From 666ea93dba377f267a2c8ecf97378a420db18383 Mon Sep 17 00:00:00 2001
From: Adam Scarr <adam@vektah.net>
Date: Sun, 6 Aug 2017 23:32:10 +1000
Subject: [PATCH] Eliminate a bunch of allocations

---
 combinator.go               | 143 ++++++++++++++-------------
 combinator_test.go          | 162 ++++++++++++++++---------------
 html/html.go                |  22 ++---
 html/html_test.go           |   5 +-
 json/json.go                |  20 ++--
 json/json_test.go           |  42 ++++----
 json/profile/cpuprofile.bat |   2 +-
 json/profile/json.go        |   4 +-
 json/profile/memprofile.bat |   2 +-
 nodes.go                    |  23 -----
 parser.go                   | 186 +++++++++++++++++-------------------
 parser_test.go              | 175 +++++++++++++++------------------
 pointer.go                  |  17 ----
 pointer_test.go             |  25 -----
 state.go                    |  45 +++++++++
 state_test.go               |  45 +++++++++
 16 files changed, 459 insertions(+), 459 deletions(-)
 delete mode 100644 nodes.go
 delete mode 100644 pointer.go
 delete mode 100644 pointer_test.go
 create mode 100644 state.go
 create mode 100644 state_test.go

diff --git a/combinator.go b/combinator.go
index 1f1317c..6197d6b 100644
--- a/combinator.go
+++ b/combinator.go
@@ -5,12 +5,13 @@ import (
 	"fmt"
 )
 
-func Nil(p Pointer) (Node, Pointer) {
-	return nil, p
+func Nil(ps *State) interface{} {
+	return nil
 }
 
-func Never(p Pointer) (Node, Pointer) {
-	return Error{p.pos, "Never matches"}, p
+func Never(ps *State) interface{} {
+	ps.ErrorHere("not anything")
+	return nil
 }
 
 func And(parsers ...Parserish) Parser {
@@ -18,23 +19,22 @@ func And(parsers ...Parserish) Parser {
 		return Nil
 	}
 
-	ps := ParsifyAll(parsers...)
+	parserfied := ParsifyAll(parsers...)
 
-	return func(p Pointer) (Node, Pointer) {
-		var nodes = make([]Node, 0, len(ps))
-		var node Node
-		newP := p
-		for _, parser := range ps {
-			node, newP = parser(newP)
-			if node == nil {
-				continue
+	return func(ps *State) interface{} {
+		var nodes = make([]interface{}, 0, len(parserfied))
+		startpos := ps.Pos
+		for _, parser := range parserfied {
+			node := parser(ps)
+			if ps.Errored() {
+				ps.Pos = startpos
+				return nil
 			}
-			if IsError(node) {
-				return node, p
+			if node != nil {
+				nodes = append(nodes, node)
 			}
-			nodes = append(nodes, node)
 		}
-		return nodes, newP
+		return nodes
 	}
 }
 
@@ -43,27 +43,26 @@ func Any(parsers ...Parserish) Parser {
 		return Nil
 	}
 
-	ps := ParsifyAll(parsers...)
+	parserfied := ParsifyAll(parsers...)
 
-	return func(p Pointer) (Node, Pointer) {
-		errors := []Error{}
-		for _, parser := range ps {
-			node, newP := parser(p)
-			if err, isErr := node.(Error); isErr {
-				errors = append(errors, err)
+	return func(ps *State) interface{} {
+		longestError := Error{}
+		startpos := ps.Pos
+		for _, parser := range parserfied {
+			node := parser(ps)
+			if ps.Errored() {
+				if ps.Error.pos > longestError.pos {
+					longestError = ps.Error
+				}
+				ps.ClearError()
 				continue
 			}
-			return node, newP
+			return node
 		}
 
-		longestError := errors[0]
-		for _, e := range errors[1:] {
-			if e.pos > longestError.pos {
-				longestError = e
-			}
-		}
-
-		return longestError, p
+		ps.Error = longestError
+		ps.Pos = startpos
+		return nil
 	}
 }
 
@@ -91,67 +90,81 @@ func manyImpl(min int, op Parserish, until Parserish, sep ...Parserish) Parser {
 		sepParser = Parsify(sep[0])
 	}
 
-	return func(p Pointer) (Node, Pointer) {
-		var node Node
-		nodes := make([]Node, 0)
-		newP := p
+	return func(ps *State) interface{} {
+		var node interface{}
+		nodes := make([]interface{}, 0, 20)
+		startpos := ps.Pos
 		for {
-			if node, _ := untilParser(newP); !IsError(node) {
+			tempPos := ps.Pos
+			node = untilParser(ps)
+			if !ps.Errored() {
+				ps.Pos = tempPos
 				if len(nodes) < min {
-					return NewError(newP.pos, "Unexpected input"), p
+					ps.Pos = startpos
+					ps.ErrorHere("something else")
+					return nil
 				}
 				break
 			}
+			ps.ClearError()
+
+			node = opParser(ps)
+			if ps.Errored() {
+				if len(nodes) < min {
+					ps.Pos = startpos
+					return nil
+				}
+				ps.ClearError()
+				break
+			}
 
-			if node, newP = opParser(newP); IsError(node) {
-				if len(nodes) < min {
-					return node, p
-				}
-				break
-			}
 			nodes = append(nodes, node)
-			if node, newP = sepParser(newP); IsError(node) {
+
+			node = sepParser(ps)
+			if ps.Errored() {
+				ps.ClearError()
 				break
 			}
 		}
-		return nodes, newP
+		return nodes
 	}
 }
 
 func Maybe(parser Parserish) Parser {
-	realParser := Parsify(parser)
+	parserfied := Parsify(parser)
 
-	return func(p Pointer) (Node, Pointer) {
-		node, newP := realParser(p)
-		if IsError(node) {
-			return nil, p
+	return func(ps *State) interface{} {
+		node := parserfied(ps)
+		if ps.Errored() {
+			ps.ClearError()
+			return nil
 		}
-		return node, newP
+
+		return node
 	}
 }
 
-func Map(parser Parserish, f func(n Node) Node) Parser {
+func Map(parser Parserish, f func(n interface{}) interface{}) Parser {
 	p := Parsify(parser)
 
-	return func(ptr Pointer) (Node, Pointer) {
-		node, newPtr := p(ptr)
-		if IsError(node) {
-			return node, ptr
+	return func(ps *State) interface{} {
+		node := p(ps)
+		if ps.Errored() {
+			return nil
 		}
-
-		return f(node), newPtr
+		return f(node)
 	}
 }
 
-func flatten(n Node) string {
+func flatten(n interface{}) interface{} {
 	if s, ok := n.(string); ok {
 		return s
 	}
 
