From 6d5965e91ad32bf47492f828d7a96a1b69f1636b Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Wed, 26 Aug 2026 19:15:00 +0200
Subject: [PATCH] feat(txn): add mvcc transactions with conflict detection

---
 src/txn.zig | 104 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)
 create mode 100644 src/txn.zig

diff --git a/src/txn.zig b/src/txn.zig
new file mode 100644
index 0000000..dd0152f
--- /dev/null
+++ b/src/txn.zig
@@ -0,0 +1,104 @@
+const std = @import("std");
+const encode = @import("encode.zig");
+const dbmod = @import("db.zig");
+const WriteBatch = @import("batch.zig").WriteBatch;
+
+const Allocator = std.mem.Allocator;
+const Kind = encode.Kind;
+const Db = dbmod.Db;
+
+const BufferedWrite = struct {
+    kind: Kind,
+    value: []u8,
+};
+
+pub const Transaction = struct {
+    db: *Db,
+    allocator: Allocator,
+    snapshotSeq: u64,
+    reads: std.StringHashMap(void),
+    writes: std.StringHashMap(BufferedWrite),
+    finished: bool,
+
+    pub fn begin(db: *Db) Transaction {
+        return .{
+            .db = db,
+            .allocator = db.allocator,
+            .snapshotSeq = if (db.nextSeq == 0) 0 else db.nextSeq - 1,
+            .reads = std.StringHashMap(void).init(db.allocator),
+            .writes = std.StringHashMap(BufferedWrite).init(db.allocator),
+            .finished = false,
+        };
+    }
+
+    pub fn deinit(self: *Transaction) void {
+        var readKeys = self.reads.keyIterator();
+        while (readKeys.next()) |key| self.allocator.free(key.*);
+        self.reads.deinit();
+
+        var writeEntries = self.writes.iterator();
+        while (writeEntries.next()) |entry| {
+            self.allocator.free(entry.key_ptr.*);
+            self.allocator.free(entry.value_ptr.value);
+        }
+        self.writes.deinit();
+    }
+
+    pub fn get(self: *Transaction, allocator: Allocator, key: []const u8) !?[]u8 {
+        if (self.writes.get(key)) |buffered| {
+            if (buffered.kind == .tombstone) return null;
+            return try allocator.dupe(u8, buffered.value);
+        }
+        try self.recordRead(key);
+        return self.db.getAsOf(allocator, key, self.snapshotSeq);
+    }
+
+    pub fn put(self: *Transaction, key: []const u8, value: []const u8) !void {
+        try self.stage(.value, key, value);
+    }
+
+    pub fn delete(self: *Transaction, key: []const u8) !void {
+        try self.stage(.tombstone, key, &.{});
+    }
+
+    fn stage(self: *Transaction, kind: Kind, key: []const u8, value: []const u8) !void {
+        const valueCopy = try self.allocator.dupe(u8, value);
+        errdefer self.allocator.free(valueCopy);
+        const gop = try self.writes.getOrPut(key);
+        if (gop.found_existing) {
+            self.allocator.free(gop.value_ptr.value);
+            gop.value_ptr.* = .{ .kind = kind, .value = valueCopy };
+            return;
+        }
+        gop.key_ptr.* = try self.allocator.dupe(u8, key);
+        gop.value_ptr.* = .{ .kind = kind, .value = valueCopy };
+    }
+
+    fn recordRead(self: *Transaction, key: []const u8) !void {
+        if (self.reads.contains(key)) return;
+        const keyCopy = try self.allocator.dupe(u8, key);
+        errdefer self.allocator.free(keyCopy);
+        try self.reads.put(keyCopy, {});
+    }
+
+    pub fn commit(self: *Transaction) !void {
+        var readKeys = self.reads.keyIterator();
+        while (readKeys.next()) |key| {
+            if (self.db.latestSeq(key.*)) |seq| {
+                if (seq > self.snapshotSeq) return error.Conflict;
+            }
+        }
+
+        var batch = WriteBatch.init(self.allocator);
+        defer batch.deinit();
+        var writeEntries = self.writes.iterator();
+        while (writeEntries.next()) |entry| {
+            switch (entry.value_ptr.kind) {
+                .value => try batch.put(entry.key_ptr.*, entry.value_ptr.value),
+                .tombstone => try batch.delete(entry.key_ptr.*),
+            }
+        }
+        try self.db.apply(&batch);
+        self.finished = true;
+    }
+};