bosun.org@v0.0.0-20210513094433-e25bc3e69a1f/cmd/bosun/conf/rule/parse/parse.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 parse builds parse trees for configurations as defined by conf.
// Clients should use that package to construct configurations rather than this
// one, which provides shared internal data structures not intended for general
// use.
package parse // import "bosun.org/cmd/bosun/conf/rule/parse"

import (
	"fmt"
	"runtime"
	"strconv"
	"strings"
)

// Tree is the representation of a single parsed configuration.
type Tree struct {
	Name string    // name of the template represented by the tree.
	Root *ListNode // top-level root of the tree.
	text string    // text parsed to create the template (or its parent)
	// Parsing only; cleared after parse.
	lex       *lexer
	token     [2]item // two-token lookahead for parser.
	peekCount int
}

// Parse returns a Tree, created by parsing the configuration described in the
// argument string. If an error is encountered, parsing stops and an empty Tree
// is returned with the error.
func Parse(name, text string) (t *Tree, err error) {
	t = New(name)
	t.text = text
	err = t.Parse(text)
	return
}

// next returns the next token.
func (t *Tree) next() item {
	if t.peekCount > 0 {
		t.peekCount--
	} else {
		t.token[0] = t.lex.nextItem()
	}
	return t.token[t.peekCount]
}

// backup backs the input stream up one token.
func (t *Tree) backup() {
	t.peekCount++
}

// backup2 backs the input stream up two tokens.
// The zeroth token is already there.
func (t *Tree) backup2(t1 item) {
	t.token[1] = t1
	t.peekCount = 2
}

// peek returns but does not consume the next token.
func (t *Tree) peek() item {
	if t.peekCount > 0 {
		return t.token[t.peekCount-1]
	}
	t.peekCount = 1
	t.token[0] = t.lex.nextItem()
	return t.token[0]
}

// Parsing.

// New allocates a new parse tree with the given name.
func New(name string) *Tree {
	return &Tree{
		Name: name,
	}
}

// ErrorContext returns a textual representation of the location of the node in the input text.
func (t *Tree) ErrorContext(n Node) (location, context string) {
	pos := int(n.Position())
	text := t.text[:pos]
	byteNum := strings.LastIndex(text, "\n")
	if byteNum == -1 {
		byteNum = pos // On first line.
	} else {
		byteNum++ // After the newline.
		byteNum = pos - byteNum
	}
	lineNum := 1 + strings.Count(text, "\n")
	context = n.String()
	context = strings.TrimSpace(context)
	context = strings.Replace(context, "\n", "\\n", -1)
	if len(context) > 20 {
		context = fmt.Sprintf("%.20s...", context)
	}
	return fmt.Sprintf("%s:%d:%d", t.Name, lineNum, byteNum), context
}

// errorf formats the error and terminates processing.
func (t *Tree) errorf(format string, args ...interface{}) {
	t.Root = nil
	format = fmt.Sprintf("parse: %s:%d: %s", t.Name, t.lex.lineNumber(), format)
	panic(fmt.Errorf(format, args...))
}

// error terminates processing.
func (t *Tree) error(err error) {
	t.errorf("%s", err)
}

// expect consumes the next token and guarantees it has the required type.
func (t *Tree) expect(expected itemType, context string) item {
	token := t.next()
	if token.typ != expected {
		t.unexpected(token, context)
	}
	return token
}

// expectOneOf consumes the next token and guarantees it has one of the required types.
func (t *Tree) expectOneOf(expected1, expected2 itemType, context string) item {
	token := t.next()
	if token.typ != expected1 && token.typ != expected2 {
		t.unexpected(token, context)
	}
	return token
}

// unexpected complains about the token and terminates processing.
func (t *Tree) unexpected(token item, context string) {
	t.errorf("unexpected %s in %s", token, context)
}

// recover is the handler that turns panics into returns from the top level of Parse.
func (t *Tree) recover(errp *error) {
	e := recover()
	if e != nil {
		if _, ok := e.(runtime.Error); ok {
			panic(e)
		}
		if t != nil {
			t.stopParse()
		}
		*errp = e.(error)
	}
	return
}

// startParse initializes the parser, using the lexer.
func (t *Tree) startParse(lex *lexer) {
	t.Root = nil
	t.lex = lex
}

// stopParse terminates parsing.
func (t *Tree) stopParse() {
	t.lex = nil
}

// Parse parses the template definition string to construct a representation of
// the template for execution. If either action delimiter string is empty, the
// default ("{{" or "}}") is used. Embedded template definitions are added to
// the treeSet map.
func (t *Tree) Parse(text string) (err error) {
	defer t.recover(&err)
	t.startParse(lex(t.Name, text))
	t.text = text
	t.Root = newList(t.peek().pos)
	t.parse(t.Root)
	t.stopParse()
	return nil
}

// parse is the top-level parser for a conf.
// It runs to EOF.
func (t *Tree) parse(root *ListNode) item {
	var n Node
	for {
		switch token := t.next(); token.typ {
		case itemIdentifier:
			switch token2 := t.next(); token2.typ {
			case itemEqual:
				t.backup2(token)
				n = t.parsePair()
			case itemIdentifier, itemSubsectionIdentifier:
				t.backup2(token)
				n = t.parseSection()
			default:
				t.unexpected(token, "input")
			}
		case itemEOF:
			if root != t.Root {
				t.unexpected(token, "input")
			}
			return token
		case itemRightDelim:
			if root == t.Root {
				t.unexpected(token, "input")
			}
			return token
		default:
			t.unexpected(token, "input")
		}
		root.append(n)
	}
}

func (t *Tree) parsePair() *PairNode {
	const context = "key=value declaration"
	token := t.expect(itemIdentifier, context)
	p := newPair(token.pos)
	p.Key = newString(token.pos, token.val, token.val)
	t.expect(itemEqual, context)
	token = t.expectOneOf(itemString, itemRawString, context)
	switch token.typ {
	case itemString:
		p.Val = newString(token.pos, token.val, token.val)
	case itemRawString:
		s, err := strconv.Unquote(token.val)
		if err != nil {
			t.error(err)
		}
		p.Val = newString(token.pos, token.val, s)
	default:
		t.unexpected(token, context)
	}
	return p
}

func (t *Tree) parseSection() *SectionNode {
	const context = "section declaration"
	token := t.expect(itemIdentifier, context)
	s := newSection(token.pos)
	start := token.pos
	s.SectionType = newString(token.pos, token.val, token.val)
	token = t.expectOneOf(itemIdentifier, itemSubsectionIdentifier, context)
	s.Name = newString(token.pos, token.val, token.val)
	t.expect(itemLeftDelim, context)
	token = t.parse(s.Nodes)
	s.RawText = t.text[start : token.pos+1]
	return s
}