvePiece::new(
            PayoutPoint {
                event_outcome: 1,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            PayoutPoint {
                event_outcome: 1000,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            true,
            2.5,
            0.0,
            1.0,
            1.0,
            4.0,
            4.0,
        )
        .expect_err("Should return error when a*d == b*c");
    }

    #[test]
    fn payout_function_to_range_outcome_test() {
        let payout_function = PayoutFunction::new(vec![
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(
                PolynomialPayoutCurvePiece::new(vec![
                    PayoutPoint {
                        event_outcome: 0,
                        outcome_payout: Amount::ZERO,
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 9,
                        outcome_payout: Amount::ZERO,
                        extra_precision: 0,
                    },
                ])
                .unwrap(),
            ),
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(
                PolynomialPayoutCurvePiece::new(vec![
                    PayoutPoint {
                        event_outcome: 9,
                        outcome_payout: Amount::ZERO,
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 10,
                        outcome_payout: Amount::from_sat(9),
                        extra_precision: 0,
                    },
                ])
                .unwrap(),
            ),
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(
                PolynomialPayoutCurvePiece::new(vec![
                    PayoutPoint {
                        event_outcome: 10,
                        outcome_payout: Amount::from_sat(9),
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 19,
                        outcome_payout: Amount::from_sat(9),
                        extra_precision: 0,
                    },
                ])
                .unwrap(),
            ),
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(
                PolynomialPayoutCurvePiece::new(vec![
                    PayoutPoint {
                        event_outcome: 19,
                        outcome_payout: Amount::from_sat(9),
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: 20,
                        outcome_payout: Amount::from_sat(10),
                        extra_precision: 0,
                    },
                ])
                .unwrap(),
            ),
            PayoutFunctionPiece::PolynomialPayoutCurvePiece(
                PolynomialPayoutCurvePiece::new(vec![
                    PayoutPoint {
                        event_outcome: 20,
                        outcome_payout: Amount::from_sat(10),
                        extra_precision: 0,
                    },
                    PayoutPoint {
                        event_outcome: u64::MAX,
                        outcome_payout: Amount::from_sat(10),
                        extra_precision: 0,
                    },
                ])
                .unwrap(),
            ),
        ])
        .unwrap();
        let expected_ranges = vec![
            RangePayout {
                start: 0,
                count: 10,
                payout: Payout {
                    offer: Amount::ZERO,
                    accept: Amount::from_sat(10),
                },
            },
            RangePayout {
                start: 10,
                count: 10,
                payout: Payout {
                    offer: Amount::from_sat(9),
                    accept: Amount::from_sat(1),
                },
            },
            RangePayout {
                start: 20,
                count: (u64::MAX - 19) as usize,
                payout: Payout {
                    offer: Amount::from_sat(10),
                    accept: Amount::ZERO,
                },
            },
        ];
        assert_eq!(
            expected_ranges,
            payout_function
                .to_range_payouts(
                    Amount::from_sat(10),
                    &RoundingIntervals {
                        intervals: vec![RoundingInterval {
                            begin_interval: 0,
                            rounding_mod: 1
                        }]
                    }
                )
                .expect("to be able to compute the range payouts.")
        );
    }

    #[test]
    fn polynomial_payout_curve_validity_test() {
        let invalid = vec![
            // Polynomial curve piece requires more than one
            vec![PayoutPoint {
                event_outcome: 0,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            }],
            // Payout point outcomes should be increasing
            vec![
                PayoutPoint {
                    event_outcome: 10,
                    outcome_payout: Amount::ZERO,
                    extra_precision: 0,
                },
                PayoutPoint {
                    event_outcome: 9,
                    outcome_payout: Amount::ZERO,
                    extra_precision: 0,
                },
            ],
        ];

        for points in invalid {
            PolynomialPayoutCurvePiece::new(points).expect_err("Invalid pieces should error");
        }
    }

    #[test]
    fn hyperbola_validity_test() {
        HyperbolaPayoutCurvePiece::new(
            PayoutPoint {
                event_outcome: 0,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            PayoutPoint {
                event_outcome: 0,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            true,
            0.0,
            0.0,
            1.0,
            2.0,
            3.0,
            4.0,
        )
        .expect_err("Payout point are not increasing should error.");
        HyperbolaPayoutCurvePiece::new(
            PayoutPoint {
                event_outcome: 0,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            PayoutPoint {
                event_outcome: 1,
                outcome_payout: Amount::ZERO,
                extra_precision: 0,
            },
            true,
            0.0,
            0.0,
            1.0,
            2.0,
            1.0,
            2.0,
        )
        .expect_err("a * b == d * c should error.");
    }

    #[test]
    fn payout_function_validity_test() {
        let invalid = vec![
            // Pieces should form a continuous function
            vec![
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece {
                    payout_points: vec![
                        PayoutPoint {
                            event_outcome: 0,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                        PayoutPoint {
                            event_outcome: 9,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                    ],
                }),
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece {
                    payout_points: vec![
                        PayoutPoint {
                            event_outcome: 11,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                        PayoutPoint {
                            event_outcome: 19,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                    ],
                }),
            ],
            vec![
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece {
                    payout_points: vec![
                        PayoutPoint {
                            event_outcome: 0,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                        PayoutPoint {
                            event_outcome: 9,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                    ],
                }),
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece {
                    payout_points: vec![
                        PayoutPoint {
                            event_outcome: 10,
                            outcome_payout: Amount::from_sat(1),
                            extra_precision: 0,
                        },
                        PayoutPoint {
                            event_outcome: 19,
                            outcome_payout: Amount::from_sat(1),
                            extra_precision: 0,
                        },
                    ],
                }),
            ],
        ];

        for pieces in invalid {
            PayoutFunction::new(pieces).expect_err("Invalid pieces should error");
        }
    }

