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// 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 json

// 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, nextValue, etc).
//
// This file starts with two simple examples using the scanner
// before diving into the scanner itself.

import "strconv"

// 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.bytes++
                if scan.step(scan, int(c)) == scanError {
                        return scan.err
                }
        }
        if scan.eof() == scanError {
                return scan.err
        }
        return nil
}

// nextValue splits data after the next whole JSON value,
// returning that value and the bytes that follow it as separate slices.
// scan is passed in for use by nextValue to avoid an allocation.
func nextValue(data []byte, scan *scanner) (value, rest []byte, err error) {
        scan.reset()
        for i, c := range data {
                v := scan.step(scan, int(c))
                if v >= scanEnd {
                        switch v {
                        case scanError:
                                return nil, nil, scan.err
                        case scanEnd:
                                return data[0:i], data[i:], nil
                        }
                }
        }
        if scan.eof() == scanError {
                return nil, nil, scan.err
        }
        return data, nil, nil
}

// 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 }

// 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, int) 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

        // 1-byte redo (see undo method)
        redo      bool
        redoCode  int
        redoState func(*scanner, int) int

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

// These values are returned by the state transition functions
// assigned to scanner.state and the method scanner.eof.
// They give details about the current state of the scan that
// callers might be interested to know about.
// It is okay to ignore the return value of any particular
// call to scanner.state: if one call returns scanError,
// every subsequent call will return scanError too.
const (
        // Continue.
        scanContinue     = 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

        // Stop.
        scanEnd   // top-level value ended *before* this byte; known to be first "stop" result
        scanError // hit an error, scanner.err.
)

// These values are stored in the parseState stack.
// They give the current state of a composite value
// being scanned.  If the parser is inside a nested value
// the parseState describes the nested state, outermost at entry 0.
const (
        parseObjectKey   = iota // parsing object key (before colon)
        parseObjectValue        // parsing object value (after colon)
        parseArrayValue         // parsing array value
)

// 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.redo = false
        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]
        s.redo = false
        if n == 0 {
                s.step = stateEndTop
                s.endTop = true
        } else {
                s.step = stateEndValue
        }
}

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

// stateBeginValueOrEmpty is the state after reading `[`.
func stateBeginValueOrEmpty(s *scanner, c int) int {
        if c <= ' ' && isSpace(rune(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 int) int {
        if c <= ' ' && isSpace(rune(c)) {
                return scanSkipSpace
        }
        switch c {
        case '{':
                s.step = stateBeginStringOrEmpty
                s.pushParseState(parseObjectKey)
                return scanBeginObject
        case '[':
                s.step = stateBeginValueOrEmpty
                s.pushParseState(parseArrayValue)
                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 int) int {
        if c <= ' ' && isSpace(rune(c)) {
                return scanSkipSpace
        }
        if c == '}' {
                n := len(s.parseState)
                s.parseState[n-1] = parseObjectValue
                return stateEndValue(s, c)
        }
        return stateBeginString(s, c)
}

// stateBeginString is the state after reading `{"key": value,`.
func stateBeginString(s *scanner, c int) int {
        if c <= ' ' && isSpace(rune(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 int) int {
        n := len(s.parseState)
        if n == 0 {
                // Completed top-level before the current byte.
                s.step = stateEndTop
                s.endTop = true
                return stateEndTop(s, c)
        }
        if c <= ' ' && isSpace(rune(c)) {
                s.step = stateEndValue
                return scanSkipSpace
        }
        ps := s.parseState[n-1]
        switch ps {
        case parseObjectKey:
                if c == ':' {
                        s.parseState[n-1] = parseObjectValue
                        s.step = stateBeginValue
                        return scanObjectKey
                }
                return s.error(c, "after object key")
        case parseObjectValue:
                if c == ',' {
                        s.parseState[n-1] = parseObjectKey
                        s.step = stateBeginString
                        return scanObjectValue
                }
                if c == '}' {
                        s.popParseState()
                        return scanEndObject
                }
                return s.error(c, "after object key:value pair")
        case parseArrayValue:
                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 int) int {
        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 int) int {
        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 int) int {
        switch c {
        case 'b', 'f', 'n', 'r', 't', '\\', '/', '"':
                s.step = stateInString
                return scanContinue
        }
        if c == '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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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")
}

// stateInStringEscU123 is the state after reading `-` during a number.
func stateNeg(s *scanner, c int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        if '0' <= c && c <= '9' {
                s.step = stateDot0
                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 int) int {
        if c == '+' {
                s.step = stateESign
                return scanContinue
        }
        if 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 int) int {
        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 int) int {
        if '0' <= c && c <= '9' {
                s.step = stateE0
                return scanContinue
        }
        return stateEndValue(s, c)
}

// stateT is the state after reading `t`.
func stateT(s *scanner, c int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        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 int) int {
        return scanError
}

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

// quoteChar formats c as a quoted character literal
func quoteChar(c int) 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] + "'"
}

// undo causes the scanner to return scanCode from the next state transition.
// This gives callers a simple 1-byte undo mechanism.
func (s *scanner) undo(scanCode int) {
        if s.redo {
                panic("json: invalid use of scanner")
        }
        s.redoCode = scanCode
        s.redoState = s.step
        s.step = stateRedo
        s.redo = true
}

// stateRedo helps implement the scanner's 1-byte undo.
func stateRedo(s *scanner, c int) int {
        s.redo = false
        s.step = s.redoState
        return s.redoCode
}