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

package lowmemjson

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

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 '"'

	RuneTypeNumberInt  // 0|[1-9][0-9]*
	RuneTypeNumberFrac // \.[0-9]*
	RuneTypeNumberExp  // [eE][-+]?[0-9]

	RuneTypeTrueT
	RuneTypeTrueR
	RuneTypeTrueU
	RuneTypeTrueE

	RuneTypeFalseF
	RuneTypeFalseA
	RuneTypeFalseL
	RuneTypeFalseS
	RuneTypeFalseE

	RuneTypeNullN
	RuneTypeNullU
	RuneTypeNullL1
	RuneTypeNullL2
)

type parseState func(rune) (RuneType, error)

type parser struct {
	err    error
	closed bool

	stack          []parseState
	stack0IsNumber bool // whether stack[0] is a number-state; affects how EOF is handled
}

// "public" API ////////////////////////////////////////////////////////////////////////////////////

func (par *parser) HandleRune(c rune) (RuneType, error) {
	if par.closed {
		return RuneTypeError, iofs.ErrClosed
	}
	if par.err != nil {
		return RuneTypeError, par.err
	}
	return par.state(c)
}

func (par *parser) HandleEOF() error {
	if par.closed {
		return iofs.ErrClosed
	}
	if par.err == nil {
		switch len(par.stack) {
		case 0:
			par.err = nil
		case 1:
			if par.stack0IsNumber {
				_, par.err = par.state('\n')
			}
			fallthrough
		default:
			par.err = io.ErrUnexpectedEOF
		}
	}
	par.closed = true
	return par.err
}

// state helpers ///////////////////////////////////////////////////////////////////////////////////

func (par *parser) pushState(state parseState, isNumber bool) {
	if len(par.stack) == 0 {
		par.stack0IsNumber = isNumber
	}
	par.stack = append(par.stack, state)
}
func (par *parser) replaceState(state parseState, isNumber bool) {
	if len(par.stack) == 1 {
		par.stack0IsNumber = isNumber
	}
	par.stack[len(par.stack)-1] = state
}
func (par *parser) popState() {
	if len(par.stack) == 1 {
		par.stack0IsNumber = false
	}
	par.stack = par.stack[:len(par.stack)-1]
}

func (par *parser) state(c rune) (RuneType, error) {
	if len(par.stack) == 0 {
		par.pushState(par.stateAny, false)
	}
	return par.stack[len(par.stack)-1](c)
}

// state: any //////////////////////////////////////////////////////////////////////////////////////

func (par *parser) stateAny(c rune) (RuneType, error) {
	switch c {
	case 0x0020, 0x000A, 0x000D, 0x0009:
		return RuneTypeSpace, nil
	case '{':
		par.replaceState(par.stateInObject, false)
		return RuneTypeObjectBeg, nil
	case '[':
		par.replaceState(par.stateInArray, false)
		return RuneTypeArrayBeg, nil
	case '"':
		par.replaceState(par.stateInString, false)
		return RuneTypeStringBeg, nil
	case '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		par.replaceState(par.stateNumberA, true)
		return par.state(c)
	case 't':
		par.replaceState(par.stateTrueT, false)
		return RuneTypeTrueT, nil
	case 'f':
		par.replaceState(par.stateFalseF, false)
		return RuneTypeFalseF, nil
	case 'n':
		par.replaceState(par.stateNullN, false)
		return RuneTypeNullN, nil
	default:
		return RuneTypeError, fmt.Errorf("any: unexpected character: %q", c)
	}
}

// state: object ///////////////////////////////////////////////////////////////////////////////////

func (par *parser) stateInObject(c rune) (RuneType, error) {
	switch c {
	case 0x0020, 0x000A, 0x000D, 0x0009:
		return RuneTypeSpace, nil
	case '"':
		par.replaceState(par.stateAfterK, false)
		par.pushState(par.stateInString, false)
		return RuneTypeStringBeg, nil
	case '}':
		par.popState()
		return RuneTypeObjectEnd, nil
	default:
		return RuneTypeError, fmt.Errorf("object: unexpected character: %q", c)
	}
}
func (par *parser) stateAfterK(c rune) (RuneType, error) {
	switch c {
	case 0x0020, 0x000A, 0x000D, 0x0009:
		return RuneTypeSpace, nil
	case ':':
		par.replaceState(par.stateAfterV, false)
		par.pushState(par.stateAny, false)
		return RuneTypeObjectColon, nil
	default:
		return RuneTypeError, fmt.Errorf("object member: unexpected character: %q", c)
	}
}
func (par *parser) stateAfterV(c rune) (RuneType, error) {
	switch c {
	case 0x0020, 0x000A, 0x000D, 0x0009:
		return RuneTypeSpace, nil
	case ',':
		par.replaceState(par.stateInObject, false)
		return RuneTypeObjectComma, nil
	case '}':
		par.popState()
		return RuneTypeObjectEnd, nil
	default:
		return RuneTypeError, fmt.Errorf("object member: unexpected character: %q", c)
	}
}

