"""Helpers for generating for loops and comprehensions.

We special case certain kinds for loops such as "for x in range(...)"
for better efficiency.  Each for loop generator class below deals one
such special case.
"""

from typing import Union, List, Optional, Tuple, Callable
from typing_extensions import Type, ClassVar

from mypy.nodes import (
    Lvalue, Expression, TupleExpr, CallExpr, RefExpr, GeneratorExpr, ARG_POS, MemberExpr, TypeAlias
)
from mypyc.ir.ops import (
    Value, BasicBlock, Integer, Branch, Register, TupleGet, TupleSet, IntOp
)
from mypyc.ir.rtypes import (
    RType, is_short_int_rprimitive, is_list_rprimitive, is_sequence_rprimitive,
    is_tuple_rprimitive, is_dict_rprimitive,
    RTuple, short_int_rprimitive, int_rprimitive
)
from mypyc.primitives.registry import CFunctionDescription
from mypyc.primitives.dict_ops import (
    dict_next_key_op, dict_next_value_op, dict_next_item_op, dict_check_size_op,
    dict_key_iter_op, dict_value_iter_op, dict_item_iter_op
)
from mypyc.primitives.list_ops import list_append_op, list_get_item_unsafe_op, new_list_set_item_op
from mypyc.primitives.set_ops import set_add_op
from mypyc.primitives.generic_ops import iter_op, next_op
from mypyc.primitives.exc_ops import no_err_occurred_op
from mypyc.irbuild.builder import IRBuilder
from mypyc.irbuild.targets import AssignmentTarget, AssignmentTargetTuple

GenFunc = Callable[[], None]


def for_loop_helper(builder: IRBuilder, index: Lvalue, expr: Expression,
                    body_insts: GenFunc, else_insts: Optional[GenFunc],
                    line: int) -> None:
    """Generate IR for a loop.

    Args:
        index: the loop index Lvalue
        expr: the expression to iterate over
        body_insts: a function that generates the body of the loop
        else_insts: a function that generates the else block instructions
    """
    # Body of the loop
    body_block = BasicBlock()
    # Block that steps to the next item
    step_block = BasicBlock()
    # Block for the else clause, if we need it
    else_block = BasicBlock()
    # Block executed after the loop
    exit_block = BasicBlock()

    # Determine where we want to exit, if our condition check fails.
    normal_loop_exit = else_block if else_insts is not None else exit_block

    for_gen = make_for_loop_generator(builder, index, expr, body_block, normal_loop_exit, line)

    builder.push_loop_stack(step_block, exit_block)
    condition_block = BasicBlock()
    builder.goto_and_activate(condition_block)

    # Add loop condition check.
    for_gen.gen_condition()

    # Generate loop body.
    builder.activate_block(body_block)
    for_gen.begin_body()
    body_insts()

    # We generate a separate step block (which might be empty).
    builder.goto_and_activate(step_block)
    for_gen.gen_step()
    # Go back to loop condition.
    builder.goto(condition_block)

    for_gen.add_cleanup(normal_loop_exit)
    builder.pop_loop_stack()

    if else_insts is not None:
        builder.activate_block(else_block)
        else_insts()
        builder.goto(exit_block)

    builder.activate_block(exit_block)


def for_loop_helper_with_index(builder: IRBuilder,
                               index: Lvalue,
                               expr: Expression,
                               expr_reg: Value,
                               body_insts: Callable[[Value], None], line: int) -> None:
    """Generate IR for a sequence iteration.

    This function only works for sequence type. Compared to for_loop_helper,
    it would feed iteration index to body_insts.

