go/ast Fprint: handle cycles in AST

Augmented ASTs may contain cycles. Keep
track of already printed objects and refer
to them with a line number instead of
printing them again.

R=rsc
CC=golang-dev
https://golang.org/cl/1998042

diff --git a/src/pkg/go/ast/print.go b/src/pkg/go/ast/print.go
index b4b3ed6672c5899ab54b2a57b30768e2e07af80d..b477ebc86c466502be9cfc5238d1f12f69f4a660 100644 (file)
--- a/src/pkg/go/ast/print.go
+++ b/src/pkg/go/ast/print.go
@@ -37,7 +37,12 @@ func NotNilFilter(_ string, value reflect.Value) bool {
 //
 func Fprint(w io.Writer, x interface{}, f FieldFilter) (n int, err os.Error) {
        // setup printer
-       p := printer{output: w, filter: f}
+       p := printer{
+               output: w,
+               filter: f,
+               ptrmap: make(map[interface{}]int),
+               last:   '\n', // force printing of line number on first line
+       }
 
        // install error handler
        defer func() {
@@ -69,9 +74,11 @@ func Print(x interface{}) (int, os.Error) {
 type printer struct {
        output  io.Writer
        filter  FieldFilter
-       written int  // number of bytes written to output
-       indent  int  // current indentation level
-       last    byte // the last byte processed by Write
+       ptrmap  map[interface{}]int // *reflect.PtrValue -> line number
+       written int                 // number of bytes written to output
+       indent  int                 // current indentation level
+       last    byte                // the last byte processed by Write
+       line    int                 // current line number
 }
 
 
@@ -87,7 +94,12 @@ func (p *printer) Write(data []byte) (n int, err os.Error) {
                        if err != nil {
                                return
                        }
+                       p.line++
                } else if p.last == '\n' {
+                       _, err = fmt.Fprintf(p.output, "%6d  ", p.line)
+                       if err != nil {
+                               return
+                       }
                        for j := p.indent; j > 0; j-- {
                                _, err = p.output.Write(indent)
                                if err != nil {
@@ -121,9 +133,8 @@ func (p *printer) printf(format string, args ...interface{}) {
 
 
 // Implementation note: Print is written for AST nodes but could be
-// used to print any acyclic data structure. It would also be easy
-// to generalize it to arbitrary data structures; such a version
-// should probably be in a different package.
+// used to print arbitrary data structures; such a version should
+// probably be in a different package.
 
 func (p *printer) print(x reflect.Value) {
        // Note: This test is only needed because AST nodes
@@ -158,7 +169,16 @@ func (p *printer) print(x reflect.Value) {
 
        case *reflect.PtrValue:
                p.printf("*")
-               p.print(v.Elem())
+               // type-checked ASTs may contain cycles - use ptrmap
+               // to keep track of objects that have been printed
+               // already and print the respective line number instead
+               ptr := v.Interface()
+               if line, exists := p.ptrmap[ptr]; exists {
+                       p.printf("(obj @ %d)", line)
+               } else {
+                       p.ptrmap[ptr] = p.line
+                       p.print(v.Elem())
+               }
 
        case *reflect.SliceValue:
                if s, ok := v.Interface().([]byte); ok {