-	if nodes, ok := n.([]Node); ok {
+	if nodes, ok := n.([]interface{}); ok {
 		sbuf := &bytes.Buffer{}
 		for _, node := range nodes {
-			sbuf.WriteString(flatten(node))
+			sbuf.WriteString(flatten(node).(string))
 		}
 		return sbuf.String()
 	}
@@ -160,7 +173,5 @@ func flatten(n Node) string {
 }
 
 func Merge(parser Parserish) Parser {
-	return Map(parser, func(n Node) Node {
-		return flatten(n)
-	})
+	return Map(parser, flatten)
 }
diff --git a/combinator_test.go b/combinator_test.go
index efee7ba..31fad1b 100644
--- a/combinator_test.go
+++ b/combinator_test.go
@@ -7,26 +7,35 @@ import (
 )
 
 func TestNil(t *testing.T) {
-	p := Pointer{"hello world", 0}
+	node, p2 := runParser("hello world", Nil)
 
-	node, p2 := Nil(p)
 	require.Equal(t, nil, node)
-	require.Equal(t, p, p2)
+	require.Equal(t, 0, p2.Pos)
+	require.False(t, p2.Errored())
+}
+
+func TestNever(t *testing.T) {
+	node, p2 := runParser("hello world", Never)
+
+	require.Equal(t, nil, node)
+	require.Equal(t, 0, p2.Pos)
+	require.True(t, p2.Errored())
 }
 
 func TestAnd(t *testing.T) {
-	p := Pointer{"hello world", 0}
+	parser := And("hello", WS, "world")
 
 	t.Run("matches sequence", func(t *testing.T) {
-		node, p2 := And("hello", WS, "world")(p)
-		require.Equal(t, []Node{"hello", "world"}, node)
+		node, p2 := runParser("hello world", parser)
+		require.Equal(t, []interface{}{"hello", "world"}, node)
 		require.Equal(t, "", p2.Get())
 	})
 
 	t.Run("returns errors", func(t *testing.T) {
-		e, p3 := And("hello", WS, "there")(p)
-		require.Equal(t, NewError(6, "Expected there"), e)
-		require.Equal(t, 0, p3.pos)
+		_, p2 := runParser("hello there", parser)
+		require.Equal(t, "world", p2.Error.Expected)
+		require.Equal(t, 6, p2.Error.pos)
+		require.Equal(t, 0, p2.Pos)
 	})
 
 	t.Run("No parsers", func(t *testing.T) {
@@ -35,44 +44,42 @@ func TestAnd(t *testing.T) {
 }
 
 func TestMaybe(t *testing.T) {
-	p := Pointer{"hello world", 0}
-
 	t.Run("matches sequence", func(t *testing.T) {
-		node, p2 := Maybe("hello")(p)
+		node, p2 := runParser("hello world", Maybe("hello"))
 		require.Equal(t, "hello", node)
 		require.Equal(t, " world", p2.Get())
 	})
 
 	t.Run("returns no errors", func(t *testing.T) {
-		e, p3 := Maybe("world")(p)
-		require.Equal(t, nil, e)
-		require.Equal(t, 0, p3.pos)
+		node, p3 := runParser("hello world", Maybe("world"))
+		require.Equal(t, nil, node)
+		require.False(t, p3.Errored())
+		require.Equal(t, 0, p3.Pos)
 	})
 }
 
 func TestAny(t *testing.T) {
-	p := Pointer{"hello world!", 0}
-
 	t.Run("Matches any", func(t *testing.T) {
-		node, p2 := Any("hello", "world")(p)
+		node, p2 := runParser("hello world!", Any("hello", "world"))
 		require.Equal(t, "hello", node)
-		require.Equal(t, 5, p2.pos)
+		require.Equal(t, 5, p2.Pos)
 	})
 
 	t.Run("Returns longest error", func(t *testing.T) {
-		err, p2 := Any(
-			Exact("nope"),
-			And(Exact("hello"), WS, Exact("world"), Exact(".")),
-			And(Exact("hello"), WS, Exact("brother")),
-		)(p)
-		require.Equal(t, NewError(11, "Expected ."), err)
-		require.Equal(t, 0, p2.pos)
+		_, p2 := runParser("hello world!", Any(
+			"nope",
+			And("hello", WS, "world", "."),
+			And("hello", WS, "brother"),
+		))
+		require.Equal(t, "offset 11: Expected .", p2.Error.Error())
+		require.Equal(t, 11, p2.Error.Pos())
+		require.Equal(t, 0, p2.Pos)
 	})
 
 	t.Run("Accepts nil matches", func(t *testing.T) {
-		node, p2 := Any(Exact("ffffff"), WS)(p)
+		node, p2 := runParser("hello world!", Any(Exact("ffffff"), WS))
 		require.Equal(t, nil, node)
-		require.Equal(t, 0, p2.pos)
+		require.Equal(t, 0, p2.Pos)
 	})
 
 	t.Run("No parsers", func(t *testing.T) {
@@ -81,87 +88,79 @@ func TestAny(t *testing.T) {
 }
 
 func TestKleene(t *testing.T) {
-	p := Pointer{"a,b,c,d,e,", 0}
-
 	t.Run("Matches sequence with sep", func(t *testing.T) {
-		node, p2 := Kleene(CharRun("abcdefg"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c", "d", "e"}, node)
-		require.Equal(t, 10, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", Kleene(Chars("a-g"), ","))
+		require.False(t, p2.Errored())
+		require.Equal(t, []interface{}{"a", "b", "c", "d", "e"}, node)
+		require.Equal(t, 10, p2.Pos)
 	})
 
 	t.Run("Matches sequence without sep", func(t *testing.T) {
-		node, p2 := Kleene(Any(CharRun("abcdefg"), Exact(",")))(p)
-		require.Equal(t, []Node{"a", ",", "b", ",", "c", ",", "d", ",", "e", ","}, node)
-		require.Equal(t, 10, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", Kleene(Any(Chars("a-g"), ",")))
+		require.Equal(t, []interface{}{"a", ",", "b", ",", "c", ",", "d", ",", "e", ","}, node)
+		require.Equal(t, 10, p2.Pos)
 	})
 
 	t.Run("Stops on error", func(t *testing.T) {
-		node, p2 := Kleene(CharRun("abc"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c"}, node)
-		require.Equal(t, 6, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", Kleene(Chars("a-c"), ","))
+		require.Equal(t, []interface{}{"a", "b", "c"}, node)
+		require.Equal(t, 6, p2.Pos)
 		require.Equal(t, "d,e,", p2.Get())
 	})
 }
 
