path: root/internal/jsonparse/parse.go

// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later

package jsonparse

import (
	"errors"
	"fmt"
	"io"
	iofs "io/fs"
	"strings"
)

var ErrParserExceededMaxDepth = errors.New("exceeded max depth")

func isHex(c rune) bool {
	return ('0' <= c && c <= '9') ||
		('a' <= c && c <= 'f') ||
		('A' <= c && c <= 'F')
}

// RuneType is the classification of a rune when parsing JSON input.
// A Parser, rather than grouping runes into tokens and classifying
// tokens, classifies runes directly.
type RuneType uint8

const (
	RuneTypeError RuneType = iota

	RuneTypeSpace // whitespace

	RuneTypeObjectBeg   // '{'
	RuneTypeObjectColon // ':'
	RuneTypeObjectComma // ','
	RuneTypeObjectEnd   // '}'

	RuneTypeArrayBeg   // '['
	RuneTypeArrayComma // ','
	RuneTypeArrayEnd   // ']'

	RuneTypeStringBeg   // opening '"'
	RuneTypeStringChar  // normal character
	RuneTypeStringEsc   // backslash
	RuneTypeStringEsc1  // single-char after a backslash
	RuneTypeStringEscU  // \uABCD : u
	RuneTypeStringEscUA // \uABCD : A
	RuneTypeStringEscUB // \uABCD : B
	RuneTypeStringEscUC // \uABCD : C
	RuneTypeStringEscUD // \uABCD : D
	RuneTypeStringEnd   // closing '"'

	RuneTypeNumberIntNeg
	RuneTypeNumberIntZero // leading zero only; non-leading zeros are IntDig, not IntZero
	RuneTypeNumberIntDig
	RuneTypeNumberFracDot
	RuneTypeNumberFracDig
	RuneTypeNumberExpE
	RuneTypeNumberExpSign
	RuneTypeNumberExpDig

	RuneTypeTrueT
	RuneTypeTrueR
	RuneTypeTrueU
	RuneTypeTrueE

	RuneTypeFalseF
	RuneTypeFalseA
	RuneTypeFalseL
	RuneTypeFalseS
	RuneTypeFalseE

	RuneTypeNullN
	RuneTypeNullU
	RuneTypeNullL1
	RuneTypeNullL2

	RuneTypeEOF

	// Not a real rune type, but used as a stack state.
	runeTypeAny
)

// GoString implements fmt.GoStringer.
//
//nolint:dupl // False positive due to similarly shaped AST.
func (t RuneType) GoString() string {
	str, ok := map[RuneType]string{
		RuneTypeError: "RuneTypeError",

		RuneTypeSpace: "RuneTypeSpace",

		RuneTypeObjectBeg:   "RuneTypeObjectBeg",
		RuneTypeObjectColon: "RuneTypeObjectColon",
		RuneTypeObjectComma: "RuneTypeObjectComma",
		RuneTypeObjectEnd:   "RuneTypeObjectEnd",

		RuneTypeArrayBeg:   "RuneTypeArrayBeg",
		RuneTypeArrayComma: "RuneTypeArrayComma",
		RuneTypeArrayEnd:   "RuneTypeArrayEnd",

		RuneTypeStringBeg:   "RuneTypeStringBeg",
		RuneTypeStringChar:  "RuneTypeStringChar",
		RuneTypeStringEsc:   "RuneTypeStringEsc",
		RuneTypeStringEsc1:  "RuneTypeStringEsc1",
		RuneTypeStringEscU:  "RuneTypeStringEscU",
		RuneTypeStringEscUA: "RuneTypeStringEscUA",
		RuneTypeStringEscUB: "RuneTypeStringEscUB",
		RuneTypeStringEscUC: "RuneTypeStringEscUC",
		RuneTypeStringEscUD: "RuneTypeStringEscUD",
		RuneTypeStringEnd:   "RuneTypeStringEnd",

