use clippy_utils::consts::{constant_simple, Constant};
use clippy_utils::diagnostics::span_lint_and_help;
use if_chain::if_chain;
use rustc_hir::{BinOpKind, Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for `0.0 / 0.0`.
    ///
    /// ### Why is this bad?
    /// It's less readable than `f32::NAN` or `f64::NAN`.
    ///
    /// ### Example
    /// ```rust
    /// let nan = 0.0f32 / 0.0;
    /// ```
    ///
    /// Use instead:
    /// ```rust
    /// let nan = f32::NAN;
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub ZERO_DIVIDED_BY_ZERO,
    complexity,
    "usage of `0.0 / 0.0` to obtain NaN instead of `f32::NAN` or `f64::NAN`"
}

declare_lint_pass!(ZeroDiv => [ZERO_DIVIDED_BY_ZERO]);

impl<'tcx> LateLintPass<'tcx> for ZeroDiv {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        // check for instances of 0.0/0.0
        if_chain! {
            if let ExprKind::Binary(ref op, left, right) = expr.kind;
            if op.node == BinOpKind::Div;
            // TODO - constant_simple does not fold many operations involving floats.
            // That's probably fine for this lint - it's pretty unlikely that someone would
            // do something like 0.0/(2.0 - 2.0), but it would be nice to warn on that case too.
            if let Some(lhs_value) = constant_simple(cx, cx.typeck_results(), left);
            if let Some(rhs_value) = constant_simple(cx, cx.typeck_results(), right);
            if Constant::F32(0.0) == lhs_value || Constant::F64(0.0) == lhs_value;
            if Constant::F32(0.0) == rhs_value || Constant::F64(0.0) == rhs_value;
            then {
                // since we're about to suggest a use of f32::NAN or f64::NAN,
                // match the precision of the literals that are given.
                let float_type = match (lhs_value, rhs_value) {
                    (Constant::F64(_), _)
                    | (_, Constant::F64(_)) => "f64",
                    _ => "f32"
                };
                span_lint_and_help(
                    cx,
                    ZERO_DIVIDED_BY_ZERO,
                    expr.span,
                    "constant division of `0.0` with `0.0` will always result in NaN",
                    None,
                    &format!(
                        "consider using `{float_type}::NAN` if you would like a constant representing NaN",
                    ),
                );
            }
        }
    }
}