1 files changed, 94 insertions, 0 deletions
diff --git a/include/wireframe/pcapng/writer.hpp b/include/wireframe/pcapng/writer.hpp
new file mode 100644
index 0000000..18f6022
--- /dev/null
+++ b/include/wireframe/pcapng/writer.hpp
@@ -0,0 +1,94 @@
+#pragma once
+
+#include <algorithm>
+#include <cstdint>
+#include <cstdio>
+#include <span>
+#include <vector>
+
+// Minimal pcapng writer: one Section Header Block, one Interface
+// Description Block, then an Enhanced Packet Block per captured packet.
+// Per-block Options are skipped entirely - they're optional in the
+// spec, and a block with none simply omits that section, so this stays
+// a valid, Wireshark-readable file without needing to hand-encode TLVs.
+//
+// Multi-byte fields are written little-endian by hand (matching the
+// 0x1A2B3C4D byte-order magic below) rather than via struct-casting,
+// for the same alignment/UB reasons as the src/wireframe/net decoders.
+namespace wireframe::pcapng {
+
+inline constexpr std::uint32_t kBlockTypeShb = 0x0A0D0D0A;
+inline constexpr std::uint32_t kBlockTypeIdb = 0x00000001;
+inline constexpr std::uint32_t kBlockTypeEpb = 0x00000006;
+inline constexpr std::uint32_t kByteOrderMagic = 0x1A2B3C4D;
+inline constexpr std::uint16_t kLinkTypeEthernet = 1;
+
+class Writer {
+public:
+    explicit Writer(std::FILE* file) : file_(file) {}
+
+    void write_section_header() {
+        std::uint8_t body[16];
+        put_u32(body + 0, kByteOrderMagic);
+        put_u16(body + 4, 1);  // major version
+        put_u16(body + 6, 0);  // minor version
+        put_u64(body + 8, 0xFFFFFFFFFFFFFFFFULL);  // section length: unknown
+        write_block(kBlockTypeShb, {body, sizeof(body)});
+    }
+
+    void write_interface_description(std::uint32_t snaplen, std::uint16_t link_type) {
+        std::uint8_t body[8];
+        put_u16(body + 0, link_type);
+        put_u16(body + 2, 0);  // reserved
+        put_u32(body + 4, snaplen);
+        write_block(kBlockTypeIdb, {body, sizeof(body)});
+    }
+
+    void write_packet(std::uint32_t interface_id, std::uint32_t ts_sec, std::uint32_t ts_usec,
+                       std::span<const unsigned char> data, std::uint32_t original_len) {
+        std::uint64_t ts_us = static_cast<std::uint64_t>(ts_sec) * 1'000'000ULL + ts_usec;
+        std::uint32_t ts_high = static_cast<std::uint32_t>(ts_us >> 32);
+        std::uint32_t ts_low = static_cast<std::uint32_t>(ts_us & 0xFFFFFFFFULL);
+
+        std::size_t padded_len = (data.size() + 3) & ~std::size_t(3);
+        std::vector<std::uint8_t> body(20 + padded_len, 0);  // tail is padding, stays zero
+        put_u32(body.data() + 0, interface_id);
+        put_u32(body.data() + 4, ts_high);
+        put_u32(body.data() + 8, ts_low);
+        put_u32(body.data() + 12, static_cast<std::uint32_t>(data.size()));
+        put_u32(body.data() + 16, original_len);
+        std::copy(data.begin(), data.end(), body.begin() + 20);
+
+        write_block(kBlockTypeEpb, body);
+    }
+
+private:
+    static void put_u16(std::uint8_t* p, std::uint16_t v) {
+        p[0] = static_cast<std::uint8_t>(v & 0xFF);
+        p[1] = static_cast<std::uint8_t>((v >> 8) & 0xFF);
+    }
+
+    static void put_u32(std::uint8_t* p, std::uint32_t v) {
+        for (int i = 0; i < 4; ++i) p[i] = static_cast<std::uint8_t>((v >> (8 * i)) & 0xFF);
+    }
+
+    static void put_u64(std::uint8_t* p, std::uint64_t v) {
+        for (int i = 0; i < 8; ++i) p[i] = static_cast<std::uint8_t>((v >> (8 * i)) & 0xFF);
+    }
+
+    void write_block(std::uint32_t type, std::span<const std::uint8_t> body) {
+        std::uint32_t total_len = static_cast<std::uint32_t>(8 + body.size() + 4);
+        std::uint8_t type_buf[4];
+        std::uint8_t len_buf[4];
+        put_u32(type_buf, type);
+        put_u32(len_buf, total_len);
+        std::fwrite(type_buf, 1, 4, file_);
+        std::fwrite(len_buf, 1, 4, file_);
+        std::fwrite(body.data(), 1, body.size(), file_);
+        std::fwrite(len_buf, 1, 4, file_);
+    }
+
+    std::FILE* file_;
+};
+
+}  // namespace wireframe::pcapng