		RuneTypeNumberIntNeg:  "RuneTypeNumberIntNeg",
		RuneTypeNumberIntZero: "RuneTypeNumberIntZero",
		RuneTypeNumberIntDig:  "RuneTypeNumberIntDig",
		RuneTypeNumberFracDot: "RuneTypeNumberFracDot",
		RuneTypeNumberFracDig: "RuneTypeNumberFracDig",
		RuneTypeNumberExpE:    "RuneTypeNumberExpE",
		RuneTypeNumberExpSign: "RuneTypeNumberExpSign",
		RuneTypeNumberExpDig:  "RuneTypeNumberExpDig",

		RuneTypeTrueT: "RuneTypeTrueT",
		RuneTypeTrueR: "RuneTypeTrueR",
		RuneTypeTrueU: "RuneTypeTrueU",
		RuneTypeTrueE: "RuneTypeTrueE",

		RuneTypeFalseF: "RuneTypeFalseF",
		RuneTypeFalseA: "RuneTypeFalseA",
		RuneTypeFalseL: "RuneTypeFalseL",
		RuneTypeFalseS: "RuneTypeFalseS",
		RuneTypeFalseE: "RuneTypeFalseE",

		RuneTypeNullN:  "RuneTypeNullN",
		RuneTypeNullU:  "RuneTypeNullU",
		RuneTypeNullL1: "RuneTypeNullL1",
		RuneTypeNullL2: "RuneTypeNullL2",

		RuneTypeEOF: "RuneTypeEOF",

		runeTypeAny: "runeTypeAny",
	}[t]
	if ok {
		return str
	}
	return fmt.Sprintf("RuneType(%d)", t)
}

// String implements fmt.Stringer.
//
//nolint:dupl // False positive due to similarly shaped AST.
func (t RuneType) String() string {
	str, ok := map[RuneType]string{
		RuneTypeError: "x",

		RuneTypeSpace: " ",

		RuneTypeObjectBeg:   "{",
		RuneTypeObjectColon: ":",
		RuneTypeObjectComma: "o",
		RuneTypeObjectEnd:   "}",

		RuneTypeArrayBeg:   "[",
		RuneTypeArrayComma: "a",
		RuneTypeArrayEnd:   "]",

		RuneTypeStringBeg:   "\"",
		RuneTypeStringChar:  "c",
		RuneTypeStringEsc:   "\\",
		RuneTypeStringEsc1:  "b",
		RuneTypeStringEscU:  "u",
		RuneTypeStringEscUA: "A",
		RuneTypeStringEscUB: "B",
		RuneTypeStringEscUC: "C",
		RuneTypeStringEscUD: "D",
		RuneTypeStringEnd:   "»",

		RuneTypeNumberIntNeg:  "-",
		RuneTypeNumberIntZero: "0",
		RuneTypeNumberIntDig:  "1",
		RuneTypeNumberFracDot: ".",
		RuneTypeNumberFracDig: "2",
		RuneTypeNumberExpE:    "e",
		RuneTypeNumberExpSign: "+",
		RuneTypeNumberExpDig:  "3",

		RuneTypeTrueT: "𝕥", // double-struck
		RuneTypeTrueR: "𝕣",
		RuneTypeTrueU: "𝕦",
		RuneTypeTrueE: "𝕖",

		RuneTypeFalseF: "𝔣", // fraktur
		RuneTypeFalseA: "𝔞",
		RuneTypeFalseL: "𝔩",
		RuneTypeFalseS: "𝔰",
		RuneTypeFalseE: "𝔢",

		RuneTypeNullN:  "ⓝ", // circled
		RuneTypeNullU:  "ⓤ",
		RuneTypeNullL1: "ⓛ",
		RuneTypeNullL2: "Ⓛ", // +uppercase

		RuneTypeEOF: "$",

		runeTypeAny: "?",
	}[t]
	if ok {
		return str
	}
	return fmt.Sprintf("<%d>", t)
}

func (t RuneType) JSONType() string {
	return map[RuneType]string{
		RuneTypeObjectBeg:     "object",
		RuneTypeArrayBeg:      "array",
		RuneTypeStringBeg:     "string",
		RuneTypeNumberIntNeg:  "number",
		RuneTypeNumberIntZero: "number",
		RuneTypeNumberIntDig:  "number",
		RuneTypeTrueT:         "true",
		RuneTypeFalseF:        "false",
		RuneTypeNullN:         "null",
		RuneTypeEOF:           "eof",
	}[t]
}

