//! Calculations and processing of numeric values.

use std::cmp;
use std::cmp::Ordering;

use az::SaturatingAs;
use typst_syntax::{Span, Spanned};
use typst_utils::{round_int_with_precision, round_with_precision};

use crate::diag::{bail, At, HintedString, SourceResult, StrResult};
use crate::foundations::{cast, func, ops, Decimal, IntoValue, Module, Scope, Value};
use crate::layout::{Angle, Fr, Length, Ratio};

/// A module with calculation definitions.
pub fn module() -> Module {
    let mut scope = Scope::new();
    scope.define_func::<abs>();
    scope.define_func::<pow>();
    scope.define_func::<exp>();
    scope.define_func::<sqrt>();
    scope.define_func::<root>();
    scope.define_func::<sin>();
    scope.define_func::<cos>();
    scope.define_func::<tan>();
    scope.define_func::<asin>();
    scope.define_func::<acos>();
    scope.define_func::<atan>();
    scope.define_func::<atan2>();
    scope.define_func::<sinh>();
    scope.define_func::<cosh>();
    scope.define_func::<tanh>();
    scope.define_func::<log>();
    scope.define_func::<ln>();
    scope.define_func::<fact>();
    scope.define_func::<perm>();
    scope.define_func::<binom>();
    scope.define_func::<gcd>();
    scope.define_func::<lcm>();
    scope.define_func::<floor>();
    scope.define_func::<ceil>();
    scope.define_func::<trunc>();
    scope.define_func::<fract>();
    scope.define_func::<round>();
    scope.define_func::<clamp>();
    scope.define_func::<min>();
    scope.define_func::<max>();
    scope.define_func::<even>();
    scope.define_func::<odd>();
    scope.define_func::<rem>();
    scope.define_func::<div_euclid>();
    scope.define_func::<rem_euclid>();
    scope.define_func::<quo>();
    scope.define_func::<norm>();
    scope.define("inf", f64::INFINITY);
    scope.define("pi", std::f64::consts::PI);
    scope.define("tau", std::f64::consts::TAU);
    scope.define("e", std::f64::consts::E);
    Module::new("calc", scope)
}

/// Calculates the absolute value of a numeric value.
///
/// ```example
/// #calc.abs(-5) \
/// #calc.abs(5pt - 2cm) \
/// #calc.abs(2fr) \
/// #calc.abs(decimal("-342.440"))
/// ```
#[func(title = "Absolute")]
pub fn abs(
    /// The value whose absolute value to calculate.
    value: ToAbs,
) -> Value {
    value.0
}

/// A value of which the absolute value can be taken.
pub struct ToAbs(Value);

cast! {
    ToAbs,
    v: i64 => Self(v.abs().into_value()),
    v: f64 => Self(v.abs().into_value()),
    v: Length => Self(Value::Length(v.try_abs()
        .ok_or("cannot take absolute value of this length")?)),
    v: Angle => Self(Value::Angle(v.abs())),
    v: Ratio => Self(Value::Ratio(v.abs())),
    v: Fr => Self(Value::Fraction(v.abs())),
    v: Decimal => Self(Value::Decimal(v.abs()))
}

/// Raises a value to some exponent.
///
/// ```example
/// #calc.pow(2, 3) \
/// #calc.pow(decimal("2.5"), 2)
/// ```
#[func(title = "Power")]
pub fn pow(
    span: Span,
    /// The base of the power.
    ///
    /// If this is a [`decimal`], the exponent can only be an [integer]($int).
    base: DecNum,
    /// The exponent of the power.
    exponent: Spanned<Num>,
) -> SourceResult<DecNum> {
    match exponent.v {
        _ if exponent.v.float() == 0.0 && base.is_zero() => {
            bail!(span, "zero to the power of zero is undefined")
        }
        Num::Int(i) if i32::try_from(i).is_err() => {
            bail!(exponent.span, "exponent is too large")
        }
        Num::Float(f) if !f.is_normal() && f != 0.0 => {
            bail!(exponent.span, "exponent may not be infinite, subnormal, or NaN")
        }
        _ => {}
    };

    match (base, exponent.v) {
        (DecNum::Int(a), Num::Int(b)) if b >= 0 => a
            .checked_pow(b as u32)
            .map(DecNum::Int)
            .ok_or_else(too_large)
            .at(span),
        (DecNum::Decimal(a), Num::Int(b)) => {
            a.checked_powi(b).map(DecNum::Decimal).ok_or_else(too_large).at(span)
        }
        (a, b) => {
            let Some(a) = a.float() else {
                return Err(cant_apply_to_decimal_and_float()).at(span);
            };

            let result = if a == std::f64::consts::E {
                b.float().exp()
            } else if a == 2.0 {
                b.float().exp2()
            } else if let Num::Int(b) = b {
                a.powi(b as i32)
            } else {
                a.powf(b.float())
            };

            if result.is_nan() {
                bail!(span, "the result is not a real number")
            }