 func TestMany(t *testing.T) {
-	p := Pointer{"a,b,c,d,e,", 0}
-
 	t.Run("Matches sequence with sep", func(t *testing.T) {
-		node, p2 := Many(CharRun("abcdefg"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c", "d", "e"}, node)
-		require.Equal(t, 10, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", Many(Chars("a-g"), Exact(",")))
+		require.Equal(t, []interface{}{"a", "b", "c", "d", "e"}, node)
+		require.Equal(t, 10, p2.Pos)
 	})
 
 	t.Run("Matches sequence without sep", func(t *testing.T) {
-		node, p2 := Many(Any(CharRun("abcdefg"), Exact(",")))(p)
-		require.Equal(t, []Node{"a", ",", "b", ",", "c", ",", "d", ",", "e", ","}, node)
-		require.Equal(t, 10, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", Many(Any(Chars("abcdefg"), Exact(","))))
+		require.Equal(t, []interface{}{"a", ",", "b", ",", "c", ",", "d", ",", "e", ","}, node)
+		require.Equal(t, 10, p2.Pos)
 	})
 
 	t.Run("Stops on error", func(t *testing.T) {
-		node, p2 := Many(CharRun("abc"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c"}, node)
-		require.Equal(t, 6, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", Many(Chars("abc"), Exact(",")))
+		require.Equal(t, []interface{}{"a", "b", "c"}, node)
+		require.Equal(t, 6, p2.Pos)
 		require.Equal(t, "d,e,", p2.Get())
 	})
 
 	t.Run("Returns error if nothing matches", func(t *testing.T) {
-		node, p2 := Many(CharRun("def"), Exact(","))(p)
-		require.Equal(t, NewError(0, "Expected some of def"), node)
-		require.Equal(t, 0, p2.pos)
+		_, p2 := runParser("a,b,c,d,e,", Many(Chars("def"), Exact(",")))
+		require.Equal(t, "offset 0: Expected def", p2.Error.Error())
 		require.Equal(t, "a,b,c,d,e,", p2.Get())
 	})
 }
 
 func TestKleeneUntil(t *testing.T) {
-	p := Pointer{"a,b,c,d,e,fg", 0}
-
 	t.Run("Matches sequence with sep", func(t *testing.T) {
-		node, p2 := KleeneUntil(CharRun("abcde"), CharRun("d"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c"}, node)
-		require.Equal(t, 6, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,fg", KleeneUntil(Chars("abcde"), "d", ","))
+		require.Equal(t, []interface{}{"a", "b", "c"}, node)
+		require.Equal(t, "d,e,fg", p2.Get())
 	})
 
 	t.Run("Breaks if separator does not match", func(t *testing.T) {
-		node, p2 := KleeneUntil(Char("abcdefg"), Char("y"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c", "d", "e", "f"}, node)
-		require.Equal(t, 11, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,fg", KleeneUntil(Chars("abcdefg", 1, 1), "y", ","))
+		require.Equal(t, []interface{}{"a", "b", "c", "d", "e", "f"}, node)
+		require.Equal(t, "g", p2.Get())
 	})
 }
 
 func TestManyUntil(t *testing.T) {
-	p := Pointer{"a,b,c,d,e,", 0}
-
 	t.Run("Matches sequence until", func(t *testing.T) {
-		node, p2 := ManyUntil(CharRun("abcdefg"), Char("d"), Exact(","))(p)
-		require.Equal(t, []Node{"a", "b", "c"}, node)
-		require.Equal(t, 6, p2.pos)
+		node, p2 := runParser("a,b,c,d,e,", ManyUntil(Chars("abcdefg"), "d", ","))
+		require.Equal(t, []interface{}{"a", "b", "c"}, node)
+		require.Equal(t, 6, p2.Pos)
 	})
 
 	t.Run("Returns error until matches early", func(t *testing.T) {
-		node, p2 := ManyUntil(CharRun("abc"), Exact("a"), Exact(","))(p)
-		require.Equal(t, NewError(0, "Unexpected input"), node)
-		require.Equal(t, 0, p2.pos)
+		_, p2 := runParser("a,b,c,d,e,", ManyUntil(Chars("abc"), "a", ","))
+		require.Equal(t, "offset 0: Expected something else", p2.Error.Error())
+		require.Equal(t, 0, p2.Pos)
 		require.Equal(t, "a,b,c,d,e,", p2.Get())
 	})
 }
@@ -171,19 +170,19 @@ type htmlTag struct {
 }
 
 func TestMap(t *testing.T) {
-	parser := Map(And("<", Range("a-zA-Z0-9"), ">"), func(n Node) Node {
-		return htmlTag{n.([]Node)[1].(string)}
+	parser := Map(And("<", Chars("a-zA-Z0-9"), ">"), func(n interface{}) interface{} {
+		return htmlTag{n.([]interface{})[1].(string)}
 	})
 
 	t.Run("sucess", func(t *testing.T) {
-		result, _ := parser(Pointer{"<html>", 0})
+		result, _ := runParser("<html>", parser)
 		require.Equal(t, htmlTag{"html"}, result)
 	})
 
 	t.Run("error", func(t *testing.T) {
-		result, ptr := parser(Pointer{"<html", 0})
-		require.Equal(t, NewError(5, "Expected >"), result)
-		require.Equal(t, 0, ptr.pos)
+		_, ps := runParser("<html", parser)
+		require.Equal(t, "offset 5: Expected >", ps.Error.Error())
+		require.Equal(t, 0, ps.Pos)
 	})
 }
 
@@ -193,14 +192,14 @@ func TestMerge(t *testing.T) {
 	parser := Merge(bracer)
 
 	t.Run("sucess", func(t *testing.T) {
-		result, _ := parser(Pointer{"((()))", 0})
+		result, _ := runParser("((()))", parser)
 		require.Equal(t, "((()))", result)
 	})
 
 	t.Run("error", func(t *testing.T) {
-		result, ptr := parser(Pointer{"((())", 0})
-		require.Equal(t, NewError(5, "Expected )"), result)
-		require.Equal(t, 0, ptr.pos)
+		_, ps := runParser("((())", parser)
+		require.Equal(t, "offset 5: Expected )", ps.Error.Error())
+		require.Equal(t, 0, ps.Pos)
 	})
 
 	require.Panics(t, func() {
@@ -209,8 +208,7 @@ func TestMerge(t *testing.T) {
 }
 
 func assertNilParser(t *testing.T, parser Parser) {
-	p := Pointer{"fff", 0}
-	node, p2 := parser(p)
+	node, p2 := runParser("fff", parser)
 	require.Equal(t, nil, node)
-	require.Equal(t, p, p2)
+	require.Equal(t, 0, p2.Pos)
 }
diff --git a/html/html.go b/html/html.go
index adee935..847af96 100644
--- a/html/html.go
+++ b/html/html.go
@@ -2,32 +2,32 @@ package html
 
 import . "github.com/vektah/goparsify"
 