// IsNumber returns whether the RuneType is one of the
// RuneTypeNumberXXX values.
func (t RuneType) IsNumber() bool {
	return RuneTypeNumberIntNeg <= t && t <= RuneTypeNumberExpDig
}

// Parser is the low-level JSON parser that powers both *Decoder and
// *ReEncoder.
type Parser struct {
	// Setting MaxError to a value greater than 0 causes
	// HandleRune to return ErrParserExceededMaxDepth if
	// objects/arrays become nested more deeply than this.
	MaxDepth int

	initialized bool

	err    error
	closed bool

	// We reuse RuneTypes to store the stack.  The base idea is:
	// stack items are "the most recently read stack-relevant
	// RuneType".
	//
	// The stack starts out with the special pseudo-RuneType
	// `runeTypeAny` that means we're willing to accept any
	// element type; an empty stack means that we have reached the
	// end of the top-level element and should accept no more
	// input except for whitespace.
	//
	// The "normal" stack-relevant RuneTypes are:
	//
	//   "\uABC    for strings
	//   -01.2e+3  for numbers
	//   𝕥𝕣𝕦       for "true"
	//   𝔣𝔞𝔩𝔰      for "false"
	//   ⓝⓤⓛ       for "null"
	//
	// Objects and arrays break the "most recently read RuneType"
	// rule; they need some special assignments:
	//
	//   {   object: waiting for key to start or '}'
	//   »   object: reading key / waiting for colon
	//   o   object: reading value / waiting for ',' or '}'
	//
	//   [   array: waiting for item to start or ']'
	//   a   array: reading item / waiting for ',' or ']'
	//
	// Within each element type, the stack item is replaced, not pushed.
	//
	// (Keep each of these examples in-sync with parse_test.go.)
	//
	// For example, given the input string
	//
	//   {"x":"y","a":"b"}
	//
	// The stack would be
	//
	//   stack   processed
	//   ?
	//   {       {
	//   »"      {"
	//   »"      {"x
	//   »       {"x"
	//   o?      {"x":
	//   o"      {"x":"
	//   o"      {"x":"y
	//   o       {"x":"y"
	//   {       {"x":"y",
	//   »"      {"x":"y","
	//   »"      {"x":"y","a
	//   »       {"x":"y","a"
	//   o?      {"x":"y","a":
	//   o"      {"x":"y","a":"
	//   o"      {"x":"y","a":"b
	//   o       {"x":"y","a":"b"
	//           {"x":"y","a":"b"}
	//
	// Or, given the input string
	//
	//   ["x","y"]
	//
	// The stack would be
	//
	//   stack   processed
	//   ?
	//   [       [
	//   a"      ["
	//   a"      ["x
	//   a       ["x"
	//   a?      ["x",
	//   a"      ["x","
	//   a"      ["x","y
	//   a       ["x","y"
	//           ["x","y"]
	stack []RuneType

	barriers []barrier
}

type barrier struct {
	stack []RuneType
}

func (par *Parser) init() {
	if !par.initialized {
		par.initialized = true
		par.pushState(runeTypeAny)
	}
}

func (par *Parser) pushState(state RuneType) RuneType {
	par.stack = append(par.stack, state)
	return state
}

func (par *Parser) replaceState(state RuneType) RuneType {
	par.stack[len(par.stack)-1] = state
	return state
}

func (par *Parser) popState() {
	par.stack = par.stack[:len(par.stack)-1]
}

func (par *Parser) stackString() string {
	par.init()
	var buf strings.Builder
	for _, s := range par.stack {
		buf.WriteString(s.String())
	}
	return buf.String()
}

func (par *Parser) depth() int {
	n := len(par.stack)
	for _, barrier := range par.barriers {
		n += len(barrier.stack)
	}
	return n
}

func (par *Parser) StackIsEmpty() bool {
	if len(par.barriers) > 0 {
		return false
	}
	if len(par.stack) == 0 {
		return true
	}
	return len(par.stack) == 1 && par.stack[0] == runeTypeAny
}

func (par *Parser) StackSize() int {
	return len(par.stack)
}

// Reset all Parser state.
func (par *Parser) Reset() {
	*par = Parser{
		MaxDepth: par.MaxDepth,
	}
}