// state: array ////////////////////////////////////////////////////////////////////////////////////

func (par *parser) stateInArray(c rune) (RuneType, error) {
	switch c {
	case 0x0020, 0x000A, 0x000D, 0x0009:
		return RuneTypeSpace, nil
	case ']':
		par.popState()
		return RuneTypeArrayEnd, nil
	default:
		par.replaceState(par.stateAfterItem, false)
		par.pushState(par.stateAny, false)
		return par.state(c)
	}
}
func (par *parser) stateAfterItem(c rune) (RuneType, error) {
	switch c {
	case 0x0020, 0x000A, 0x000D, 0x0009:
		return RuneTypeSpace, nil
	case ',':
		par.replaceState(par.stateInArray, false)
		return RuneTypeArrayComma, nil
	case ']':
		par.popState()
		return RuneTypeArrayEnd, nil
	default:
		return RuneTypeError, fmt.Errorf("array: unexpected character: %q", c)
	}
}

// state: string ///////////////////////////////////////////////////////////////////////////////////

func (par *parser) stateInString(c rune) (RuneType, error) {
	switch {
	case c == '\\':
		par.replaceState(par.stateInEsc, false)
		return 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)
	}
}
func (par *parser) stateInEsc(c rune) (RuneType, error) {
	switch c {
	case '"', '\\', '/', 'b', 'f', 'n', 'r', 't':
		par.replaceState(par.stateInString, false)
		return RuneTypeStringEsc1, nil
	case 'u':
		par.replaceState(par.stateInEscU, false)
		return RuneTypeStringEscU, nil
	default:
		return RuneTypeError, fmt.Errorf("string backslash sequence: unexpected character: %q", c)
	}
}
func (par *parser) _stateInEscU(c rune, typ RuneType, nxt parseState) (RuneType, error) {
	switch {
	case ('0' <= c && c <= '9') || ('a' <= c && c <= 'f') || ('A' <= c && c <= 'F'):
		par.replaceState(nxt, false)
		return typ, nil
	default:
		return RuneTypeError, fmt.Errorf("string unicode sequence: unexpected character: %q", c)
	}
}
func (par *parser) stateInEscU(c rune) (RuneType, error) {
	return par._stateInEscU(c, RuneTypeStringEscUA, par.stateInEscUA)
}
func (par *parser) stateInEscUA(c rune) (RuneType, error) {
	return par._stateInEscU(c, RuneTypeStringEscUB, par.stateInEscUB)
}
func (par *parser) stateInEscUB(c rune) (RuneType, error) {
	return par._stateInEscU(c, RuneTypeStringEscUC, par.stateInEscUC)
}
func (par *parser) stateInEscUC(c rune) (RuneType, error) {
	return par._stateInEscU(c, RuneTypeStringEscUD, par.stateInString)
}

// state: 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─H─╯
//                                ╰────<─────╯                 ╰────<─────╯  │     │   │     │ ╰────<───╯
//                                                                           ╰─"E"─╯   ╰─"-"─╯
//                                                                                     │     │
//                                                                                     ╰─"+"─╯
//
// Which state we're at is the 'X' in 'stateNumberX'.