    Args:
        index: the loop index Lvalue
        expr: the expression to iterate over
        body_insts: a function that generates the body of the loop.
                    It needs a index as parameter.
    """
    assert is_sequence_rprimitive(expr_reg.type)
    target_type = builder.get_sequence_type(expr)

    body_block = BasicBlock()
    step_block = BasicBlock()
    exit_block = BasicBlock()
    condition_block = BasicBlock()

    for_gen = ForSequence(builder, index, body_block, exit_block, line, False)
    for_gen.init(expr_reg, target_type, reverse=False)

    builder.push_loop_stack(step_block, exit_block)

    builder.goto_and_activate(condition_block)
    for_gen.gen_condition()

    builder.activate_block(body_block)
    for_gen.begin_body()
    body_insts(builder.read(for_gen.index_target))

    builder.goto_and_activate(step_block)
    for_gen.gen_step()
    builder.goto(condition_block)

    for_gen.add_cleanup(exit_block)
    builder.pop_loop_stack()

    builder.activate_block(exit_block)


def sequence_from_generator_preallocate_helper(
        builder: IRBuilder,
        gen: GeneratorExpr,
        empty_op_llbuilder: Callable[[Value, int], Value],
        set_item_op: CFunctionDescription) -> Optional[Value]:
    """Generate a new tuple or list from a simple generator expression.

    Currently we only optimize for simplest generator expression, which means that
    there is no condition list in the generator and only one original sequence with
    one index is allowed.

    e.g.  (1) tuple(f(x) for x in a_list/a_tuple)
          (2) list(f(x) for x in a_list/a_tuple)
          (3) [f(x) for x in a_list/a_tuple]
    RTuple as an original sequence is not supported yet.

    Args:
        empty_op_llbuilder: A function that can generate an empty sequence op when
            passed in length. See `new_list_op_with_length` and `new_tuple_op_with_length`
            for detailed implementation.
        set_item_op: A primitive that can modify an arbitrary position of a sequence.
            The op should have three arguments:
                - Self
                - Target position
                - New Value
            See `new_list_set_item_op` and `new_tuple_set_item_op` for detailed
            implementation.
    """
    if len(gen.sequences) == 1 and len(gen.indices) == 1 and len(gen.condlists[0]) == 0:
        rtype = builder.node_type(gen.sequences[0])
        if is_list_rprimitive(rtype) or is_tuple_rprimitive(rtype):
            sequence = builder.accept(gen.sequences[0])
            length = builder.builder.builtin_len(sequence, gen.line, use_pyssize_t=True)
            target_op = empty_op_llbuilder(length, gen.line)

            def set_item(item_index: Value) -> None:
                e = builder.accept(gen.left_expr)
                builder.call_c(set_item_op, [target_op, item_index, e], gen.line)

            for_loop_helper_with_index(builder, gen.indices[0], gen.sequences[0], sequence,
                                       set_item, gen.line)

            return target_op
    return None


def translate_list_comprehension(builder: IRBuilder, gen: GeneratorExpr) -> Value:
    # Try simplest list comprehension, otherwise fall back to general one
    val = sequence_from_generator_preallocate_helper(
        builder, gen,
        empty_op_llbuilder=builder.builder.new_list_op_with_length,
        set_item_op=new_list_set_item_op)
    if val is not None:
        return val

    list_ops = builder.new_list_op([], gen.line)
    loop_params = list(zip(gen.indices, gen.sequences, gen.condlists))

    def gen_inner_stmts() -> None:
        e = builder.accept(gen.left_expr)
        builder.call_c(list_append_op, [list_ops, e], gen.line)

    comprehension_helper(builder, loop_params, gen_inner_stmts, gen.line)
    return list_ops


def translate_set_comprehension(builder: IRBuilder, gen: GeneratorExpr) -> Value:
    set_ops = builder.new_set_op([], gen.line)
    loop_params = list(zip(gen.indices, gen.sequences, gen.condlists))

    def gen_inner_stmts() -> None:
        e = builder.accept(gen.left_expr)
        builder.call_c(set_add_op, [set_ops, e], gen.line)

    comprehension_helper(builder, loop_params, gen_inner_stmts, gen.line)
    return set_ops


def comprehension_helper(builder: IRBuilder,
                         loop_params: List[Tuple[Lvalue, Expression, List[Expression]]],
                         gen_inner_stmts: Callable[[], None],
                         line: int) -> None:
    """Helper function for list comprehensions.

