package rbuf

// copyright (c) 2014, Jason E. Aten
// license: MIT

// Some text from the Golang standard library doc is adapted and
// reproduced in fragments below to document the expected behaviors
// of the interface functions Read()/Write()/ReadFrom()/WriteTo() that
// are implemented here. Those descriptions (see
// http://golang.org/pkg/io/#Reader for example) are
// copyright 2010 The Go Authors.

import "io"

// FixedSizeRingBuf:
//
//    a fixed-size circular ring buffer. Yes, just what is says.
//
//    We keep a pair of ping/pong buffers so that we can linearize
//    the circular buffer into a contiguous slice if need be.
//
// For efficiency, a FixedSizeRingBuf may be vastly preferred to
// a bytes.Buffer. The ReadWithoutAdvance(), Advance(), and Adopt()
// methods are all non-standard methods written for speed.
//
// For an I/O heavy application, I have replaced bytes.Buffer with
// FixedSizeRingBuf and seen memory consumption go from 8GB to 25MB.
// Yes, that is a 300x reduction in memory footprint. Everything ran
// faster too.
//
// Note that Bytes(), while inescapable at times, is expensive: avoid
// it if possible. Instead it is better to use the FixedSizeRingBuf.Readable
// member to get the number of bytes available. Bytes() is expensive because
// it may copy the back and then the front of a wrapped buffer A[Use]
// into A[1-Use] in order to get a contiguous slice. If possible use ContigLen()
// first to get the size that can be read without copying, Read() that
// amount, and then Read() a second time -- to avoid the copy. See
// BytesTwo() for a method that does this for you.
//
type FixedSizeRingBuf struct {
	A        [2][]byte // a pair of ping/pong buffers. Only one is active.
	Use      int       // which A buffer is in active use, 0 or 1
	N        int       // MaxViewInBytes, the size of A[0] and A[1] in bytes.
	Beg      int       // start of data in A[Use]
	Readable int       // number of bytes available to read in A[Use]
}

// ContigLen gets the length of the largest read that we can provide to a contiguous slice
// without an extra linearizing copy of all bytes internally.
func (b *FixedSizeRingBuf) ContigLen() int {
	extent := b.Beg + b.Readable
	firstContigLen := intMin(extent, b.N) - b.Beg
	return firstContigLen
}

// constructor. NewFixedSizeRingBuf will allocate internally
// two buffers of size maxViewInBytes.
func NewFixedSizeRingBuf(maxViewInBytes int) *FixedSizeRingBuf {
	n := maxViewInBytes
	r := &FixedSizeRingBuf{
		Use: 0, // 0 or 1, whichever is actually in use at the moment.
		// If we are asked for Bytes() and we wrap, linearize into the other.

		N:        n,
		Beg:      0,
		Readable: 0,
	}
	r.A[0] = make([]byte, n, n)
	r.A[1] = make([]byte, n, n)

	return r
}

// from the standard library description of Bytes():
// Bytes() returns a slice of the contents of the unread portion of the buffer.
// If the caller changes the contents of the
// returned slice, the contents of the buffer will change provided there
//  are no intervening method calls on the Buffer.
//
// The largest slice Bytes ever returns is bounded above by the maxViewInBytes
// value used when calling NewFixedSizeRingBuf().
func (b *FixedSizeRingBuf) Bytes() []byte {

	extent := b.Beg + b.Readable
	if extent <= b.N {
		// we fit contiguously in this buffer without wrapping to the other
		return b.A[b.Use][b.Beg:(b.Beg + b.Readable)]
	}

