From cd6d6f97a8f765134a56385ad7737a5b35734671 Mon Sep 17 00:00:00 2001
From: Vikram Rangnekar <vicram@gmail.com>
Date: Sun, 16 Jun 2019 20:51:36 -0400
Subject: [PATCH] Reduce alllocations done by the stack

---
 psql/bench.8   |  6 +++---
 qcode/qcode.go | 13 ++++++------
 util/stack.go  | 54 ++++++++++++++++++++++++--------------------------
 3 files changed, 36 insertions(+), 37 deletions(-)

diff --git a/psql/bench.8 b/psql/bench.8
index 014931c..d42c64f 100644
--- a/psql/bench.8
+++ b/psql/bench.8
@@ -1,7 +1,7 @@
 goos: darwin
 goarch: amd64
 pkg: github.com/dosco/super-graph/psql
-BenchmarkCompile-8           	  100000	     15246 ns/op	    2823 B/op	      53 allocs/op
-BenchmarkCompileParallel-8   	  300000	      4559 ns/op	    2848 B/op	      53 allocs/op
+BenchmarkCompile-8           	  100000	     15092 ns/op	    3562 B/op	      36 allocs/op
+BenchmarkCompileParallel-8   	  300000	      4684 ns/op	    3592 B/op	      36 allocs/op
 PASS
-ok  	github.com/dosco/super-graph/psql	3.116s
+ok  	github.com/dosco/super-graph/psql	3.133s
diff --git a/qcode/qcode.go b/qcode/qcode.go
index d61519d..0335482 100644
--- a/qcode/qcode.go
+++ b/qcode/qcode.go
@@ -390,16 +390,15 @@ func (com *Compiler) compileArgs(sel *Select, args []Arg) error {
 	return nil
 }
 
-func (com *Compiler) compileArgObj(arg *Arg) (*Exp, error) {
+func (com *Compiler) compileArgObj(st *util.Stack, arg *Arg) (*Exp, error) {
 	if arg.Val.Type != nodeObj {
 		return nil, fmt.Errorf("expecting an object")
 	}
 
-	return com.compileArgNode(arg.Val, true)
+	return com.compileArgNode(st, arg.Val, true)
 }
 
-func (com *Compiler) compileArgNode(node *Node, usePool bool) (*Exp, error) {
-	st := util.NewStack()
+func (com *Compiler) compileArgNode(st *util.Stack, node *Node, usePool bool) (*Exp, error) {
 	var root *Exp
 
 	if node == nil || len(node.Children) == 0 {
@@ -521,9 +520,10 @@ func (com *Compiler) compileArgSearch(sel *Select, arg *Arg) error {
 }
 
 func (com *Compiler) compileArgWhere(sel *Select, arg *Arg) error {
+	st := util.NewStack()
 	var err error
 
-	ex, err := com.compileArgObj(arg)
+	ex, err := com.compileArgObj(st, arg)
 	if err != nil {
 		return err
 	}
@@ -864,6 +864,7 @@ func pushChild(st *util.Stack, exp *Exp, node *Node) {
 func compileFilter(filter []string) (*Exp, error) {
 	var fl *Exp
 	com := &Compiler{}
+	st := util.NewStack()
 
 	if len(filter) == 0 {
 		return &Exp{Op: OpNop}, nil
@@ -874,7 +875,7 @@ func compileFilter(filter []string) (*Exp, error) {
 		if err != nil {
 			return nil, err
 		}
-		f, err := com.compileArgNode(node, false)
+		f, err := com.compileArgNode(st, node, false)
 		if err != nil {
 			return nil, err
 		}
diff --git a/util/stack.go b/util/stack.go
index 06f5709..fd88f3e 100644
--- a/util/stack.go
+++ b/util/stack.go
@@ -1,49 +1,47 @@
 package util
 
-type (
-	Stack struct {
-		top    *StackNode
-		length int
-	}
-	StackNode struct {
-		value interface{}
-		prev  *StackNode
-	}
-)
+type Stack struct {
+	stA [20]interface{}
+	st  []interface{}
+	top int
+}
 
 // Create a new Stack
 func NewStack() *Stack {
-	return &Stack{nil, 0}
+	s := &Stack{top: -1}
+	s.st = s.stA[:0]
+	return s
 }
 
 // Return the number of items in the Stack
-func (this *Stack) Len() int {
-	return this.length
+func (s *Stack) Len() int {
+	return (s.top + 1)
 }
 
 // View the top item on the Stack
-func (this *Stack) Peek() interface{} {
-	if this.length == 0 {
-		return nil
+func (s *Stack) Peek() interface{} {
+	if s.top == -1 {
+		return -1
 	}
-	return this.top.value
+	return s.st[s.top]
 }
 
 // Pop the top item of the Stack and return it
-func (this *Stack) Pop() interface{} {
-	if this.length == 0 {
-		return nil
+func (s *Stack) Pop() interface{} {
+	if s.top == -1 {
+		return -1
 	}
 
-	n := this.top
-	this.top = n.prev
-	this.length--
-	return n.value
+	s.top--
+	return s.st[(s.top + 1)]
 }
 
 // Push a value onto the top of the Stack
-func (this *Stack) Push(value interface{}) {
-	n := &StackNode{value, this.top}
-	this.top = n
-	this.length++
+func (s *Stack) Push(value interface{}) {
+	s.top++
+	if len(s.st) <= s.top {
+		s.st = append(s.st, value)
+	} else {
+		s.st[s.top] = value
+	}
 }