1 files changed, 39 insertions, 0 deletions
diff --git a/cryptfs/intrablock.go b/cryptfs/intrablock.go
new file mode 100644
index 0000000..78fd140
--- /dev/null
+++ b/cryptfs/intrablock.go
@@ -0,0 +1,39 @@
+package cryptfs
+
+// intraBlock identifies a part of a file block
+type intraBlock struct {
+	BlockNo uint64  // Block number in file
+	Offset  uint64  // Offset into block plaintext
+	Length  uint64  // Length of data from this block
+	fs    *CryptFS
+}
+
+// isPartial - is the block partial? This means we have to do read-modify-write.
+func (ib *intraBlock) IsPartial() bool {
+	if ib.Offset > 0 || ib.Length < ib.fs.plainBS {
+		return true
+	}
+	return false
+}
+
+// CiphertextRange - get byte range in ciphertext file corresponding to BlockNo
+// (complete block)
+func (ib *intraBlock) CiphertextRange() (offset uint64, length uint64) {
+	return ib.BlockNo * ib.fs.cipherBS, ib.fs.cipherBS
+}
+
+// PlaintextRange - get byte range in plaintext corresponding to BlockNo
+// (complete block)
+func (ib *intraBlock) PlaintextRange() (offset uint64, length uint64) {
+	return ib.BlockNo * ib.fs.plainBS, ib.fs.plainBS
+}
+
+// CropBlock - crop a potentially larger plaintext block down to the relevant part
+func (ib *intraBlock) CropBlock(d []byte) []byte{
+	lenHave := len(d)
+	lenWant := int(ib.Offset+ib.Length)
+	if lenHave < lenWant {
+		return d[ib.Offset:lenHave]
+	}
+	return d[ib.Offset:lenWant]
+}