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

package lowmemjson

import (
	"unicode/utf8"
)

// 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)
	}
}