// PushReadBarrier causes the parser to emit EOF once the end of the
// element that is started by the current top-of-stack is reached,
// until this is un-done with PopBarrier.  It essentially turns the
// parser in to a sub-parser.
//
// PushReadBarrier may only be called at the beginning of an element,
// whether that be
//
//   - runeTypeAny
//   - RuneTypeObjectBeg
//   - RuneTypeArrayBeg
//   - RuneTypeStringBeg
//   - RuneTypeNumberIntNeg, RuneTypeNumberIntZero, RuneTypeNumberIntDig
//   - RuneTypeTrueT
//   - RuneTypeFalseF
//   - RuneTypeNullN
func (par *Parser) PushReadBarrier() {
	// Sanity checking.
	par.init()
	if len(par.stack) == 0 {
		panic(errors.New("should not happen: illegal PushReadBarrier call: empty stack"))
	}
	curState := par.stack[len(par.stack)-1]
	switch curState {
	case runeTypeAny,
		RuneTypeObjectBeg,
		RuneTypeArrayBeg,
		RuneTypeStringBeg,
		RuneTypeNumberIntNeg, RuneTypeNumberIntZero, RuneTypeNumberIntDig,
		RuneTypeTrueT,
		RuneTypeFalseF,
		RuneTypeNullN:
		// OK
	default:
		panic(fmt.Errorf("should not happen: illegal PushReadBarrier call: %q", curState))
	}
	// Actually push.
	par.barriers = append(par.barriers, barrier{
		stack: par.stack[:len(par.stack)-1],
	})
	par.stack = []RuneType{curState}
}

// PushWriteBarrier causes the parser to emit EOF once the end of the
// about-to-start element is reached, until this is un-done with
// PopBarrier.  It essentially turns the parser in to a sub-parser.
//
// PushWriteBarrier may only be called at the places where an element
// of any type may start:
//
//   - runeTypeAny for top-level and object-value elements
//   - RuneTypeArrayBeg for array-item elements
//
// PushWriteBarrier signals intent to write an element; if it is
// called in a place where an element is optional (at the beginning of
// an array), it becomes a syntax error to not write the element.
func (par *Parser) PushWriteBarrier() {
	par.init()
	if len(par.stack) == 0 {
		panic(errors.New("should not happen: illegal PushWriteBarrier call: empty stack"))
	}
	switch par.stack[len(par.stack)-1] {
	case runeTypeAny:
		par.popState()
		par.barriers = append(par.barriers, barrier{
			stack: par.stack,
		})
		par.stack = []RuneType{runeTypeAny}
	case RuneTypeArrayBeg:
		par.replaceState(RuneTypeArrayComma)
		par.barriers = append(par.barriers, barrier{
			stack: par.stack,
		})
		par.stack = []RuneType{runeTypeAny}
	default:
		panic(fmt.Errorf("should not happen: illegal PushWriteBarrier call: %q", par.stack[len(par.stack)-1]))
	}
}

// PopBarrier reverses a call to PushReadBarrier or PushWriteBarrier.
func (par *Parser) PopBarrier() {
	if len(par.barriers) == 0 {
		panic(errors.New("should not happen: illegal PopBarrier call: empty barrier stack"))
	}
	barrier := par.barriers[len(par.barriers)-1]
	par.barriers = par.barriers[:len(par.barriers)-1]
	par.closed = false
	par.stack = append(barrier.stack, par.stack...)
}

