added

vendor/github.com/glycerine/zygomys/zygo/printstate.go

@@ -0,0 +1,100 @@
+package zygo
+
+import (
+	"fmt"
+)
+
+// begin supporting SexpString structs
+
+// PrintState threads the state of display through SexpString() and Show() calls,
+// to give pretty-printing indentation and to avoid infinite looping on
+// cyclic data structures.
+type PrintState struct {
+	Indent int
+	Seen   Seen
+}
+
+func (ps *PrintState) SetSeen(x interface{}, name string) {
+	if ps == nil {
+		panic("can't SetSeen on a nil PrintState")
+	}
+	ps.Seen[x] = struct{}{}
+}
+
+func (ps *PrintState) GetSeen(x interface{}) bool {
+	if ps == nil {
+		return false
+	}
+	_, ok := ps.Seen[x]
+	return ok
+}
+
+func (ps *PrintState) GetIndent() int {
+	if ps == nil {
+		return 0
+	}
+	return ps.Indent
+}
+
+func (ps *PrintState) AddIndent(addme int) *PrintState {
+	if ps == nil {
+		return &PrintState{
+			Indent: addme,
+			Seen:   NewSeen(),
+		}
+	}
+	return &PrintState{
+		Indent: ps.Indent + addme,
+		Seen:   ps.Seen,
+	}
+}
+
+func NewPrintState() *PrintState {
+	return &PrintState{
+		Seen: NewSeen(),
+	}
+}
+
+func NewPrintStateWithIndent(indent int) *PrintState {
+	return &PrintState{
+		Indent: indent,
+		Seen:   NewSeen(),
+	}
+}
+
+func (ps *PrintState) Clear() {
+	ps.Indent = 0
+	ps.Seen = NewSeen()
+}
+
+func (ps *PrintState) Dump() {
+	fmt.Printf("ps Dump: ")
+	if ps == nil {
+		fmt.Printf("nil\n")
+		return
+	}
+	for k, v := range ps.Seen {
+		fmt.Printf("ps Dump: %p   -- %v\n", k, v)
+	}
+	fmt.Printf("\n")
+}
+
+// Seen tracks if a value has already been displayed, to
+// detect and avoid cycles.
+//
+/* Q: How to do garbage-collection safe graph traversal in a graph of Go objects?
+
+A: "Instead of converting the pointer to a uintptr, just store the pointer
+itself in a map[interface{}]bool.  If you encounter the same pointer
+again, you will get the same map entry.  The GC must guarantee that
+using pointers as map keys will work even if the pointers move."
+
+- Ian Lance Taylor on golang-nuts (2016 June 20).
+*/
+type Seen map[interface{}]struct{}
+
+func NewSeen() Seen {
+	return Seen(make(map[interface{}]struct{}))
+}
+
+// end supporting SexpString structs