Simplified push method; updated first and max slice sizes

README.md

@@ -1,6 +1,6 @@
 # stack [![Build Status](https://travis-ci.com/ef-ds/stack.svg?branch=master)](https://travis-ci.com/ef-ds/stack) [![codecov](https://codecov.io/gh/ef-ds/stack/branch/master/graph/badge.svg)](https://codecov.io/gh/ef-ds/stack) [![Go Report Card](https://goreportcard.com/badge/github.com/ef-ds/stack)](https://goreportcard.com/report/github.com/ef-ds/stack)  [![GoDoc](https://godoc.org/github.com/ef-ds/stack?status.svg)](https://godoc.org/github.com/ef-ds/stack)
 
-Package stack implements a very fast and efficient general purpose Last-In-First-Out (LIFO) stack data structure that is specifically optimized to perform when used by Microservices and serverless services running in production environments. Internally, stack stores the elements in a dynamic growing semi-circular doubly linked list of arrays.
+Package stack implements a very fast and efficient general purpose Last-In-First-Out (LIFO) stack data structure that is specifically optimized to perform when used by Microservices and serverless services running in production environments. Internally, stack stores the elements in a dynamic growing semi-circular inverted singly linked list of arrays.
 
 
 ## Install
@@ -64,7 +64,7 @@ See the [benchmark tests](https://github.com/ef-ds/stack-bench-tests/blob/master
 
 
 ## Performance
-Stack has constant time (O(1)) on all its operations (Push/Pop/Back/Len). It's not amortized constant because stack never copies more than 256 (maxInternalSliceSize/sliceGrowthFactor) items and when it expands or grow, it never does so by more than 1024 (maxInternalSliceSize) items in a single operation.
+Stack has constant time (O(1)) on all its operations (Push/Pop/Back/Len). It's not amortized constant because stack never copies more than 256 (maxInternalSliceSize/2) items and when it expands or grow, it never does so by more than 512 (maxInternalSliceSize) items in a single operation.
 
 Stack offers either the best or very competitive performance across all test sets, suites and ranges.
 
@@ -74,9 +74,9 @@ See [performance](https://github.com/ef-ds/stack-bench-tests/blob/master/PERFORM
 
 
 ## Design
-The Efficient Data Structures (ef-ds) stack employs a new, modern stack design: a semi-circular shaped, linked slices design.
+The Efficient Data Structures (ef-ds) stack employs a new, modern stack design: a dynamic growing semi-circular inverted singly linked list of slices.
 
-That means the [LIFO stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)) is a [doubly-linked list](https://en.wikipedia.org/wiki/Doubly_linked_list) where each node value is a fixed size [slice](https://tour.golang.org/moretypes/7). It is semi-circular in shape because the first node in the linked list points to itself, but the last one points to nil.
+That means the [LIFO stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)) is a [singly-linked list](https://en.wikipedia.org/wiki/Singly_linked_list) where each node value is a fixed size [slice](https://tour.golang.org/moretypes/7). It is inverted singly linked list because each node points only to the previous one (instead of next) and it is semi-circular in shape because the first node in the linked list points to itself, but the last one points to nil.
 
 ![ns/op](testdata/stack.jpg?raw=true "stack Design")
 
@@ -151,7 +151,7 @@ One sofware engineer can't change the world him/herself, but a whole bunch of us
 
 
 ## Competition
-We're extremely interested in improving stack. Please let us know your suggestions for possible improvements and if you know of other high performance stacks not tested here, let us know and we're very glad to benchmark them.
+We're extremely interested in improving stack and we're on an endless quest for better efficiency and more performance. Please let us know your suggestions for possible improvements and if you know of other high performance stacks not tested here, let us know and we're very glad to benchmark them.
 
 
 ## Releases

stack.go

@@ -26,29 +26,16 @@ package stack
 
 const (
 	// firstSliceSize holds the size of the first slice.
-	firstSliceSize = 4
-
-	// sliceGrowthFactor determines by how much and how fast the first internal
-	// slice should grow. A growth factor of 4, firstSliceSize = 4 and
-	// maxInternalSliceSize = 256, the first slice will start with size 4,
-	// then 16 (4*4), then 64 (16*4), then 256 (64*4), then 1024 (256*4).
-	// The growth factor should be tweaked together with firstSliceSize and
-	// maxInternalSliceSize and for maximum efficiency.
-	// sliceGrowthFactor only applies to the very first slice creates. All other
-	// subsequent slices are created with fixed size of maxInternalSliceSize.
-	sliceGrowthFactor = 4
+	firstSliceSize = 8
 
 	// maxInternalSliceSize holds the maximum size of each internal slice.
-	maxInternalSliceSize = 1024
+	maxInternalSliceSize = 512
 )
 
 // Stack implements an unbounded, dynamically growing Last-In-First-Out (LIFO)
 // stack data structure.
 // The zero value for stack is an empty stack ready to use.
 type Stack struct {
-	// Head points to the first node of the linked list.
-	head *node
-
 	// Tail points to the last node of the linked list.
 	// In an empty stack, head and tail points to the same node.
 	tail *node
@@ -63,9 +50,6 @@ type node struct {
 	// v holds the list of user added values in this node.
 	v []interface{}
 
-	// n points to the next node in the linked list.
-	n *node
-
 	// p points to the previous node in the linked list.
 	p *node
 }
@@ -99,31 +83,14 @@ func (s *Stack) Back() (interface{}, bool) {
 // Push adds value v to the the back of the stack.
 // The complexity is O(1).
 func (s *Stack) Push(v interface{}) {
-	switch {
-	case s.head == nil:
-		// No nodes present yet.
-		h := &node{v: make([]interface{}, 0, firstSliceSize)}
-		h.p = h
-		s.head = h
-		s.tail = h
-	case len(s.tail.v) < cap(s.tail.v):
-		// There's room in the tail slice.
-	case cap(s.tail.v) < maxInternalSliceSize:
-		// We're on the first slice and it hasn't grown large enough yet.
-		l := len(s.tail.v)
-		nv := make([]interface{}, l, l*sliceGrowthFactor)
-		copy(nv, s.tail.v)
-		s.tail.v = nv
-	case s.tail.n != nil:
-		// There's at least one unused slice between head and tail nodes.
-		n := s.tail.n
-		s.tail = n
-	default:
-		// No available nodes, so make one.
-		n := &node{v: make([]interface{}, 0, maxInternalSliceSize)}
-		n.p = s.tail
-		s.tail.n = n
-		s.tail = n
+	if s.tail == nil {
+		s.tail = &node{v: make([]interface{}, 0, firstSliceSize)}
+		s.tail.p = s.tail
+	} else if len(s.tail.v) >= maxInternalSliceSize {
+		s.tail = &node{
+			v: make([]interface{}, 0, maxInternalSliceSize),
+			p: s.tail,
+		}
 	}
 	s.len++
 	s.tail.v = append(s.tail.v, v)
@@ -137,16 +104,15 @@ func (s *Stack) Pop() (interface{}, bool) {
 	if s.len == 0 {
 		return nil, false
 	}
+
 	s.len--
 	tp := len(s.tail.v) - 1
 	vp := &s.tail.v[tp]
 	v := *vp
 	*vp = nil // Avoid memory leaks
 	s.tail.v = s.tail.v[:tp]
 	if tp <= 0 {
-		// Move to the previous slice as all elements
-		// in the current one were removed.
-		s.tail = s.tail.p
+		s.tail = s.tail.p // Move to the previous slice.
 	}
 	return v, true
 }