feat(stats): add database statistics

src/stats.zig

@@ -0,0 +1,48 @@
+const std = @import("std");
+
+pub const maxTrackedLevels = 8;
+
+pub const LevelStat = struct {
+    tables: usize = 0,
+    entries: u64 = 0,
+    physicalBytes: u64 = 0,
+};
+
+pub const Stats = struct {
+    memtableEntries: usize = 0,
+    memtableBytes: usize = 0,
+    sstableCount: usize = 0,
+    liveEntries: u64 = 0,
+    physicalBytes: u64 = 0,
+    cacheHits: u64 = 0,
+    cacheMisses: u64 = 0,
+    nextSeq: u64 = 0,
+    levels: [maxTrackedLevels]LevelStat = [_]LevelStat{.{}} ** maxTrackedLevels,
+
+    pub fn cacheHitRate(self: Stats) f64 {
+        const total = self.cacheHits + self.cacheMisses;
+        if (total == 0) return 0;
+        return @as(f64, @floatFromInt(self.cacheHits)) / @as(f64, @floatFromInt(total));
+    }
+
+    pub fn deepestLevel(self: Stats) usize {
+        var deepest: usize = 0;
+        for (self.levels, 0..) |level, i| {
+            if (level.tables > 0) deepest = i;
+        }
+        return deepest;
+    }
+
+    pub fn print(self: Stats, writer: anytype) !void {
+        try writer.print("memtable    entries={d} bytes={d}\n", .{ self.memtableEntries, self.memtableBytes });
+        try writer.print("sstables    count={d} entries={d} physicalBytes={d}\n", .{ self.sstableCount, self.liveEntries, self.physicalBytes });
+        try writer.print("nextSeq     {d}\n", .{self.nextSeq});
+        try writer.print("blockCache  hits={d} misses={d} hitRate={d:.3}\n", .{ self.cacheHits, self.cacheMisses, self.cacheHitRate() });
+        var i: usize = 0;
+        const deepest = self.deepestLevel();
+        while (i <= deepest) : (i += 1) {
+            const level = self.levels[i];
+            try writer.print("  L{d}        tables={d} entries={d} physicalBytes={d}\n", .{ i, level.tables, level.entries, level.physicalBytes });
+        }
+    }
+};