// number: integer-part ////////////////////////////////////////////////////////
func (par *parser) stateNumberA(c rune) (RuneType, error) { // start
	switch c {
	case '-':
		par.replaceState(par.stateNumberB, true)
		return RuneTypeNumberInt, nil
	case '0':
		par.replaceState(par.stateNumberC, true)
		return RuneTypeNumberInt, nil
	case '1', '2', '3', '4', '5', '6', '7', '8', '9':
		par.replaceState(par.stateNumberD, true)
		return RuneTypeNumberInt, nil
	default:
		return RuneTypeError, fmt.Errorf("number: unexpected character: %q", c)
	}
}
func (par *parser) stateNumberB(c rune) (RuneType, error) { // got a leading "-"
	switch c {
	case '0':
		par.replaceState(par.stateNumberC, true)
		return RuneTypeNumberInt, nil
	case '1', '2', '3', '4', '5', '6', '7', '8', '9':
		par.replaceState(par.stateNumberD, true)
		return RuneTypeNumberInt, nil
	default:
		return RuneTypeError, fmt.Errorf("number: unexpected character: %q", c)
	}
}
func (par *parser) stateNumberC(c rune) (RuneType, error) { // ready for the fraction or exponent part to start
	switch c {
	case '.':
		par.replaceState(par.stateNumberE, true)
		return RuneTypeNumberFrac, nil
	case 'e', 'E':
		par.replaceState(par.stateNumberG, true)
		return RuneTypeNumberExp, nil
	default:
		par.popState()
		return par.state(c)
	}
}
func (par *parser) stateNumberD(c rune) (RuneType, error) { // in the integer part
	switch c {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		return RuneTypeNumberInt, nil
	case '.':
		par.replaceState(par.stateNumberE, true)
		return RuneTypeNumberFrac, nil
	case 'e', 'E':
		par.replaceState(par.stateNumberG, true)
		return RuneTypeNumberExp, nil
	default:
		par.popState()
		return par.state(c)
	}
}

// number: fraction-part ///////////////////////////////////////////////////////
func (par *parser) stateNumberE(c rune) (RuneType, error) { // got a ".", ready to read a number for the fraction part
	switch c {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		par.replaceState(par.stateNumberF, true)
		return RuneTypeNumberFrac, nil
	default:
		return RuneTypeError, fmt.Errorf("number: unexpected character: %q", c)
	}
}
func (par *parser) stateNumberF(c rune) (RuneType, error) { // in the fraction part
	switch c {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		return RuneTypeNumberFrac, nil
	case 'e', 'E':
		par.replaceState(par.stateNumberG, true)
		return RuneTypeNumberExp, nil
	default:
		par.popState()
		return par.state(c)
	}
}

// number: exponent-part ///////////////////////////////////////////////////////
func (par *parser) stateNumberG(c rune) (RuneType, error) { // got a leading "e"
	switch c {
	case '-', '+', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		par.replaceState(par.stateNumberH, true)
		return RuneTypeNumberExp, nil
	default:
		return RuneTypeError, fmt.Errorf("number: unexpected character: %c", c)
	}
}
func (par *parser) stateNumberH(c rune) (RuneType, error) { // in the exponent's number part
	switch c {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		return RuneTypeNumberExp, nil
	default:
		par.popState()
		return par.state(c)
	}
}

// state: literals /////////////////////////////////////////////////////////////////////////////////

func (par *parser) l(c rune, full string, exp rune, typ RuneType, nxt parseState) (RuneType, error) {
	if c != exp {
		return RuneTypeError, fmt.Errorf("%s: unexpected character: %q", full, c)
	}
	if nxt == nil {
		par.popState()
	} else {
		par.replaceState(nxt, false)
	}
	return typ, nil
}

func (par *parser) stateTrueT(c rune) (RuneType, error) {
	return par.l(c, "true", 'r', RuneTypeTrueR, par.stateTrueR)
}
func (par *parser) stateTrueR(c rune) (RuneType, error) {
	return par.l(c, "true", 'u', RuneTypeTrueU, par.stateTrueU)
}
func (par *parser) stateTrueU(c rune) (RuneType, error) {
	return par.l(c, "true", 'e', RuneTypeTrueR, nil)
}

func (par *parser) stateFalseF(c rune) (RuneType, error) {
	return par.l(c, "false", 'a', RuneTypeFalseA, par.stateFalseA)
}
func (par *parser) stateFalseA(c rune) (RuneType, error) {
	return par.l(c, "false", 'l', RuneTypeFalseL, par.stateFalseL)
}
func (par *parser) stateFalseL(c rune) (RuneType, error) {
	return par.l(c, "false", 's', RuneTypeFalseS, par.stateFalseS)
}
func (par *parser) stateFalseS(c rune) (RuneType, error) {
	return par.l(c, "false", 'e', RuneTypeFalseE, nil)
}

func (par *parser) stateNullN(c rune) (RuneType, error) {
	return par.l(c, "null", 'u', RuneTypeNullU, par.stateNullU)
}
func (par *parser) stateNullU(c rune) (RuneType, error) {
	return par.l(c, "null", 'l', RuneTypeNullL1, par.stateNullL)
}
func (par *parser) stateNullL(c rune) (RuneType, error) {
	return par.l(c, "null", 'l', RuneTypeNullL2, nil)
}