perf: the console's constants move to flash, -378 B and -214 B of RAM

8382 -> 8004 B of flash, 725 -> 511 B of RAM on a part that has 2048.

The command names were a std::to_array of string_view, and on a Harvard
machine that is the worst of both: the characters land in .data and so does
the table's own pointer-and-length pair for each of them, so the firmware
carried 216 B of RAM for thirteen words that never change - and paid for them
in flash too, since .data is copied out of an initialiser image at startup.

They are one NUL-separated blob in flash now, walked with lpm. Separators
rather than an offset table, because an offset table is the RAM this exists to
give back; the names sit in it in match order, so the walk that finds a name
is the same walk that compares it and measures it. Flash falls further than
RAM does: the initialiser image and the two tables were 216 B of it, and the
blob is 87.

The header said the names "cannot" be in flash because they are matched at run
time. Being matched at run time is not a reason to be in RAM on a machine with
two address spaces - only being *written* is, and nothing writes these.

Two smaller things came with it. `reset`'s exact-match rule was a bool on
every entry to protect one; it is an index found by searching the list, so
reordering the commands cannot move the protection onto a different one. And
`version` was the last string_view left, holding its own characters and a
pointer to them.

The matching is now pinned rather than assumed: lookup() is constexpr and the
battery asserts the load-bearing order the README documents - `s` is show and
not statistics, `st` is statistics, no abbreviation of `reset` resolves, and
`helpful` is not `help`. Red-checked by claiming `s` is statistics.

