use crate::def::Def;
use crate::expr::Expr;
use crate::expr::Recursive;
use roc_collections::all::SendMap;
use roc_module::ident::TagName;
use roc_module::operator::CalledVia;
use roc_module::symbol::Symbol;
use roc_region::all::{Located, Region};
use roc_types::subs::{VarStore, Variable};

/// Some builtins cannot be constructed in code gen alone, and need to be defined
/// as separate Roc defs. For example, List.get has this type:
///
/// List.get : List elem, Int -> Result elem [ OutOfBounds ]*
///
/// Because this returns an open tag union for its Err type, it's not possible
/// for code gen to return a hardcoded value for OutOfBounds. For example,
/// if this Result unifies to [ Foo, OutOfBounds ] then OutOfBOunds will
/// get assigned the number 1 (because Foo got 0 alphabetically), whereas
/// if it unifies to [ OutOfBounds, Qux ] then OutOfBounds will get the number 0.
///
/// Getting these numbers right requires having List.get participate in the
/// normal type-checking and monomorphization processes. As such, this function
/// returns a normal def for List.get, which performs a bounds check and then
/// delegates to the compiler-internal List.getUnsafe function to do the actual
/// lookup (if the bounds check passed). That internal function is hardcoded in code gen,
/// which works fine because it doesn't involve any open tag unions.
pub fn builtin_defs(var_store: &VarStore) -> Vec<Def> {
    vec![
        list_get(var_store),
        list_first(var_store),
        int_div(var_store),
    ]
}

/// List.get : List elem, Int -> Result elem [ OutOfBounds ]*
fn list_get(var_store: &VarStore) -> Def {
    use crate::expr::Expr::*;

    defn(
        Symbol::LIST_GET,
        vec![Symbol::LIST_GET_ARG_LIST, Symbol::LIST_GET_ARG_INDEX],
        var_store,
        // Perform a bounds check. If it passes, delegate to List.#getUnsafe
        If {
            cond_var: var_store.fresh(),
            branch_var: var_store.fresh(),
            branches: vec![(
                // if-condition
                no_region(
                    // index < List.len list
                    call(
                        Symbol::NUM_LT,
                        vec![
                            Var(Symbol::LIST_GET_ARG_INDEX),
                            call(
                                Symbol::LIST_LEN,
                                vec![Var(Symbol::LIST_GET_ARG_LIST)],
                                var_store,
                            ),
                        ],
                        var_store,
                    ),
                ),
                // then-branch
                no_region(
                    // Ok
                    tag(
                        "Ok",
                        vec![
                            // List.getUnsafe list index
                            Call(
                                Box::new((
                                    var_store.fresh(),
                                    no_region(Var(Symbol::LIST_GET_UNSAFE)),
                                    var_store.fresh(),
                                )),
                                vec![
                                    (var_store.fresh(), no_region(Var(Symbol::LIST_GET_ARG_LIST))),
                                    (
                                        var_store.fresh(),
                                        no_region(Var(Symbol::LIST_GET_ARG_INDEX)),
                                    ),
                                ],
                                CalledVia::Space,
                            ),
                        ],
                        var_store,
                    ),
                ),
            )],
            final_else: Box::new(
                // else-branch
                no_region(
                    // Err
                    tag(
                        "Err",
                        vec![tag("OutOfBounds", Vec::new(), var_store)],
                        var_store,
                    ),
                ),
            ),
        },
    )
}

/// Int.div : Int, Int -> Result Int [ DivByZero ]*
fn int_div(var_store: &VarStore) -> Def {
    use crate::expr::Expr::*;
    use crate::pattern::Pattern::*;

    let args = vec![
        (
            var_store.fresh(),
            no_region(Identifier(Symbol::INT_DIV_ARG_NUMERATOR)),
        ),
        (
            var_store.fresh(),
            no_region(Identifier(Symbol::INT_DIV_ARG_DENOMINATOR)),
        ),
    ];