    Args:
        loop_params: a list of (index, expr, [conditions]) tuples defining nested loops:
            - "index" is the Lvalue indexing that loop;
            - "expr" is the expression for the object to be iterated over;
            - "conditions" is a list of conditions, evaluated in order with short-circuiting,
                that must all be true for the loop body to be executed
        gen_inner_stmts: function to generate the IR for the body of the innermost loop
    """
    def handle_loop(loop_params: List[Tuple[Lvalue, Expression, List[Expression]]]) -> None:
        """Generate IR for a loop.

        Given a list of (index, expression, [conditions]) tuples, generate IR
        for the nested loops the list defines.
        """
        index, expr, conds = loop_params[0]
        for_loop_helper(builder, index, expr,
                        lambda: loop_contents(conds, loop_params[1:]),
                        None, line)

    def loop_contents(
            conds: List[Expression],
            remaining_loop_params: List[Tuple[Lvalue, Expression, List[Expression]]],
    ) -> None:
        """Generate the body of the loop.

        Args:
            conds: a list of conditions to be evaluated (in order, with short circuiting)
                to gate the body of the loop
            remaining_loop_params: the parameters for any further nested loops; if it's empty
                we'll instead evaluate the "gen_inner_stmts" function
        """
        # Check conditions, in order, short circuiting them.
        for cond in conds:
            cond_val = builder.accept(cond)
            cont_block, rest_block = BasicBlock(), BasicBlock()
            # If the condition is true we'll skip the continue.
            builder.add_bool_branch(cond_val, rest_block, cont_block)
            builder.activate_block(cont_block)
            builder.nonlocal_control[-1].gen_continue(builder, cond.line)
            builder.goto_and_activate(rest_block)

        if remaining_loop_params:
            # There's another nested level, so the body of this loop is another loop.
            return handle_loop(remaining_loop_params)
        else:
            # We finally reached the actual body of the generator.
            # Generate the IR for the inner loop body.
            gen_inner_stmts()

    handle_loop(loop_params)


def is_range_ref(expr: RefExpr) -> bool:
    return (expr.fullname == 'builtins.range'
            or isinstance(expr.node, TypeAlias) and expr.fullname == 'six.moves.xrange')


def make_for_loop_generator(builder: IRBuilder,
                            index: Lvalue,
                            expr: Expression,
                            body_block: BasicBlock,
                            loop_exit: BasicBlock,
                            line: int,
                            nested: bool = False) -> 'ForGenerator':
    """Return helper object for generating a for loop over an iterable.

    If "nested" is True, this is a nested iterator such as "e" in "enumerate(e)".
    """

    rtyp = builder.node_type(expr)
    if is_sequence_rprimitive(rtyp):
        # Special case "for x in <list>".
        expr_reg = builder.accept(expr)
        target_type = builder.get_sequence_type(expr)

        for_list = ForSequence(builder, index, body_block, loop_exit, line, nested)
        for_list.init(expr_reg, target_type, reverse=False)
        return for_list

    if is_dict_rprimitive(rtyp):
        # Special case "for k in <dict>".
        expr_reg = builder.accept(expr)
        target_type = builder.get_dict_key_type(expr)

        for_dict = ForDictionaryKeys(builder, index, body_block, loop_exit, line, nested)
        for_dict.init(expr_reg, target_type)
        return for_dict

    if (isinstance(expr, CallExpr)
            and isinstance(expr.callee, RefExpr)):
        if (is_range_ref(expr.callee)
                and (len(expr.args) <= 2
                     or (len(expr.args) == 3
                         and builder.extract_int(expr.args[2]) is not None))
                and set(expr.arg_kinds) == {ARG_POS}):
            # Special case "for x in range(...)".
            # We support the 3 arg form but only for int literals, since it doesn't
            # seem worth the hassle of supporting dynamically determining which
            # direction of comparison to do.
            if len(expr.args) == 1:
                start_reg = Integer(0)  # type: Value
                end_reg = builder.accept(expr.args[0])
            else:
                start_reg = builder.accept(expr.args[0])
                end_reg = builder.accept(expr.args[1])
            if len(expr.args) == 3:
                step = builder.extract_int(expr.args[2])
                assert step is not None
                if step == 0:
                    builder.error("range() step can't be zero", expr.args[2].line)
            else:
                step = 1

