exp/template/html: defines a parse context for a subset of HTML.

This defines just enough context to distinguish HTML URI attributes
from parsed character data.

It does not affect any public module API as I thought I would get
early comment on style for defining enumerations and tables.

R=rsc, r, nigeltao, r
CC=golang-dev
https://golang.org/cl/4906043

src/pkg/exp/template/html/context.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package html
+
+import (
+	"fmt"
+)
+
+// context describes the state an HTML parser must be in when it reaches the
+// portion of HTML produced by evaluating a particular template node.
+//
+// The zero value of type context is the start context for a template that
+// produces an HTML fragment as defined at
+// http://www.w3.org/TR/html5/the-end.html#parsing-html-fragments
+// where the context element is null.
+type context struct {
+	state state
+	delim delim
+}
+
+func (c context) String() string {
+	return fmt.Sprintf("context{state: %s, delim: %s", c.state, c.delim)
+}
+
+// eq is true if the two contexts are identical field-wise.
+func (c context) eq(d context) bool {
+	return c.state == d.state && c.delim == d.delim
+}
+
+// state describes a high-level HTML parser state.
+//
+// It bounds the top of the element stack, and by extension the HTML
+// insertion mode, but also contains state that does not correspond to
+// anything in the HTML5 parsing algorithm because a single token 
+// production in the HTML grammar may contain embedded actions in a template.
+// For instance, the quoted HTML attribute produced by
+//     <div title="Hello {{.World}}">
+// is a single token in HTML's grammar but in a template spans several nodes.
+type state uint8
+
+const (
+	// statePCDATA is parsed character data.  An HTML parser is in
+	// this state when its parse position is outside an HTML tag,
+	// directive, comment, and special element body.
+	statePCDATA state = iota
+	// stateTag occurs before an HTML attribute or the end of a tag.
+	stateTag
+	// stateURI occurs inside an HTML attribute whose content is a URI.
+	stateURI
+	// stateError is an infectious error state outside any valid
+	// HTML/CSS/JS construct.
+	stateError
+)
+
+var stateNames = [...]string{
+	statePCDATA: "statePCDATA",
+	stateTag:    "stateTag",
+	stateURI:    "stateURI",
+	stateError:  "stateError",
+}
+
+func (s state) String() string {
+	if uint(s) < uint(len(stateNames)) {
+		return stateNames[s]
+	}
+	return fmt.Sprintf("illegal state %d", uint(s))
+}
+
+// delim is the delimiter that will end the current HTML attribute.
+type delim uint8
+
+const (
+	// delimNone occurs outside any attribute.
+	delimNone delim = iota
+	// delimDoubleQuote occurs when a double quote (") closes the attribute.
+	delimDoubleQuote
+	// delimSingleQuote occurs when a single quote (') closes the attribute.
+	delimSingleQuote
+	// delimSpaceOrTagEnd occurs when a space or right angle bracket (>)
+	// closes the attribute.
+	delimSpaceOrTagEnd
+)
+
+var delimNames = [...]string{
+	delimNone:          "delimNone",
+	delimDoubleQuote:   "delimDoubleQuote",
+	delimSingleQuote:   "delimSingleQuote",
+	delimSpaceOrTagEnd: "delimSpaceOrTagEnd",
+}
+
+func (d delim) String() string {
+	if uint(d) < uint(len(delimNames)) {
+		return delimNames[d]
+	}
+	return fmt.Sprintf("illegal delim %d", uint(d))
+}