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unify.go
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unify.go
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// Copyright 2016 The OPA Authors. All rights reserved.
// Use of this source code is governed by an Apache2
// license that can be found in the LICENSE file.
package ast
func isRefSafe(ref Ref, safe VarSet) bool {
switch head := ref[0].Value.(type) {
case Var:
return safe.Contains(head)
case Call:
return isCallSafe(head, safe)
default:
for v := range ref[0].Vars() {
if !safe.Contains(v) {
return false
}
}
return true
}
}
func isCallSafe(call Call, safe VarSet) bool {
vis := NewVarVisitor().WithParams(SafetyCheckVisitorParams)
vis.Walk(call)
unsafe := vis.Vars().Diff(safe)
return len(unsafe) == 0
}
// Unify returns a set of variables that will be unified when the equality expression defined by
// terms a and b is evaluated. The unifier assumes that variables in the VarSet safe are already
// unified.
func Unify(safe VarSet, a *Term, b *Term) VarSet {
u := &unifier{
safe: safe,
unified: VarSet{},
unknown: map[Var]VarSet{},
}
u.unify(a, b)
return u.unified
}
type unifier struct {
safe VarSet
unified VarSet
unknown map[Var]VarSet
}
func (u *unifier) isSafe(x Var) bool {
return u.safe.Contains(x) || u.unified.Contains(x)
}
func (u *unifier) unify(a *Term, b *Term) {
switch a := a.Value.(type) {
case Var:
switch b := b.Value.(type) {
case Var:
if u.isSafe(b) {
u.markSafe(a)
} else if u.isSafe(a) {
u.markSafe(b)
} else {
u.markUnknown(a, b)
u.markUnknown(b, a)
}
case *Array, Object:
u.unifyAll(a, b)
case Ref:
if isRefSafe(b, u.safe) {
u.markSafe(a)
}
case Call:
if isCallSafe(b, u.safe) {
u.markSafe(a)
}
default:
u.markSafe(a)
}
case Ref:
if isRefSafe(a, u.safe) {
switch b := b.Value.(type) {
case Var:
u.markSafe(b)
case *Array, Object:
u.markAllSafe(b)
}
}
case Call:
if isCallSafe(a, u.safe) {
switch b := b.Value.(type) {
case Var:
u.markSafe(b)
case *Array, Object:
u.markAllSafe(b)
}
}
case *ArrayComprehension:
switch b := b.Value.(type) {
case Var:
u.markSafe(b)
case *Array:
u.markAllSafe(b)
}
case *ObjectComprehension:
switch b := b.Value.(type) {
case Var:
u.markSafe(b)
case *object:
u.markAllSafe(b)
}
case *SetComprehension:
switch b := b.Value.(type) {
case Var:
u.markSafe(b)
}
case *Array:
switch b := b.Value.(type) {
case Var:
u.unifyAll(b, a)
case *ArrayComprehension, *ObjectComprehension, *SetComprehension:
u.markAllSafe(a)
case Ref:
if isRefSafe(b, u.safe) {
u.markAllSafe(a)
}
case Call:
if isCallSafe(b, u.safe) {
u.markAllSafe(a)
}
case *Array:
if a.Len() == b.Len() {
for i := 0; i < a.Len(); i++ {
u.unify(a.Elem(i), b.Elem(i))
}
}
}
case *object:
switch b := b.Value.(type) {
case Var:
u.unifyAll(b, a)
case Ref:
if isRefSafe(b, u.safe) {
u.markAllSafe(a)
}
case Call:
if isCallSafe(b, u.safe) {
u.markAllSafe(a)
}
case *object:
if a.Len() == b.Len() {
_ = a.Iter(func(k, v *Term) error {
if v2 := b.Get(k); v2 != nil {
u.unify(v, v2)
}
return nil
}) // impossible to return error
}
}
default:
switch b := b.Value.(type) {
case Var:
u.markSafe(b)
}
}
}
func (u *unifier) markAllSafe(x Value) {
vis := u.varVisitor()
vis.Walk(x)
for v := range vis.Vars() {
u.markSafe(v)
}
}
func (u *unifier) markSafe(x Var) {
u.unified.Add(x)
// Add dependencies of 'x' to safe set
vs := u.unknown[x]
delete(u.unknown, x)
for v := range vs {
u.markSafe(v)
}
// Add dependants of 'x' to safe set if they have no more
// dependencies.
for v, deps := range u.unknown {
if deps.Contains(x) {
delete(deps, x)
if len(deps) == 0 {
u.markSafe(v)
}
}
}
}
func (u *unifier) markUnknown(a, b Var) {
if _, ok := u.unknown[a]; !ok {
u.unknown[a] = NewVarSet()
}
u.unknown[a].Add(b)
}
func (u *unifier) unifyAll(a Var, b Value) {
if u.isSafe(a) {
u.markAllSafe(b)
} else {
vis := u.varVisitor()
vis.Walk(b)
unsafe := vis.Vars().Diff(u.safe).Diff(u.unified)
if len(unsafe) == 0 {
u.markSafe(a)
} else {
for v := range unsafe {
u.markUnknown(a, v)
}
}
}
}
func (u *unifier) varVisitor() *VarVisitor {
return NewVarVisitor().WithParams(VarVisitorParams{
SkipRefHead: true,
SkipObjectKeys: true,
SkipClosures: true,
})
}