	// wrap into the other buffer
	src := b.Use
	dest := 1 - b.Use

	n := copy(b.A[dest], b.A[src][b.Beg:])
	n += copy(b.A[dest][n:], b.A[src][0:(extent%b.N)])

	b.Use = dest
	b.Beg = 0

	return b.A[b.Use][:n]
}

// BytesTwo returns all readable bytes, but in two separate slices,
// to avoid copying. The two slices are from the same buffer, but
// are not contiguous. Either or both may be empty slices.
func (b *FixedSizeRingBuf) BytesTwo(makeCopy bool) (first []byte, second []byte) {

	extent := b.Beg + b.Readable
	if extent <= b.N {
		// we fit contiguously in this buffer without wrapping to the other.
		// Let second stay an empty slice.
		return b.A[b.Use][b.Beg:(b.Beg + b.Readable)], second
	}

	return b.A[b.Use][b.Beg:b.N], b.A[b.Use][0:(extent % b.N)]
}

// Read():
//
// from bytes.Buffer.Read(): Read reads the next len(p) bytes
// from the buffer or until the buffer is drained. The return
// value n is the number of bytes read. If the buffer has no data
// to return, err is io.EOF (unless len(p) is zero); otherwise it is nil.
//
//  from the description of the Reader interface,
//     http://golang.org/pkg/io/#Reader
//
/*
Reader is the interface that wraps the basic Read method.

Read reads up to len(p) bytes into p. It returns the number
of bytes read (0 <= n <= len(p)) and any error encountered.
Even if Read returns n < len(p), it may use all of p as scratch
space during the call. If some data is available but not
len(p) bytes, Read conventionally returns what is available
instead of waiting for more.

When Read encounters an error or end-of-file condition after
successfully reading n > 0 bytes, it returns the number of bytes
read. It may return the (non-nil) error from the same call or
return the error (and n == 0) from a subsequent call. An instance
of this general case is that a Reader returning a non-zero number
of bytes at the end of the input stream may return
either err == EOF or err == nil. The next Read should
return 0, EOF regardless.

Callers should always process the n > 0 bytes returned before
considering the error err. Doing so correctly handles I/O errors
that happen after reading some bytes and also both of the
allowed EOF behaviors.

Implementations of Read are discouraged from returning a zero
byte count with a nil error, and callers should treat that
situation as a no-op.
*/
//
func (b *FixedSizeRingBuf) Read(p []byte) (n int, err error) {
	return b.ReadAndMaybeAdvance(p, true)
}

// ReadWithoutAdvance(): if you want to Read the data and leave
// it in the buffer, so as to peek ahead for example.
func (b *FixedSizeRingBuf) ReadWithoutAdvance(p []byte) (n int, err error) {
	return b.ReadAndMaybeAdvance(p, false)
}

func (b *FixedSizeRingBuf) ReadAndMaybeAdvance(p []byte, doAdvance bool) (n int, err error) {
	if len(p) == 0 {
		return 0, nil
	}
	if b.Readable == 0 {
		return 0, io.EOF
	}
	extent := b.Beg + b.Readable
	if extent <= b.N {
		n += copy(p, b.A[b.Use][b.Beg:extent])
	} else {
		n += copy(p, b.A[b.Use][b.Beg:b.N])
		if n < len(p) {
			n += copy(p[n:], b.A[b.Use][0:(extent%b.N)])
		}
	}
	if doAdvance {
		b.Advance(n)
	}
	return
}

//
// Write writes len(p) bytes from p to the underlying data stream.
// It returns the number of bytes written from p (0 <= n <= len(p))
// and any error encountered that caused the write to stop early.
// Write must return a non-nil error if it returns n < len(p).
//
func (b *FixedSizeRingBuf) Write(p []byte) (n int, err error) {
	for {
		if len(p) == 0 {
			// nothing (left) to copy in; notice we shorten our
			// local copy p (below) as we read from it.
			return
		}

		writeCapacity := b.N - b.Readable
		if writeCapacity <= 0 {
			// we are all full up already.
			return n, io.ErrShortWrite
		}
		if len(p) > writeCapacity {
			err = io.ErrShortWrite
			// leave err set and
			// keep going, write what we can.
		}

		writeStart := (b.Beg + b.Readable) % b.N

		upperLim := intMin(writeStart+writeCapacity, b.N)

		k := copy(b.A[b.Use][writeStart:upperLim], p)

		n += k
		b.Readable += k
		p = p[k:]

		// we can fill from b.A[b.Use][0:something] from
		// p's remainder, so loop
	}
}