-func Parse(input string) (result Node, remaining string, err error) {
+func Parse(input string) (result interface{}, remaining string, err error) {
 	return ParseString(tag, input)
 }
 
 type Tag struct {
 	Name       string
 	Attributes map[string]string
-	Body       []Node
+	Body       []interface{}
 }
 
 var (
 	tag Parser
 
-	identifier = Merge(And(Range("a-z", 1, 1), Range("a-zA-Z0-9", 0)))
-	text       = CharRunUntil("<>")
+	identifier = Merge(And(Chars("a-z", 1, 1), Chars("a-zA-Z0-9", 0)))
+	text       = NotChars("<>")
 
 	element  = Any(text, &tag)
 	elements = Kleene(element)
 	//attr := And(identifier, equal, String())
 	attr  = And(WS, identifier, WS, "=", WS, Any(String('"'), String('\'')))
-	attrs = Map(Kleene(attr, WS), func(node Node) Node {
-		nodes := node.([]Node)
+	attrs = Map(Kleene(attr, WS), func(node interface{}) interface{} {
+		nodes := node.([]interface{})
 		attr := map[string]string{}
 
 		for _, attrNode := range nodes {
-			attrNodes := attrNode.([]Node)
+			attrNodes := attrNode.([]interface{})
 			attr[attrNodes[0].(string)] = attrNodes[2].(string)
 		}
 
@@ -39,13 +39,13 @@ var (
 )
 
 func init() {
-	tag = Map(And(tstart, elements, tend), func(node Node) Node {
-		nodes := node.([]Node)
-		openTag := nodes[0].([]Node)
+	tag = Map(And(tstart, elements, tend), func(node interface{}) interface{} {
+		nodes := node.([]interface{})
+		openTag := nodes[0].([]interface{})
 		return Tag{
 			Name:       openTag[1].(string),
 			Attributes: openTag[2].(map[string]string),
-			Body:       nodes[1].([]Node),
+			Body:       nodes[1].([]interface{}),
 		}
 
 	})
diff --git a/html/html_test.go b/html/html_test.go
index defdc10..f2f8e6f 100644
--- a/html/html_test.go
+++ b/html/html_test.go
@@ -4,14 +4,13 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/require"
-	. "github.com/vektah/goparsify"
 )
 
 func TestParse(t *testing.T) {
 	result, _, err := Parse(`<body>hello <p color="blue">world</p></body>`)
 	require.NoError(t, err)
-	require.Equal(t, Tag{Name: "body", Attributes: map[string]string{}, Body: []Node{
+	require.Equal(t, Tag{Name: "body", Attributes: map[string]string{}, Body: []interface{}{
 		"hello ",
-		Tag{Name: "p", Attributes: map[string]string{"color": "blue"}, Body: []Node{"world"}},
+		Tag{Name: "p", Attributes: map[string]string{"color": "blue"}, Body: []interface{}{"world"}},
 	}}, result)
 }
diff --git a/json/json.go b/json/json.go
index 3aa5163..60c5a3f 100644
--- a/json/json.go
+++ b/json/json.go
@@ -9,15 +9,15 @@ import (
 var (
 	value Parser
 
-	array = Map(And(WS, "[", Kleene(&value, And(WS, ",")), "]"), func(n Node) Node {
-		return n.([]Node)[1].([]Node)
+	array = Map(And(WS, "[", Kleene(&value, And(WS, ",")), "]"), func(n interface{}) interface{} {
+		return n.([]interface{})[1].([]interface{})
 	})
 	properties = Kleene(And(WS, String('"'), WS, ":", WS, &value), ",")
-	object     = Map(And(WS, "{", WS, properties, WS, "}"), func(n Node) Node {
+	object     = Map(And(WS, "{", WS, properties, WS, "}"), func(n interface{}) interface{} {
 		ret := map[string]interface{}{}
 
-		for _, prop := range n.([]Node)[1].([]Node) {
-			vals := prop.([]Node)
+		for _, prop := range n.([]interface{})[1].([]interface{}) {
+			vals := prop.([]interface{})
 			if len(vals) == 3 {
 				ret[vals[0].(string)] = vals[2]
 			} else {
@@ -28,20 +28,20 @@ var (
 		return ret
 	})
 
-	_null = Map(And(WS, "null"), func(n Node) Node {
+	_null = Map(And(WS, "null"), func(n interface{}) interface{} {
 		return nil
 	})
 
-	_true = Map(And(WS, "true"), func(n Node) Node {
+	_true = Map(And(WS, "true"), func(n interface{}) interface{} {
 		return true
 	})
 
-	_false = Map(And(WS, "false"), func(n Node) Node {
+	_false = Map(And(WS, "false"), func(n interface{}) interface{} {
 		return false
 	})
 
-	Y = Map(And(&value, WS), func(n Node) Node {
-		nodes := n.([]Node)
+	Y = Map(And(&value, WS), func(n interface{}) interface{} {
+		nodes := n.([]interface{})
 		if len(nodes) > 0 {
 			return nodes[0]
 		}
diff --git a/json/json_test.go b/json/json_test.go
index 7f0ab9f..84fdd5c 100644
--- a/json/json_test.go
+++ b/json/json_test.go
@@ -1,10 +1,11 @@
 package json
 
 import (
+	stdlibJson "encoding/json"
 	"testing"
 
+	parsecJson "github.com/prataprc/goparsec/json"
 	"github.com/stretchr/testify/require"
-	. "github.com/vektah/goparsify"
 )
 
 func TestUnmarshal(t *testing.T) {
@@ -29,7 +30,7 @@ func TestUnmarshal(t *testing.T) {
 	t.Run("array", func(t *testing.T) {
 		result, err := Unmarshal(`[true, null, false]`)
 		require.NoError(t, err)
-		require.Equal(t, []Node{true, nil, false}, result)
+		require.Equal(t, []interface{}{true, nil, false}, result)
 	})
 
 	t.Run("object", func(t *testing.T) {
@@ -41,16 +42,16 @@ func TestUnmarshal(t *testing.T) {
 
 const benchmarkString = `{"true":true, "false":false, "null": null}`
 
-//func BenchmarkUnmarshalParsec(b *testing.B) {
-//	bytes := []byte(benchmarkString)
-//
-//	for i := 0; i < b.N; i++ {
-//		scanner := parsecJson.NewJSONScanner(bytes)
-//		_, remaining := parsecJson.Y(scanner)
-//
-//		require.True(b, remaining.Endof())
-//	}
-//}
+func BenchmarkUnmarshalParsec(b *testing.B) {
+	bytes := []byte(benchmarkString)
+
+	for i := 0; i < b.N; i++ {
+		scanner := parsecJson.NewJSONScanner(bytes)
+		_, remaining := parsecJson.Y(scanner)
+
+		require.True(b, remaining.Endof())
+	}
+}
 
 func BenchmarkUnmarshalParsify(b *testing.B) {
 	for i := 0; i < b.N; i++ {
@@ -59,12 +60,11 @@ func BenchmarkUnmarshalParsify(b *testing.B) {
 	}
 }
 
