From d5e25376382f3211b5a0887e84e859e12b0fdcd6 Mon Sep 17 00:00:00 2001
From: Kevin Wang <kevmo314@gmail.com>
Date: Sat, 20 Jan 2024 16:30:39 -0500
Subject: [PATCH] feat: add pagefile api (#47)

---
 pkg/btree/pagefile.go      | 31 +++++++++++++++++++
 pkg/btree/pagefile_test.go | 62 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+)
 create mode 100644 pkg/btree/pagefile.go
 create mode 100644 pkg/btree/pagefile_test.go

diff --git a/pkg/btree/pagefile.go b/pkg/btree/pagefile.go
new file mode 100644
index 00000000..24b0bb55
--- /dev/null
+++ b/pkg/btree/pagefile.go
@@ -0,0 +1,31 @@
+package btree
+
+import "io"
+
+type PageFile struct {
+	io.ReadWriteSeeker
+	PageSize int
+}
+
+func (pf *PageFile) Seek(offset int64, whence int) (int64, error) {
+	if offset == 0 && whence == io.SeekEnd {
+		// Seek to the end of the file
+		offset, err := pf.ReadWriteSeeker.Seek(0, io.SeekEnd)
+		if err != nil {
+			return 0, err
+		}
+		// If the offset is not a multiple of the page size, we need to pad the file
+		// with zeros to the next page boundary.
+		if pf.PageSize > 0 && offset%int64(pf.PageSize) != 0 {
+			// Calculate the number of bytes to pad
+			pad := int64(pf.PageSize) - (offset % int64(pf.PageSize))
+			// Write the padding
+			if _, err := pf.Write(make([]byte, pad)); err != nil {
+				return 0, err
+			}
+			return offset + pad, nil
+		}
+		return offset, nil
+	}
+	return pf.ReadWriteSeeker.Seek(offset, whence)
+}
diff --git a/pkg/btree/pagefile_test.go b/pkg/btree/pagefile_test.go
new file mode 100644
index 00000000..b2a010d8
--- /dev/null
+++ b/pkg/btree/pagefile_test.go
@@ -0,0 +1,62 @@
+package btree
+
+import (
+	"io"
+	"testing"
+)
+
+func TestPageFile(t *testing.T) {
+	t.Run("no page size behaves like regular ReadWriteSeeker", func(t *testing.T) {
+		buf := newSeekableBuffer()
+		pf := &PageFile{
+			ReadWriteSeeker: buf,
+		}
+		if _, err := pf.Seek(0, io.SeekStart); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := pf.Write([]byte("hello")); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := pf.Seek(0, io.SeekStart); err != nil {
+			t.Fatal(err)
+		}
+		b := make([]byte, 5)
+		if _, err := pf.Read(b); err != nil {
+			t.Fatal(err)
+		}
+		if string(b) != "hello" {
+			t.Fatalf("expected %q, got %q", "hello", string(b))
+		}
+	})
+
+	t.Run("page size allocates pages on seek end", func(t *testing.T) {
+		buf := newSeekableBuffer()
+		pf := &PageFile{
+			ReadWriteSeeker: buf,
+			PageSize:        16,
+		}
+		if _, err := pf.Seek(0, io.SeekEnd); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := pf.Write([]byte("hello")); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := pf.Seek(0, io.SeekStart); err != nil {
+			t.Fatal(err)
+		}
+		b := make([]byte, 5)
+		if _, err := pf.Read(b); err != nil {
+			t.Fatal(err)
+		}
+		if string(b) != "hello" {
+			t.Fatalf("expected %q, got %q", "hello", string(b))
+		}
+		n, err := pf.Seek(0, io.SeekEnd)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if n != 16 {
+			t.Fatalf("expected %d, got %d", 16, n)
+		}
+	})
+}