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use core::fmt::{self, Write};

use crate::{
    path::Path,
    update::Set,
    value::{Num, ValueOrRef},
};

/// Represents a [DynamoDB math operation][1] used as a part of an update expression.
///
/// See also: [`Path::math`]
///
/// [1]: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.SET.IncrementAndDecrement
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Math {
    pub(crate) dst: Path,
    pub(crate) src: Option<Path>,
    op: MathOp,
    pub(crate) num: ValueOrRef,
}

/// A [math operation][1] to modify a field and assign the updated value
/// to another (possibly different) field.
///
/// See also: [`Path::math`]
///
/// [1]: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.SET.IncrementAndDecrement
impl Math {
    pub fn builder<T>(dst: T) -> Builder
    where
        T: Into<Path>,
    {
        Builder {
            dst: dst.into(),
            src: None,
        }
    }

    /// Add an additional action to this `SET` statement.
    ///
    /// ```
    /// use dynamodb_expression::{Num, Path};
    /// # use pretty_assertions::assert_eq;
    ///
    /// let set = Path::new_name("foo").math().add(7)
    ///     .and(Path::new_name("bar").assign("a value"));
    /// assert_eq!(r#"SET foo = foo + 7, bar = "a value""#, set.to_string());
    /// ```
    pub fn and<T>(self, action: T) -> Set
    where
        T: Into<Set>,
    {
        Set::from(self).and(action)
    }
}

impl fmt::Display for Math {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let Self { dst, src, op, num } = self;

        // If no source field is specified, default to using the destination field.
        let src = src.as_ref().unwrap_or(dst);

        write!(f, "{dst} = {src} {op} {num}")
    }
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum MathOp {
    Add,
    Sub,
}

impl fmt::Debug for MathOp {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self, f)
    }
}

impl fmt::Display for MathOp {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_char(match self {
            MathOp::Add => '+',
            MathOp::Sub => '-',
        })
    }
}

/// See: [`Path::math`]
#[must_use = "Consume this `Builder` by using its `.add()` or `.sub()` methods"]
#[derive(Debug, Clone)]
pub struct Builder {
    dst: Path,
    src: Option<Path>,
}

impl Builder {
    /// Sets the source field to read the initial value from.
    /// Defaults to the destination field.
    pub fn src<T>(mut self, src: T) -> Self
    where
        T: Into<Path>,
    {
        self.src = Some(src.into());

        self
    }

    /// Sets addition as the operation to perform.
    #[allow(clippy::should_implement_trait)]
    pub fn add<T>(self, num: T) -> Math
    where
        T: Into<Num>,
    {
        self.with_op(MathOp::Add, num)
    }

    /// Sets subtraction as the operation to perform.
    #[allow(clippy::should_implement_trait)]
    pub fn sub<T>(self, num: T) -> Math
    where
        T: Into<Num>,
    {
        self.with_op(MathOp::Sub, num)
    }

    fn with_op<T>(self, op: MathOp, num: T) -> Math
    where
        T: Into<Num>,
    {
        let Self { dst, src } = self;

        Math {
            dst,
            src,
            op,
            num: num.into().into(),
        }
    }
}