-//
-//func BenchmarkUnmarshalStdlib(b *testing.B) {
-//	bytes := []byte(benchmarkString)
-//	var result interface{}
-//	for i := 0; i < b.N; i++ {
-//		err := stdlibJson.Unmarshal(bytes, &result)
-//		require.NoError(b, err)
-//	}
-//}
+func BenchmarkUnmarshalStdlib(b *testing.B) {
+	bytes := []byte(benchmarkString)
+	var result interface{}
+	for i := 0; i < b.N; i++ {
+		err := stdlibJson.Unmarshal(bytes, &result)
+		require.NoError(b, err)
+	}
+}
diff --git a/json/profile/cpuprofile.bat b/json/profile/cpuprofile.bat
index 2899eb3..c293b5e 100644
--- a/json/profile/cpuprofile.bat
+++ b/json/profile/cpuprofile.bat
@@ -1,3 +1,3 @@
 go build
 profile.exe -cpuprofile cpu.out
-go tool pprof --inuse_objects profile.exe cpu.out
+go tool pprof profile.exe cpu.out
diff --git a/json/profile/json.go b/json/profile/json.go
index b94848b..a03d047 100644
--- a/json/profile/json.go
+++ b/json/profile/json.go
@@ -31,11 +31,13 @@ func main() {
 			}
 		}()
 	}
+	max := 1000000
 	if *memprofile != "" {
 		runtime.MemProfileRate = 1
+		max = 10000
 	}
 
-	for i := 0; i < 10000; i++ {
+	for i := 0; i < max; i++ {
 		_, err := json.Unmarshal(`{"true":true, "false":false, "null": null}`)
 		if err != nil {
 			panic(err)
diff --git a/json/profile/memprofile.bat b/json/profile/memprofile.bat
index a9c935e..71873ac 100644
--- a/json/profile/memprofile.bat
+++ b/json/profile/memprofile.bat
@@ -1,3 +1,3 @@
 go build
 profile.exe -memprofile mem.out
-go tool pprof --inuse_objects profile.exe mem.out
+go tool pprof profile.exe mem.out
diff --git a/nodes.go b/nodes.go
deleted file mode 100644
index 29dd58f..0000000
--- a/nodes.go
+++ /dev/null
@@ -1,23 +0,0 @@
-package parsec
-
-import "fmt"
-
-type Node interface {
-}
-
-type Error struct {
-	pos     int
-	Message string
-}
-
-func (e Error) Pos() int      { return e.pos }
-func (e Error) Error() string { return fmt.Sprintf("offset %d: %s", e.pos, e.Message) }
-
-func NewError(pos int, message string) Error {
-	return Error{pos, message}
-}
-
-func IsError(n interface{}) bool {
-	_, isErr := n.(Error)
-	return isErr
-}
diff --git a/parser.go b/parser.go
index 7599bf1..98f59ce 100644
--- a/parser.go
+++ b/parser.go
@@ -7,7 +7,7 @@ import (
 	"unicode/utf8"
 )
 
-type Parser func(Pointer) (Node, Pointer)
+type Parser func(*State) interface{}
 
 // Parserish types are any type that can be turned into a Parser by Parsify
 // These currently include *Parser and string literals.
@@ -27,13 +27,13 @@ type Parserish interface{}
 
 func Parsify(p Parserish) Parser {
 	switch p := p.(type) {
-	case func(Pointer) (Node, Pointer):
+	case func(*State) interface{}:
 		return Parser(p)
 	case Parser:
 		return p
 	case *Parser:
 		// Todo: Maybe capture this stack and on nil show it? Is there a good error library to do this?
-		return func(ptr Pointer) (Node, Pointer) {
+		return func(ptr *State) interface{} {
 			return (*p)(ptr)
 		}
 	case string:
@@ -51,80 +51,34 @@ func ParsifyAll(parsers ...Parserish) []Parser {
 	return ret
 }
 
-func ParseString(parser Parserish, input string) (result Node, remaining string, err error) {
+func ParseString(parser Parserish, input string) (result interface{}, remaining string, err error) {
 	p := Parsify(parser)
-	result, pointer := p(Pointer{input, 0})
+	ps := &State{input, 0, Error{}}
+	result = p(ps)
 
-	if err, isErr := result.(Error); isErr {
-		return nil, pointer.Get(), err
+	if ps.Error.Expected != "" {
+		return nil, ps.Get(), ps.Error
 	}
 
-	return result, pointer.Get(), nil
+	return result, ps.Get(), nil
 }
 
 func Exact(match string) Parser {
-	return func(p Pointer) (Node, Pointer) {
-		if !strings.HasPrefix(p.Get(), match) {
-			return NewError(p.pos, "Expected "+match), p
+	return func(ps *State) interface{} {
+		if !strings.HasPrefix(ps.Get(), match) {
+			ps.ErrorHere(match)
+			return nil
 		}
 
-		return match, p.Advance(len(match))
+		ps.Advance(len(match))
+
+		return match
 	}
 }
 
-func Char(match string) Parser {
-	return func(p Pointer) (Node, Pointer) {
-		r, w := utf8.DecodeRuneInString(p.Get())
-
-		if !strings.ContainsRune(match, r) {
-			return NewError(p.pos, "Expected one of "+string(match)), p
-
-		}
-		return string(r), p.Advance(w)
-	}
-}
-
-func CharRun(match string) Parser {
-	return func(p Pointer) (Node, Pointer) {
-		matched := 0
-		for p.pos+matched < len(p.input) {
-			r, w := utf8.DecodeRuneInString(p.input[p.pos+matched:])
-			if !strings.ContainsRune(match, r) {
-				break
-			}
-			matched += w
-		}
-
-		if matched == 0 {
-			return NewError(p.pos, "Expected some of "+match), p
-		}
-
-		return p.input[p.pos : p.pos+matched], p.Advance(matched)
-	}
-}
-
-func CharRunUntil(match string) Parser {
-	return func(p Pointer) (Node, Pointer) {
-		matched := 0
-		for p.pos+matched < len(p.input) {
-			r, w := utf8.DecodeRuneInString(p.input[p.pos+matched:])
-			if strings.ContainsRune(match, r) {
-				break
-			}
-			matched += w
-		}
-
-		if matched == 0 {
-			return NewError(p.pos, "Expected some of "+match), p
-		}
-
-		return p.input[p.pos : p.pos+matched], p.Advance(matched)
-	}
-}
-
-func Range(r string, repetition ...int) Parser {
-	min := int(1)
-	max := int(-1)
+func parseRepetition(defaultMin, defaultMax int, repetition ...int) (min int, max int) {
+	min = defaultMin
+	max = defaultMax
 	switch len(repetition) {
 	case 0:
 	case 1:
@@ -135,39 +89,67 @@ func Range(r string, repetition ...int) Parser {
 	default:
 		panic(fmt.Errorf("Dont know what %d repetion args mean", len(repetition)))
 	}
+	return min, max
+}
 
