// Copyright 2010 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 scanner

// JSON value parser state machine.
// Just about at the limit of what is reasonable to write by hand.
// Some parts are a bit tedious, but overall it nicely factors out the
// otherwise common code from the multiple scanning functions
// in this package (Compact, Indent, checkValid, etc).
//
import "strconv"

// A scanner is a JSON scanning state machine.
// Callers call scan.reset() and then pass bytes in one at a time
// by calling scan.step(&scan, c) for each byte.
// The return value, referred to as an opcode, tells the
// caller about significant parsing events like beginning
// and ending literals, objects, and arrays, so that the
// caller can follow along if it wishes.
// The return value scanEnd indicates that a single top-level
// JSON value has been completed, *before* the byte that
// just got passed in.  (The indication must be delayed in order
// to recognize the end of numbers: is 123 a whole value or
// the beginning of 12345e+6?).
type scanner struct {
	// The step is a func to be called to execute the next transition.
	// Also tried using an integer constant and a single func
	// with a switch, but using the func directly was 10% faster
	// on a 64-bit Mac Mini, and it's nicer to read.
	step func(*scanner, byte) int

	// Reached end of top-level value.
	endTop bool

	// Stack of what we're in the middle of - array values, object keys, object values.
	parseState []int

	// Error that happened, if any.
	err error

	// total bytes consumed, updated by decoder.Decode
	bytes int64
}

// reset prepares the scanner for use.
// It must be called before calling s.step.
func (s *scanner) reset() {
	s.step = stateBeginValue
	s.parseState = s.parseState[0:0]
	s.err = nil
	s.endTop = false
}

// eof tells the scanner that the end of input has been reached.
// It returns a scan status just as s.step does.
func (s *scanner) eof() int {
	if s.err != nil {
		return scanError
	}
	if s.endTop {
		return scanEnd
	}
	s.step(s, ' ')
	if s.endTop {
		return scanEnd
	}
	if s.err == nil {
		s.err = &SyntaxError{"unexpected end of JSON input", s.bytes}
	}
	return scanError
}

// pushParseState pushes a new parse state p onto the parse stack.
func (s *scanner) pushParseState(p int) {
	s.parseState = append(s.parseState, p)
}

// popParseState pops a parse state (already obtained) off the stack
// and updates s.step accordingly.
func (s *scanner) popParseState() {
	n := len(s.parseState) - 1
	s.parseState = s.parseState[0:n]
	if n == 0 {
		s.step = stateEndTop
		s.endTop = true
	} else {
		s.step = stateEndValue
	}
}

// stateError is the state after reaching a syntax error,
// such as after reading `[1}` or `5.1.2`.
func stateError(s *scanner, c byte) int {
	return scanError
}

// error records an error and switches to the error state.
func (s *scanner) error(c byte, context string) int {
	s.step = stateError
	s.err = &SyntaxError{"invalid character " + quoteChar(c) + " " + context, s.bytes}
	return scanError
}

// A SyntaxError is a description of a JSON syntax error.
type SyntaxError struct {
	msg    string // description of error
	Offset int64  // error occurred after reading Offset bytes
}

func (e *SyntaxError) Error() string { return e.msg }

// quoteChar formats c as a quoted character literal
func quoteChar(c byte) string {
	// special cases - different from quoted strings
	if c == '\'' {
		return `'\''`
	}
	if c == '"' {
		return `'"'`
	}

	// use quoted string with different quotation marks
	s := strconv.Quote(string(c))
	return "'" + s[1:len(s)-1] + "'"
}