1 files changed, 51 insertions, 0 deletions
diff --git a/internal/contentenc/intrablock.go b/internal/contentenc/intrablock.go
new file mode 100644
index 0000000..330b980
--- /dev/null
+++ b/internal/contentenc/intrablock.go
@@ -0,0 +1,51 @@
+package contentenc
+
+// intraBlock identifies a part of a file block
+type intraBlock struct {
+	BlockNo uint64 // Block number in file
+	Skip    uint64 // Offset into block plaintext
+	Length  uint64 // Length of data from this block
+	fs      *ContentEnc
+}
+
+// isPartial - is the block partial? This means we have to do read-modify-write.
+func (ib *intraBlock) IsPartial() bool {
+	if ib.Skip > 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.fs.BlockNoToCipherOff(ib.BlockNo), ib.fs.cipherBS
+}
+
+// PlaintextRange - get byte range in plaintext corresponding to BlockNo
+// (complete block)
+func (ib *intraBlock) PlaintextRange() (offset uint64, length uint64) {
+	return ib.fs.BlockNoToPlainOff(ib.BlockNo), 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.Skip + ib.Length)
+	if lenHave < lenWant {
+		return d[ib.Skip:lenHave]
+	}
+	return d[ib.Skip:lenWant]
+}
+
+// Ciphertext range corresponding to the sum of all "blocks" (complete blocks)
+func (ib *intraBlock) JointCiphertextRange(blocks []intraBlock) (offset uint64, length uint64) {
+	firstBlock := blocks[0]
+	lastBlock := blocks[len(blocks)-1]
+
+	offset = ib.fs.BlockNoToCipherOff(firstBlock.BlockNo)
+	offsetLast := ib.fs.BlockNoToCipherOff(lastBlock.BlockNo)
+	length = offsetLast + ib.fs.cipherBS - offset
+
+	return offset, length
+}