-	runes := []rune(r)
-	if len(runes)%3 != 0 {
-		panic("ranges should be in the form a-z0-9")
-	}
+// parseMatcher turns a string in the format a-f01234A-F into:
+//   - a set string of matches string(01234)
+//   - a set of ranges [][]rune{{'a', 'f'}, {'A', 'F'}}
+func parseMatcher(matcher string) (matches string, ranges [][]rune) {
+	runes := []rune(matcher)
 
-	var ranges [][]rune
-	for i := 0; i < len(runes); i += 3 {
-		start := runes[i]
-		end := runes[i+2]
-		if start <= end {
-			ranges = append(ranges, []rune{start, end})
+	for i := 0; i < len(runes); i++ {
+
+		if i+2 < len(runes) && runes[i+1] == '-' {
+			start := runes[i]
+			end := runes[i+2]
+			if start <= end {
+				ranges = append(ranges, []rune{start, end})
+			} else {
+				ranges = append(ranges, []rune{end, start})
+			}
+		} else if i+1 < len(runes) && runes[i] == '\\' {
+			matches += string(runes[i+1])
 		} else {
-			ranges = append(ranges, []rune{end, start})
+			matches += string(runes[i])
 		}
+
 	}
 
-	return func(p Pointer) (Node, Pointer) {
+	return matches, ranges
+}
+
+func Chars(matcher string, repetition ...int) Parser {
+	return charsImpl(matcher, false, repetition...)
+}
+
+func NotChars(matcher string, repetition ...int) Parser {
+	return charsImpl(matcher, true, repetition...)
+}
+
+func charsImpl(matcher string, stopOn bool, repetition ...int) Parser {
+	min, max := parseRepetition(1, -1, repetition...)
+	matches, ranges := parseMatcher(matcher)
+
+	return func(ps *State) interface{} {
 		matched := 0
-		for p.pos+matched < len(p.input) {
+		for ps.Pos+matched < len(ps.Input) {
 			if max != -1 && matched >= max {
 				break
 			}
 
-			r, w := utf8.DecodeRuneInString(p.input[p.pos+matched:])
+			r, w := utf8.DecodeRuneInString(ps.Input[ps.Pos+matched:])
 
-			anyMatched := false
-			for _, rng := range ranges {
-				if r >= rng[0] && r <= rng[1] {
-					anyMatched = true
+			anyMatched := strings.ContainsRune(matches, r)
+			if !anyMatched {
+				for _, rng := range ranges {
+					if r >= rng[0] && r <= rng[1] {
+						anyMatched = true
+					}
 				}
 			}
-			if !anyMatched {
+
+			if anyMatched == stopOn {
 				break
 			}
 
@@ -175,47 +157,55 @@ func Range(r string, repetition ...int) Parser {
 		}
 
 		if matched < min {
-			return NewError(p.pos+matched, fmt.Sprintf("Expected at least %d more of %s", min-matched, r)), p
+			ps.ErrorHere(matcher)
+			return nil
 		}
 
-		return p.input[p.pos : p.pos+matched], p.Advance(matched)
+		result := ps.Input[ps.Pos : ps.Pos+matched]
+		ps.Advance(matched)
+		return result
 	}
 }
 
-func WS(p Pointer) (Node, Pointer) {
-	_, p2 := CharRun("\t\n\v\f\r \x85\xA0")(p)
-	return nil, p2
+var ws = Chars("\t\n\v\f\r \x85\xA0", 0)
+
+func WS(ps *State) interface{} {
+	ws(ps)
+	return nil
 }
 
 func String(quote rune) Parser {
-	return func(p Pointer) (Node, Pointer) {
+	return func(ps *State) interface{} {
 		var r rune
 		var w int
-		r, w = utf8.DecodeRuneInString(p.input[p.pos:])
+		var matched int
+		r, matched = utf8.DecodeRuneInString(ps.Input[ps.Pos:])
 		if r != quote {
-			return NewError(p.pos, `Expected "`), p
+			ps.ErrorHere("\"")
+			return nil
 		}
 
-		matched := w
 		result := &bytes.Buffer{}
 
-		for p.pos+matched < len(p.input) {
-			r, w = utf8.DecodeRuneInString(p.input[p.pos+matched:])
+		for ps.Pos+matched < len(ps.Input) {
+			r, w = utf8.DecodeRuneInString(ps.Input[ps.Pos+matched:])
 			matched += w
 
 			if r == '\\' {
-				r, w = utf8.DecodeRuneInString(p.input[p.pos+matched:])
+				r, w = utf8.DecodeRuneInString(ps.Input[ps.Pos+matched:])
 				result.WriteRune(r)
 				matched += w
 				continue
 			}
 
 			if r == quote {
-				return result.String(), p.Advance(matched)
+				ps.Advance(matched)
+				return result.String()
 			}
 			result.WriteRune(r)
 		}
 
-		return NewError(p.pos, "Unterminated string"), p
+		ps.ErrorHere("\"")
+		return nil
 	}
 }
diff --git a/parser_test.go b/parser_test.go
index 4d60781..f6122ab 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -7,31 +7,29 @@ import (
 )
 
 func TestParsify(t *testing.T) {
-	p := Pointer{"ffooo", 0}
 
 	t.Run("strings", func(t *testing.T) {
-		node, _ := Parsify("ff")(p)
-		require.Equal(t, "ff", node)
+		require.Equal(t, "ff", Parsify("ff")(InputString("ffooo")))
 	})
 
 	t.Run("parsers", func(t *testing.T) {
-		node, _ := Parsify(CharRun("f"))(p)
-		require.Equal(t, "ff", node)
+		require.Equal(t, "ff", Parsify(Chars("f"))(InputString("ffooo")))
 	})
 
 	t.Run("parser funcs", func(t *testing.T) {
-		node, _ := Parsify(func(p Pointer) (Node, Pointer) {
-			return "hello", p
-		})(p)
+		node := Parsify(func(p *State) interface{} {
+			return "hello"
+		})(InputString("ffooo"))
+
 		require.Equal(t, "hello", node)
 	})
 
 	t.Run("*parsers", func(t *testing.T) {
 		var parser Parser
 		parserfied := Parsify(&parser)
-		parser = CharRun("f")
+		parser = Chars("f")
 