            Ok(DecNum::Float(result))
        }
    }
}

/// Raises a value to some exponent of e.
///
/// ```example
/// #calc.exp(1)
/// ```
#[func(title = "Exponential")]
pub fn exp(
    span: Span,
    /// The exponent of the power.
    exponent: Spanned<Num>,
) -> SourceResult<f64> {
    match exponent.v {
        Num::Int(i) if i32::try_from(i).is_err() => {
            bail!(exponent.span, "exponent is too large")
        }
        Num::Float(f) if !f.is_normal() && f != 0.0 => {
            bail!(exponent.span, "exponent may not be infinite, subnormal, or NaN")
        }
        _ => {}
    }

    let result = exponent.v.float().exp();
    if result.is_nan() {
        bail!(span, "the result is not a real number")
    }

    Ok(result)
}

/// Calculates the square root of a number.
///
/// ```example
/// #calc.sqrt(16) \
/// #calc.sqrt(2.5)
/// ```
#[func(title = "Square Root")]
pub fn sqrt(
    /// The number whose square root to calculate. Must be non-negative.
    value: Spanned<Num>,
) -> SourceResult<f64> {
    if value.v.float() < 0.0 {
        bail!(value.span, "cannot take square root of negative number");
    }
    Ok(value.v.float().sqrt())
}

/// Calculates the real nth root of a number.
///
/// If the number is negative, then n must be odd.
///
/// ```example
/// #calc.root(16.0, 4) \
/// #calc.root(27.0, 3)
/// ```
#[func]
pub fn root(
    /// The expression to take the root of
    radicand: f64,
    /// Which root of the radicand to take
    index: Spanned<i64>,
) -> SourceResult<f64> {
    if index.v == 0 {
        bail!(index.span, "cannot take the 0th root of a number");
    } else if radicand < 0.0 {
        if index.v % 2 == 0 {
            bail!(
                index.span,
                "negative numbers do not have a real nth root when n is even"
            );
        } else {
            Ok(-(-radicand).powf(1.0 / index.v as f64))
        }
    } else {
        Ok(radicand.powf(1.0 / index.v as f64))
    }
}

/// Calculates the sine of an angle.
///
/// When called with an integer or a float, they will be interpreted as
/// radians.
///
/// ```example
/// #calc.sin(1.5) \
/// #calc.sin(90deg)
/// ```
#[func(title = "Sine")]
pub fn sin(
    /// The angle whose sine to calculate.
    angle: AngleLike,
) -> f64 {
    match angle {
        AngleLike::Angle(a) => a.sin(),
        AngleLike::Int(n) => (n as f64).sin(),
        AngleLike::Float(n) => n.sin(),
    }
}

/// Calculates the cosine of an angle.
///
/// When called with an integer or a float, they will be interpreted as
/// radians.
///
/// ```example
/// #calc.cos(1.5) \
/// #calc.cos(90deg)
/// ```
#[func(title = "Cosine")]
pub fn cos(
    /// The angle whose cosine to calculate.
    angle: AngleLike,
) -> f64 {
    match angle {
        AngleLike::Angle(a) => a.cos(),
        AngleLike::Int(n) => (n as f64).cos(),
        AngleLike::Float(n) => n.cos(),
    }
}

/// Calculates the tangent of an angle.
///
/// When called with an integer or a float, they will be interpreted as
/// radians.
///
/// ```example
/// #calc.tan(1.5) \
/// #calc.tan(90deg)
/// ```
#[func(title = "Tangent")]
pub fn tan(
    /// The angle whose tangent to calculate.
    angle: AngleLike,
) -> f64 {
    match angle {
        AngleLike::Angle(a) => a.tan(),
        AngleLike::Int(n) => (n as f64).tan(),
        AngleLike::Float(n) => n.tan(),
    }
}

/// Calculates the arcsine of a number.
///
/// ```example
/// #calc.asin(0) \
/// #calc.asin(1)
/// ```
#[func(title = "Arcsine")]
pub fn asin(
    /// The number whose arcsine to calculate. Must be between -1 and 1.
    value: Spanned<Num>,
) -> SourceResult<Angle> {
    let val = value.v.float();
    if val < -1.0 || val > 1.0 {
        bail!(value.span, "value must be between -1 and 1");
    }
    Ok(Angle::rad(val.asin()))
}

/// Calculates the arccosine of a number.
///
/// ```example
/// #calc.acos(0) \
/// #calc.acos(1)
/// ```
#[func(title = "Arccosine")]
pub fn acos(
    /// The number whose arcsine to calculate. Must be between -1 and 1.
    value: Spanned<Num>,
) -> SourceResult<Angle> {
    let val = value.v.float();
    if val < -1.0 || val > 1.0 {
        bail!(value.span, "value must be between -1 and 1");
    }
    Ok(Angle::rad(val.acos()))
}

