package cursor

import (
	"io"
)

// Cursor reads data from a specific spot in a data source.
type Cursor[Datum any] interface {
	// I almost parameterized Cursor by its implementation (i.e. the Curiously
	// Recurring Template Pattern), but then each parser would need that parameter.
	// That might work well in a language with much stronger type inference, but
	// not in Go. The upside would have been that for each implementation Impl,
	// Impl.Read could have returned an unboxed Impl, which would have slightly
	// simplified testing and maybe slightly reduced allocs.

	// Read fill dst with data from this Cursor's position in the underlying
	// source. It returns the number of data it read and a new Cursor for
	// the position at which the read ended, or an error if the read failed.
	// All calls to a given Cursor will return data from the same position.
	// If n < len(dst) or if the cursor's position is at the end of the data source,
	// Read will return an error explaining why it read fewer bytes than requested.
	// If the error was due to the cursor reaching the end of the data source,
	// err will be io.EOF.
	Read(dst []Datum) (n uint64, next Cursor[Datum], err error)

	// Pos returns the Cursor's position within the source.
	Pos() uint64

	// At returns a new cursor at the position pos.
	At(pos uint64) Cursor[Datum]
}

type SliceCursor[Datum any] struct {
	data   []Datum
	offset uint64
}

func NewSlice[Datum any](data []Datum) SliceCursor[Datum] {
	return SliceCursor[Datum]{
		data:   data,
		offset: 0,
	}
}

func (sc SliceCursor[Datum]) Read(dst []Datum) (n uint64, next Cursor[Datum], err error) {
	if sc.offset == uint64(len(sc.data)) {
		return 0, sc, io.EOF
	}
	copied := copy(dst, sc.data[sc.offset:])
	if copied < len(dst) {
		err = io.EOF
	}
	n = uint64(copied)
	sc.offset += n
	return n, sc, err
}

func (sc SliceCursor[Datum]) Pos() uint64 {
	return sc.offset
}

func (sc SliceCursor[Datum]) At(pos uint64) Cursor[Datum] {
	sc.offset = pos
	return sc
}

type ReaderAtCursor struct {
	r   io.ReaderAt
	pos uint64
}

func NewReaderAt(r io.ReaderAt) ReaderAtCursor {
	return ReaderAtCursor{r: r}
}

func (rac ReaderAtCursor) Read(dst []byte) (uint64, Cursor[byte], error) {
	n, err := rac.r.ReadAt(dst, int64(rac.pos))
	if n > 0 {
		rac.pos += uint64(n)
	}
	return uint64(n), rac, err
}

func (rac ReaderAtCursor) Pos() uint64 {
	return rac.pos
}

func (rac ReaderAtCursor) At(pos uint64) Cursor[byte] {
	rac.pos = pos
	return rac
}