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

package lowmemjson

import (
	"encoding/json"
	"io"
	"reflect"
	"unicode/utf8"
)

const Tab = "\t"

const hex = "0123456789abcdef"

func hex2int[T interface{ byte | rune }](c T) (byte, bool) {
	switch {
	case '0' <= c && c <= '9':
		return byte(c) - '0', true
	case 'a' <= c && c <= 'f':
		return byte(c) - 'a' + 10, true
	case 'A' <= c && c <= 'F':
		return byte(c) - 'A' + 10, true
	default:
		return 0, false
	}
}

var (
	numberType    = reflect.TypeOf(json.Number(""))
	byteType      = reflect.TypeOf(byte(0))
	byteSliceType = reflect.TypeOf(([]byte)(nil))
)

// generic I/O /////////////////////////////////////////////////////////////////

func writeByte(w io.Writer, c byte) error {
	if br, ok := w.(interface{ WriteByte(byte) error }); ok {
		return br.WriteByte(c)
	}
	var buf [1]byte
	buf[0] = c
	if _, err := w.Write(buf[:]); err != nil {
		return err
	}
	return nil
}

func writeRune(w io.Writer, c rune) (int, error) {
	if rw, ok := w.(interface{ WriteRune(rune) (int, error) }); ok {
		return rw.WriteRune(c)
	}
	var buf [utf8.UTFMax]byte
	n := utf8.EncodeRune(buf[:], c)
	return w.Write(buf[:n])
}

// JSON string encoding ////////////////////////////////////////////////////////

type BackslashEscapeMode uint8

const (
	BackslashEscapeNone = BackslashEscapeMode(iota)
	BackslashEscapeShort
	BackslashEscapeUnicode
)

type BackslashEscaper = func(rune, BackslashEscapeMode) BackslashEscapeMode

func EscapePreserve(_ rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	return wasEscaped
}

func EscapeJSSafe(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	// JSON is notionally a JS subset, but that's not actually
	// true.
	//
	// http://timelessrepo.com/json-isnt-a-javascript-subset
	switch c {
	case '\u2028', '\u2029':
		return BackslashEscapeUnicode
	default:
		return wasEscaped
	}
}

func EscapeHTMLSafe(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	switch c {
	case '&', '<', '>':
		return BackslashEscapeUnicode
	default:
		return EscapeJSSafe(c, wasEscaped)
	}
}

func EscapeDefault(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	switch c {
	case '\b', '\f', utf8.RuneError:
		return BackslashEscapeUnicode
	default:
		return EscapeHTMLSafe(c, wasEscaped)
	}
}

func EscapeDefaultNonHTMLSafe(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	switch c {
	case '\b', '\f', utf8.RuneError:
		return BackslashEscapeUnicode
	default:
		return EscapeJSSafe(c, wasEscaped)
	}
}

func writeStringUnicodeEscape(w io.Writer, c rune) (int, error) {
	buf := [6]byte{
		'\\',
		'u',
		hex[(c>>12)&0xf],
		hex[(c>>8)&0xf],
		hex[(c>>4)&0xf],
		hex[(c>>0)&0xf],
	}
	return w.Write(buf[:])
}
func writeStringShortEscape(w io.Writer, c rune) (int, error) {
	var b byte
	switch c {
	case '"', '\\', '/':
		b = byte(c)
	case '\b':
		b = 'b'
	case '\f':
		b = 'f'
	case '\n':
		b = 'n'
	case '\r':
		b = 'r'
	case '\t':
		b = 't'
	default:
		panic("should not happen")
	}
	buf := [2]byte{'\\', b}
	return w.Write(buf[:])
}
func writeStringChar(w io.Writer, c rune, wasEscaped BackslashEscapeMode, escaper BackslashEscaper) (int, error) {
	if escaper == nil {
		escaper = EscapeDefault
	}
	switch escaper(c, wasEscaped) {
	case BackslashEscapeNone:
		switch {
		case c < 0x0020: // override, gotta escape these
			switch c {
			case '\b', '\f', '\n', '\r', '\t': // short-escape if possible
				return writeStringShortEscape(w, c)
			default:
				return writeStringUnicodeEscape(w, c)
			}
		case c == '"' || c == '\\': // override, gotta escape these
			return writeStringShortEscape(w, c)
		default: // obey
			return writeRune(w, c)
		}
	case BackslashEscapeShort:
		switch c {
		case '"', '\\', '/', '\b', '\f', '\n', '\r', '\t': // obey
			return writeStringShortEscape(w, c)
		default: // override, can't short-escape these
			return writeRune(w, c)
		}
	case BackslashEscapeUnicode:
		switch {
		case c > 0xFFFF: // override, can't escape these (TODO: unless we use UTF-16 surrogates?)
			return writeRune(w, c)
		default: // obey
			return writeStringUnicodeEscape(w, c)
		}
	default:
		panic("escaper returned an invalid escape mode")
	}
}