/// Calculates the arctangent of a number.
///
/// ```example
/// #calc.atan(0) \
/// #calc.atan(1)
/// ```
#[func(title = "Arctangent")]
pub fn atan(
    /// The number whose arctangent to calculate.
    value: Num,
) -> Angle {
    Angle::rad(value.float().atan())
}

/// Calculates the four-quadrant arctangent of a coordinate.
///
/// The arguments are `(x, y)`, not `(y, x)`.
///
/// ```example
/// #calc.atan2(1, 1) \
/// #calc.atan2(-2, -3)
/// ```
#[func(title = "Four-quadrant Arctangent")]
pub fn atan2(
    /// The X coordinate.
    x: Num,
    /// The Y coordinate.
    y: Num,
) -> Angle {
    Angle::rad(f64::atan2(y.float(), x.float()))
}

/// Calculates the hyperbolic sine of a hyperbolic angle.
///
/// ```example
/// #calc.sinh(0) \
/// #calc.sinh(1.5)
/// ```
#[func(title = "Hyperbolic Sine")]
pub fn sinh(
    /// The hyperbolic angle whose hyperbolic sine to calculate.
    value: f64,
) -> f64 {
    value.sinh()
}

/// Calculates the hyperbolic cosine of a hyperbolic angle.
///
/// ```example
/// #calc.cosh(0) \
/// #calc.cosh(1.5)
/// ```
#[func(title = "Hyperbolic Cosine")]
pub fn cosh(
    /// The hyperbolic angle whose hyperbolic cosine to calculate.
    value: f64,
) -> f64 {
    value.cosh()
}

/// Calculates the hyperbolic tangent of an hyperbolic angle.
///
/// ```example
/// #calc.tanh(0) \
/// #calc.tanh(1.5)
/// ```
#[func(title = "Hyperbolic Tangent")]
pub fn tanh(
    /// The hyperbolic angle whose hyperbolic tangent to calculate.
    value: f64,
) -> f64 {
    value.tanh()
}

/// Calculates the logarithm of a number.
///
/// If the base is not specified, the logarithm is calculated in base 10.
///
/// ```example
/// #calc.log(100)
/// ```
#[func(title = "Logarithm")]
pub fn log(
    span: Span,
    /// The number whose logarithm to calculate. Must be strictly positive.
    value: Spanned<Num>,
    /// The base of the logarithm. May not be zero.
    #[named]
    #[default(Spanned::new(10.0, Span::detached()))]
    base: Spanned<f64>,
) -> SourceResult<f64> {
    let number = value.v.float();
    if number <= 0.0 {
        bail!(value.span, "value must be strictly positive")
    }

    if !base.v.is_normal() {
        bail!(base.span, "base may not be zero, NaN, infinite, or subnormal")
    }

    let result = if base.v == std::f64::consts::E {
        number.ln()
    } else if base.v == 2.0 {
        number.log2()
    } else if base.v == 10.0 {
        number.log10()
    } else {
        number.log(base.v)
    };

    if result.is_infinite() || result.is_nan() {
        bail!(span, "the result is not a real number")
    }

    Ok(result)
}

/// Calculates the natural logarithm of a number.
///
/// ```example
/// #calc.ln(calc.e)
/// ```
#[func(title = "Natural Logarithm")]
pub fn ln(
    span: Span,
    /// The number whose logarithm to calculate. Must be strictly positive.
    value: Spanned<Num>,
) -> SourceResult<f64> {
    let number = value.v.float();
    if number <= 0.0 {
        bail!(value.span, "value must be strictly positive")
    }

    let result = number.ln();
    if result.is_infinite() {
        bail!(span, "result close to -inf")
    }

    Ok(result)
}

/// Calculates the factorial of a number.
///
/// ```example
/// #calc.fact(5)
/// ```
#[func(title = "Factorial")]
pub fn fact(
    /// The number whose factorial to calculate. Must be non-negative.
    number: u64,
) -> StrResult<i64> {
    Ok(fact_impl(1, number).ok_or_else(too_large)?)
}

/// Calculates a permutation.
///
/// Returns the `k`-permutation of `n`, or the number of ways to choose `k`
/// items from a set of `n` with regard to order.
///
/// ```example
/// $ "perm"(n, k) &= n!/((n - k)!) \
///   "perm"(5, 3) &= #calc.perm(5, 3) $
/// ```
#[func(title = "Permutation")]
pub fn perm(
    /// The base number. Must be non-negative.
    base: u64,
    /// The number of permutations. Must be non-negative.
    numbers: u64,
) -> StrResult<i64> {
    // By convention.
    if base < numbers {
        return Ok(0);
    }

