diff --git a/src/batch.zig b/src/batch.zig
index 76aa581..b096ef4 100644
--- a/src/batch.zig
+++ b/src/batch.zig
@@ -44,3 +44,19 @@ pub const WriteBatch = struct {
         return self.ops.items.len;
     }
 };
+
+test "write batch accumulates typed ops" {
+    const allocator = std.testing.allocator;
+    var batch = WriteBatch.init(allocator);
+    defer batch.deinit();
+
+    try batch.put("alpha", "one");
+    try batch.put("bravo", "two");
+    try batch.delete("charlie");
+
+    try std.testing.expectEqual(@as(usize, 3), batch.count());
+    try std.testing.expectEqual(Kind.value, batch.ops.items[0].kind);
+    try std.testing.expectEqualStrings("bravo", batch.ops.items[1].key);
+    try std.testing.expectEqual(Kind.tombstone, batch.ops.items[2].kind);
+    try std.testing.expectEqual(@as(usize, 0), batch.ops.items[2].value.len);
+}
diff --git a/src/bloom.zig b/src/bloom.zig
index f6359c5..4a348ba 100644
--- a/src/bloom.zig
+++ b/src/bloom.zig
@@ -76,3 +76,40 @@ pub const Bloom = struct {
         };
     }
 };
+
+test "bloom no false negatives" {
+    const allocator = std.testing.allocator;
+    var bloom = try Bloom.init(allocator, 1000, 0.01);
+    defer bloom.deinit();
+
+    var buf: [32]u8 = undefined;
+    var i: usize = 0;
+    while (i < 1000) : (i += 1) {
+        const key = try std.fmt.bufPrint(&buf, "key-{d}", .{i});
+        bloom.add(key);
+    }
+    i = 0;
+    while (i < 1000) : (i += 1) {
+        const key = try std.fmt.bufPrint(&buf, "key-{d}", .{i});
+        try std.testing.expect(bloom.mayContain(key));
+    }
+}
+
+test "bloom serialize roundtrip" {
+    const allocator = std.testing.allocator;
+    var bloom = try Bloom.init(allocator, 100, 0.01);
+    defer bloom.deinit();
+    bloom.add("alpha");
+    bloom.add("bravo");
+
+    var out = std.ArrayList(u8).init(allocator);
+    defer out.deinit();
+    try bloom.serialize(encode.ByteWriter.init(&out));
+
+    var restored = try Bloom.deserialize(allocator, out.items);
+    defer restored.deinit();
+    try std.testing.expect(restored.mayContain("alpha"));
+    try std.testing.expect(restored.mayContain("bravo"));
+    try std.testing.expectEqual(bloom.bitCount, restored.bitCount);
+    try std.testing.expectEqual(bloom.hashCount, restored.hashCount);
+}
diff --git a/src/cache.zig b/src/cache.zig
index 38b943e..f9f8024 100644
--- a/src/cache.zig
+++ b/src/cache.zig
@@ -119,3 +119,51 @@ pub const BlockCache = struct {
         if (self.tail == null) self.tail = node;
     }
 };
+
+test "block cache stores and serves" {
+    const allocator = std.testing.allocator;
+    var cache = BlockCache.init(allocator, 1024);
+    defer cache.deinit();
+
+    const key = BlockKey{ .fileId = 1, .offset = 0 };
+    try std.testing.expectEqual(@as(?[]const u8, null), cache.get(key));
+    try std.testing.expectEqual(@as(u64, 1), cache.stats.misses);
+
+    try cache.put(key, "block-bytes");
+    const got = cache.get(key).?;
+    try std.testing.expectEqualStrings("block-bytes", got);
+    try std.testing.expectEqual(@as(u64, 1), cache.stats.hits);
+}
+
+test "block cache evicts least recently used" {
+    const allocator = std.testing.allocator;
+    var cache = BlockCache.init(allocator, 20);
+    defer cache.deinit();
+
+    const a = BlockKey{ .fileId = 1, .offset = 0 };
+    const b = BlockKey{ .fileId = 1, .offset = 100 };
+    const c = BlockKey{ .fileId = 1, .offset = 200 };
+
+    try cache.put(a, "0123456789");
+    try cache.put(b, "0123456789");
+    _ = cache.get(a);
+    try cache.put(c, "0123456789");
+
+    try std.testing.expect(cache.get(b) == null);
+    try std.testing.expect(cache.get(a) != null);
+    try std.testing.expect(cache.get(c) != null);
+}
+
+test "block cache keeps most recent under tiny capacity" {
+    const allocator = std.testing.allocator;
+    var cache = BlockCache.init(allocator, 1);
+    defer cache.deinit();
+
+    const a = BlockKey{ .fileId = 2, .offset = 0 };
+    const b = BlockKey{ .fileId = 2, .offset = 50 };
+    try cache.put(a, "big-payload");
+    try cache.put(b, "big-payload");
+
+    try std.testing.expect(cache.get(a) == null);
+    try std.testing.expect(cache.get(b) != null);
+}
diff --git a/src/compress.zig b/src/compress.zig
index b8c1eac..3eb858b 100644
--- a/src/compress.zig
+++ b/src/compress.zig
@@ -96,3 +96,74 @@ pub fn decode(allocator: Allocator, codec: Codec, payload: []const u8, originalL