            for_range = ForRange(builder, index, body_block, loop_exit, line, nested)
            for_range.init(start_reg, end_reg, step)
            return for_range

        elif (expr.callee.fullname == 'builtins.enumerate'
                and len(expr.args) == 1
                and expr.arg_kinds == [ARG_POS]
                and isinstance(index, TupleExpr)
                and len(index.items) == 2):
            # Special case "for i, x in enumerate(y)".
            lvalue1 = index.items[0]
            lvalue2 = index.items[1]
            for_enumerate = ForEnumerate(builder, index, body_block, loop_exit, line,
                                         nested)
            for_enumerate.init(lvalue1, lvalue2, expr.args[0])
            return for_enumerate

        elif (expr.callee.fullname == 'builtins.zip'
                and len(expr.args) >= 2
                and set(expr.arg_kinds) == {ARG_POS}
                and isinstance(index, TupleExpr)
                and len(index.items) == len(expr.args)):
            # Special case "for x, y in zip(a, b)".
            for_zip = ForZip(builder, index, body_block, loop_exit, line, nested)
            for_zip.init(index.items, expr.args)
            return for_zip

        if (expr.callee.fullname == 'builtins.reversed'
                and len(expr.args) == 1
                and expr.arg_kinds == [ARG_POS]
                and is_sequence_rprimitive(builder.node_type(expr.args[0]))):
            # Special case "for x in reversed(<list>)".
            expr_reg = builder.accept(expr.args[0])
            target_type = builder.get_sequence_type(expr)

            for_list = ForSequence(builder, index, body_block, loop_exit, line, nested)
            for_list.init(expr_reg, target_type, reverse=True)
            return for_list
    if (isinstance(expr, CallExpr)
            and isinstance(expr.callee, MemberExpr)
            and not expr.args):
        # Special cases for dictionary iterator methods, like dict.items().
        rtype = builder.node_type(expr.callee.expr)
        if (is_dict_rprimitive(rtype)
                and expr.callee.name in ('keys', 'values', 'items')):
            expr_reg = builder.accept(expr.callee.expr)
            for_dict_type = None  # type: Optional[Type[ForGenerator]]
            if expr.callee.name == 'keys':
                target_type = builder.get_dict_key_type(expr.callee.expr)
                for_dict_type = ForDictionaryKeys
            elif expr.callee.name == 'values':
                target_type = builder.get_dict_value_type(expr.callee.expr)
                for_dict_type = ForDictionaryValues
            else:
                target_type = builder.get_dict_item_type(expr.callee.expr)
                for_dict_type = ForDictionaryItems
            for_dict_gen = for_dict_type(builder, index, body_block, loop_exit, line, nested)
            for_dict_gen.init(expr_reg, target_type)
            return for_dict_gen

    # Default to a generic for loop.
    expr_reg = builder.accept(expr)
    for_obj = ForIterable(builder, index, body_block, loop_exit, line, nested)
    item_type = builder._analyze_iterable_item_type(expr)
    item_rtype = builder.type_to_rtype(item_type)
    for_obj.init(expr_reg, item_rtype)
    return for_obj


class ForGenerator:
    """Abstract base class for generating for loops."""

    def __init__(self,
                 builder: IRBuilder,
                 index: Lvalue,
                 body_block: BasicBlock,
                 loop_exit: BasicBlock,
                 line: int,
                 nested: bool) -> None:
        self.builder = builder
        self.index = index
        self.body_block = body_block
        self.line = line
        # Some for loops need a cleanup block that we execute at exit. We
        # create a cleanup block if needed. However, if we are generating a for
        # loop for a nested iterator, such as "e" in "enumerate(e)", the
        # outermost generator should generate the cleanup block -- we don't
        # need to do it here.
        if self.need_cleanup() and not nested:
            # Create a new block to handle cleanup after loop exit.
            self.loop_exit = BasicBlock()
        else:
            # Just use the existing loop exit block.
            self.loop_exit = loop_exit

    def need_cleanup(self) -> bool:
        """If this returns true, we need post-loop cleanup."""
        return False

    def add_cleanup(self, exit_block: BasicBlock) -> None:
        """Add post-loop cleanup, if needed."""
        if self.need_cleanup():
            self.builder.activate_block(self.loop_exit)
            self.gen_cleanup()
            self.builder.goto(exit_block)

    def gen_condition(self) -> None:
        """Generate check for loop exit (e.g. exhaustion of iteration)."""

