// Copyright (c) 2009 The Go Authors. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//    * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//    * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
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// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// JSON value parser state machine.
// This package is almost entirely copied from the Go stdlib.
// Changes made to it permit users of the package to tell
// if some slice of bytes is a valid beginning of a json string.
package json

import "fmt"

type (
	context    int
	scanStatus int
)

const (
	contextKey context = iota
	contextObj
	contextArr

	scanContinue     scanStatus = iota // uninteresting byte
	scanBeginLiteral                   // end implied by next result != scanContinue
	scanBeginObject                    // begin object
	scanObjectKey                      // just finished object key (string)
	scanObjectValue                    // just finished non-last object value
	scanEndObject                      // end object (implies scanObjectValue if possible)
	scanBeginArray                     // begin array
	scanArrayValue                     // just finished array value
	scanEndArray                       // end array (implies scanArrayValue if possible)
	scanSkipSpace                      // space byte; can skip; known to be last "continue" result
	scanEnd                            // top-level value ended *before* this byte; known to be first "stop" result
	scanError                          // hit an error, scanner.err.
)

type (
	scanner struct {
		step     func(*scanner, byte) scanStatus
		contexts []context
		endTop   bool
		err      error
		index    int
	}
)

// Scan returns the number of bytes scanned and if there was any error
// in trying to reach the end of data
func Scan(data []byte) (int, error) {
	s := &scanner{}
	_ = checkValid(data, s)
	return s.index, s.err
}

// checkValid verifies that data is valid JSON-encoded data.
// scan is passed in for use by checkValid to avoid an allocation.
func checkValid(data []byte, scan *scanner) error {
	scan.reset()
	for _, c := range data {
		scan.index++
		if scan.step(scan, c) == scanError {
			return scan.err
		}
	}
	if scan.eof() == scanError {
		return scan.err
	}
	return nil
}

func isSpace(c byte) bool {
	return c == ' ' || c == '\t' || c == '\r' || c == '\n'
}

func (s *scanner) reset() {
	s.step = stateBeginValue
	s.contexts = s.contexts[0:0]
	s.err = nil
}

// 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() scanStatus {
	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 = fmt.Errorf("unexpected end of JSON input")
	}
	return scanError
}

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

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

// stateBeginValueOrEmpty is the state after reading `[`.
func stateBeginValueOrEmpty(s *scanner, c byte) scanStatus {
	if c <= ' ' && isSpace(c) {
		return scanSkipSpace
	}
	if c == ']' {
		return stateEndValue(s, c)
	}
	return stateBeginValue(s, c)
}

// stateBeginValue is the state at the beginning of the input.
func stateBeginValue(s *scanner, c byte) scanStatus {
	if c <= ' ' && isSpace(c) {
		return scanSkipSpace
	}
	switch c {
	case '{':
		s.step = stateBeginStringOrEmpty
		s.pushParseState(contextKey)
		return scanBeginObject
	case '[':
		s.step = stateBeginValueOrEmpty
		s.pushParseState(contextArr)
		return scanBeginArray
	case '"':
		s.step = stateInString
		return scanBeginLiteral
	case '-':
		s.step = stateNeg
		return scanBeginLiteral
	case '0': // beginning of 0.123
		s.step = state0
		return scanBeginLiteral
	case 't': // beginning of true
		s.step = stateT
		return scanBeginLiteral
	case 'f': // beginning of false
		s.step = stateF
		return scanBeginLiteral
	case 'n': // beginning of null
		s.step = stateN
		return scanBeginLiteral
	}
	if '1' <= c && c <= '9' { // beginning of 1234.5
		s.step = state1
		return scanBeginLiteral
	}
	return s.error(c, "looking for beginning of value")
}

// stateBeginStringOrEmpty is the state after reading `{`.
func stateBeginStringOrEmpty(s *scanner, c byte) scanStatus {
	if c <= ' ' && isSpace(c) {
		return scanSkipSpace
	}
	if c == '}' {
		n := len(s.contexts)
		s.contexts[n-1] = contextObj
		return stateEndValue(s, c)
	}
	return stateBeginString(s, c)
}