// HandleEOF feeds EOF to the Parser.  The returned RuneType is either
// RuneTypeEOF or RuneTypeError.
//
// An error is returned if and only if the RuneType is RuneTypeError.
// Returns io/fs.ErrClosed if .HandleEOF() has previously been called
// (and .Reset() has not been called since).
//
// Once RuneTypeError or RuneTypeEOF has been returned, it will keep
// being returned from both .HandleRune(c) and .HandleEOF() until
// .Reset() is called.
//
// RuneTypeEOF indicates that a complete JSON document has been read.
func (par *Parser) HandleEOF() (RuneType, error) {
	if par.closed {
		return RuneTypeError, iofs.ErrClosed
	}
	defer func() {
		par.closed = true
	}()
	if par.err != nil {
		return RuneTypeError, par.err
	}
	par.init()
	switch len(par.stack) {
	case 0:
		return RuneTypeEOF, nil
	case 1:
		switch {
		case par.stack[0].IsNumber():
			if _, err := par.HandleRune('\n'); err == nil {
				return RuneTypeEOF, nil
			}
		case par.stack[0] == runeTypeAny:
			par.err = io.EOF
			return RuneTypeError, par.err
		}
		fallthrough
	default:
		par.err = io.ErrUnexpectedEOF
		return RuneTypeError, par.err
	}
}

