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

package lowmemjson

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

	"git.lukeshu.com/go/lowmemjson/internal"
)

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 ////////////////////////////////////////////////////////

// BackslashEscapeMode identifies one of the three ways that a
// character may be represented in a JSON string:
//
//   - literally (no backslash escaping)
//
//   - as a short "well-known" `\X` backslash sequence (where `X` is a
//     single-character)
//
//   - as a long Unicode `\uXXXX` backslash sequence
type BackslashEscapeMode uint8

const (
	BackslashEscapeNone BackslashEscapeMode = iota
	BackslashEscapeShort
	BackslashEscapeUnicode
)

// A BackslashEscaper controls how a ReEncoder emits a character in a
// JSON string.  The `rune` argument is the character being
// considered, and the `BackslashEscapeMode` argument is how it was
// originally encoded in the input.
//
// The ReEncoder will panic if a BackslashEscaper returns an unknown
// BackslashEscapeMode.
type BackslashEscaper = func(rune, BackslashEscapeMode) BackslashEscapeMode

// EscapePreserve is a BackslashEscaper that preserves the original
// input escaping.
func EscapePreserve(_ rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	return wasEscaped
}

// EscapeJSSafe is a BackslashEscaper that escapes strings such that
// the JSON safe to embed in JS; it otherwise preserves the original
// input escaping.
//
// JSON is notionally a JS subset, but that's not actually true; so
// more conservative backslash-escaping is necessary to safely embed
// it in JS.  http://timelessrepo.com/json-isnt-a-javascript-subset
func EscapeJSSafe(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	switch c {
	case '\u2028', '\u2029':
		return BackslashEscapeUnicode
	default:
		return wasEscaped
	}
}

// EscapeHTMLSafe is a BackslashEscaper that escapes strings such that
// the JSON is safe to embed in HTML; it otherwise preserves the
// original input escaping.
func EscapeHTMLSafe(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	switch c {
	case '&', '<', '>':
		return BackslashEscapeUnicode
	default:
		return EscapeJSSafe(c, wasEscaped)
	}
}

// EscapeDefault is a BackslashEscaper that mimics the default
// behavior of encoding/json.
//
// It is like EscapeHTMLSafe, but also uses long Unicode `\uXXXX`
// sequences for `\b`, `\f`, and the `\uFFFD` Unicode replacement
// character.
//
// A ReEncoder uses EscapeDefault if a BackslashEscaper is not
// specified.
func EscapeDefault(c rune, wasEscaped BackslashEscapeMode) BackslashEscapeMode {
	switch c {
	case '\b', '\f', utf8.RuneError:
		return BackslashEscapeUnicode
	default:
		return EscapeHTMLSafe(c, wasEscaped)
	}
}

// EscapeDefaultNonHTMLSafe is a BackslashEscaper that mimics the
// default behavior of an encoding/json.Encoder that has had
// SetEscapeHTML(false) called on it.
//
// It is like EscapeJSSafe, but also uses long Unicode `\uXXXX`
// sequences for `\b`, `\f`, and the `\uFFFD` Unicode replacement
// character.
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',
		internal.Hex[(c>>12)&0xf],
		internal.Hex[(c>>8)&0xf],
		internal.Hex[(c>>4)&0xf],
		internal.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")
	}
}