// stateBeginString is the state after reading `{"key": value,`.
func stateBeginString(s *scanner, c byte) scanStatus {
	if c <= ' ' && isSpace(c) {
		return scanSkipSpace
	}
	if c == '"' {
		s.step = stateInString
		return scanBeginLiteral
	}
	return s.error(c, "looking for beginning of object key string")
}

// stateEndValue is the state after completing a value,
// such as after reading `{}` or `true` or `["x"`.
func stateEndValue(s *scanner, c byte) scanStatus {
	n := len(s.contexts)
	if n == 0 {
		// Completed top-level before the current byte.
		s.step = stateEndTop
		s.endTop = true
		return stateEndTop(s, c)
	}
	if c <= ' ' && isSpace(c) {
		s.step = stateEndValue
		return scanSkipSpace
	}
	ps := s.contexts[n-1]
	switch ps {
	case contextKey:
		if c == ':' {
			s.contexts[n-1] = contextObj
			s.step = stateBeginValue
			return scanObjectKey
		}
		return s.error(c, "after object key")
	case contextObj:
		if c == ',' {
			s.contexts[n-1] = contextKey
			s.step = stateBeginString
			return scanObjectValue
		}
		if c == '}' {
			s.popParseState()
			return scanEndObject
		}
		return s.error(c, "after object key:value pair")
	case contextArr:
		if c == ',' {
			s.step = stateBeginValue
			return scanArrayValue
		}
		if c == ']' {
			s.popParseState()
			return scanEndArray
		}
		return s.error(c, "after array element")
	}
	return s.error(c, "")
}

// stateEndTop is the state after finishing the top-level value,
// such as after reading `{}` or `[1,2,3]`.
// Only space characters should be seen now.
func stateEndTop(s *scanner, c byte) scanStatus {
	if c != ' ' && c != '\t' && c != '\r' && c != '\n' {
		// Complain about non-space byte on next call.
		s.error(c, "after top-level value")
	}
	return scanEnd
}

// stateInString is the state after reading `"`.
func stateInString(s *scanner, c byte) scanStatus {
	if c == '"' {
		s.step = stateEndValue
		return scanContinue
	}
	if c == '\\' {
		s.step = stateInStringEsc
		return scanContinue
	}
	if c < 0x20 {
		return s.error(c, "in string literal")
	}
	return scanContinue
}

// stateInStringEsc is the state after reading `"\` during a quoted string.
func stateInStringEsc(s *scanner, c byte) scanStatus {
	switch c {
	case 'b', 'f', 'n', 'r', 't', '\\', '/', '"':
		s.step = stateInString
		return scanContinue
	case 'u':
		s.step = stateInStringEscU
		return scanContinue
	}
	return s.error(c, "in string escape code")
}

// stateInStringEscU is the state after reading `"\u` during a quoted string.
func stateInStringEscU(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
		s.step = stateInStringEscU1
		return scanContinue
	}
	// numbers
	return s.error(c, "in \\u hexadecimal character escape")
}

// stateInStringEscU1 is the state after reading `"\u1` during a quoted string.
func stateInStringEscU1(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
		s.step = stateInStringEscU12
		return scanContinue
	}
	// numbers
	return s.error(c, "in \\u hexadecimal character escape")
}

// stateInStringEscU12 is the state after reading `"\u12` during a quoted string.
func stateInStringEscU12(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
		s.step = stateInStringEscU123
		return scanContinue
	}
	// numbers
	return s.error(c, "in \\u hexadecimal character escape")
}

// stateInStringEscU123 is the state after reading `"\u123` during a quoted string.
func stateInStringEscU123(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F' {
		s.step = stateInString
		return scanContinue
	}
	// numbers
	return s.error(c, "in \\u hexadecimal character escape")
}

// stateNeg is the state after reading `-` during a number.
func stateNeg(s *scanner, c byte) scanStatus {
	if c == '0' {
		s.step = state0
		return scanContinue
	}
	if '1' <= c && c <= '9' {
		s.step = state1
		return scanContinue
	}
	return s.error(c, "in numeric literal")
}

// state1 is the state after reading a non-zero integer during a number,
// such as after reading `1` or `100` but not `0`.
func state1(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' {
		s.step = state1
		return scanContinue
	}
	return state0(s, c)
}