-		node, _ := parserfied(p)
+		node := parserfied(InputString("ffooo"))
 		require.Equal(t, "ff", node)
 	})
 
@@ -43,122 +41,77 @@ func TestParsify(t *testing.T) {
 func TestParsifyAll(t *testing.T) {
 	parsers := ParsifyAll("ff", "gg")
 
-	result, _ := parsers[0](Pointer{"ffooo", 0})
+	result := parsers[0](InputString("ffooo"))
 	require.Equal(t, "ff", result)
 
-	result, _ = parsers[1](Pointer{"ffooo", 0})
-	require.Equal(t, NewError(0, "Expected gg"), result)
+	result = parsers[1](InputString("ffooo"))
+	require.Equal(t, nil, result)
 }
 
 func TestExact(t *testing.T) {
-	p := Pointer{"fooo", 0}
-
 	t.Run("success", func(t *testing.T) {
-		node, p2 := Exact("fo")(p)
+		node, ps := runParser("foobar", Exact("fo"))
 		require.Equal(t, "fo", node)
-		require.Equal(t, p.Advance(2), p2)
+		require.Equal(t, "obar", ps.Get())
 	})
 
 	t.Run("error", func(t *testing.T) {
-		node, p2 := Exact("bar")(p)
-		require.Equal(t, NewError(0, "Expected bar"), node)
-		require.Equal(t, 0, p2.pos)
+		_, ps := runParser("foobar", Exact("bar"))
+		require.Equal(t, "bar", ps.Error.Expected)
+		require.Equal(t, 0, ps.Pos)
 	})
 }
 
-func TestChar(t *testing.T) {
-	p := Pointer{"foobar", 0}
-
-	t.Run("success", func(t *testing.T) {
-		node, p2 := Char("fo")(p)
-		require.Equal(t, "f", node)
-		require.Equal(t, p.Advance(1), p2)
-	})
-
-	t.Run("error", func(t *testing.T) {
-		node, p2 := Char("bar")(p)
-		require.Equal(t, NewError(0, "Expected one of bar"), node)
-		require.Equal(t, 0, p2.pos)
-	})
-}
-
-func TestCharRun(t *testing.T) {
-	p := Pointer{"foobar", 0}
-
-	t.Run("success", func(t *testing.T) {
-		node, p2 := CharRun("fo")(p)
-		require.Equal(t, "foo", node)
-		require.Equal(t, p.Advance(3), p2)
-	})
-
-	t.Run("error", func(t *testing.T) {
-		node, p2 := CharRun("bar")(p)
-		require.Equal(t, NewError(0, "Expected some of bar"), node)
-		require.Equal(t, 0, p2.pos)
-	})
-}
-
-func TestCharUntil(t *testing.T) {
-	p := Pointer{"foobar", 0}
-
-	t.Run("success", func(t *testing.T) {
-		node, p2 := CharRunUntil("z")(p)
-		require.Equal(t, "foobar", node)
-		require.Equal(t, p.Advance(6), p2)
-	})
-
-	t.Run("error", func(t *testing.T) {
-		node, p2 := CharRunUntil("f")(p)
-		require.Equal(t, NewError(0, "Expected some of f"), node)
-		require.Equal(t, 0, p2.pos)
-	})
-}
-
-func TestWS(t *testing.T) {
-	p := Pointer{"  fooo", 0}
-
-	node, p2 := WS(p)
-	require.Equal(t, nil, node)
-	require.Equal(t, p.Advance(2), p2)
-}
-
-func TestRange(t *testing.T) {
+func TestChars(t *testing.T) {
 	t.Run("full match", func(t *testing.T) {
-		node, p := Range("a-z")(Pointer{"foobar", 0})
+		node, ps := runParser("foobar", Chars("a-z"))
 		require.Equal(t, "foobar", node)
-		require.Equal(t, "", p.Get())
+		require.Equal(t, "", ps.Get())
+		require.False(t, ps.Errored())
 	})
 
 	t.Run("partial match", func(t *testing.T) {
-		node, p := Range("1-4d-a")(Pointer{"a1b2c3d4efg", 0})
+		node, ps := runParser("a1b2c3d4efg", Chars("1-4d-a"))
 		require.Equal(t, "a1b2c3d4", node)
-		require.Equal(t, "efg", p.Get())
+		require.Equal(t, "efg", ps.Get())
+		require.False(t, ps.Errored())
 	})
 
 	t.Run("limited match", func(t *testing.T) {
-		node, p := Range("1-4d-a", 1, 2)(Pointer{"a1b2c3d4efg", 0})
+		node, ps := runParser("a1b2c3d4efg", Chars("1-4d-a", 1, 2))
 		require.Equal(t, "a1", node)
-		require.Equal(t, "b2c3d4efg", p.Get())
+		require.Equal(t, "b2c3d4efg", ps.Get())
+		require.False(t, ps.Errored())
 	})
 
 	t.Run("no match", func(t *testing.T) {
-		node, p := Range("0-9")(Pointer{"ffffff", 0})
-		require.Equal(t, NewError(0, "Expected at least 1 more of 0-9"), node)
-		require.Equal(t, 0, p.pos)
+		_, ps := runParser("ffffff", Chars("0-9"))
+		require.Equal(t, "offset 0: Expected 0-9", ps.Error.Error())
+		require.Equal(t, 0, ps.Pos)
 	})
 
 	t.Run("no match with min", func(t *testing.T) {
-		node, p := Range("0-9", 4)(Pointer{"ffffff", 0})
-		require.Equal(t, NewError(0, "Expected at least 4 more of 0-9"), node)
-		require.Equal(t, 0, p.pos)
+		_, ps := runParser("ffffff", Chars("0-9", 4))
+		require.Equal(t, "0-9", ps.Error.Expected)
+		require.Equal(t, 0, ps.Pos)
+	})
+
+	t.Run("test exact matches", func(t *testing.T) {
+		node, ps := runParser("aaff", Chars("abcd"))
+		require.Equal(t, "aa", node)
+		require.Equal(t, 2, ps.Pos)
+		require.False(t, ps.Errored())
+	})
+
+	t.Run("test not matches", func(t *testing.T) {
+		node, ps := runParser("aaff", NotChars("ff"))
+		require.Equal(t, "aa", node)
+		require.Equal(t, 2, ps.Pos)
+		require.False(t, ps.Errored())
 	})
 
 	require.Panics(t, func() {
-		Range("abcd")
-	})
-
-	require.Panics(t, func() {
-		Range("a-b", 1, 2, 3)
+		Chars("a-b", 1, 2, 3)
 	})
 }
 