//
// WriteAndMaybeOverwriteOldestData always consumes the full
// buffer p, even if that means blowing away the oldest
// unread bytes in the ring to make room. In reality, only the last
// min(len(p),b.N) bytes of p will end up being written to the ring.
//
// This allows the ring to act as a record of the most recent
// b.N bytes of data -- a kind of temporal LRU cache, so the
// speak. The linux kernel's dmesg ring buffer is similar.
//
func (b *FixedSizeRingBuf) WriteAndMaybeOverwriteOldestData(p []byte) (n int, err error) {
	writeCapacity := b.N - b.Readable
	if len(p) > writeCapacity {
		b.Advance(len(p) - writeCapacity)
	}
	startPos := 0
	if len(p) > b.N {
		startPos = len(p) - b.N
	}
	n, err = b.Write(p[startPos:])
	if err != nil {
		return n, err
	}
	return len(p), nil
}

// WriteTo and ReadFrom avoid intermediate allocation and copies.

// WriteTo avoids intermediate allocation and copies.
// WriteTo writes data to w until there's no more data to write
// or when an error occurs. The return value n is the number of
// bytes written. Any error encountered during the write is also returned.
func (b *FixedSizeRingBuf) WriteTo(w io.Writer) (n int64, err error) {

	if b.Readable == 0 {
		return 0, io.EOF
	}

	extent := b.Beg + b.Readable
	firstWriteLen := intMin(extent, b.N) - b.Beg
	secondWriteLen := b.Readable - firstWriteLen
	if firstWriteLen > 0 {
		m, e := w.Write(b.A[b.Use][b.Beg:(b.Beg + firstWriteLen)])
		n += int64(m)
		b.Advance(m)

		if e != nil {
			return n, e
		}
		// all bytes should have been written, by definition of
		// Write method in io.Writer
		if m != firstWriteLen {
			return n, io.ErrShortWrite
		}
	}
	if secondWriteLen > 0 {
		m, e := w.Write(b.A[b.Use][0:secondWriteLen])
		n += int64(m)
		b.Advance(m)

		if e != nil {
			return n, e
		}
		// all bytes should have been written, by definition of
		// Write method in io.Writer
		if m != secondWriteLen {
			return n, io.ErrShortWrite
		}
	}

	return n, nil
}

// ReadFrom avoids intermediate allocation and copies.
// ReadFrom() reads data from r until EOF or error. The return value n
// is the number of bytes read. Any error except io.EOF encountered
// during the read is also returned.
func (b *FixedSizeRingBuf) ReadFrom(r io.Reader) (n int64, err error) {
	for {
		writeCapacity := b.N - b.Readable
		if writeCapacity <= 0 {
			// we are all full
			return n, nil
		}
		writeStart := (b.Beg + b.Readable) % b.N
		upperLim := intMin(writeStart+writeCapacity, b.N)

		m, e := r.Read(b.A[b.Use][writeStart:upperLim])
		n += int64(m)
		b.Readable += m
		if e == io.EOF {
			return n, nil
		}
		if e != nil {
			return n, e
		}
	}
}

// Reset quickly forgets any data stored in the ring buffer. The
// data is still there, but the ring buffer will ignore it and
// overwrite those buffers as new data comes in.
func (b *FixedSizeRingBuf) Reset() {
	b.Beg = 0
	b.Readable = 0
	b.Use = 0
}

// Advance(): non-standard, but better than Next(),
// because we don't have to unwrap our buffer and pay the cpu time
// for the copy that unwrapping may need.
// Useful in conjuction/after ReadWithoutAdvance() above.
func (b *FixedSizeRingBuf) Advance(n int) {
	if n <= 0 {
		return
	}
	if n > b.Readable {
		n = b.Readable
	}
	b.Readable -= n
	b.Beg = (b.Beg + n) % b.N
}