    Ok(fact_impl(base - numbers + 1, base).ok_or_else(too_large)?)
}

/// Calculates the product of a range of numbers. Used to calculate
/// permutations. Returns None if the result is larger than `i64::MAX`
fn fact_impl(start: u64, end: u64) -> Option<i64> {
    // By convention
    if end + 1 < start {
        return Some(0);
    }

    let real_start: u64 = cmp::max(1, start);
    let mut count: u64 = 1;
    for i in real_start..=end {
        count = count.checked_mul(i)?;
    }

    count.try_into().ok()
}

/// Calculates a binomial coefficient.
///
/// Returns the `k`-combination of `n`, or the number of ways to choose `k`
/// items from a set of `n` without regard to order.
///
/// ```example
/// #calc.binom(10, 5)
/// ```
#[func(title = "Binomial")]
pub fn binom(
    /// The upper coefficient. Must be non-negative.
    n: u64,
    /// The lower coefficient. Must be non-negative.
    k: u64,
) -> StrResult<i64> {
    Ok(binom_impl(n, k).ok_or_else(too_large)?)
}

/// Calculates a binomial coefficient, with `n` the upper coefficient and `k`
/// the lower coefficient. Returns `None` if the result is larger than
/// `i64::MAX`
fn binom_impl(n: u64, k: u64) -> Option<i64> {
    if k > n {
        return Some(0);
    }

    // By symmetry
    let real_k = cmp::min(n - k, k);
    if real_k == 0 {
        return Some(1);
    }

    let mut result: u64 = 1;
    for i in 0..real_k {
        result = result.checked_mul(n - i)?.checked_div(i + 1)?;
    }

    result.try_into().ok()
}

/// Calculates the greatest common divisor of two integers.
///
/// ```example
/// #calc.gcd(7, 42)
/// ```
#[func(title = "Greatest Common Divisor")]
pub fn gcd(
    /// The first integer.
    a: i64,
    /// The second integer.
    b: i64,
) -> i64 {
    let (mut a, mut b) = (a, b);
    while b != 0 {
        let temp = b;
        b = a % b;
        a = temp;
    }

    a.abs()
}

/// Calculates the least common multiple of two integers.
///
/// ```example
/// #calc.lcm(96, 13)
/// ```
#[func(title = "Least Common Multiple")]
pub fn lcm(
    /// The first integer.
    a: i64,
    /// The second integer.
    b: i64,
) -> StrResult<i64> {
    if a == b {
        return Ok(a.abs());
    }

    Ok(a.checked_div(gcd(a, b))
        .and_then(|gcd| gcd.checked_mul(b))
        .map(|v| v.abs())
        .ok_or_else(too_large)?)
}

/// Rounds a number down to the nearest integer.
///
/// If the number is already an integer, it is returned unchanged.
///
/// Note that this function will always return an [integer]($int), and will
/// error if the resulting [`float`] or [`decimal`] is larger than the maximum
/// 64-bit signed integer or smaller than the minimum for that type.
///
/// ```example
/// #calc.floor(500.1)
/// #assert(calc.floor(3) == 3)
/// #assert(calc.floor(3.14) == 3)
/// #assert(calc.floor(decimal("-3.14")) == -4)
/// ```
#[func]
pub fn floor(
    /// The number to round down.
    value: DecNum,
) -> StrResult<i64> {
    match value {
        DecNum::Int(n) => Ok(n),
        DecNum::Float(n) => Ok(crate::foundations::convert_float_to_int(n.floor())
            .map_err(|_| too_large())?),
        DecNum::Decimal(n) => Ok(i64::try_from(n.floor()).map_err(|_| too_large())?),
    }
}

/// Rounds a number up to the nearest integer.
///
/// If the number is already an integer, it is returned unchanged.
///
/// Note that this function will always return an [integer]($int), and will
/// error if the resulting [`float`] or [`decimal`] is larger than the maximum
/// 64-bit signed integer or smaller than the minimum for that type.
///
/// ```example
/// #calc.ceil(500.1)
/// #assert(calc.ceil(3) == 3)
/// #assert(calc.ceil(3.14) == 4)
/// #assert(calc.ceil(decimal("-3.14")) == -3)
/// ```
#[func]
pub fn ceil(
    /// The number to round up.
    value: DecNum,
) -> StrResult<i64> {
    match value {
        DecNum::Int(n) => Ok(n),
        DecNum::Float(n) => Ok(crate::foundations::convert_float_to_int(n.ceil())
            .map_err(|_| too_large())?),
        DecNum::Decimal(n) => Ok(i64::try_from(n.ceil()).map_err(|_| too_large())?),
    }
}