// HandleRune feeds a Unicode rune to the Parser.
//
// An error is returned if and only if the RuneType is RuneTypeError.
// Returns io/fs.ErrClosed if .HandleEOF() has previously been called
// (and .Reset() has not been called since).
//
// Once RuneTypeError or RuneTypeEOF has been returned, it will keep
// being returned from both .HandleRune(c) and .HandleEOF() until
// .Reset() is called.
//
// RuneTypeEOF indicates that the rune cannot be appended to the JSON
// document; a new JSON document must be started in order to process
// that rune.
func (par *Parser) HandleRune(c rune) (RuneType, error) {
	if par.closed {
		return RuneTypeError, iofs.ErrClosed
	}
	if par.err != nil {
		return RuneTypeError, par.err
	}
	par.init()
	if len(par.stack) == 0 {
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		default:
			return RuneTypeEOF, nil
		}
	}
	switch par.stack[len(par.stack)-1] {
	// any /////////////////////////////////////////////////////////////////////////////////////
	case runeTypeAny:
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		case '{':
			if par.MaxDepth > 0 && par.depth() > par.MaxDepth {
				return RuneTypeError, ErrParserExceededMaxDepth
			}
			return par.replaceState(RuneTypeObjectBeg), nil
		case '[':
			if par.MaxDepth > 0 && par.depth() > par.MaxDepth {
				return RuneTypeError, ErrParserExceededMaxDepth
			}
			return par.replaceState(RuneTypeArrayBeg), nil
		case '"':
			return par.replaceState(RuneTypeStringBeg), nil
		case '-':
			return par.replaceState(RuneTypeNumberIntNeg), nil
		case '0':
			return par.replaceState(RuneTypeNumberIntZero), nil
		case '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberIntDig), nil
		case 't':
			return par.replaceState(RuneTypeTrueT), nil
		case 'f':
			return par.replaceState(RuneTypeFalseF), nil
		case 'n':
			return par.replaceState(RuneTypeNullN), nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q looking for beginning of value", c)
		}
	// object //////////////////////////////////////////////////////////////////////////////////
	case RuneTypeObjectBeg: // waiting for key to start or '}'
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		case '"':
			par.replaceState(RuneTypeStringEnd)
			return par.pushState(RuneTypeStringBeg), nil
		case '}':
			par.popState()
			return RuneTypeObjectEnd, nil
		default:
			return RuneTypeError, fmt.Errorf("object: unexpected character: %q", c)
		}
	case RuneTypeStringEnd: // waiting for ':'
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		case ':':
			par.replaceState(RuneTypeObjectComma)
			par.pushState(runeTypeAny)
			return RuneTypeObjectColon, nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q after object key", c)
		}
	case RuneTypeObjectComma: // waiting for ',' or '}'
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		case ',':
			par.replaceState(RuneTypeObjectBeg)
			return RuneTypeObjectComma, nil
		case '}':
			par.popState()
			return RuneTypeObjectEnd, nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q after object key:value pair", c)
		}
	// array ///////////////////////////////////////////////////////////////////////////////////
	case RuneTypeArrayBeg: // waiting for item to start or ']'
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		case ']':
			par.popState()
			return RuneTypeArrayEnd, nil
		default:
			par.replaceState(RuneTypeArrayComma)
			par.pushState(runeTypeAny)
			return par.HandleRune(c)
		}
	case RuneTypeArrayComma: // waiting for ',' or ']'
		switch c {
		case 0x0020, 0x000A, 0x000D, 0x0009:
			return RuneTypeSpace, nil
		case ',':
			par.pushState(runeTypeAny)
			return RuneTypeArrayComma, nil
		case ']':
			par.popState()
			return RuneTypeArrayEnd, nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q after array element", c)
		}
	// string //////////////////////////////////////////////////////////////////////////////////
	case RuneTypeStringBeg: // waiting for char or '"'
		switch {
		case c == '\\':
			return par.replaceState(RuneTypeStringEsc), nil
		case c == '"':
			par.popState()
			return RuneTypeStringEnd, nil
		case 0x0020 <= c && c <= 0x10FFFF:
			return RuneTypeStringChar, nil
		default:
			return RuneTypeError, fmt.Errorf("string: unexpected character: %q", c)
		}
	case RuneTypeStringEsc: // waiting for escape char
		switch c {
		case '"', '\\', '/', 'b', 'f', 'n', 'r', 't':
			par.replaceState(RuneTypeStringBeg)
			return RuneTypeStringEsc1, nil
		case 'u':
			return par.replaceState(RuneTypeStringEscU), nil
		default:
			return RuneTypeError, fmt.Errorf("string backslash sequence: unexpected character: %q", c)
		}
	case RuneTypeStringEscU:
		if !isHex(c) {
			return RuneTypeError, fmt.Errorf("string unicode sequence: unexpected character: %q", c)
		}
		return par.replaceState(RuneTypeStringEscUA), nil
	case RuneTypeStringEscUA:
		if !isHex(c) {
			return RuneTypeError, fmt.Errorf("string unicode sequence: unexpected character: %q", c)
		}
		return par.replaceState(RuneTypeStringEscUB), nil
	case RuneTypeStringEscUB:
		if !isHex(c) {
			return RuneTypeError, fmt.Errorf("string unicode sequence: unexpected character: %q", c)
		}
		return par.replaceState(RuneTypeStringEscUC), nil
	case RuneTypeStringEscUC:
		if !isHex(c) {
			return RuneTypeError, fmt.Errorf("string unicode sequence: unexpected character: %q", c)
		}
		par.replaceState(RuneTypeStringBeg)
		return RuneTypeStringEscUD, nil
	// number //////////////////////////////////////////////////////////////////////////////////