// Adopt(): non-standard.
//
// For efficiency's sake, (possibly) take ownership of
// already allocated slice offered in me.
//
// If me is large we will adopt it, and we will potentially then
// write to the me buffer.
// If we already have a bigger buffer, copy me into the existing
// buffer instead.
func (b *FixedSizeRingBuf) Adopt(me []byte) {
	n := len(me)
	if n > b.N {
		b.A[0] = me
		b.A[1] = make([]byte, n, n)
		b.N = n
		b.Use = 0
		b.Beg = 0
		b.Readable = n
	} else {
		// we already have a larger buffer, reuse it.
		copy(b.A[0], me)
		b.Use = 0
		b.Beg = 0
		b.Readable = n
	}
}

func intMax(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func intMin(a, b int) int {
	if a < b {
		return a
	} else {
		return b
	}
}

func (f *FixedSizeRingBuf) Avail() int {
	return f.Readable
}

// First returns the earliest index, or -1 if
// the ring is empty
func (f *FixedSizeRingBuf) First() int {
	if f.Readable == 0 {
		return -1
	}
	return f.Beg
}

// Next returns the index of the element after
// from, or -1 if no more. returns -2 if erroneous
// input (bad from).
func (f *FixedSizeRingBuf) Nextpos(from int) int {
	if from >= f.N || from < 0 {
		return -2
	}
	if f.Readable == 0 {
		return -1
	}

	last := f.Last()
	if from == last {
		return -1
	}
	a0, a1, b0, b1 := f.LegalPos()
	switch {
	case from >= a0 && from < a1:
		return from + 1
	case from == a1:
		return b0 // can be -1
	case from >= b0 && from < b1:
		return from + 1
	case from == b1:
		return -1
	}
	return -1
}

// LegalPos returns the legal index positions,
// [a0,aLast] and [b0,bLast] inclusive, where the
// [a0,aLast] holds the first FIFO ordered segment,
// and the [b0,bLast] holds the second ordered segment,
// if any.
// A position of -1 means the segment is not used,
// perhaps because b.Readable is zero, or because
// the second segment [b0,bLast] is not in use (when
// everything fits in the first [a0,aLast] segment).
//
func (b *FixedSizeRingBuf) LegalPos() (a0, aLast, b0, bLast int) {
	a0 = -1
	aLast = -1
	b0 = -1
	bLast = -1
	if b.Readable == 0 {
		return
	}
	a0 = b.Beg
	last := b.Beg + b.Readable - 1
	if last < b.N {
		aLast = last
		return
	}
	aLast = b.N - 1
	b0 = 0
	bLast = last % b.N
	return
}

// Prevpos returns the index of the element before
// from, or -1 if no more and from is the
// first in the ring. Returns -2 on bad
// from position.
func (f *FixedSizeRingBuf) Prevpos(from int) int {
	if from >= f.N || from < 0 {
		return -2
	}
	if f.Readable == 0 {
		return -1
	}
	if from == f.Beg {
		return -1
	}
	a0, a1, b0, b1 := f.LegalPos()
	switch {
	case from == a0:
		return -1
	case from > a0 && from <= a1:
		return from - 1
	case from == b0:
		return a1
	case from > b0 && from <= b1:
		return from - 1
	}
	return -1
}

// Last returns the index of the last element,
// or -1 if the ring is empty.
func (f *FixedSizeRingBuf) Last() int {
	if f.Readable == 0 {
		return -1
	}

	last := f.Beg + f.Readable - 1
	if last < f.N {
		// we fit without wrapping
		return last
	}

	return last % f.N
}

// Kth presents the contents of the
// ring as a strictly linear sequence,
// so the user doesn't need to think
// about modular arithmetic. Here k indexes from
// [0, f.Readable-1], assuming f.Avail()
// is greater than 0. Kth() returns an
// actual index where the logical k-th
// element, starting from f.Beg, resides.
// f.Beg itself lives at k = 0. If k is
// out of bounds, or the ring is empty,
// -1 is returned.
func (f *FixedSizeRingBuf) Kth(k int) int {
	if f.Readable == 0 || k < 0 || k >= f.Readable {
		return -1
	}
	return (f.Beg + k) % f.N
}

// DeleteMostRecentBytes trims back the last n bytes written.
func (f *FixedSizeRingBuf) DeleteMostRecentBytes(n int) {
	if n <= 0 {
		return
	}
	if n >= f.Readable {
		f.Readable = 0
		return
	}
	f.Readable -= n
}