/// Returns the integer part of a number.
///
/// If the number is already an integer, it is returned unchanged.
///
/// Note that this function will always return an [integer]($int), and will
/// error if the resulting [`float`] or [`decimal`] is larger than the maximum
/// 64-bit signed integer or smaller than the minimum for that type.
///
/// ```example
/// #calc.trunc(15.9)
/// #assert(calc.trunc(3) == 3)
/// #assert(calc.trunc(-3.7) == -3)
/// #assert(calc.trunc(decimal("8493.12949582390")) == 8493)
/// ```
#[func(title = "Truncate")]
pub fn trunc(
    /// The number to truncate.
    value: DecNum,
) -> StrResult<i64> {
    match value {
        DecNum::Int(n) => Ok(n),
        DecNum::Float(n) => Ok(crate::foundations::convert_float_to_int(n.trunc())
            .map_err(|_| too_large())?),
        DecNum::Decimal(n) => Ok(i64::try_from(n.trunc()).map_err(|_| too_large())?),
    }
}

/// Returns the fractional part of a number.
///
/// If the number is an integer, returns `0`.
///
/// ```example
/// #calc.fract(-3.1)
/// #assert(calc.fract(3) == 0)
/// #assert(calc.fract(decimal("234.23949211")) == decimal("0.23949211"))
/// ```
#[func(title = "Fractional")]
pub fn fract(
    /// The number to truncate.
    value: DecNum,
) -> DecNum {
    match value {
        DecNum::Int(_) => DecNum::Int(0),
        DecNum::Float(n) => DecNum::Float(n.fract()),
        DecNum::Decimal(n) => DecNum::Decimal(n.fract()),
    }
}

/// Rounds a number to the nearest integer away from zero.
///
/// Optionally, a number of decimal places can be specified.
///
/// If the number of digits is negative, its absolute value will indicate the
/// amount of significant integer digits to remove before the decimal point.
///
/// Note that this function will return the same type as the operand. That is,
/// applying `round` to a [`float`] will return a `float`, and to a [`decimal`],
/// another `decimal`. You may explicitly convert the output of this function to
/// an integer with [`int`], but note that such a conversion will error if the
/// `float` or `decimal` is larger than the maximum 64-bit signed integer or
/// smaller than the minimum integer.
///
/// In addition, this function can error if there is an attempt to round beyond
/// the maximum or minimum integer or `decimal`. If the number is a `float`,
/// such an attempt will cause `{float.inf}` or `{-float.inf}` to be returned
/// for maximum and minimum respectively.
///
/// ```example
/// #calc.round(3.1415, digits: 2)
/// #assert(calc.round(3) == 3)
/// #assert(calc.round(3.14) == 3)
/// #assert(calc.round(3.5) == 4.0)
/// #assert(calc.round(3333.45, digits: -2) == 3300.0)
/// #assert(calc.round(-48953.45, digits: -3) == -49000.0)
/// #assert(calc.round(3333, digits: -2) == 3300)
/// #assert(calc.round(-48953, digits: -3) == -49000)
/// #assert(calc.round(decimal("-6.5")) == decimal("-7"))
/// #assert(calc.round(decimal("7.123456789"), digits: 6) == decimal("7.123457"))
/// #assert(calc.round(decimal("3333.45"), digits: -2) == decimal("3300"))
/// #assert(calc.round(decimal("-48953.45"), digits: -3) == decimal("-49000"))
/// ```
#[func]
pub fn round(
    /// The number to round.
    value: DecNum,
    /// If positive, the number of decimal places.
    ///
    /// If negative, the number of significant integer digits that should be
    /// removed before the decimal point.
    #[named]
    #[default(0)]
    digits: i64,
) -> StrResult<DecNum> {
    match value {
        DecNum::Int(n) => Ok(DecNum::Int(
            round_int_with_precision(n, digits.saturating_as::<i16>())
                .ok_or_else(too_large)?,
        )),
        DecNum::Float(n) => {
            Ok(DecNum::Float(round_with_precision(n, digits.saturating_as::<i16>())))
        }
        DecNum::Decimal(n) => Ok(DecNum::Decimal(
            n.round(digits.saturating_as::<i32>()).ok_or_else(too_large)?,
        )),
    }
}