    def begin_body(self) -> None:
        """Generate ops at the beginning of the body (if needed)."""

    def gen_step(self) -> None:
        """Generate stepping to the next item (if needed)."""

    def gen_cleanup(self) -> None:
        """Generate post-loop cleanup (if needed)."""

    def load_len(self, expr: Union[Value, AssignmentTarget]) -> Value:
        """A helper to get collection length, used by several subclasses."""
        return self.builder.builder.builtin_len(self.builder.read(expr, self.line), self.line)


class ForIterable(ForGenerator):
    """Generate IR for a for loop over an arbitrary iterable (the normal case)."""

    def need_cleanup(self) -> bool:
        # Create a new cleanup block for when the loop is finished.
        return True

    def init(self, expr_reg: Value, target_type: RType) -> None:
        # Define targets to contain the expression, along with the iterator that will be used
        # for the for-loop. If we are inside of a generator function, spill these into the
        # environment class.
        builder = self.builder
        iter_reg = builder.call_c(iter_op, [expr_reg], self.line)
        builder.maybe_spill(expr_reg)
        self.iter_target = builder.maybe_spill(iter_reg)
        self.target_type = target_type

    def gen_condition(self) -> None:
        # We call __next__ on the iterator and check to see if the return value
        # is NULL, which signals either the end of the Iterable being traversed
        # or an exception being raised. Note that Branch.IS_ERROR checks only
        # for NULL (an exception does not necessarily have to be raised).
        builder = self.builder
        line = self.line
        self.next_reg = builder.call_c(next_op, [builder.read(self.iter_target, line)], line)
        builder.add(Branch(self.next_reg, self.loop_exit, self.body_block, Branch.IS_ERROR))

    def begin_body(self) -> None:
        # Assign the value obtained from __next__ to the
        # lvalue so that it can be referenced by code in the body of the loop.
        builder = self.builder
        line = self.line
        # We unbox here so that iterating with tuple unpacking generates a tuple based
        # unpack instead of an iterator based one.
        next_reg = builder.coerce(self.next_reg, self.target_type, line)
        builder.assign(builder.get_assignment_target(self.index), next_reg, line)

    def gen_step(self) -> None:
        # Nothing to do here, since we get the next item as part of gen_condition().
        pass

    def gen_cleanup(self) -> None:
        # We set the branch to go here if the conditional evaluates to true. If
        # an exception was raised during the loop, then err_reg wil be set to
        # True. If no_err_occurred_op returns False, then the exception will be
        # propagated using the ERR_FALSE flag.
        self.builder.call_c(no_err_occurred_op, [], self.line)


def unsafe_index(
    builder: IRBuilder, target: Value, index: Value, line: int
) -> Value:
    """Emit a potentially unsafe index into a target."""
    # This doesn't really fit nicely into any of our data-driven frameworks
    # since we want to use __getitem__ if we don't have an unsafe version,
    # so we just check manually.
    if is_list_rprimitive(target.type):
        return builder.call_c(list_get_item_unsafe_op, [target, index], line)
    else:
        return builder.gen_method_call(target, '__getitem__', [index], None, line)


class ForSequence(ForGenerator):
    """Generate optimized IR for a for loop over a sequence.

    Supports iterating in both forward and reverse.
    """

    def init(self, expr_reg: Value, target_type: RType, reverse: bool) -> None:
        builder = self.builder
        self.reverse = reverse
        # Define target to contain the expression, along with the index that will be used
        # for the for-loop. If we are inside of a generator function, spill these into the
        # environment class.
        self.expr_target = builder.maybe_spill(expr_reg)
        if not reverse:
            index_reg = Integer(0)  # type: Value
        else:
            index_reg = builder.binary_op(self.load_len(self.expr_target),
                                          Integer(1), '-', self.line)
        self.index_target = builder.maybe_spill_assignable(index_reg)
        self.target_type = target_type

