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// Copyright 2010 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 main
import (
"go/ast"
"go/token"
"reflect"
)
type simplifier struct {
hasDotImport bool // package file contains: import . "some/import/path"
}
func (s *simplifier) Visit(node ast.Node) ast.Visitor {
switch n := node.(type) {
case *ast.CompositeLit:
// array, slice, and map composite literals may be simplified
outer := n
var eltType ast.Expr
switch typ := outer.Type.(type) {
case *ast.ArrayType:
eltType = typ.Elt
case *ast.MapType:
eltType = typ.Value
}
if eltType != nil {
typ := reflect.ValueOf(eltType)
for i, x := range outer.Elts {
px := &outer.Elts[i]
// look at value of indexed/named elements
if t, ok := x.(*ast.KeyValueExpr); ok {
x = t.Value
px = &t.Value
}
ast.Walk(s, x) // simplify x
// if the element is a composite literal and its literal type
// matches the outer literal's element type exactly, the inner
// literal type may be omitted
if inner, ok := x.(*ast.CompositeLit); ok {
if match(nil, typ, reflect.ValueOf(inner.Type)) {
inner.Type = nil
}
}
// if the outer literal's element type is a pointer type *T
// and the element is & of a composite literal of type T,
// the inner &T may be omitted.
if ptr, ok := eltType.(*ast.StarExpr); ok {
if addr, ok := x.(*ast.UnaryExpr); ok && addr.Op == token.AND {
if inner, ok := addr.X.(*ast.CompositeLit); ok {
if match(nil, reflect.ValueOf(ptr.X), reflect.ValueOf(inner.Type)) {
inner.Type = nil // drop T
*px = inner // drop &
}
}
}
}
}
// node was simplified - stop walk (there are no subnodes to simplify)
return nil
}
case *ast.SliceExpr:
// a slice expression of the form: s[a:len(s)]
// can be simplified to: s[a:]
// if s is "simple enough" (for now we only accept identifiers)
if s.hasDotImport {
// if dot imports are present, we cannot be certain that an
// unresolved "len" identifier refers to the predefined len()
break
}
if s, _ := n.X.(*ast.Ident); s != nil && s.Obj != nil {
// the array/slice object is a single, resolved identifier
if call, _ := n.High.(*ast.CallExpr); call != nil && len(call.Args) == 1 && !call.Ellipsis.IsValid() {
// the high expression is a function call with a single argument
if fun, _ := call.Fun.(*ast.Ident); fun != nil && fun.Name == "len" && fun.Obj == nil {
// the function called is "len" and it is not locally defined; and
// because we don't have dot imports, it must be the predefined len()
if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Obj == s.Obj {
// the len argument is the array/slice object
n.High = nil
}
}
}
}
// Note: We could also simplify slice expressions of the form s[0:b] to s[:b]
// but we leave them as is since sometimes we want to be very explicit
// about the lower bound.
case *ast.RangeStmt:
// a range of the form: for x, _ = range v {...}
// can be simplified to: for x = range v {...}
if ident, _ := n.Value.(*ast.Ident); ident != nil && ident.Name == "_" {
n.Value = nil
}
}
return s
}
func simplify(f *ast.File) {
var s simplifier
// determine if f contains dot imports
for _, imp := range f.Imports {
if imp.Name != nil && imp.Name.Name == "." {
s.hasDotImport = true
break
}
}
ast.Walk(&s, f)
}
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