/// Clamps a number between a minimum and maximum value.
///
/// ```example
/// #calc.clamp(5, 0, 4)
/// #assert(calc.clamp(5, 0, 10) == 5)
/// #assert(calc.clamp(5, 6, 10) == 6)
/// #assert(calc.clamp(decimal("5.45"), 2, decimal("45.9")) == decimal("5.45"))
/// #assert(calc.clamp(decimal("5.45"), decimal("6.75"), 12) == decimal("6.75"))
/// ```
#[func]
pub fn clamp(
    span: Span,
    /// The number to clamp.
    value: DecNum,
    /// The inclusive minimum value.
    min: DecNum,
    /// The inclusive maximum value.
    max: Spanned<DecNum>,
) -> SourceResult<DecNum> {
    // Ignore if there are incompatible types (decimal and float) since that
    // will cause `apply3` below to error before calling clamp, avoiding a
    // panic.
    if min
        .apply2(max.v, |min, max| max < min, |min, max| max < min, |min, max| max < min)
        .unwrap_or(false)
    {
        bail!(max.span, "max must be greater than or equal to min")
    }

    value
        .apply3(min, max.v, i64::clamp, f64::clamp, Decimal::clamp)
        .ok_or_else(cant_apply_to_decimal_and_float)
        .at(span)
}

/// Determines the minimum of a sequence of values.
///
/// ```example
/// #calc.min(1, -3, -5, 20, 3, 6) \
/// #calc.min("typst", "is", "cool")
/// ```
#[func(title = "Minimum")]
pub fn min(
    span: Span,
    /// The sequence of values from which to extract the minimum.
    /// Must not be empty.
    #[variadic]
    values: Vec<Spanned<Value>>,
) -> SourceResult<Value> {
    minmax(span, values, Ordering::Less)
}

/// Determines the maximum of a sequence of values.
///
/// ```example
/// #calc.max(1, -3, -5, 20, 3, 6) \
/// #calc.max("typst", "is", "cool")
/// ```
#[func(title = "Maximum")]
pub fn max(
    span: Span,
    /// The sequence of values from which to extract the maximum.
    /// Must not be empty.
    #[variadic]
    values: Vec<Spanned<Value>>,
) -> SourceResult<Value> {
    minmax(span, values, Ordering::Greater)
}

/// Find the minimum or maximum of a sequence of values.
fn minmax(
    span: Span,
    values: Vec<Spanned<Value>>,
    goal: Ordering,
) -> SourceResult<Value> {
    let mut iter = values.into_iter();
    let Some(Spanned { v: mut extremum, .. }) = iter.next() else {
        bail!(span, "expected at least one value");
    };

    for Spanned { v, span } in iter {
        let ordering = ops::compare(&v, &extremum).at(span)?;
        if ordering == goal {
            extremum = v;
        }
    }

    Ok(extremum)
}

/// Determines whether an integer is even.
///
/// ```example
/// #calc.even(4) \
/// #calc.even(5) \
/// #range(10).filter(calc.even)
/// ```
#[func]
pub fn even(
    /// The number to check for evenness.
    value: i64,
) -> bool {
    value % 2 == 0
}

/// Determines whether an integer is odd.
///
/// ```example
/// #calc.odd(4) \
/// #calc.odd(5) \
/// #range(10).filter(calc.odd)
/// ```
#[func]
pub fn odd(
    /// The number to check for oddness.
    value: i64,
) -> bool {
    value % 2 != 0
}

/// Calculates the remainder of two numbers.
///
/// The value `calc.rem(x, y)` always has the same sign as `x`, and is smaller
/// in magnitude than `y`.
///
/// This can error if given a [`decimal`] input and the dividend is too small in
/// magnitude compared to the divisor.
///
/// ```example
/// #calc.rem(7, 3) \
/// #calc.rem(7, -3) \
/// #calc.rem(-7, 3) \
/// #calc.rem(-7, -3) \
/// #calc.rem(1.75, 0.5)
/// ```
#[func(title = "Remainder")]
pub fn rem(
    span: Span,
    /// The dividend of the remainder.
    dividend: DecNum,
    /// The divisor of the remainder.
    divisor: Spanned<DecNum>,
) -> SourceResult<DecNum> {
    if divisor.v.is_zero() {
        bail!(divisor.span, "divisor must not be zero");
    }

    dividend
        .apply2(
            divisor.v,
            |a, b| Some(DecNum::Int(a % b)),
            |a, b| Some(DecNum::Float(a % b)),
            |a, b| a.checked_rem(b).map(DecNum::Decimal),
        )
        .ok_or_else(cant_apply_to_decimal_and_float)
        .at(span)?
        .ok_or("dividend too small compared to divisor")
        .at(span)
}

