// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package types2

import (
	"cmd/compile/internal/syntax"
	"fmt"
)

// An Alias represents an alias type.
// Whether or not Alias types are created is controlled by the
// gotypesalias setting with the GODEBUG environment variable.
// For gotypesalias=1, alias declarations produce an Alias type.
// Otherwise, the alias information is only in the type name,
// which points directly to the actual (aliased) type.
type Alias struct {
	obj     *TypeName      // corresponding declared alias object
	orig    *Alias         // original, uninstantiated alias
	tparams *TypeParamList // type parameters, or nil
	targs   *TypeList      // type arguments, or nil
	fromRHS Type           // RHS of type alias declaration; may be an alias
	actual  Type           // actual (aliased) type; never an alias
}

// NewAlias creates a new Alias type with the given type name and rhs.
// rhs must not be nil.
func NewAlias(obj *TypeName, rhs Type) *Alias {
	alias := (*Checker)(nil).newAlias(obj, rhs)
	// Ensure that alias.actual is set (#65455).
	alias.cleanup()
	return alias
}

// Obj returns the type name for the declaration defining the alias type a.
// For instantiated types, this is same as the type name of the origin type.
func (a *Alias) Obj() *TypeName { return a.orig.obj }

func (a *Alias) String() string { return TypeString(a, nil) }

// Underlying returns the [underlying type] of the alias type a, which is the
// underlying type of the aliased type. Underlying types are never Named,
// TypeParam, or Alias types.
//
// [underlying type]: https://go.dev/ref/spec#Underlying_types.
func (a *Alias) Underlying() Type { return unalias(a).Underlying() }

// Origin returns the generic Alias type of which a is an instance.
// If a is not an instance of a generic alias, Origin returns a.
func (a *Alias) Origin() *Alias { return a.orig }

// TypeParams returns the type parameters of the alias type a, or nil.
// A generic Alias and its instances have the same type parameters.
func (a *Alias) TypeParams() *TypeParamList { return a.tparams }

// SetTypeParams sets the type parameters of the alias type a.
// The alias a must not have type arguments.
func (a *Alias) SetTypeParams(tparams []*TypeParam) {
	assert(a.targs == nil)
	a.tparams = bindTParams(tparams)
}

// TypeArgs returns the type arguments used to instantiate the Alias type.
// If a is not an instance of a generic alias, the result is nil.
func (a *Alias) TypeArgs() *TypeList { return a.targs }

// Rhs returns the type R on the right-hand side of an alias
// declaration "type A = R", which may be another alias.
func (a *Alias) Rhs() Type { return a.fromRHS }

// Unalias returns t if it is not an alias type;
// otherwise it follows t's alias chain until it
// reaches a non-alias type which is then returned.
// Consequently, the result is never an alias type.
func Unalias(t Type) Type {
	if a0, _ := t.(*Alias); a0 != nil {
		return unalias(a0)
	}
	return t
}

func unalias(a0 *Alias) Type {
	if a0.actual != nil {
		return a0.actual
	}
	var t Type
	for a := a0; a != nil; a, _ = t.(*Alias) {
		t = a.fromRHS
	}
	if t == nil {
		panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
	}

	// Memoize the type only if valid.
	// In the presence of unfinished cyclic declarations, Unalias
	// would otherwise latch the invalid value (#66704).
	// TODO(adonovan): rethink, along with checker.typeDecl's use
	// of Invalid to mark unfinished aliases.
	if t != Typ[Invalid] {
		a0.actual = t
	}

	return t
}

// asNamed returns t as *Named if that is t's
// actual type. It returns nil otherwise.
func asNamed(t Type) *Named {
	n, _ := Unalias(t).(*Named)
	return n
}

// newAlias creates a new Alias type with the given type name and rhs.
// rhs must not be nil.
func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
	assert(rhs != nil)
	a := new(Alias)
	a.obj = obj
	a.orig = a
	a.fromRHS = rhs
	if obj.typ == nil {
		obj.typ = a
	}

	// Ensure that a.actual is set at the end of type checking.
	if check != nil {
		check.needsCleanup(a)
	}

	return a
}

// newAliasInstance creates a new alias instance for the given origin and type
// arguments, recording pos as the position of its synthetic object (for error
// reporting).
func (check *Checker) newAliasInstance(pos syntax.Pos, orig *Alias, targs []Type, expanding *Named, ctxt *Context) *Alias {
	assert(len(targs) > 0)
	obj := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
	rhs := check.subst(pos, orig.fromRHS, makeSubstMap(orig.TypeParams().list(), targs), expanding, ctxt)
	res := check.newAlias(obj, rhs)
	res.orig = orig
	res.tparams = orig.tparams
	res.targs = newTypeList(targs)
	return res
}

func (a *Alias) cleanup() {
	// Ensure a.actual is set before types are published,
	// so Unalias is a pure "getter", not a "setter".
	actual := Unalias(a)

	if actual == Typ[Invalid] {
		// We don't set a.actual to Typ[Invalid] during type checking,
		// as it may indicate that the RHS is not fully set up.
		a.actual = actual
	}
}
