62 lines
2.9 KiB
Swift
62 lines
2.9 KiB
Swift
extension PureXML.Parsing.ByteDecoder {
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/// The JIS X 0208 index (pointer -> code point), assembled from the vendored
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/// parts. `0xFFFF` marks an unmapped pointer.
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static let jis0208: [UInt16] = jis0208Part1 + jis0208Part2 + jis0208Part3
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/// The Shift-JIS decoder (the WHATWG `Shift_JIS` algorithm). Single bytes cover
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/// ASCII and half-width katakana; a lead byte introduces a two-byte sequence
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/// resolved through the JIS X 0208 index.
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enum ShiftJIS {
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/// Decodes a whole Shift-JIS byte buffer.
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static func decode(_ bytes: ArraySlice<UInt8>) -> String {
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var scalars = String.UnicodeScalarView()
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var lead: UInt8 = 0
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for byte in bytes {
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if lead != 0 {
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let saved = lead
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lead = 0
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if let scalar = twoByteScalar(lead: saved, trail: byte) {
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scalars.append(scalar)
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} else {
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scalars.append("\u{FFFD}")
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if byte <= 0x7F { scalars.append(Unicode.Scalar(byte)) } // reprocess ASCII trail
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}
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} else if let single = singleByteScalar(byte) {
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scalars.append(single)
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} else if isLead(byte) {
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lead = byte
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} else {
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scalars.append("\u{FFFD}")
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}
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}
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if lead != 0 { scalars.append("\u{FFFD}") }
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return String(scalars)
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}
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/// The scalar for a byte that is not part of a two-byte sequence: ASCII
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/// (`0x00`-`0x80`) or half-width katakana (`0xA1`-`0xDF`), else nil.
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static func singleByteScalar(_ byte: UInt8) -> Unicode.Scalar? {
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if byte <= 0x80 { return Unicode.Scalar(byte) }
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if (0xA1 ... 0xDF).contains(byte) { return Unicode.Scalar(0xFF61 + UInt32(byte) - 0xA1) }
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return nil
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}
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/// Whether a byte introduces a two-byte sequence.
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static func isLead(_ byte: UInt8) -> Bool {
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(0x81 ... 0x9F).contains(byte) || (0xE0 ... 0xFC).contains(byte)
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}
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/// The scalar for a `(lead, trail)` pair, or nil when the pair is invalid
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/// or unmapped.
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static func twoByteScalar(lead: UInt8, trail: UInt8) -> Unicode.Scalar? {
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guard (0x40 ... 0x7E).contains(trail) || (0x80 ... 0xFC).contains(trail) else { return nil }
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let offset = lead < 0xA0 ? 0x81 : 0xC1
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let trailOffset = trail < 0x7F ? 0x40 : 0x41
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let pointer = (Int(lead) - offset) * 188 + Int(trail) - trailOffset
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if (8836 ... 10715).contains(pointer) { return Unicode.Scalar(0xE000 + pointer - 8836) } // EUDC private use
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guard pointer >= 0, pointer < jis0208.count, jis0208[pointer] != 0xFFFF else { return nil }
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return Unicode.Scalar(UInt32(jis0208[pointer]))
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}
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}
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}
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