/// Performs euclidean division of two numbers.
///
/// The result of this computation is that of a division rounded to the integer
/// `{n}` such that the dividend is greater than or equal to `{n}` times the divisor.
///
/// ```example
/// #calc.div-euclid(7, 3) \
/// #calc.div-euclid(7, -3) \
/// #calc.div-euclid(-7, 3) \
/// #calc.div-euclid(-7, -3) \
/// #calc.div-euclid(1.75, 0.5) \
/// #calc.div-euclid(decimal("1.75"), decimal("0.5"))
/// ```
#[func(title = "Euclidean Division")]
pub fn div_euclid(
    span: Span,
    /// The dividend of the division.
    dividend: DecNum,
    /// The divisor of the division.
    divisor: Spanned<DecNum>,
) -> SourceResult<DecNum> {
    if divisor.v.is_zero() {
        bail!(divisor.span, "divisor must not be zero");
    }

    dividend
        .apply2(
            divisor.v,
            |a, b| Some(DecNum::Int(a.div_euclid(b))),
            |a, b| Some(DecNum::Float(a.div_euclid(b))),
            |a, b| a.checked_div_euclid(b).map(DecNum::Decimal),
        )
        .ok_or_else(cant_apply_to_decimal_and_float)
        .at(span)?
        .ok_or_else(too_large)
        .at(span)
}

/// This calculates the least nonnegative remainder of a division.
///
/// Warning: Due to a floating point round-off error, the remainder may equal
/// the absolute value of the divisor if the dividend is much smaller in
/// magnitude than the divisor and the dividend is negative. This only applies
/// for floating point inputs.
///
/// In addition, this can error if given a [`decimal`] input and the dividend is
/// too small in magnitude compared to the divisor.
///
/// ```example
/// #calc.rem-euclid(7, 3) \
/// #calc.rem-euclid(7, -3) \
/// #calc.rem-euclid(-7, 3) \
/// #calc.rem-euclid(-7, -3) \
/// #calc.rem-euclid(1.75, 0.5) \
/// #calc.rem-euclid(decimal("1.75"), decimal("0.5"))
/// ```
#[func(title = "Euclidean Remainder", keywords = ["modulo", "modulus"])]
pub fn rem_euclid(
    span: Span,
    /// The dividend of the remainder.
    dividend: DecNum,
    /// The divisor of the remainder.
    divisor: Spanned<DecNum>,
) -> SourceResult<DecNum> {
    if divisor.v.is_zero() {
        bail!(divisor.span, "divisor must not be zero");
    }

    dividend
        .apply2(
            divisor.v,
            |a, b| Some(DecNum::Int(a.rem_euclid(b))),
            |a, b| Some(DecNum::Float(a.rem_euclid(b))),
            |a, b| a.checked_rem_euclid(b).map(DecNum::Decimal),
        )
        .ok_or_else(cant_apply_to_decimal_and_float)
        .at(span)?
        .ok_or("dividend too small compared to divisor")
        .at(span)
}

/// Calculates the quotient (floored division) of two numbers.
///
/// Note that this function will always return an [integer]($int), and will
/// error if the resulting [`float`] or [`decimal`] is larger than the maximum
/// 64-bit signed integer or smaller than the minimum for that type.
///
/// ```example
/// $ "quo"(a, b) &= floor(a/b) \
///   "quo"(14, 5) &= #calc.quo(14, 5) \
///   "quo"(3.46, 0.5) &= #calc.quo(3.46, 0.5) $
/// ```
#[func(title = "Quotient")]
pub fn quo(
    span: Span,
    /// The dividend of the quotient.
    dividend: DecNum,
    /// The divisor of the quotient.
    divisor: Spanned<DecNum>,
) -> SourceResult<i64> {
    if divisor.v.is_zero() {
        bail!(divisor.span, "divisor must not be zero");
    }

    let divided = dividend
        .apply2(
            divisor.v,
            |a, b| Some(DecNum::Int(a / b)),
            |a, b| Some(DecNum::Float(a / b)),
            |a, b| a.checked_div(b).map(DecNum::Decimal),
        )
        .ok_or_else(cant_apply_to_decimal_and_float)
        .at(span)?
        .ok_or_else(too_large)
        .at(span)?;

    floor(divided).at(span)
}

/// Calculates the p-norm of a sequence of values.
///
/// ```example
/// #calc.norm(1, 2, -3, 0.5) \
/// #calc.norm(p: 3, 1, 2)
/// ```
#[func(title = "𝑝-Norm")]
pub fn norm(
    /// The p value to calculate the p-norm of.
    #[named]
    #[default(Spanned::new(2.0, Span::detached()))]
    p: Spanned<f64>,
    /// The sequence of values from which to calculate the p-norm.
    /// Returns `0.0` if empty.
    #[variadic]
    values: Vec<f64>,
) -> SourceResult<f64> {
    if p.v <= 0.0 {
        bail!(p.span, "p must be greater than zero");
    }

    // Create an iterator over the absolute values.
    let abs = values.into_iter().map(f64::abs);

    Ok(if p.v.is_infinite() {
        // When p is infinity, the p-norm is the maximum of the absolute values.
        abs.max_by(|a, b| a.total_cmp(b)).unwrap_or(0.0)
    } else {
        abs.map(|v| v.powf(p.v)).sum::<f64>().powf(1.0 / p.v)
    })
}

