feat(codec): add CodecException, ByteWriter and Crc32C

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

@@ -0,0 +1,113 @@
+package kestrel.store.codec
+
+import kestrel.KestrelException
+
+class CodecException(message: String) : KestrelException(message)
+
+/** Growable little-endian-free byte sink. Multi-byte integers are written big-endian. */
+class ByteWriter(initialCapacity: Int = 32) {
+    private var buffer = ByteArray(initialCapacity)
+    private var size = 0
+
+    val length: Int get() = size
+
+    private fun ensure(extra: Int) {
+        if (size + extra <= buffer.size) return
+        var newCapacity = buffer.size * 2
+        while (newCapacity < size + extra) newCapacity *= 2
+        buffer = buffer.copyOf(newCapacity)
+    }
+
+    fun writeByte(value: Int): ByteWriter {
+        ensure(1)
+        buffer[size++] = value.toByte()
+        return this
+    }
+
+    fun writeBytes(bytes: ByteArray): ByteWriter {
+        ensure(bytes.size)
+        System.arraycopy(bytes, 0, buffer, size, bytes.size)
+        size += bytes.size
+        return this
+    }
+
+    fun writeFixedLong(value: Long): ByteWriter {
+        ensure(8)
+        for (i in 7 downTo 0) {
+            buffer[size++] = ((value ushr (i * 8)) and 0xFF).toByte()
+        }
+        return this
+    }
+
+    fun writeFixedDouble(value: Double): ByteWriter = writeFixedLong(java.lang.Double.doubleToLongBits(value))
+
+    /** Unsigned LEB128 varint. */
+    fun writeVarInt(value: Long): ByteWriter {
+        var remaining = value
+        while (true) {
+            val byte = (remaining and 0x7F).toInt()
+            remaining = remaining ushr 7
+            if (remaining == 0L) {
+                writeByte(byte)
+                return this
+            }
+            writeByte(byte or 0x80)
+        }
+    }
+
+    /** Zigzag-encoded signed varint, safe for negative numbers. */
+    fun writeZigZag(value: Long): ByteWriter = writeVarInt((value shl 1) xor (value shr 63))
+
+    fun writeLengthPrefixed(bytes: ByteArray): ByteWriter {
+        writeVarInt(bytes.size.toLong())
+        writeBytes(bytes)
+        return this
+    }
+
+    fun toByteArray(): ByteArray = buffer.copyOf(size)
+}
+
+class ByteReader(private val data: ByteArray, private var position: Int = 0) {
+    val remaining: Int get() = data.size - position
+
+    fun hasRemaining(): Boolean = position < data.size
+
+    fun readByte(): Int {
+        if (position >= data.size) throw CodecException("unexpected end of buffer")
+        return data[position++].toInt() and 0xFF
+    }
+
+    fun readBytes(count: Int): ByteArray {
+        if (position + count > data.size) throw CodecException("unexpected end of buffer")
+        val slice = data.copyOfRange(position, position + count)
+        position += count
+        return slice
+    }
+
+    fun readFixedLong(): Long {
+        var result = 0L
+        repeat(8) { result = (result shl 8) or readByte().toLong() }
+        return result
+    }
+
+    fun readFixedDouble(): Double = java.lang.Double.longBitsToDouble(readFixedLong())
+
+    fun readVarInt(): Long {
+        var result = 0L
+        var shift = 0
+        while (true) {
+            val byte = readByte()
+            result = result or ((byte.toLong() and 0x7F) shl shift)
+            if (byte and 0x80 == 0) return result
+            shift += 7
+            if (shift > 63) throw CodecException("varint too long")
+        }
+    }
+
+    fun readZigZag(): Long {
+        val encoded = readVarInt()
+        return (encoded ushr 1) xor -(encoded and 1)
+    }
+
+    fun readLengthPrefixed(): ByteArray = readBytes(readVarInt().toInt())
+}

src/main/kotlin/kestrel/store/codec/Crc32C.kt

@@ -0,0 +1,14 @@
+package kestrel.store.codec
+
+import java.util.zip.CRC32C
+
+/** Castagnoli CRC-32 checksums used to detect corruption of on-disk pages and records. */
+object Crc32C {
+    fun of(bytes: ByteArray): Int = of(bytes, 0, bytes.size)
+
+    fun of(bytes: ByteArray, offset: Int, length: Int): Int {
+        val crc = CRC32C()
+        crc.update(bytes, offset, length)
+        return crc.value.toInt()
+    }
+}