@@ -181,26 +134,48 @@ func TestParseString(t *testing.T) {
 
 func TestString(t *testing.T) {
 	t.Run("test basic match", func(t *testing.T) {
-		result, p := String('"')(Pointer{`"hello"`, 0})
+		result, p := runParser(`"hello"`, String('"'))
 		require.Equal(t, `hello`, result)
 		require.Equal(t, "", p.Get())
 	})
 
 	t.Run("test non match", func(t *testing.T) {
-		result, p := String('"')(Pointer{`1`, 0})
-		require.Equal(t, NewError(0, `Expected "`), result)
+		_, p := runParser(`1`, String('"'))
+		require.Equal(t, `"`, p.Error.Expected)
 		require.Equal(t, `1`, p.Get())
 	})
 
 	t.Run("test unterminated string", func(t *testing.T) {
-		result, p := String('"')(Pointer{`"hello `, 0})
-		require.Equal(t, NewError(0, `Unterminated string`), result)
+		_, p := runParser(`"hello `, String('"'))
+		require.Equal(t, `"`, p.Error.Expected)
 		require.Equal(t, `"hello `, p.Get())
 	})
 
 	t.Run("test escaping", func(t *testing.T) {
-		result, p := String('"')(Pointer{`"hello \"world\""`, 0})
+		result, p := runParser(`"hello \"world\""`, String('"'))
 		require.Equal(t, `hello "world"`, result)
 		require.Equal(t, ``, p.Get())
 	})
 }
+
+func TestWS(t *testing.T) {
+	t.Run("consumes all whitespace", func(t *testing.T) {
+		result, p := runParser("    asdf", WS)
+		require.Equal(t, nil, result)
+		require.Equal(t, "asdf", p.Get())
+		require.False(t, p.Errored())
+	})
+
+	t.Run("never errors", func(t *testing.T) {
+		result, p := runParser("asdf", WS)
+		require.Equal(t, nil, result)
+		require.Equal(t, "asdf", p.Get())
+		require.False(t, p.Errored())
+	})
+}
+
+func runParser(input string, parser Parser) (interface{}, *State) {
+	ps := InputString(input)
+	result := parser(ps)
+	return result, ps
+}
diff --git a/pointer.go b/pointer.go
deleted file mode 100644
index 7727833..0000000
--- a/pointer.go
+++ /dev/null
@@ -1,17 +0,0 @@
-package parsec
-
-type Pointer struct {
-	input string
-	pos   int
-}
-
-func (p Pointer) Advance(i int) Pointer {
-	return Pointer{p.input, p.pos + i}
-}
-
-func (p Pointer) Get() string {
-	if p.pos > len(p.input) {
-		return ""
-	}
-	return p.input[p.pos:]
-}
diff --git a/pointer_test.go b/pointer_test.go
deleted file mode 100644
index 6b432f0..0000000
--- a/pointer_test.go
+++ /dev/null
@@ -1,25 +0,0 @@
-package parsec
-
-import (
-	"testing"
-
-	"github.com/stretchr/testify/require"
-)
-
-func TestPointer(t *testing.T) {
-	p := Pointer{"fooo", 0}
-
-	t.Run("Advances", func(t *testing.T) {
-		p2 := p.Advance(2)
-		require.Equal(t, Pointer{"fooo", 2}, p2)
-		require.Equal(t, Pointer{"fooo", 0}, p)
-		require.Equal(t, Pointer{"fooo", 3}, p2.Advance(1))
-	})
-
-	t.Run("Get", func(t *testing.T) {
-		require.Equal(t, "fooo", p.Get())
-		require.Equal(t, "ooo", p.Advance(1).Get())
-		require.Equal(t, "", p.Advance(4).Get())
-		require.Equal(t, "", p.Advance(10).Get())
-	})
-}
diff --git a/state.go b/state.go
new file mode 100644
index 0000000..f8df3f1
--- /dev/null
+++ b/state.go
@@ -0,0 +1,45 @@
+package parsec
+
+import "fmt"
+
+type Error struct {
+	pos      int
+	Expected string
+}
+
+func (e Error) Pos() int      { return e.pos }
+func (e Error) Error() string { return fmt.Sprintf("offset %d: Expected %s", e.pos, e.Expected) }
+
+type State struct {
+	Input string
+	Pos   int
+	Error Error
+}
+
+func (s *State) Advance(i int) {
+	s.Pos += i
+}
+
+func (s *State) Get() string {
+	if s.Pos > len(s.Input) {
+		return ""
+	}
+	return s.Input[s.Pos:]
+}
+
+func (s *State) ErrorHere(expected string) {
+	s.Error.pos = s.Pos
+	s.Error.Expected = expected
+}
+
+func (s *State) ClearError() {
+	s.Error.Expected = ""
+}
+
+func (s *State) Errored() bool {
+	return s.Error.Expected != ""
+}
+
+func InputString(input string) *State {
+	return &State{Input: input}
+}
diff --git a/state_test.go b/state_test.go
new file mode 100644
index 0000000..fe04a58
--- /dev/null
+++ b/state_test.go
@@ -0,0 +1,45 @@
+package parsec
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestState_Advance(t *testing.T) {
+	ps := InputString("fooo")
+	require.Equal(t, 0, ps.Pos)
+	ps.Advance(2)
+	require.Equal(t, 2, ps.Pos)
+	ps.Advance(1)
+	require.Equal(t, 3, ps.Pos)
+}
+
+func TestState_Get(t *testing.T) {
+	ps := InputString("fooo")
+	require.Equal(t, "fooo", ps.Get())
+	ps.Advance(1)
+	require.Equal(t, "ooo", ps.Get())
+	ps.Advance(4)
+	require.Equal(t, "", ps.Get())
+	ps.Advance(10)
+	require.Equal(t, "", ps.Get())
+}
+
+func TestState_Errors(t *testing.T) {
+	ps := InputString("fooo")
+
+	ps.ErrorHere("hello")
+	require.Equal(t, "offset 0: Expected hello", ps.Error.Error())
+	require.Equal(t, 0, ps.Error.Pos())
+	require.True(t, ps.Errored())
+
+	ps.ClearError()
+	require.False(t, ps.Errored())
+
+	ps.Advance(2)
+	ps.ErrorHere("hello2")
+	require.Equal(t, "offset 2: Expected hello2", ps.Error.Error())
+	require.Equal(t, 2, ps.Error.Pos())
+	require.True(t, ps.Errored())
+}