feat(store): add reverse range iterator and manifest loading

src/db.zig

@@ -755,3 +755,133 @@ pub const RangeIterator = struct {
     }
 };
 
+const ReverseSource = union(enum) {
+    mem: memtable.ReverseCursor,
+    sst: sstable.ReverseCursor,
+
+    fn valid(self: *ReverseSource) bool {
+        return switch (self.*) {
+            .mem => |*c| c.valid(),
+            .sst => |*c| c.valid(),
+        };
+    }
+
+    fn record(self: *ReverseSource) Record {
+        return switch (self.*) {
+            .mem => |*c| c.record(),
+            .sst => |*c| c.record(),
+        };
+    }
+
+    fn advance(self: *ReverseSource) void {
+        switch (self.*) {
+            .mem => |*c| c.advance(),
+            .sst => |*c| c.advance(),
+        }
+    }
+
+    fn deinit(self: *ReverseSource) void {
+        switch (self.*) {
+            .mem => {},
+            .sst => |*c| c.deinit(),
+        }
+    }
+};
+
+pub const ReverseRangeIterator = struct {
+    resultAllocator: Allocator,
+    arena: std.heap.ArenaAllocator,
+    cursors: []ReverseSource,
+    start: ?[]u8,
+    startInclusive: bool,
+    maxSeq: u64,
+
+    pub fn next(self: *ReverseRangeIterator) !?Entry {
+        while (true) {
+            var maxKey: ?[]const u8 = null;
+            for (self.cursors) |*c| {
+                if (c.valid()) {
+                    const key = c.record().key;
+                    if (maxKey == null or order(key, maxKey.?) == .gt) maxKey = key;
+                }
+            }
+            if (maxKey == null) return null;
+            if (self.start) |bound| {
+                const past = if (self.startInclusive) order(maxKey.?, bound) == .lt else order(maxKey.?, bound) != .gt;
+                if (past) return null;
+            }
+
+            var winner: ?Record = null;
+            for (self.cursors) |*c| {
+                if (c.valid() and order(c.record().key, maxKey.?) == .eq) {
+                    const rec = c.record();
+                    if (rec.seq <= self.maxSeq and (winner == null or rec.seq > winner.?.seq)) winner = rec;
+                    c.advance();
+                }
+            }
+            if (winner == null) continue;
+            if (winner.?.kind == .tombstone) continue;
+
+            const arena = self.arena.allocator();
+            return Entry{
+                .key = try arena.dupe(u8, winner.?.key),
+                .value = try arena.dupe(u8, winner.?.value),
+            };
+        }
+    }
+
+    pub fn deinit(self: *ReverseRangeIterator) void {
+        self.arena.deinit();
+        for (self.cursors) |*c| c.deinit();
+        if (self.start) |bound| self.resultAllocator.free(bound);
+        self.resultAllocator.free(self.cursors);
+    }
+};
+
+fn rangesOverlap(aMin: []const u8, aMax: []const u8, bMin: []const u8, bMax: []const u8) bool {
+    return order(aMin, bMax) != .gt and order(bMin, aMax) != .gt;
+}
+
+fn loadManifest(allocator: Allocator, dir: std.fs.Dir) !std.AutoHashMap(u64, usize) {
+    var map = std.AutoHashMap(u64, usize).init(allocator);
+    errdefer map.deinit();
+
+    const file = dir.openFile(manifestName, .{}) catch |err| switch (err) {
+        error.FileNotFound => return map,
+        else => return err,
+    };
+    defer file.close();
+    const data = try file.readToEndAlloc(allocator, std.math.maxInt(usize));
+    defer allocator.free(data);
+    if (data.len < 8) return map;
+
+    const storedCrc = encode.readU32(data, 0);
+    const body = data[4..];
+    if (encode.crc(body) != storedCrc) return map;
+
+    const count = encode.readU32(body, 0);
+    var pos: usize = 4;
+    var i: u32 = 0;
+    while (i < count) : (i += 1) {
+        if (pos + 12 > body.len) break;
+        const fileId = encode.readU64(body, pos);
+        const level = encode.readU32(body, pos + 8);
+        try map.put(fileId, level);
+        pos += 12;
+    }
+    return map;
+}
+
+fn sstName(buf: []u8, id: u64) ![]const u8 {
+    return std.fmt.bufPrint(buf, "sst-{d:0>10}.cbt", .{id});
+}
+
+fn parseSstId(name: []const u8) ?u64 {
+    const prefix = "sst-";
+    const suffix = ".cbt";
+    if (!std.mem.startsWith(u8, name, prefix)) return null;
+    if (!std.mem.endsWith(u8, name, suffix)) return null;
+    const digits = name[prefix.len .. name.len - suffix.len];
+    return std.fmt.parseInt(u64, digits, 10) catch null;
+}
+