    def gen_condition(self) -> None:
        builder = self.builder
        line = self.line
        # TODO: Don't reload the length each time when iterating an immutable sequence?
        if self.reverse:
            # If we are iterating in reverse order, we obviously need
            # to check that the index is still positive. Somewhat less
            # obviously we still need to check against the length,
            # since it could shrink out from under us.
            comparison = builder.binary_op(builder.read(self.index_target, line),
                                           Integer(0), '>=', line)
            second_check = BasicBlock()
            builder.add_bool_branch(comparison, second_check, self.loop_exit)
            builder.activate_block(second_check)
        # For compatibility with python semantics we recalculate the length
        # at every iteration.
        len_reg = self.load_len(self.expr_target)
        comparison = builder.binary_op(builder.read(self.index_target, line), len_reg, '<', line)
        builder.add_bool_branch(comparison, self.body_block, self.loop_exit)

    def begin_body(self) -> None:
        builder = self.builder
        line = self.line
        # Read the next list item.
        value_box = unsafe_index(
            builder,
            builder.read(self.expr_target, line),
            builder.read(self.index_target, line),
            line
        )
        assert value_box
        # We coerce to the type of list elements here so that
        # iterating with tuple unpacking generates a tuple based
        # unpack instead of an iterator based one.
        builder.assign(builder.get_assignment_target(self.index),
                       builder.coerce(value_box, self.target_type, line), line)

    def gen_step(self) -> None:
        # Step to the next item.
        builder = self.builder
        line = self.line
        step = 1 if not self.reverse else -1
        add = builder.int_op(short_int_rprimitive,
                             builder.read(self.index_target, line),
                             Integer(step), IntOp.ADD, line)
        builder.assign(self.index_target, add, line)


class ForDictionaryCommon(ForGenerator):
    """Generate optimized IR for a for loop over dictionary keys/values.

    The logic is pretty straightforward, we use PyDict_Next() API wrapped in
    a tuple, so that we can modify only a single register. The layout of the tuple:
      * f0: are there more items (bool)
      * f1: current offset (int)
      * f2: next key (object)
      * f3: next value (object)
    For more info see https://docs.python.org/3/c-api/dict.html#c.PyDict_Next.

    Note that for subclasses we fall back to generic PyObject_GetIter() logic,
    since they may override some iteration methods in subtly incompatible manner.
    The fallback logic is implemented in CPy.h via dynamic type check.
    """
    dict_next_op = None  # type: ClassVar[CFunctionDescription]
    dict_iter_op = None  # type: ClassVar[CFunctionDescription]

    def need_cleanup(self) -> bool:
        # Technically, a dict subclass can raise an unrelated exception
        # in __next__(), so we need this.
        return True

    def init(self, expr_reg: Value, target_type: RType) -> None:
        builder = self.builder
        self.target_type = target_type

        # We add some variables to environment class, so they can be read across yield.
        self.expr_target = builder.maybe_spill(expr_reg)
        offset = Integer(0)
        self.offset_target = builder.maybe_spill_assignable(offset)
        self.size = builder.maybe_spill(self.load_len(self.expr_target))

        # For dict class (not a subclass) this is the dictionary itself.
        iter_reg = builder.call_c(self.dict_iter_op, [expr_reg], self.line)
        self.iter_target = builder.maybe_spill(iter_reg)

    def gen_condition(self) -> None:
        """Get next key/value pair, set new offset, and check if we should continue."""
        builder = self.builder
        line = self.line
        self.next_tuple = self.builder.call_c(
            self.dict_next_op, [builder.read(self.iter_target, line),
                                builder.read(self.offset_target, line)], line)

        # Do this here instead of in gen_step() to minimize variables in environment.
        new_offset = builder.add(TupleGet(self.next_tuple, 1, line))
        builder.assign(self.offset_target, new_offset, line)

        should_continue = builder.add(TupleGet(self.next_tuple, 0, line))
        builder.add(
            Branch(should_continue, self.body_block, self.loop_exit, Branch.BOOL)
        )

    def gen_step(self) -> None:
        """Check that dictionary didn't change size during iteration.

