import sequtils

type
    Buffer*[T] = object ## Uses T as internal data buffer
        data*: T
        offset*: int
    SeqBuffer*[T] = Buffer[seq[T]] ## Uses seq[T] as internal data buffer

template `[]`*[T](b: Buffer[T], i: int): auto = b.data[b.offset + i]
template `[]=`*[S, T](b: var Buffer[S], i: int, v: T) = b.data[b.offset + i] = v

proc init*[T](b: var Buffer[T], d: T) =
    shallowCopy(b.data, d)
    b.offset = 0

proc initBuffer*[T](d: T): Buffer[T] =
    shallowCopy(result.data, d)

proc subbuffer*[T](b: Buffer[T], offset: int): Buffer[T] =
    shallowCopy(result.data, b.data)
    result.offset = b.offset + offset

template isNil*[T](b: Buffer[T]): bool = b.data.isNil

template copyElements*[T](dst: var Buffer[T], src: Buffer[T], count: int) =
    when defined(js):
        for i in 0 ..< count: dst[i] = src[i]
    else:
        copyMem(addr dst[dst.offset], unsafeAddr src[src.offset], count * sizeof(dst[0]))

template zeroMem*[T](dst: var Buffer[T]) =
    when defined(js):
        applyIt(dst.data, type(dst[0])(0))
    else:
        zeroMem(addr dst.data[dst.offset], sizeof(dst[0]) * (dst.data.len - dst.offset))

when isMainModule:
    var buf: SeqBuffer[uint8]
    buf.init(newSeq[uint8](1024))
    buf[1] = 5
    buf.offset = 1
    echo buf[0].int

    var strBuf = initBuffer(newString(20))
    strBuf[1] = 'A'
    let subBuf = strBuf.subbuffer(1)
    echo subBuf[0] # Should print A