use rustc::lint::*;
use rustc::hir::*;
use utils::span_lint;

/// **What it does:** Detects classic underflow/overflow checks.
///
/// **Why is this bad?** Most classic C underflow/overflow checks will fail in
/// Rust. Users can use functions like `overflowing_*` and `wrapping_*` instead.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// a + b < a
/// ```

declare_lint! {
    pub OVERFLOW_CHECK_CONDITIONAL,
    Warn,
    "overflow checks inspired by C which are likely to panic"
}

#[derive(Copy, Clone)]
pub struct OverflowCheckConditional;

impl LintPass for OverflowCheckConditional {
    fn get_lints(&self) -> LintArray {
        lint_array!(OVERFLOW_CHECK_CONDITIONAL)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for OverflowCheckConditional {
    // a + b < a, a > a + b, a < a - b, a - b > a
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        if_let_chain! {[
            let Expr_::ExprBinary(ref op, ref first, ref second) = expr.node,
            let Expr_::ExprBinary(ref op2, ref ident1, ref ident2) = first.node,
            let Expr_::ExprPath(QPath::Resolved(_, ref path1)) = ident1.node,
            let Expr_::ExprPath(QPath::Resolved(_, ref path2)) = ident2.node,
            let Expr_::ExprPath(QPath::Resolved(_, ref path3)) = second.node,
            &path1.segments[0] == &path3.segments[0] || &path2.segments[0] == &path3.segments[0],
            cx.tables.expr_ty(ident1).is_integral(),
            cx.tables.expr_ty(ident2).is_integral()
        ], {
            if let BinOp_::BiLt = op.node {
                if let BinOp_::BiAdd = op2.node {
                    span_lint(cx, OVERFLOW_CHECK_CONDITIONAL, expr.span,
                        "You are trying to use classic C overflow conditions that will fail in Rust.");
                }
            }
            if let BinOp_::BiGt = op.node {
                if let BinOp_::BiSub = op2.node {
                    span_lint(cx, OVERFLOW_CHECK_CONDITIONAL, expr.span,
                        "You are trying to use classic C underflow conditions that will fail in Rust.");
                }
            }
        }}

        if_let_chain! {[
            let Expr_::ExprBinary(ref op, ref first, ref second) = expr.node,
            let Expr_::ExprBinary(ref op2, ref ident1, ref ident2) = second.node,
            let Expr_::ExprPath(QPath::Resolved(_, ref path1)) = ident1.node,
            let Expr_::ExprPath(QPath::Resolved(_, ref path2)) = ident2.node,
            let Expr_::ExprPath(QPath::Resolved(_, ref path3)) = first.node,
            &path1.segments[0] == &path3.segments[0] || &path2.segments[0] == &path3.segments[0],
            cx.tables.expr_ty(ident1).is_integral(),
            cx.tables.expr_ty(ident2).is_integral()
        ], {
            if let BinOp_::BiGt = op.node {
                if let BinOp_::BiAdd = op2.node {
                    span_lint(cx, OVERFLOW_CHECK_CONDITIONAL, expr.span,
                        "You are trying to use classic C overflow conditions that will fail in Rust.");
                }
            }
            if let BinOp_::BiLt = op.node {
                if let BinOp_::BiSub = op2.node {
                    span_lint(cx, OVERFLOW_CHECK_CONDITIONAL, expr.span,
                        "You are trying to use classic C underflow conditions that will fail in Rust.");
                }
            }
        }}
    }
}