        Raise RuntimeError if it is not the case to match CPython behavior.
        """
        builder = self.builder
        line = self.line
        # Technically, we don't need a new primitive for this, but it is simpler.
        builder.call_c(dict_check_size_op,
                       [builder.read(self.expr_target, line),
                        builder.read(self.size, line)], line)

    def gen_cleanup(self) -> None:
        # Same as for generic ForIterable.
        self.builder.call_c(no_err_occurred_op, [], self.line)


class ForDictionaryKeys(ForDictionaryCommon):
    """Generate optimized IR for a for loop over dictionary keys."""
    dict_next_op = dict_next_key_op
    dict_iter_op = dict_key_iter_op

    def begin_body(self) -> None:
        builder = self.builder
        line = self.line

        # Key is stored at the third place in the tuple.
        key = builder.add(TupleGet(self.next_tuple, 2, line))
        builder.assign(builder.get_assignment_target(self.index),
                       builder.coerce(key, self.target_type, line), line)


class ForDictionaryValues(ForDictionaryCommon):
    """Generate optimized IR for a for loop over dictionary values."""
    dict_next_op = dict_next_value_op
    dict_iter_op = dict_value_iter_op

    def begin_body(self) -> None:
        builder = self.builder
        line = self.line

        # Value is stored at the third place in the tuple.
        value = builder.add(TupleGet(self.next_tuple, 2, line))
        builder.assign(builder.get_assignment_target(self.index),
                       builder.coerce(value, self.target_type, line), line)


class ForDictionaryItems(ForDictionaryCommon):
    """Generate optimized IR for a for loop over dictionary items."""
    dict_next_op = dict_next_item_op
    dict_iter_op = dict_item_iter_op

    def begin_body(self) -> None:
        builder = self.builder
        line = self.line

        key = builder.add(TupleGet(self.next_tuple, 2, line))
        value = builder.add(TupleGet(self.next_tuple, 3, line))

        # Coerce just in case e.g. key is itself a tuple to be unpacked.
        assert isinstance(self.target_type, RTuple)
        key = builder.coerce(key, self.target_type.types[0], line)
        value = builder.coerce(value, self.target_type.types[1], line)

        target = builder.get_assignment_target(self.index)
        if isinstance(target, AssignmentTargetTuple):
            # Simpler code for common case: for k, v in d.items().
            if len(target.items) != 2:
                builder.error("Expected a pair for dict item iteration", line)
            builder.assign(target.items[0], key, line)
            builder.assign(target.items[1], value, line)
        else:
            rvalue = builder.add(TupleSet([key, value], line))
            builder.assign(target, rvalue, line)


class ForRange(ForGenerator):
    """Generate optimized IR for a for loop over an integer range."""

    def init(self, start_reg: Value, end_reg: Value, step: int) -> None:
        builder = self.builder
        self.start_reg = start_reg
        self.end_reg = end_reg
        self.step = step
        self.end_target = builder.maybe_spill(end_reg)
        if is_short_int_rprimitive(start_reg.type) and is_short_int_rprimitive(end_reg.type):
            index_type = short_int_rprimitive
        else:
            index_type = int_rprimitive
        index_reg = Register(index_type)
        builder.assign(index_reg, start_reg, -1)
        self.index_reg = builder.maybe_spill_assignable(index_reg)
        # Initialize loop index to 0. Assert that the index target is assignable.
        self.index_target = builder.get_assignment_target(
            self.index)  # type: Union[Register, AssignmentTarget]
        builder.assign(self.index_target, builder.read(self.index_reg, self.line), self.line)

    def gen_condition(self) -> None:
        builder = self.builder
        line = self.line
        # Add loop condition check.
        cmp = '<' if self.step > 0 else '>'
        comparison = builder.binary_op(builder.read(self.index_reg, line),
                                       builder.read(self.end_target, line), cmp, line)
        builder.add_bool_branch(comparison, self.body_block, self.loop_exit)

    def gen_step(self) -> None:
        builder = self.builder
        line = self.line

