SwiftNES/Sources/Data/Rom.swift

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struct Rom {
fileprivate let PRG_ROM_PAGE_SIZE = 16384
fileprivate let CHR_ROM_PAGE_SIZE = 8192
var program: [UInt8]
var character: [UInt8]
var mapper: UInt8
var screenMirror: Mirroring
init(_ raw: [UInt8]) throws {
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guard raw[0..<4] == [0x4E, 0x45, 0x53, 0x1A] else { throw HeaderParseError.notINES("File is not in iNES file format.") }
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mapper = (raw[7] & 0b1111_0000) | (raw[6] >> 4)
let inesVer = (raw[7] >> 2) & 0b11
guard inesVer == 0 else { throw HeaderParseError.iNes2("iNES2.0 format not supported.") }
let fourScreen = raw[6] & 0b1000 != 0
let vertMirroring = raw[6] & 0b1 != 0
screenMirror = switch (fourScreen, vertMirroring) {
case (true, _):
Mirroring.fourScreen
case (false, true):
Mirroring.vertical
case (false, false):
Mirroring.horizontal
}
let programSize = Int(raw[4]) * PRG_ROM_PAGE_SIZE
let characterSize = Int(raw[5]) * CHR_ROM_PAGE_SIZE
let skipTrainer = raw[6] & 0b100 != 0
let programStart = 16 + (skipTrainer ? 512 : 0)
let characterStart = programStart + programSize
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program = Array(raw[programStart..<(programStart + programSize)])
character = Array(raw[characterStart..<(characterStart + characterSize)])
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}
}
enum Mirroring {
case vertical
case horizontal
case fourScreen
}
enum HeaderParseError: Error {
case notINES(String)
case iNes2(String)
}