// Converts the prefix of buf to a UTF8 string. If buf terminates in the middle
// of a character, returns the remaining bytes of the partial character in a
// new buffer. Note! This assumes that the prefix of buf *is* valid UTF8--it
// only examines the bytes of the last character in the buffer and assumes it
// will find an initial byte before the start of the buffer.
export function toPartialUtf8String(buf: Buffer): { str: string; buf: Buffer } {
    if (buf.length === 0) {
        return { str: '', buf: Buffer.of() }
    }
    let lastValidByteOffsetExclusive = buf.length
    if ((buf[lastValidByteOffsetExclusive - 1] & 0x80) !== 0) {
        // Multi-byte character. Count additional trailing bytes. UTF8 trailing
        // bytes have the bit pattern 10??_????.
        let numBytes = 1
        while ((buf[lastValidByteOffsetExclusive - numBytes] & 0xc0) === 0x80) {
            numBytes++
        }
        // Scrutinize the initial byte to see if the encoding is complete.
        // Characters of N bytes encode the length in the first character.
        // The high order N bits are set, and the next bit is clear. For
        // example a 4-byte character starts with 1111_0???.
        const byte = buf[lastValidByteOffsetExclusive - numBytes]
        const mask = 0xff ^ ((1 << (7 - numBytes)) - 1)
        const value = numBytes === 6 ? 0xfc : mask ^ (1 << (7 - numBytes))
        if ((byte & mask) !== value) {
            // The trailing bytes are incomplete; don't decode them.
            lastValidByteOffsetExclusive -= numBytes
        }
    }
    return {
        str: buf.slice(0, lastValidByteOffsetExclusive).toString('utf8'),
        buf: Buffer.from(buf.slice(lastValidByteOffsetExclusive)),
    }
}