Repository: apple/swift-nio
File: Sources/NIOCore/ByteBuffer-hexdump.swift (Swift)

Code before:
        var hexString = ""
        hexString.reserveCapacity(self.readableBytes * 3)

        for byte in self.readableBytesView {
            hexString += String(byte: byte, padding: 2)
            hexString += " "
        }

        return String(hexString.dropLast())
    }

    /// Return a `String` of hexadecimal digits of bytes in the Buffer.,
    /// in a format that's compatible with `xxd -r -p`, but clips the output to the max length of `limit` bytes.
    /// If the dump contains more than the `limit` bytes, this function will return the first `limit/2`
    /// and the last `limit/2` of that, replacing the rest with `...`, i.e. `01 02 03 ... 09 11 12`.
    ///
    /// - parameters:
    ///     - limit: The maximum amount of bytes presented in the dump.
    func hexDumpShort(limit: Int) -> String {
        guard self.readableBytes > limit else {
            return self.hexDumpShort()
        }

        var buffer = self
        let front = buffer.readSlice(length: limit / 2)!
        buffer.moveReaderIndex(forwardBy: buffer.readableBytes - limit / 2)
        let back = buffer.readSlice(length: buffer.readableBytes)!

        let startHex = front.hexDumpShort()
        let endHex = back.hexDumpShort()
        return startHex + " ... " + endHex
    }

    // Dump hexdump -C compatible format without the last line containing the byte length of the buffer
    func _hexDumpLines(offset: Int) -> String {
        var result = ""
        // Each hex dump line represents 16 bytes, and takes up to 79 characters including the \n in the end.
        // Plus the last line of the hex dump which is just 8 characters.
        result.reserveCapacity(self.readableBytes / 16 * 79 + 8)

        var buffer = self
        // We're going to move the lineOffset index forward, so copy it into a variable
        var lineOffset = offset

        while buffer.readableBytes > 0 {
            let slice = buffer.readSlice(length: min(16, buffer.readableBytes))!
            let lineLength = slice.readableBytes

            // Left column of the hex dump signifies the offset from the beginning of the dumped buffer
            // and is separated from the next column with two spaces.
            result += String(byte: lineOffset, padding: 8)

Review comment (suggestion):
It might be cleaner to use `moveReaderIndex(to:)` with a computation from `writerIndex`. Up to you though.

Code after:
    private func hexDumpPlain() -> String {
        var hexString = ""
        hexString.reserveCapacity(self.readableBytes * 3)

        for byte in self.readableBytesView {
            hexString += String(byte, radix: 16, padding: 2)
            hexString += " "
        }

        return String(hexString.dropLast())
    }

    /// Return a `String` of space delimited hexadecimal digits of the readable bytes in the buffer,
    /// in a format that's compatible with `xxd -r -p`, but clips the output to the max length of `maxBytes` bytes.
    /// If the dump contains more than the `maxBytes` bytes, this function will return the first `maxBytes/2`
    /// and the last `maxBytes/2` of that, replacing the rest with `...`, i.e. `01 02 03 ... 09 11 12`.
    ///
    /// - parameters:
    ///     - maxBytes: The maximum amount of bytes presented in the dump.
    private func hexDumpPlain(maxBytes: Int) -> String {
        // If the buffer length fits in the max bytes limit in the hex dump, just dump the whole thing.
        if self.readableBytes <= maxBytes {
            return self.hexDump(format: .plain)
        }

        var buffer = self

        // Safe to force-unwrap because we just checked readableBytes is > maxBytes above.
        let front = buffer.readSlice(length: maxBytes / 2)!
        buffer.moveReaderIndex(to: buffer.writerIndex - maxBytes / 2)
        let back = buffer.readSlice(length: buffer.readableBytes)!

        let startHex = front.hexDumpPlain()
        let endHex = back.hexDumpPlain()
        return startHex + " ... " + endHex
    }

    /// Returns a `String` containing a detailed hex dump of this buffer.
    /// Intended to be used internally in ``hexDump(format:)``
    /// - parameters:
    ///     - lineOffset: an offset from the beginning of the outer buffer that is being dumped. It's used to print the line offset in hexdump -C format.
    ///     - paddingBefore: the amount of space to pad before the first byte dumped on this line, used in center and right columns.
    ///     - paddingAfter: the amount of sapce to pad after the last byte on this line, used in center and right columns.
    private func _hexDumpLine(lineOffset: Int, paddingBefore: Int = 0, paddingAfter: Int = 0) -> String {
        // Each line takes 78 visible characters + \n
        var result = ""
        result.reserveCapacity(79)

        // Left column of the hex dump signifies the offset from the beginning of the dumped buffer
        // and is separated from the next column with two spaces.
        result += String(lineOffset, radix: 16, padding: 8)