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// Copyright 2014 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 analysis
// This file computes the "implements" relation over all pairs of
// named types in the program. (The mark-up is done by typeinfo.go.)
// TODO(adonovan): do we want to report implements(C, I) where C and I
// belong to different packages and at least one is not exported?
import (
"go/types"
"sort"
"golang.org/x/tools/go/types/typeutil"
)
// computeImplements computes the "implements" relation over all pairs
// of named types in allNamed.
func computeImplements(cache *typeutil.MethodSetCache, allNamed []*types.Named) map[*types.Named]implementsFacts {
// Information about a single type's method set.
type msetInfo struct {
typ types.Type
mset *types.MethodSet
mask1, mask2 uint64
}
initMsetInfo := func(info *msetInfo, typ types.Type) {
info.typ = typ
info.mset = cache.MethodSet(typ)
for i := 0; i < info.mset.Len(); i++ {
name := info.mset.At(i).Obj().Name()
info.mask1 |= 1 << methodBit(name[0])
info.mask2 |= 1 << methodBit(name[len(name)-1])
}
}
// satisfies(T, U) reports whether type T satisfies type U.
// U must be an interface.
//
// Since there are thousands of types (and thus millions of
// pairs of types) and types.Assignable(T, U) is relatively
// expensive, we compute assignability directly from the
// method sets. (At least one of T and U must be an
// interface.)
//
// We use a trick (thanks gri!) related to a Bloom filter to
// quickly reject most tests, which are false. For each
// method set, we precompute a mask, a set of bits, one per
// distinct initial byte of each method name. Thus the mask
// for io.ReadWriter would be {'R','W'}. AssignableTo(T, U)
// cannot be true unless mask(T)&mask(U)==mask(U).
//
// As with a Bloom filter, we can improve precision by testing
// additional hashes, e.g. using the last letter of each
// method name, so long as the subset mask property holds.
//
// When analyzing the standard library, there are about 1e6
// calls to satisfies(), of which 0.6% return true. With a
// 1-hash filter, 95% of calls avoid the expensive check; with
// a 2-hash filter, this grows to 98.2%.
satisfies := func(T, U *msetInfo) bool {
return T.mask1&U.mask1 == U.mask1 &&
T.mask2&U.mask2 == U.mask2 &&
containsAllIdsOf(T.mset, U.mset)
}
// Information about a named type N, and perhaps also *N.
type namedInfo struct {
isInterface bool
base msetInfo // N
ptr msetInfo // *N, iff N !isInterface
}
var infos []namedInfo
// Precompute the method sets and their masks.
for _, N := range allNamed {
var info namedInfo
initMsetInfo(&info.base, N)
_, info.isInterface = N.Underlying().(*types.Interface)
if !info.isInterface {
initMsetInfo(&info.ptr, types.NewPointer(N))
}
if info.base.mask1|info.ptr.mask1 == 0 {
continue // neither N nor *N has methods
}
infos = append(infos, info)
}
facts := make(map[*types.Named]implementsFacts)
// Test all pairs of distinct named types (T, U).
// TODO(adonovan): opt: compute (U, T) at the same time.
for t := range infos {
T := &infos[t]
var to, from, fromPtr []types.Type
for u := range infos {
if t == u {
continue
}
U := &infos[u]
switch {
case T.isInterface && U.isInterface:
if satisfies(&U.base, &T.base) {
to = append(to, U.base.typ)
}
if satisfies(&T.base, &U.base) {
from = append(from, U.base.typ)
}
case T.isInterface: // U concrete
if satisfies(&U.base, &T.base) {
to = append(to, U.base.typ)
} else if satisfies(&U.ptr, &T.base) {
to = append(to, U.ptr.typ)
}
case U.isInterface: // T concrete
if satisfies(&T.base, &U.base) {
from = append(from, U.base.typ)
} else if satisfies(&T.ptr, &U.base) {
fromPtr = append(fromPtr, U.base.typ)
}
}
}
// Sort types (arbitrarily) to avoid nondeterminism.
sort.Sort(typesByString(to))
sort.Sort(typesByString(from))
sort.Sort(typesByString(fromPtr))
facts[T.base.typ.(*types.Named)] = implementsFacts{to, from, fromPtr}
}
return facts
}
type implementsFacts struct {
to []types.Type // named or ptr-to-named types assignable to interface T
from []types.Type // named interfaces assignable from T
fromPtr []types.Type // named interfaces assignable only from *T
}
type typesByString []types.Type
func (p typesByString) Len() int { return len(p) }
func (p typesByString) Less(i, j int) bool { return p[i].String() < p[j].String() }
func (p typesByString) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
// methodBit returns the index of x in [a-zA-Z], or 52 if not found.
func methodBit(x byte) uint64 {
switch {
case 'a' <= x && x <= 'z':
return uint64(x - 'a')
case 'A' <= x && x <= 'Z':
return uint64(26 + x - 'A')
}
return 52 // all other bytes
}
// containsAllIdsOf reports whether the method identifiers of T are a
// superset of those in U. If U belongs to an interface type, the
// result is equal to types.Assignable(T, U), but is cheaper to compute.
//
// TODO(gri): make this a method of *types.MethodSet.
//
func containsAllIdsOf(T, U *types.MethodSet) bool {
t, tlen := 0, T.Len()
u, ulen := 0, U.Len()
for t < tlen && u < ulen {
tMeth := T.At(t).Obj()
uMeth := U.At(u).Obj()
tId := tMeth.Id()
uId := uMeth.Id()
if tId > uId {
// U has a method T lacks: fail.
return false
}
if tId < uId {
// T has a method U lacks: ignore it.
t++
continue
}
// U and T both have a method of this Id. Check types.
if !types.Identical(tMeth.Type(), uMeth.Type()) {
return false // type mismatch
}
u++
t++
}
return u == ulen
}