    let body = If {
        branch_var: var_store.fresh(),
        cond_var: var_store.fresh(),
        branches: vec![(
            // if-condition
            no_region(
                // Int.eq denominator 0
                call(
                    Symbol::INT_NEQ_I64,
                    vec![
                        Var(Symbol::INT_DIV_ARG_DENOMINATOR),
                        (Int(var_store.fresh(), 0)),
                    ],
                    var_store,
                ),
            ),
            // denominator was not zero
            no_region(
                // Ok (Int.#divUnsafe numerator denominator)
                tag(
                    "Ok",
                    vec![
                        // Int.#divUnsafe numerator denominator
                        call(
                            Symbol::INT_DIV_UNSAFE,
                            vec![
                                (Var(Symbol::INT_DIV_ARG_NUMERATOR)),
                                (Var(Symbol::INT_DIV_ARG_DENOMINATOR)),
                            ],
                            var_store,
                        ),
                    ],
                    var_store,
                ),
            ),
        )],
        final_else: Box::new(
            // denominator was zero
            no_region(tag(
                "Err",
                vec![tag("DivByZero", Vec::new(), var_store)],
                var_store,
            )),
        ),
    };

    let expr = Closure(
        var_store.fresh(),
        Symbol::INT_DIV,
        Recursive::NotRecursive,
        args,
        Box::new((no_region(body), var_store.fresh())),
    );

    Def {
        loc_pattern: no_region(Identifier(Symbol::INT_DIV)),
        loc_expr: no_region(expr),
        expr_var: var_store.fresh(),
        pattern_vars: SendMap::default(),
        annotation: None,
    }
}

/// List.first : List elem -> Result elem [ ListWasEmpty ]*
fn list_first(var_store: &VarStore) -> Def {
    use crate::expr::Expr::*;

    defn(
        Symbol::LIST_FIRST,
        vec![Symbol::LIST_FIRST_ARG],
        var_store,
        // Perform a bounds check. If it passes, delegate to List.getUnsafe.
        If {
            // TODO Use "when" instead of "if" so that we can have False be the first branch.
            // We want that for branch prediction; usually we expect the list to be nonempty.
            cond_var: var_store.fresh(),
            branch_var: var_store.fresh(),
            branches: vec![(
                // if-condition
                no_region(
                    // List.isEmpty list
                    call(
                        Symbol::LIST_IS_EMPTY,
                        vec![Var(Symbol::LIST_FIRST_ARG)],
                        var_store,
                    ),
                ),
                // list was empty
                no_region(
                    // Err ListWasEmpty
                    tag(
                        "Err",
                        vec![tag("ListWasEmpty", Vec::new(), var_store)],
                        var_store,
                    ),
                ),
            )],
            final_else: Box::new(
                // list was not empty
                no_region(
                    // Ok (List.#getUnsafe list 0)
                    tag(
                        "Ok",
                        vec![
                            // List.#getUnsafe list 0
                            call(
                                Symbol::LIST_GET_UNSAFE,
                                vec![(Var(Symbol::LIST_FIRST_ARG)), (Int(var_store.fresh(), 0))],
                                var_store,
                            ),
                        ],
                        var_store,
                    ),
                ),
            ),
        },
    )
}

#[inline(always)]
fn no_region<T>(value: T) -> Located<T> {
    Located {
        region: Region::zero(),
        value,
    }
}

#[inline(always)]
fn tag(name: &'static str, args: Vec<Expr>, var_store: &VarStore) -> Expr {
    Expr::Tag {
        variant_var: var_store.fresh(),
        ext_var: var_store.fresh(),
        name: TagName::Global(name.into()),
        arguments: args
            .into_iter()
            .map(|expr| (var_store.fresh(), no_region(expr)))
            .collect::<Vec<(Variable, Located<Expr>)>>(),
    }
}

#[inline(always)]
fn call(symbol: Symbol, args: Vec<Expr>, var_store: &VarStore) -> Expr {
    Expr::Call(
        Box::new((
            var_store.fresh(),
            no_region(Expr::Var(symbol)),
            var_store.fresh(),
        )),
        args.into_iter()
            .map(|expr| (var_store.fresh(), no_region(expr)))
            .collect::<Vec<(Variable, Located<Expr>)>>(),
        CalledVia::Space,
    )
}

#[inline(always)]
fn defn(fn_name: Symbol, args: Vec<Symbol>, var_store: &VarStore, body: Expr) -> Def {
    use crate::expr::Expr::*;
    use crate::pattern::Pattern::*;

    let closure_args = args
        .into_iter()
        .map(|symbol| (var_store.fresh(), no_region(Identifier(symbol))))
        .collect();

    let expr = Closure(
        var_store.fresh(),
        fn_name,
        Recursive::NotRecursive,
        closure_args,
        Box::new((no_region(body), var_store.fresh())),
    );

    Def {
        loc_pattern: no_region(Identifier(fn_name)),
        loc_expr: no_region(expr),
        expr_var: var_store.fresh(),
        pattern_vars: SendMap::default(),
        annotation: None,
    }
}