feat(codec): add TupleCodec and encode

src/main/kotlin/kestrel/store/codec/TupleCodec.kt

@@ -0,0 +1,103 @@
+package kestrel.store.codec
+
+import java.math.BigDecimal
+import java.math.BigInteger
+import java.time.LocalDate
+import kestrel.storage.Value
+
+/**
+ * Encodes a row (list of [Value]) as a self-describing byte payload: a leading
+ * null bitmap followed by a type-tagged, variable-length payload for every
+ * non-null column. Used to store row bodies on disk pages, independent of the
+ * order-preserving key encoding in [KeyCodec].
+ */
+object TupleCodec {
+    private const val TAG_BOOLEAN = 1
+    private const val TAG_INTEGER = 2
+    private const val TAG_REAL = 3
+    private const val TAG_TEXT = 4
+    private const val TAG_DECIMAL = 5
+    private const val TAG_DATE = 6
+
+    fun encode(values: List<Value>): ByteArray {
+        val writer = ByteWriter()
+        writer.writeVarInt(values.size.toLong())
+        writer.writeBytes(nullBitmap(values))
+        for (value in values) {
+            if (value.isNull) continue
+            when (value) {
+                is Value.BooleanValue -> {
+                    writer.writeByte(TAG_BOOLEAN)
+                    writer.writeByte(if (value.value) 1 else 0)
+                }
+                is Value.IntegerValue -> {
+                    writer.writeByte(TAG_INTEGER)
+                    writer.writeZigZag(value.value)
+                }
+                is Value.RealValue -> {
+                    writer.writeByte(TAG_REAL)
+                    writer.writeFixedDouble(value.value)
+                }
+                is Value.TextValue -> {
+                    writer.writeByte(TAG_TEXT)
+                    writer.writeLengthPrefixed(value.value.toByteArray(Charsets.UTF_8))
+                }
+                is Value.DecimalValue -> {
+                    writer.writeByte(TAG_DECIMAL)
+                    writer.writeZigZag(value.value.scale().toLong())
+                    writer.writeLengthPrefixed(value.value.unscaledValue().toByteArray())
+                }
+                is Value.DateValue -> {
+                    writer.writeByte(TAG_DATE)
+                    writer.writeZigZag(value.value.toEpochDay())
+                }
+                Value.NullValue -> Unit
+            }
+        }
+        return writer.toByteArray()
+    }
+
+    fun decode(bytes: ByteArray): List<Value> {
+        val reader = ByteReader(bytes)
+        val columnCount = reader.readVarInt().toInt()
+        val bitmapBytes = reader.readBytes(bitmapSize(columnCount))
+        val result = ArrayList<Value>(columnCount)
+        for (index in 0 until columnCount) {
+            if (isNullBit(bitmapBytes, index)) {
+                result.add(Value.NullValue)
+                continue
+            }
+            result.add(
+                when (val tag = reader.readByte()) {
+                    TAG_BOOLEAN -> Value.BooleanValue(reader.readByte() != 0)
+                    TAG_INTEGER -> Value.IntegerValue(reader.readZigZag())
+                    TAG_REAL -> Value.RealValue(reader.readFixedDouble())
+                    TAG_TEXT -> Value.TextValue(String(reader.readLengthPrefixed(), Charsets.UTF_8))
+                    TAG_DECIMAL -> {
+                        val scale = reader.readZigZag().toInt()
+                        val unscaled = BigInteger(reader.readLengthPrefixed())
+                        Value.DecimalValue(BigDecimal(unscaled, scale))
+                    }
+                    TAG_DATE -> Value.DateValue(LocalDate.ofEpochDay(reader.readZigZag()))
+                    else -> throw CodecException("unknown tuple tag $tag")
+                }
+            )
+        }
+        return result
+    }
+
+    private fun bitmapSize(columnCount: Int): Int = (columnCount + 7) / 8
+
+    private fun nullBitmap(values: List<Value>): ByteArray {
+        val bitmap = ByteArray(bitmapSize(values.size))
+        for (index in values.indices) {
+            if (values[index].isNull) {
+                bitmap[index / 8] = (bitmap[index / 8].toInt() or (1 shl (index % 8))).toByte()
+            }
+        }
+        return bitmap
+    }
+
+    private fun isNullBit(bitmap: ByteArray, index: Int): Boolean =
+        (bitmap[index / 8].toInt() shr (index % 8)) and 1 == 1
+}