import Foundation

private let tinyThumbnailHeaderPattern = Data(base64Encoded: "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")
private let tinyThumbnailFooterPattern = Data(base64Encoded: "/9k=")

public func decodeTinyThumbnail(data: Data) -> Data? {
    if data.count < 3 {
        return nil
    }
    guard let tinyThumbnailHeaderPattern = tinyThumbnailHeaderPattern, let tinyThumbnailFooterPattern = tinyThumbnailFooterPattern else {
        return nil
    }
    var version: UInt8 = 0
    data.copyBytes(to: &version, count: 1)
    if version != 1 {
        return nil
    }
    var width: UInt8 = 0
    var height: UInt8 = 0
    data.copyBytes(to: &width, from: 1 ..< 2)
    data.copyBytes(to: &height, from: 2 ..< 3)
    
    var resultData = Data()
    resultData.append(tinyThumbnailHeaderPattern)
    resultData.append(data.subdata(in: 3 ..< data.count))
    resultData.append(tinyThumbnailFooterPattern)
    resultData.withUnsafeMutableBytes { buffer -> Void in
        guard let resultBytes = buffer.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
            return
        }
        resultBytes[164] = width
        resultBytes[166] = height
    }
    return resultData
}