From c0603b1c306e71cb80d8071d62400ad261c229b7 Mon Sep 17 00:00:00 2001
From: Brandon Dyck <brandon@dyck.us>
Date: Wed, 25 Sep 2024 16:34:31 -0600
Subject: [PATCH] Added a generic ReaderAt

---
 cursor/cursor.go      | 35 ++++++++++++++++++++++++++++-------
 cursor/cursor_test.go |  2 +-
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/cursor/cursor.go b/cursor/cursor.go
index 446cdaa..f682216 100644
--- a/cursor/cursor.go
+++ b/cursor/cursor.go
@@ -3,6 +3,7 @@
 package cursor
 
 import (
+	"errors"
 	"io"
 )
 
@@ -32,6 +33,26 @@ type Cursor[Datum any] interface {
 	At(pos uint64) Cursor[Datum]
 }
 
+type ReaderAt[T any] interface {
+	ReadAt(p []T, off int64) (n int, err error)
+}
+
+type SliceReaderAt[T any] []T
+
+func (s SliceReaderAt[T]) ReadAt(dst []T, off int64) (n int, err error) {
+	if off < 0 {
+		return 0, errors.New("SliceReaderAt.ReadAt: negative offset")
+	}
+	if off >= int64(len(s)) {
+		return 0, io.EOF
+	}
+	n = copy(dst, s[off:])
+	if n < len(dst) {
+		err = io.EOF
+	}
+	return n, err
+}
+
 type SliceCursor[Datum any] struct {
 	data   []Datum
 	offset uint64
@@ -66,16 +87,16 @@ func (sc SliceCursor[Datum]) At(pos uint64) Cursor[Datum] {
 	return sc
 }
 
-type ReaderAtCursor struct {
-	r   io.ReaderAt
+type ReaderAtCursor[T any] struct {
+	r   ReaderAt[T]
 	pos uint64
 }
 
-func NewReaderAt(r io.ReaderAt) ReaderAtCursor {
-	return ReaderAtCursor{r: r}
+func NewReaderAt[T any](r ReaderAt[T]) ReaderAtCursor[T] {
+	return ReaderAtCursor[T]{r: r}
 }
 
-func (rac ReaderAtCursor) Read(dst []byte) (uint64, Cursor[byte], error) {
+func (rac ReaderAtCursor[T]) Read(dst []T) (uint64, Cursor[T], error) {
 	n, err := rac.r.ReadAt(dst, int64(rac.pos))
 	if n > 0 {
 		rac.pos += uint64(n)
@@ -83,11 +104,11 @@ func (rac ReaderAtCursor) Read(dst []byte) (uint64, Cursor[byte], error) {
 	return uint64(n), rac, err
 }
 
-func (rac ReaderAtCursor) Pos() uint64 {
+func (rac ReaderAtCursor[T]) Pos() uint64 {
 	return rac.pos
 }
 
-func (rac ReaderAtCursor) At(pos uint64) Cursor[byte] {
+func (rac ReaderAtCursor[T]) At(pos uint64) Cursor[T] {
 	rac.pos = pos
 	return rac
 }
diff --git a/cursor/cursor_test.go b/cursor/cursor_test.go
index 7b4c422..08f0e2b 100644
--- a/cursor/cursor_test.go
+++ b/cursor/cursor_test.go
@@ -105,7 +105,7 @@ func TestSliceCursor(t *testing.T) {
 }
 
 func TestReaderAtCursor(t *testing.T) {
-	testCursor(t, func(b []byte) cursor.ReaderAtCursor {
+	testCursor(t, func(b []byte) cursor.ReaderAtCursor[byte] {
 		return cursor.NewReaderAt(bytes.NewReader(b))
 	})
 	t.Run("Read returns an error if the ReaderAt fails", rapid.MakeCheck(func(t *rapid.T) {