    #[test]
    fn rounding_intervals_valid_is_valid() {
        let rounding_intervals = RoundingIntervals {
            intervals: vec![
                RoundingInterval {
                    begin_interval: 0,
                    rounding_mod: 1,
                },
                RoundingInterval {
                    begin_interval: 10,
                    rounding_mod: 1,
                },
            ],
        };

        rounding_intervals.validate().expect("it to be valid.");
    }

    #[test]
    fn rounding_intervals_empty_is_invalid() {
        let rounding_intervals = RoundingIntervals { intervals: vec![] };

        rounding_intervals
            .validate()
            .expect_err("should not accept empty rounding intervals");
    }

    #[test]
    fn rounding_intervals_does_not_start_at_zero_is_invalid() {
        let rounding_intervals = RoundingIntervals {
            intervals: vec![
                RoundingInterval {
                    begin_interval: 1,
                    rounding_mod: 1,
                },
                RoundingInterval {
                    begin_interval: 10,
                    rounding_mod: 1,
                },
            ],
        };

        rounding_intervals
            .validate()
            .expect_err("should not accept rounding interval not starting at zero");
    }

    #[test]
    fn rounding_intervals_not_strictly_increasing_is_invalid() {
        let rounding_intervals = RoundingIntervals {
            intervals: vec![
                RoundingInterval {
                    begin_interval: 0,
                    rounding_mod: 1,
                },
                RoundingInterval {
                    begin_interval: 10,
                    rounding_mod: 1,
                },
                RoundingInterval {
                    begin_interval: 10,
                    rounding_mod: 1,
                },
            ],
        };

        rounding_intervals
            .validate()
            .expect_err("should not accept rounding intervals not strictly increasing");
    }

    #[test]
    fn should_not_round_above_total_collateral() {
        let rounding_intervals = RoundingIntervals {
            intervals: vec![RoundingInterval {
                begin_interval: 0,
                rounding_mod: 25,
            }],
        };

        let payout_function = PayoutFunction {
            payout_function_pieces: vec![
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece {
                    payout_points: vec![
                        PayoutPoint {
                            event_outcome: 0,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                        PayoutPoint {
                            event_outcome: 500,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                    ],
                }),
                PayoutFunctionPiece::PolynomialPayoutCurvePiece(PolynomialPayoutCurvePiece {
                    payout_points: vec![
                        PayoutPoint {
                            event_outcome: 500,
                            outcome_payout: Amount::ZERO,
                            extra_precision: 0,
                        },
                        PayoutPoint {
                            event_outcome: 1048575,
                            outcome_payout: Amount::from_sat(7513),
                            extra_precision: 0,
                        },
                    ],
                }),
            ],
        };

        payout_function
            .to_range_payouts(Amount::from_sat(75130), &rounding_intervals)
            .expect("To be able to compute the range payouts");
    }

    #[test]
    fn floating_point_error_doesnt_fail() {
        let function = PayoutFunction::new(vec![PayoutFunctionPiece::PolynomialPayoutCurvePiece(
            PolynomialPayoutCurvePiece::new(vec![
                PayoutPoint {
                    event_outcome: 22352,
                    outcome_payout: Amount::ZERO,
                    extra_precision: 0,
                },
                PayoutPoint {
                    event_outcome: 55881,
                    outcome_payout: Amount::from_sat(87455),
                    extra_precision: 0,
                },
            ])
            .unwrap(),
        )])
        .unwrap();

        let rounding_mod = 1;

        let rounding_intervals = RoundingIntervals {
            intervals: vec![RoundingInterval {
                begin_interval: 0,
                rounding_mod,
            }],
        };

        function
            .to_range_payouts(Amount::from_sat(87455), &rounding_intervals)
            .expect("Not to fail");
    }

    #[test]
    fn monotonic_increasing_payout_curve_is_valid() {
        let polynomial = PolynomialPayoutCurvePiece {
            payout_points: vec![
                PayoutPoint {
                    event_outcome: 0,
                    outcome_payout: Amount::from_sat(1),
                    extra_precision: 0,
                },
                PayoutPoint {
                    event_outcome: 2,
                    outcome_payout: Amount::from_sat(5),
                    extra_precision: 0,
                },
            ],
        };

        assert_eq!(polynomial.evaluate(0), 1.0);
        assert_eq!(polynomial.evaluate(2), 5.0);
    }

    #[test]
    fn monotonic_decreasing_payout_curve_is_valid() {
        let polynomial = PolynomialPayoutCurvePiece {
            payout_points: vec![
                PayoutPoint {
                    event_outcome: 0,
                    outcome_payout: Amount::from_sat(10),
                    extra_precision: 0,
                },
                PayoutPoint {
                    event_outcome: 1,
                    outcome_payout: Amount::from_sat(8),
                    extra_precision: 0,
                },
            ],
        };

        assert_eq!(polynomial.evaluate(0), 10.0);
        assert_eq!(polynomial.evaluate(1), 8.0);
    }
}