	//
	// Here's a flattened drawing of the syntax diagram from www.json.org :
	//
	//      [------------ integer ----------][-- fraction ---][-------- exponent -------]
	//     >─╮─────╭─╮─"0"───────╭─────────╭──╮─────────────╭──╮───────────────────────╭─>
	//       │     │ │           │         │  │             │  │                       │
	//       ╰─"-"─╯ ╰─digit 1-9─╯─╭digit╮─╯  ╰─"."─╭digit╮─╯  ╰─"e"─╭─╮─────╭─╭digit╮─╯
	//                             ╰──<──╯          ╰──<──╯    │     │ │     │ ╰──<──╯
	//                                                         ╰─"E"─╯ ╰─"-"─╯
	//                                                                 │     │
	//                                                                 ╰─"+"─╯
	//
	// Now here it is slightly redrawn, and with each distinct state our
	// parser can be in marked with a single-capital-letter:
	//
	//        [-------------- integer ------------][--------- fraction --------][--------- exponent ---------]
	//     >─A─╮───────╭──╮─"0"─────────C─╭─────────╮──────────────────╭─────────╮──────────────────────────╭─>
	//         │       │  │               │         │                  │         │                          │
	//         ╰─"-"─B─╯  ╰─digit 1-9─╭─D─╯─digit╮  ╰─"."─E─digit──╭─F─╯─digit╮  ╰─"e"─╭─G─╮─────╭─╭digit─I─╯
	//                                ╰────<─────╯                 ╰────<─────╯  │     │   │     H ╰────<───╯
	//                                                                           ╰─"E"─╯   ╰─"-"─╯
	//                                                                                     │     │
	//                                                                                     ╰─"+"─╯
	//
	// You may notice that each of these states may be uniquely identified
	// by the last-read RuneType:
	//
	//     A = (nothing yet)
	//     B = IntNeg
	//     C = IntZero
	//     D = IntDig
	//     E = FracDot
	//     F = FracDig
	//     G = ExpE
	//     H = ExpSign
	//     I = ExpDig
	//
	// The 'A' state is part of the runeTypeAny case above, and
	// the remainder follow:
	case RuneTypeNumberIntNeg: // B
		switch c {
		case '0':
			return par.replaceState(RuneTypeNumberIntZero), nil
		case '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberIntDig), nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q in numeric literal", c)
		}
	case RuneTypeNumberIntZero: // C
		switch c {
		case '.':
			return par.replaceState(RuneTypeNumberFracDot), nil
		case 'e', 'E':
			return par.replaceState(RuneTypeNumberExpE), nil
		default:
			par.popState()
			return par.HandleRune(c)
		}
	case RuneTypeNumberIntDig: // D
		switch c {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberIntDig), nil
		case '.':
			return par.replaceState(RuneTypeNumberFracDot), nil
		case 'e', 'E':
			return par.replaceState(RuneTypeNumberExpE), nil
		default:
			par.popState()
			return par.HandleRune(c)
		}
	case RuneTypeNumberFracDot: // E
		switch c {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberFracDig), nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q in numeric literal", c)
		}
	case RuneTypeNumberFracDig: // F
		switch c {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberFracDig), nil
		case 'e', 'E':
			return par.replaceState(RuneTypeNumberExpE), nil
		default:
			par.popState()
			return par.HandleRune(c)
		}
	case RuneTypeNumberExpE: // G
		switch c {
		case '-', '+':
			return par.replaceState(RuneTypeNumberExpSign), nil
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberExpDig), nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q in numeric literal", c)
		}
	case RuneTypeNumberExpSign: // H
		switch c {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberExpDig), nil
		default:
			return RuneTypeError, fmt.Errorf("invalid character %q in numeric literal", c)
		}
	case RuneTypeNumberExpDig: // I
		switch c {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			return par.replaceState(RuneTypeNumberExpDig), nil
		default:
			par.popState()
			return par.HandleRune(c)
		}
	// literals ////////////////////////////////////////////////////////////////////////////////
	// true
	case RuneTypeTrueT:
		return par.expectRune(c, 'r', RuneTypeTrueR, "true", false)
	case RuneTypeTrueR:
		return par.expectRune(c, 'u', RuneTypeTrueU, "true", false)
	case RuneTypeTrueU:
		return par.expectRune(c, 'e', RuneTypeTrueE, "true", true)
	// false
	case RuneTypeFalseF:
		return par.expectRune(c, 'a', RuneTypeFalseA, "false", false)
	case RuneTypeFalseA:
		return par.expectRune(c, 'l', RuneTypeFalseL, "false", false)
	case RuneTypeFalseL:
		return par.expectRune(c, 's', RuneTypeFalseS, "false", false)
	case RuneTypeFalseS:
		return par.expectRune(c, 'e', RuneTypeFalseE, "false", true)
	// null
	case RuneTypeNullN:
		return par.expectRune(c, 'u', RuneTypeNullU, "null", false)
	case RuneTypeNullU:
		return par.expectRune(c, 'l', RuneTypeNullL1, "null", false)
	case RuneTypeNullL1:
		return par.expectRune(c, 'l', RuneTypeNullL2, "null", true)
	default:
		panic(fmt.Errorf(`should not happen: invalid stack: "%s"`, par.stackString()))
	}
}

func (par *Parser) expectRune(c, exp rune, typ RuneType, context string, pop bool) (RuneType, error) {
	if c != exp {
		return RuneTypeError, fmt.Errorf("invalid character %q in literal %s (expecting %q)", c, context, exp)
	}
	if pop {
		par.popState()
		return typ, nil
	} else {
		return par.replaceState(typ), nil
	}
}