diff options
Diffstat (limited to 'src/encoding/json/scanner.go')
-rw-r--r-- | src/encoding/json/scanner.go | 610 |
1 files changed, 610 insertions, 0 deletions
diff --git a/src/encoding/json/scanner.go b/src/encoding/json/scanner.go new file mode 100644 index 0000000..4c43f5f --- /dev/null +++ b/src/encoding/json/scanner.go @@ -0,0 +1,610 @@ +// 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, etc). +// +// This file starts with two simple examples using the scanner +// before diving into the scanner itself. + +import ( + "strconv" + "sync" +) + +// Valid reports whether data is a valid JSON encoding. +func Valid(data []byte) bool { + scan := newScanner() + defer freeScanner(scan) + return checkValid(data, scan) == nil +} + +// checkValid verifies that data is valid JSON-encoded data. +// scan is passed in for use by checkValid to avoid an allocation. +// checkValid returns nil or a SyntaxError. +func checkValid(data []byte, scan *scanner) error { + scan.reset() + for _, c := range data { + scan.bytes++ + if scan.step(scan, c) == scanError { + return scan.err + } + } + if scan.eof() == scanError { + return scan.err + } + return nil +} + +// A SyntaxError is a description of a JSON syntax error. +// Unmarshal will return a SyntaxError if the JSON can't be parsed. +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, 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 (and deliberately + // not set to zero by scan.reset) + bytes int64 +} + +var scannerPool = sync.Pool{ + New: func() any { + return &scanner{} + }, +} + +func newScanner() *scanner { + scan := scannerPool.Get().(*scanner) + // scan.reset by design doesn't set bytes to zero + scan.bytes = 0 + scan.reset() + return scan +} + +func freeScanner(scan *scanner) { + // Avoid hanging on to too much memory in extreme cases. + if len(scan.parseState) > 1024 { + scan.parseState = nil + } + scannerPool.Put(scan) +} + +// 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 +) + +// This limits the max nesting depth to prevent stack overflow. +// This is permitted by https://tools.ietf.org/html/rfc7159#section-9 +const maxNestingDepth = 10000 + +// 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. +// an error state is returned if maxNestingDepth was exceeded, otherwise successState is returned. +func (s *scanner) pushParseState(c byte, newParseState int, successState int) int { + s.parseState = append(s.parseState, newParseState) + if len(s.parseState) <= maxNestingDepth { + return successState + } + return s.error(c, "exceeded max depth") +} + +// 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 + } +} + +func isSpace(c byte) bool { + return c <= ' ' && (c == ' ' || c == '\t' || c == '\r' || c == '\n') +} + +// stateBeginValueOrEmpty is the state after reading `[`. +func stateBeginValueOrEmpty(s *scanner, c byte) int { + if 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) int { + if isSpace(c) { + return scanSkipSpace + } + switch c { + case '{': + s.step = stateBeginStringOrEmpty + return s.pushParseState(c, parseObjectKey, scanBeginObject) + case '[': + s.step = stateBeginValueOrEmpty + return s.pushParseState(c, parseArrayValue, 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) int { + if isSpace(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 byte) int { + if 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) 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 isSpace(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 byte) int { + if !isSpace(c) { + // 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) 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 byte) int { + 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) 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 byte) 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 byte) 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 byte) 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") +} + +// stateNeg is the state after reading `-` during a number. +func stateNeg(s *scanner, c byte) 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 byte) 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 byte) 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 byte) 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 byte) int { + 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) int { + 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) 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 byte) int { + if '0' <= c && c <= '9' { + return scanContinue + } + return stateEndValue(s, c) +} + +// stateT is the state after reading `t`. +func stateT(s *scanner, c byte) 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 byte) 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 byte) 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 byte) 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 byte) 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 byte) 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 byte) 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 byte) 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 byte) 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 byte) 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 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 +} + +// 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] + "'" +} |