/// A value which can be passed to functions that work with integers and floats.
#[derive(Debug, Copy, Clone)]
pub enum Num {
    Int(i64),
    Float(f64),
}

impl Num {
    fn float(self) -> f64 {
        match self {
            Self::Int(v) => v as f64,
            Self::Float(v) => v,
        }
    }
}

cast! {
    Num,
    self => match self {
        Self::Int(v) => v.into_value(),
        Self::Float(v) => v.into_value(),
    },
    v: i64 => Self::Int(v),
    v: f64 => Self::Float(v),
}

/// A value which can be passed to functions that work with integers, floats,
/// and decimals.
#[derive(Debug, Copy, Clone)]
pub enum DecNum {
    Int(i64),
    Float(f64),
    Decimal(Decimal),
}

impl DecNum {
    /// Checks if this number is equivalent to zero.
    fn is_zero(self) -> bool {
        match self {
            Self::Int(i) => i == 0,
            Self::Float(f) => f == 0.0,
            Self::Decimal(d) => d.is_zero(),
        }
    }

    /// If this `DecNum` holds an integer or float, returns a float.
    /// Otherwise, returns `None`.
    fn float(self) -> Option<f64> {
        match self {
            Self::Int(i) => Some(i as f64),
            Self::Float(f) => Some(f),
            Self::Decimal(_) => None,
        }
    }

    /// If this `DecNum` holds an integer or decimal, returns a decimal.
    /// Otherwise, returns `None`.
    fn decimal(self) -> Option<Decimal> {
        match self {
            Self::Int(i) => Some(Decimal::from(i)),
            Self::Float(_) => None,
            Self::Decimal(d) => Some(d),
        }
    }

    /// Tries to apply a function to two decimal or numeric arguments.
    ///
    /// Fails with `None` if one is a float and the other is a decimal.
    fn apply2<T>(
        self,
        other: Self,
        int: impl FnOnce(i64, i64) -> T,
        float: impl FnOnce(f64, f64) -> T,
        decimal: impl FnOnce(Decimal, Decimal) -> T,
    ) -> Option<T> {
        match (self, other) {
            (Self::Int(a), Self::Int(b)) => Some(int(a, b)),
            (Self::Decimal(a), Self::Decimal(b)) => Some(decimal(a, b)),
            (Self::Decimal(a), Self::Int(b)) => Some(decimal(a, Decimal::from(b))),
            (Self::Int(a), Self::Decimal(b)) => Some(decimal(Decimal::from(a), b)),
            (a, b) => Some(float(a.float()?, b.float()?)),
        }
    }

    /// Tries to apply a function to three decimal or numeric arguments.
    ///
    /// Fails with `None` if one is a float and the other is a decimal.
    fn apply3(
        self,
        other: Self,
        third: Self,
        int: impl FnOnce(i64, i64, i64) -> i64,
        float: impl FnOnce(f64, f64, f64) -> f64,
        decimal: impl FnOnce(Decimal, Decimal, Decimal) -> Decimal,
    ) -> Option<Self> {
        match (self, other, third) {
            (Self::Int(a), Self::Int(b), Self::Int(c)) => Some(Self::Int(int(a, b, c))),
            (Self::Decimal(a), b, c) => {
                Some(Self::Decimal(decimal(a, b.decimal()?, c.decimal()?)))
            }
            (a, Self::Decimal(b), c) => {
                Some(Self::Decimal(decimal(a.decimal()?, b, c.decimal()?)))
            }
            (a, b, Self::Decimal(c)) => {
                Some(Self::Decimal(decimal(a.decimal()?, b.decimal()?, c)))
            }
            (a, b, c) => Some(Self::Float(float(a.float()?, b.float()?, c.float()?))),
        }
    }
}

cast! {
    DecNum,
    self => match self {
        Self::Int(v) => v.into_value(),
        Self::Float(v) => v.into_value(),
        Self::Decimal(v) => v.into_value(),
    },
    v: i64 => Self::Int(v),
    v: f64 => Self::Float(v),
    v: Decimal => Self::Decimal(v),
}

/// A value that can be passed to a trigonometric function.
pub enum AngleLike {
    Int(i64),
    Float(f64),
    Angle(Angle),
}

cast! {
    AngleLike,
    v: i64 => Self::Int(v),
    v: f64 => Self::Float(v),
    v: Angle => Self::Angle(v),
}

/// The error message when the result is too large to be represented.
#[cold]
fn too_large() -> &'static str {
    "the result is too large"
}

/// The hinted error message when trying to apply an operation to decimal and
/// float operands.
#[cold]
fn cant_apply_to_decimal_and_float() -> HintedString {
    HintedString::new("cannot apply this operation to a decimal and a float".into())
        .with_hint(
            "if loss of precision is acceptable, explicitly cast the \
             decimal to a float with `float(value)`",
        )
}
