flatudp: the signal-list codec, with a hand-computed decode check

src/source/flatudp/flatudp_signals.hpp

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+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace udpscope::flatudp {
+
+/// The numeric value is the `SignalInfo::type_code` (the order of the client library's `UDPS_T_*`).
+enum class Type : std::uint8_t { U8 = 0, I8, U16, I16, U32, I32, U64, I64, F32, F64 };
+
+constexpr std::uint32_t kMaxCount = 4096;
+constexpr std::size_t kMaxDatagram = 65507;  ///< the largest UDP payload over IPv4
+
+/// One signal of the sender's datagram: `count` consecutive values of `type`.
+struct Row {
+  std::string name;
+  Type type = Type::F32;
+  std::uint32_t count = 1;
+  bool operator==(const Row&) const = default;
+};
+
+std::size_t type_size(Type t);
+const char* type_name(Type t);  ///< "uint8" ... "float64"
+bool parse_type(const std::string& s, Type& out);
+
+/// The bytes one datagram must have: the sum of `type_size * count`.
+std::size_t datagram_size(const std::vector<Row>& rows);
+/// True when no row is a vector. Such a list can be batched, several datagrams to a frame.
+bool all_scalar(const std::vector<Row>& rows);
+
+/// `name:type[,name:type[count]...]`; a count of 1 is written without brackets.
+std::string format(const std::vector<Row>& rows);
+/// The inverse of `format`. Whitespace around names, types and counts is ignored. An empty text
+/// is an empty list (and not an error: `problem` reports it). Returns an error in words, empty
+/// on success; `rows` is empty after a failure.
+std::string parse(const std::string& text, std::vector<Row>& rows);
+
+/// The first reason this list cannot describe a datagram, in words for the user; empty when valid.
+std::string problem(const std::vector<Row>& rows);
+
+/// Decodes one datagram of `datagram_size(rows)` bytes to doubles, row by row, byte-swapping when
+/// `big_endian` differs from the host. Appends to `cols[s]`; the caller clears them between uses.
+void decode_datagram(const std::vector<Row>& rows, const std::uint8_t* data, bool big_endian,
+                     std::vector<std::vector<double>>& cols);
+
+}  // namespace udpscope::flatudp