KtIrc/src/main/kotlin/com/dmdirc/ktirc/io/MessageParser.kt

package com.dmdirc.ktirc.io

/**
 * Parses a message received from an IRC server.
 *
 * IRC messages consist of:
 *
 * - Optionally, IRCv3 message tags. Identified with an '@' character
 * - Optionally, a prefix, identified with an ':' character
 * - A command in the form of a consecutive sequence of letters or exactly three numbers
 * - Some number of space-separated parameters
 * - Optionally, a final 'trailing' parameter prefixed with a ':' character
 *
 * For example:
 *
 * @aaa=bbb;ccc;example.com/ddd=eee :nick!ident@host.com PRIVMSG #someChannel :This is a test message
 * ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^ ^^^^^^^ ^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^
 * IRCv3 tags                       Prefix               Cmd     Param #1     Trailing parameter
 *
 * or:
 *
 * PING 12345678
 * ^^^^ ^^^^^^^^
 * Cmd  Param #1
 */
class MessageParser {

    companion object {
        private const val AT = '@'.toByte()
        private const val COLON = ':'.toByte()
    }

    fun parse(message: ByteArray) = CursorByteArray(message).run {
        IrcMessage(takeTags(), takePrefix(), String(takeWord()), takeParams().toList())
    }

    /**
     * Attempts to read IRCv3 tags from the message.
     */
    private fun CursorByteArray.takeTags() = takeOptionalPrefixedSection(AT)

    /**
     * Attempts to read a prefix from the message.
     */
    private fun CursorByteArray.takePrefix() = takeOptionalPrefixedSection(COLON)

    /**
     * Read a single parameter from the message. If the parameter is a trailing one, the entire message will be
     * consumed.
     */
    private fun CursorByteArray.takeParam() = when (peek()) {
        COLON -> takeRemaining(skip = 1)
        else -> takeWord()
    }

    /**
     * Reads all remaining parameters from the message.
     */
    private fun CursorByteArray.takeParams() = sequence {
        while (!exhausted()) {
            yield(takeParam())
        }
    }

    private fun CursorByteArray.takeOptionalPrefixedSection(prefix: Byte) = when {
        exhausted() -> null
        peek() == prefix -> takeWord(skip = 1)
        else -> null
    }

}

class IrcMessage(val tags: ByteArray?, val prefix: ByteArray?, val command: String, val params: List<ByteArray>)

/**
 * A ByteArray with a 'cursor' that tracks the current read position.
 */
internal class CursorByteArray(private val data: ByteArray, var cursor: Int = 0) {

    companion object {
        private const val SPACE = ' '.toByte()
    }

    /**
     * Returns whether or not the cursor has reached the end of the array.
     */
    fun exhausted() = cursor >= data.size

    /**
     * Returns the next byte in the array without advancing the cursor.
     *
     * @throws ArrayIndexOutOfBoundsException If the array is [exhausted].
     */
    @Throws(ArrayIndexOutOfBoundsException::class)
    fun peek() = data[cursor]

    /**
     * Returns the next "word" in the byte array - that is, all non-space characters up until the next space.
     *
     * After calling this method, the cursor will be advanced to the start of the next word (i.e., it will skip over
     * any number of space characters).
     *
     * @param skip Number of bytes to omit from the start of the word
     */
    fun takeWord(skip: Int = 0) = data.sliceArray(cursor + skip until seekTo { it == SPACE }).apply { seekTo { it != SPACE } }

    /**
     * Takes all remaining bytes from the cursor until the end of the array.
     *
     * @param skip Number of bytes to omit from the start of the remainder
     */
    fun takeRemaining(skip: Int = 0) = data.sliceArray(cursor + skip until data.size).apply { cursor = data.size }

    private fun seekTo(matcher: (Byte) -> Boolean): Int {
        while (!exhausted() && !matcher(peek())) {
            cursor++
        }
        return cursor
    }

}