reader.go | 23 +++++++++++++++++------
 t.go      |  2 +-

diff --git a/reader.go b/reader.go
index c8af14640bb813e98eabeca5305aa02532ffaba1..a8264ed1d637250db21705180757ec3e4e4ffbdc 100644
--- a/reader.go
+++ b/reader.go
@@ -22,7 +22,7 @@ 	SetReadahead(int64)
 	// If non-nil, the provided function is called when the implementation needs to know the
 	// readahead for the current reader. Calls occur during Reads and Seeks, and while the Client is
 	// locked.
-	SetReadaheadFunc(func() int64)
+	SetReadaheadFunc(ReadaheadFunc)
 	// Don't wait for pieces to complete and be verified. Read calls return as soon as they can when
 	// the underlying chunks become available.
 	SetResponsive()
@@ -32,6 +32,14 @@ // Piece range by piece index, [begin, end).
 type pieceRange struct {
 	begin, end pieceIndex
 }
+
+type ReadaheadContext struct {
+	ContiguousReadStartPos int64
+	CurrentPos             int64
+}
+
+// Returns the desired readahead for a Reader.
+type ReadaheadFunc func(ReadaheadContext) int64
 
 type reader struct {
 	t *Torrent
@@ -39,7 +47,7 @@ 	// Adjust the read/seek window to handle Readers locked to File extents and the like.
 	offset, length int64
 
 	// Function to dynamically calculate readahead. If nil, readahead is static.
-	readaheadFunc func() int64
+	readaheadFunc ReadaheadFunc
 
 	// Required when modifying pos and readahead.
 	mu sync.Locker
@@ -79,7 +87,7 @@ 	r.posChanged()
 	r.mu.Unlock()
 }
 
-func (r *reader) SetReadaheadFunc(f func() int64) {
+func (r *reader) SetReadaheadFunc(f ReadaheadFunc) {
 	r.mu.Lock()
 	r.readaheadFunc = f
 	r.posChanged()
@@ -116,7 +124,10 @@ // Calculates the pieces this reader wants downloaded, ignoring the cached value at r.pieces.
 func (r *reader) piecesUncached() (ret pieceRange) {
 	ra := r.readahead
 	if r.readaheadFunc != nil {
-		ra = r.readaheadFunc()
+		ra = r.readaheadFunc(ReadaheadContext{
+			ContiguousReadStartPos: r.contiguousReadStartPos,
+			CurrentPos:             r.pos,
+		})
 	}
 	if ra < 1 {
 		// Needs to be at least 1, because [x, x) means we don't want
@@ -303,6 +314,6 @@ 	r.t.logger.Log(m.Skip(1))
 }
 
 // Implementation inspired by https://news.ycombinator.com/item?id=27019613.
-func (r *reader) defaultReadaheadFunc() int64 {
-	return r.pos - r.contiguousReadStartPos
+func defaultReadaheadFunc(r ReadaheadContext) int64 {
+	return r.CurrentPos - r.ContiguousReadStartPos
 }
diff --git a/t.go b/t.go
index 580a3bc91c9cf789156431c9a24f405e8d77da90..d861e14e979cf85eb65faafc2ba7eca4e7878da8 100644
--- a/t.go
+++ b/t.go
@@ -42,7 +42,7 @@ 		t:      t,
 		offset: offset,
 		length: length,
 	}
-	r.readaheadFunc = r.defaultReadaheadFunc
+	r.readaheadFunc = defaultReadaheadFunc
 	t.addReader(&r)
 	return &r
 }