        # Increment index register. If the range is known to fit in short ints, use
        # short ints.
        if (is_short_int_rprimitive(self.start_reg.type)
                and is_short_int_rprimitive(self.end_reg.type)):
            new_val = builder.int_op(short_int_rprimitive,
                                     builder.read(self.index_reg, line),
                                     Integer(self.step), IntOp.ADD, line)

        else:
            new_val = builder.binary_op(
                builder.read(self.index_reg, line), Integer(self.step), '+', line)
        builder.assign(self.index_reg, new_val, line)
        builder.assign(self.index_target, new_val, line)


class ForInfiniteCounter(ForGenerator):
    """Generate optimized IR for a for loop counting from 0 to infinity."""

    def init(self) -> None:
        builder = self.builder
        # Create a register to store the state of the loop index and
        # initialize this register along with the loop index to 0.
        zero = Integer(0)
        self.index_reg = builder.maybe_spill_assignable(zero)
        self.index_target = builder.get_assignment_target(
            self.index)  # type: Union[Register, AssignmentTarget]
        builder.assign(self.index_target, zero, self.line)

    def gen_step(self) -> None:
        builder = self.builder
        line = self.line
        # We can safely assume that the integer is short, since we are not going to wrap
        # around a 63-bit integer.
        # NOTE: This would be questionable if short ints could be 32 bits.
        new_val = builder.int_op(short_int_rprimitive,
                                 builder.read(self.index_reg, line),
                                 Integer(1), IntOp.ADD, line)
        builder.assign(self.index_reg, new_val, line)
        builder.assign(self.index_target, new_val, line)


class ForEnumerate(ForGenerator):
    """Generate optimized IR for a for loop of form "for i, x in enumerate(it)"."""

    def need_cleanup(self) -> bool:
        # The wrapped for loop might need cleanup. This might generate a
        # redundant cleanup block, but that's okay.
        return True

    def init(self, index1: Lvalue, index2: Lvalue, expr: Expression) -> None:
        # Count from 0 to infinity (for the index lvalue).
        self.index_gen = ForInfiniteCounter(
            self.builder,
            index1,
            self.body_block,
            self.loop_exit,
            self.line, nested=True)
        self.index_gen.init()
        # Iterate over the actual iterable.
        self.main_gen = make_for_loop_generator(
            self.builder,
            index2,
            expr,
            self.body_block,
            self.loop_exit,
            self.line, nested=True)

    def gen_condition(self) -> None:
        # No need for a check for the index generator, since it's unconditional.
        self.main_gen.gen_condition()

    def begin_body(self) -> None:
        self.index_gen.begin_body()
        self.main_gen.begin_body()

    def gen_step(self) -> None:
        self.index_gen.gen_step()
        self.main_gen.gen_step()

    def gen_cleanup(self) -> None:
        self.index_gen.gen_cleanup()
        self.main_gen.gen_cleanup()


class ForZip(ForGenerator):
    """Generate IR for a for loop of form `for x, ... in zip(a, ...)`."""

    def need_cleanup(self) -> bool:
        # The wrapped for loops might need cleanup. We might generate a
        # redundant cleanup block, but that's okay.
        return True

    def init(self, indexes: List[Lvalue], exprs: List[Expression]) -> None:
        assert len(indexes) == len(exprs)
        # Condition check will require multiple basic blocks, since there will be
        # multiple conditions to check.
        self.cond_blocks = [BasicBlock() for _ in range(len(indexes) - 1)] + [self.body_block]
        self.gens = []  # type: List[ForGenerator]
        for index, expr, next_block in zip(indexes, exprs, self.cond_blocks):
            gen = make_for_loop_generator(
                self.builder,
                index,
                expr,
                next_block,
                self.loop_exit,
                self.line, nested=True)
            self.gens.append(gen)

    def gen_condition(self) -> None:
        for i, gen in enumerate(self.gens):
            gen.gen_condition()
            if i < len(self.gens) - 1:
                self.builder.activate_block(self.cond_blocks[i])

    def begin_body(self) -> None:
        for gen in self.gens:
            gen.begin_body()

    def gen_step(self) -> None:
        for gen in self.gens:
            gen.gen_step()

    def gen_cleanup(self) -> None:
        for gen in self.gens:
            gen.gen_cleanup()
