diff --git a/src/test/kotlin/kestrel/store/codec/ByteCursorTest.kt b/src/test/kotlin/kestrel/store/codec/ByteCursorTest.kt
new file mode 100644
index 0000000..26c43f1
--- /dev/null
+++ b/src/test/kotlin/kestrel/store/codec/ByteCursorTest.kt
@@ -0,0 +1,73 @@
+package kestrel.store.codec
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class ByteCursorTest {
+    @Test
+    fun writesAndReadsVarIntsAcrossByteBoundaries() {
+        val samples = listOf(0L, 1L, 127L, 128L, 300L, 16384L, Int.MAX_VALUE.toLong(), Long.MAX_VALUE)
+        val writer = ByteWriter()
+        for (sample in samples) writer.writeVarInt(sample)
+        val reader = ByteReader(writer.toByteArray())
+        for (sample in samples) assertEquals(sample, reader.readVarInt())
+        assertFalse(reader.hasRemaining())
+    }
+
+    @Test
+    fun zigZagRoundTripsNegativeAndPositiveValues() {
+        val samples = listOf(0L, -1L, 1L, -64L, 64L, Long.MIN_VALUE, Long.MAX_VALUE)
+        val writer = ByteWriter()
+        for (sample in samples) writer.writeZigZag(sample)
+        val reader = ByteReader(writer.toByteArray())
+        for (sample in samples) assertEquals(sample, reader.readZigZag())
+    }
+
+    @Test
+    fun negativeNumbersUseFewerBytesThanUnsignedVarIntWouldViaZigZag() {
+        val writer = ByteWriter()
+        writer.writeZigZag(-1L)
+        assertEquals(1, writer.length)
+    }
+
+    @Test
+    fun fixedWidthLongAndDoubleRoundTrip() {
+        val writer = ByteWriter()
+        writer.writeFixedLong(-42L)
+        writer.writeFixedDouble(3.14159)
+        val reader = ByteReader(writer.toByteArray())
+        assertEquals(-42L, reader.readFixedLong())
+        assertEquals(3.14159, reader.readFixedDouble())
+    }
+
+    @Test
+    fun lengthPrefixedBytesRoundTrip() {
+        val writer = ByteWriter()
+        writer.writeLengthPrefixed(byteArrayOf(1, 2, 3))
+        writer.writeLengthPrefixed(ByteArray(0))
+        writer.writeByte(9)
+        val reader = ByteReader(writer.toByteArray())
+        assertTrue(reader.readLengthPrefixed().contentEquals(byteArrayOf(1, 2, 3)))
+        assertTrue(reader.readLengthPrefixed().contentEquals(ByteArray(0)))
+        assertEquals(9, reader.readByte())
+    }
+
+    @Test
+    fun readingPastEndThrows() {
+        val reader = ByteReader(byteArrayOf(1))
+        reader.readByte()
+        assertFailsWith<CodecException> { reader.readByte() }
+    }
+
+    @Test
+    fun growsBufferBeyondInitialCapacity() {
+        val writer = ByteWriter(2)
+        repeat(500) { writer.writeByte(it and 0xFF) }
+        assertEquals(500, writer.length)
+        val reader = ByteReader(writer.toByteArray())
+        repeat(500) { assertEquals(it and 0xFF, reader.readByte()) }
+    }
+}
diff --git a/src/test/kotlin/kestrel/store/codec/Crc32CTest.kt b/src/test/kotlin/kestrel/store/codec/Crc32CTest.kt
new file mode 100644
index 0000000..4312025
--- /dev/null
+++ b/src/test/kotlin/kestrel/store/codec/Crc32CTest.kt
@@ -0,0 +1,33 @@
+package kestrel.store.codec
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotEquals
+
+class Crc32CTest {
+    @Test
+    fun sameBytesProduceSameChecksum() {
+        val a = "hello kestrel".toByteArray()
+        val b = "hello kestrel".toByteArray()
+        assertEquals(Crc32C.of(a), Crc32C.of(b))
+    }
+
+    @Test
+    fun differentBytesProduceDifferentChecksum() {
+        assertNotEquals(Crc32C.of("hello".toByteArray()), Crc32C.of("hellp".toByteArray()))
+    }
+
+    @Test
+    fun detectsSingleBitFlip() {
+        val original = ByteArray(4096) { (it % 251).toByte() }
+        val corrupted = original.copyOf()
+        corrupted[2000] = (corrupted[2000].toInt() xor 0x01).toByte()
+        assertNotEquals(Crc32C.of(original), Crc32C.of(corrupted))
+    }
+
+    @Test
+    fun offsetAndLengthCoverOnlyTheRequestedSlice() {
+        val buffer = "xxhelloxx".toByteArray()
+        assertEquals(Crc32C.of("hello".toByteArray()), Crc32C.of(buffer, 2, 5))
+    }
+}
diff --git a/src/test/kotlin/kestrel/store/codec/KeyCodecTest.kt b/src/test/kotlin/kestrel/store/codec/KeyCodecTest.kt
new file mode 100644
index 0000000..95fcfed
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