Both modes byte-identical, ten tests green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -66,7 +66,7 @@ cmake --preset atmega328p-generated
 cmake --build --preset atmega328p-generated
 ```
 
-The firmware is **8382 B** of flash, byte-identical between the generated and
+The firmware is **8004 B** of flash, byte-identical between the generated and
 reflect modes, and `ctest` holds it to that number.
 
 ## Atmel Studio

src/terminal.hpp

@@ -18,6 +18,33 @@
 // until Ctrl+C, and Ctrl+C abandons a half-typed line anywhere else.
 namespace app {
 
+// Commands, in the order they are matched - which is the order the original
+// firmware matched them in, and that order is load-bearing. An abbreviation
+// resolves to the *first* entry it prefixes, so `s` is show (not statistics,
+// not set) exactly as it always was, and anything appended to this list cannot
+// steal an abbreviation that already meant something else.
+//
+// A function rather than a variable, so the list exists only while the flash
+// blob below is being built and never as storage of its own.
+consteval auto command_names()
+{
+	return std::to_array<std::string_view>({
+	    "help",
+	    "show",
+	    "curve",
+	    "monitor",
+	    "bootloader",
+	    "uptime",
+	    "statistics",
+	    "histogram",
+	    "reset",
+	    "set",
+	    "auto",
+	    "version",
+	    "save",
+	});
+}
+
 class terminal {
 	static constexpr char ctrl_c = 0x03;
 	static constexpr char backspace = 0x08;
@@ -28,31 +55,48 @@ class terminal {
 	static inline bool m_monitoring = false;
 	static inline std::uint64_t m_last_monitor = 0;
 
-	// Commands, in the order they are matched - which is the order the original
+	static constexpr std::uint8_t command_count = command_names().size();
-	// firmware matched them in, and that order is load-bearing. An abbreviation
-	// resolves to the *first* entry it prefixes, so `s` is show (not statistics,
-	// not set) exactly as it always was, and anything appended to this list
-	// cannot steal an abbreviation that already meant something else.
-	struct command {
-		std::string_view name;
-		bool exact; // reset only: an abbreviation must not be able to wipe data
-	};
 
-	static constexpr auto commands = std::to_array<command>({
+	static constexpr std::size_t name_bytes = [] {
-	    {"help", false},
+		std::size_t total = 0;
-	    {"show", false},
+		for (auto one : command_names()) {
-	    {"curve", false},
+			total += one.size() + 1;
-	    {"monitor", false},
+		}
-	    {"bootloader", false},
+		return total;
-	    {"uptime", false},
+	}();
-	    {"statistics", false},
+
-	    {"histogram", false},
+	// The names as one NUL-separated blob in flash. Matched at run time, which
-	    {"reset", true},
+	// is not a reason to sit in RAM on a machine with two address spaces: a
-	    {"set", false},
+	// table of `string_view` puts the characters *and* its own pointers there,
-	    {"auto", false},
+	// and on this part that was 216 B of 2 KB spent on data that never changes.
-	    {"version", false},
+	// Separators rather than an offset table, because an offset table is the
-	    {"save", false},
+	// RAM this exists to give back.
-	});
+	static constexpr auto names_data = [] {
+		std::array<char, name_bytes> chars{};
+		std::size_t at = 0;
+		for (auto one : command_names()) {
+			for (char c : one) {
+				chars[at++] = c;
+			}
+			chars[at++] = '\0';
+		}
+		return chars;
+	}();
+
+	static constexpr avr::flash_table<names_data> names{};
+
+	// `reset` must be typed in full - an abbreviation must not be able to wipe
+	// data. Found in the list rather than written as an index, so reordering the
+	// commands cannot move the protection onto a different one.
+	static constexpr std::uint8_t exact_command = [] {
+		const auto list = command_names();
+		for (std::uint8_t i = 0; i < list.size(); ++i) {
+			if (list[i] == "reset") {
+				return i;
+			}
+		}
+		return static_cast<std::uint8_t>(list.size());
+	}();
 
 	// Column the descriptions' colons line up in, counted from the start of the
 	// name. The longest name is `bootloader` at 10, so 12 leaves it a space and
@@ -66,20 +110,8 @@ class terminal {
 
 	// `name ....: ` - the dotted label the original used everywhere it printed a
 	// list of things, which is what makes a column of values readable without
-	// counting spaces. One renderer for all three users; only the column differs.
+	// counting spaces. Its width comes from the type, so the padding needs no
-	static void label(std::string_view text, std::uint8_t column)
+	// hand-counted constant.
-	{
-		serial << text << ' ';
-		for (auto i = text.size() + 1; i < column; ++i) {
-			serial << '.';
-		}
-		serial << ": "_P;
-	}
-
-	// The same, for a label that is a literal rather than a command name: it
-	// stays in flash, and its width comes from the type, so the padding needs no
-	// hand-counted constant. The command names cannot use this - they are
-	// string_views because they are matched at run time.
 	template <typename Flash>
 	static void label(Flash text, std::uint8_t column)
 	{
@@ -90,9 +122,21 @@ class terminal {
 		serial << ": "_P;
 	}
 
-	static void help_row(std::string_view name)
+	// The same shape for a command name, whose width is not a type but the walk
+	// to its separator - so it is counted as it is printed rather than measured
+	// first, which would read the blob twice.
+	static void help_row(std::uint8_t which)
 	{
-		label(name, help_column);
+		auto at = name_start(which);
+		std::uint8_t width = 0;
+		for (char c; (c = names.at(at + width)) != '\0'; ++width) {
+			serial << c;
+		}
+		serial << ' ';
+		for (std::uint8_t i = width + 1; i < help_column; ++i) {
+			serial << '.';
+		}
+		serial << ": "_P;
 	}
 
 	// Quarter- C as a signed decimal with a two-digit fraction. The sign is taken
@@ -110,31 +154,31 @@ class terminal {
 	static void help()
 	{
 		serial << "\r\nFanTemp "_P << version << " command overview\r\n"_P;