         .rle => rleDecompress(allocator, payload, originalLen),
     };
 }
+
+fn expectRoundtrip(allocator: Allocator, input: []const u8) !void {
+    const compressed = try rleCompress(allocator, input);
+    defer allocator.free(compressed);
+    const restored = try rleDecompress(allocator, compressed, input.len);
+    defer allocator.free(restored);
+    try std.testing.expectEqualSlices(u8, input, restored);
+}
+
+test "rle roundtrip on runs literals and mixed" {
+    const allocator = std.testing.allocator;
+    try expectRoundtrip(allocator, "");
+    try expectRoundtrip(allocator, "a");
+    try expectRoundtrip(allocator, "aaaaaaaaaa");
+    try expectRoundtrip(allocator, "abcdefghij");
+    try expectRoundtrip(allocator, "aaabbbcccdddeeefff");
+    try expectRoundtrip(allocator, "aaaaaXbbbbbYcccccZ");
+    try expectRoundtrip(allocator, &[_]u8{0} ** 300);
+    try expectRoundtrip(allocator, &[_]u8{0xff} ** 300);
+}
+
+test "rle roundtrip on random data" {
+    const allocator = std.testing.allocator;
+    var prng = std.Random.DefaultPrng.init(0x1234abcd);
+    const random = prng.random();
+    var buf: [4096]u8 = undefined;
+    var trial: usize = 0;
+    while (trial < 40) : (trial += 1) {
+        const len = random.intRangeAtMost(usize, 0, buf.len);
+        for (buf[0..len]) |*byte| byte.* = random.intRangeAtMost(u8, 0, 5);
+        try expectRoundtrip(allocator, buf[0..len]);
+    }
+}
+
+test "rle compresses repetitive payload" {
+    const allocator = std.testing.allocator;
+    const payload = "x" ** 1000;
+    const compressed = try rleCompress(allocator, payload);
+    defer allocator.free(compressed);
+    try std.testing.expect(compressed.len < payload.len / 10);
+}
+
+test "encode picks smaller representation" {
+    const allocator = std.testing.allocator;
+
+    const compressible = "y" ** 500;
+    const enc1 = try encode(allocator, compressible);
+    defer enc1.deinit(allocator);
+    try std.testing.expectEqual(Codec.rle, enc1.codec);
+    const back1 = try decode(allocator, enc1.codec, enc1.bytes, compressible.len);
+    defer allocator.free(back1);
+    try std.testing.expectEqualSlices(u8, compressible, back1);
+
+    var incompressible: [256]u8 = undefined;
+    var prng = std.Random.DefaultPrng.init(0xfeed);
+    prng.random().bytes(&incompressible);
+    const enc2 = try encode(allocator, &incompressible);
+    defer enc2.deinit(allocator);
+    try std.testing.expectEqual(Codec.none, enc2.codec);
+    const back2 = try decode(allocator, enc2.codec, enc2.bytes, incompressible.len);
+    defer allocator.free(back2);
+    try std.testing.expectEqualSlices(u8, &incompressible, back2);
+}
+
+test "decompress rejects truncated stream" {
+    const allocator = std.testing.allocator;
+    const payload = "z" ** 200;
+    const compressed = try rleCompress(allocator, payload);
+    defer allocator.free(compressed);
+    try std.testing.expectError(error.CorruptBlock, rleDecompress(allocator, compressed[0 .. compressed.len - 1], payload.len));
+}
diff --git a/src/root.zig b/src/root.zig
index 6170fda..8c0ad79 100644
--- a/src/root.zig
+++ b/src/root.zig
@@ -17,3 +17,17 @@ pub const BlockCache = @import("cache.zig").BlockCache;
 pub const compress = @import("compress.zig");
 pub const Transaction = @import("txn.zig").Transaction;
 pub const Stats = @import("stats.zig").Stats;
+
+test {
+    _ = @import("encode.zig");
+    _ = @import("bloom.zig");
+    _ = @import("memtable.zig");
+    _ = @import("wal.zig");
+    _ = @import("sstable.zig");
+    _ = @import("cache.zig");
+    _ = @import("batch.zig");
+    _ = @import("compress.zig");
+    _ = @import("txn.zig");
+    _ = @import("stats.zig");
+    _ = @import("db.zig");
+}
diff --git a/src/stats.zig b/src/stats.zig
index ef6716f..64cc336 100644
--- a/src/stats.zig
+++ b/src/stats.zig
@@ -46,3 +46,20 @@ pub const Stats = struct {
         }
     }
 };
+
+test "stats hit rate and deepest level" {
+    var stats = Stats{};
+    stats.cacheHits = 3;
+    stats.cacheMisses = 1;
+    try std.testing.expectApproxEqAbs(@as(f64, 0.75), stats.cacheHitRate(), 1e-9);
+
+    stats.levels[0].tables = 2;
+    stats.levels[2].tables = 1;
+    try std.testing.expectEqual(@as(usize, 2), stats.deepestLevel());
+}
+
+test "empty stats hit rate is zero" {
+    const stats = Stats{};
+    try std.testing.expectEqual(@as(f64, 0), stats.cacheHitRate());
+    try std.testing.expectEqual(@as(usize, 0), stats.deepestLevel());
+}