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_chain! {
            if let Expr_::ExprBinary(ref op, ref first, ref second) = expr.node;
            if let Expr_::ExprBinary(ref op2, ref ident1, ref ident2) = first.node;
            if let Expr_::ExprPath(QPath::Resolved(_, ref path1)) = ident1.node;
            if let Expr_::ExprPath(QPath::Resolved(_, ref path2)) = ident2.node;
            if let Expr_::ExprPath(QPath::Resolved(_, ref path3)) = second.node;
            if path1.segments[0] == path3.segments[0] || path2.segments[0] == path3.segments[0];
            if cx.tables.expr_ty(ident1).is_integral();
            if cx.tables.expr_ty(ident2).is_integral();
            then {
                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_chain! {
            if let Expr_::ExprBinary(ref op, ref first, ref second) = expr.node;
            if let Expr_::ExprBinary(ref op2, ref ident1, ref ident2) = second.node;
            if let Expr_::ExprPath(QPath::Resolved(_, ref path1)) = ident1.node;
            if let Expr_::ExprPath(QPath::Resolved(_, ref path2)) = ident2.node;
            if let Expr_::ExprPath(QPath::Resolved(_, ref path3)) = first.node;
            if path1.segments[0] == path3.segments[0] || path2.segments[0] == path3.segments[0];
            if cx.tables.expr_ty(ident1).is_integral();
            if cx.tables.expr_ty(ident2).is_integral();
            then {
                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.");
                    }
                }
            }
        }
    }
}