// state0 is the state after reading `0` during a number.
func state0(s *scanner, c byte) scanStatus {
	if c == '.' {
		s.step = stateDot
		return scanContinue
	}
	if c == 'e' || c == 'E' {
		s.step = stateE
		return scanContinue
	}
	return stateEndValue(s, c)
}

// stateDot is the state after reading the integer and decimal point in a number,
// such as after reading `1.`.
func stateDot(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' {
		s.step = stateDot0
		return scanContinue
	}
	return s.error(c, "after decimal point in numeric literal")
}

// stateDot0 is the state after reading the integer, decimal point, and subsequent
// digits of a number, such as after reading `3.14`.
func stateDot0(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' {
		return scanContinue
	}
	if c == 'e' || c == 'E' {
		s.step = stateE
		return scanContinue
	}
	return stateEndValue(s, c)
}

// stateE is the state after reading the mantissa and e in a number,
// such as after reading `314e` or `0.314e`.
func stateE(s *scanner, c byte) scanStatus {
	if c == '+' || c == '-' {
		s.step = stateESign
		return scanContinue
	}
	return stateESign(s, c)
}

// stateESign is the state after reading the mantissa, e, and sign in a number,
// such as after reading `314e-` or `0.314e+`.
func stateESign(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' {
		s.step = stateE0
		return scanContinue
	}
	return s.error(c, "in exponent of numeric literal")
}

// stateE0 is the state after reading the mantissa, e, optional sign,
// and at least one digit of the exponent in a number,
// such as after reading `314e-2` or `0.314e+1` or `3.14e0`.
func stateE0(s *scanner, c byte) scanStatus {
	if '0' <= c && c <= '9' {
		return scanContinue
	}
	return stateEndValue(s, c)
}

// stateT is the state after reading `t`.
func stateT(s *scanner, c byte) scanStatus {
	if c == 'r' {
		s.step = stateTr
		return scanContinue
	}
	return s.error(c, "in literal true (expecting 'r')")
}

// stateTr is the state after reading `tr`.
func stateTr(s *scanner, c byte) scanStatus {
	if c == 'u' {
		s.step = stateTru
		return scanContinue
	}
	return s.error(c, "in literal true (expecting 'u')")
}

// stateTru is the state after reading `tru`.
func stateTru(s *scanner, c byte) scanStatus {
	if c == 'e' {
		s.step = stateEndValue
		return scanContinue
	}
	return s.error(c, "in literal true (expecting 'e')")
}

// stateF is the state after reading `f`.
func stateF(s *scanner, c byte) scanStatus {
	if c == 'a' {
		s.step = stateFa
		return scanContinue
	}
	return s.error(c, "in literal false (expecting 'a')")
}

// stateFa is the state after reading `fa`.
func stateFa(s *scanner, c byte) scanStatus {
	if c == 'l' {
		s.step = stateFal
		return scanContinue
	}
	return s.error(c, "in literal false (expecting 'l')")
}

// stateFal is the state after reading `fal`.
func stateFal(s *scanner, c byte) scanStatus {
	if c == 's' {
		s.step = stateFals
		return scanContinue
	}
	return s.error(c, "in literal false (expecting 's')")
}

// stateFals is the state after reading `fals`.
func stateFals(s *scanner, c byte) scanStatus {
	if c == 'e' {
		s.step = stateEndValue
		return scanContinue
	}
	return s.error(c, "in literal false (expecting 'e')")
}

// stateN is the state after reading `n`.
func stateN(s *scanner, c byte) scanStatus {
	if c == 'u' {
		s.step = stateNu
		return scanContinue
	}
	return s.error(c, "in literal null (expecting 'u')")
}

// stateNu is the state after reading `nu`.
func stateNu(s *scanner, c byte) scanStatus {
	if c == 'l' {
		s.step = stateNul
		return scanContinue
	}
	return s.error(c, "in literal null (expecting 'l')")
}

// stateNul is the state after reading `nul`.
func stateNul(s *scanner, c byte) scanStatus {
	if c == 'l' {
		s.step = stateEndValue
		return scanContinue
	}
	return s.error(c, "in literal null (expecting 'l')")
}

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

// error records an error and switches to the error state.
func (s *scanner) error(c byte, context string) scanStatus {
	s.step = stateError
	s.err = fmt.Errorf("invalid character <<%c>> %s", c, context)
	return scanError
}