-		help_row(commands[0].name);
+		help_row(0);
 		serial << "prints this help message\r\n"_P;
-		help_row(commands[1].name);
+		help_row(1);
 		serial << "shows current temperature and fan speed\r\n"_P;
-		help_row(commands[2].name);
+		help_row(2);
 		serial << "shows mapping from temperature to fan speed\r\n"_P;
-		help_row(commands[3].name);
+		help_row(3);
 		serial << "loops the show command until Ctrl+C is pressed\r\n"_P;
-		help_row(commands[4].name);
+		help_row(4);
 		serial << "enters the bootloader\r\n"_P;
-		help_row(commands[5].name);
+		help_row(5);
 		serial << "shows system uptime\r\n"_P;
-		help_row(commands[6].name);
+		help_row(6);
 		serial << "prints overall statistics like min and max temp\r\n"_P;
-		help_row(commands[7].name);
+		help_row(7);
 		serial << "prints a histogram of the temperature\r\n"_P;
-		help_row(commands[8].name);
+		help_row(8);
 		serial << "resets statistics to 0 in EEPROM and RAM (no abbreviation)\r\n"_P;
-		help_row(commands[9].name);
+		help_row(9);
 		serial << "sets the fan speed to the provided value, 0-100\r\n"_P;
-		help_row(commands[10].name);
+		help_row(10);
 		serial << "turns on automatic fan control\r\n"_P;
-		help_row(commands[11].name);
+		help_row(11);
 		serial << "displays firmware version\r\n"_P;
-		help_row(commands[12].name);
+		help_row(12);
 		serial << "writes the statistics to EEPROM now\r\n"_P;
 		serial << "commands may be abbreviated: 'up' is uptime\r\n"_P;
 	}
@@ -259,25 +303,58 @@ class terminal {
 		}
 	}
 
-	// Abbreviations: the input matches a command when it is a non-empty prefix
+	// Where the given command's name begins in the blob. A walk over the
-	// of it. `reset` is the exception and must be typed in full.
+	// separators, for the two callers that need to start at one name.
-	//
+	static constexpr std::uint8_t name_start(std::uint8_t which)
-	// starts_with, not substr: substr throws std::out_of_range, and one
-	// potentially-throwing call is enough to pull in std::terminate, which does
-	// not exist in a freestanding AVR build. The link fails rather than the
-	// firmware, so this is a build-time trap rather than a runtime one - but it
-	// is a trap, and the whole file avoids substr for that reason.
-	static bool matches(std::string_view input, const command &c)
 	{
-		if (input.empty()) {
+		std::uint8_t at = 0;
-			return false;
+		for (std::uint8_t i = 0; i < which; ++i) {
+			while (names.at(at) != '\0') {
+				++at;
 			}
-		if (c.exact) {
+			++at;
-			return input == c.name;
 		}
-		return c.name.starts_with(input);
+		return at;
 	}
 
+  public:
+	// Which command a word names, or `command_count` for none. The input
+	// matches when it is a non-empty prefix of a name; `reset` is the exception
+	// and must be typed in full.
+	//
+	// One pass over the blob answers both, because the names sit in it in match
+	// order: each name is compared as it is walked, and the walk to its
+	// separator is also what measures it.
+	//
+	// Public and `constexpr` so the battery can pin it: the match order is
+	// load-bearing, an abbreviation must never reach `reset`, and reading the
+	// names from flash must not have changed either.
+	static constexpr std::uint8_t lookup(std::string_view input)
+	{
+		if (input.empty()) {
+			return command_count;
+		}
+		std::uint8_t at = 0;
+		for (std::uint8_t i = 0; i < command_count; ++i) {
+			std::uint8_t length = 0;
+			bool prefix = true;
+			for (char c; (c = names.at(at + length)) != '\0'; ++length) {
+				if (length < input.size() && input[length] != c) {
+					prefix = false;
+				}
+			}
+			const bool whole = input.size() == length;
+			if (prefix && input.size() <= length && (i != exact_command || whole)) {
+				return i;
+			}
+			at += length + 1;
+		}
+		return command_count;
+	}
+
+	static constexpr std::uint8_t no_command = command_count;
+
+  private:
 	static void dispatch(std::string_view input)
 	{
 		// A line that overflowed the buffer is not a command - it is the tail of
@@ -299,13 +376,7 @@ class terminal {
 			return;
 		}
 
-		std::uint8_t which = commands.size();
+		const auto which = lookup(word);
-		for (std::uint8_t i = 0; i < commands.size(); ++i) {
-			if (matches(word, commands[i])) {
-				which = i;
-				break;
-			}
-		}
 
 		switch (which) {
 		case 0:
@@ -377,7 +448,10 @@ class terminal {
 	}
 
   public:
-	static constexpr std::string_view version = "v2.2";
+	// A flash string, like every other literal the console prints: as a
+	// `string_view` it was the last constant left in RAM, holding its own
+	// characters and a pointer to them.
+	static constexpr auto version = "v2.2"_P;
 	static constexpr std::uint8_t bar_max = 100;
 
 	// Columns the dotted labels' colons land in, and the curve's span. All four

test/CMakeLists.txt

@@ -2,7 +2,7 @@ libavr_format_test()
 
 # The image must stay below the boot section at 0x7e00, and the README states
 # what it measures; a library advance that moves it says nothing on its own.
-libavr_size_claim_test(fantemp 8382)
+libavr_size_claim_test(fantemp 8004)
 
 # The README says the two modes emit the same image, and only a tree with
 # both built can say whether they do.

test/consteval.cpp

@@ -6,6 +6,7 @@
 #include <cstdint>
 
 #include "curve.hpp"
+#include "terminal.hpp"
 #include "thermistor.hpp"
 
 namespace {
@@ -81,4 +82,39 @@ consteval bool thermistor_falls()
 }
 static_assert(thermistor_falls());
 
+// The console's one parsing rule, read out of the flash blob the names live
+// in. Every case here is a property of the *order* the names sit in, so this
+// is what an edit to that list has to answer to.
+using app::terminal;
+
+// Full names, and the ends of the list - a walk that miscounts a separator
+// lands on a neighbour rather than failing, so both ends are named.
+static_assert(terminal::lookup("help") == 0);
+static_assert(terminal::lookup("save") == 12);
+static_assert(terminal::lookup("bootloader") == 4);
+
+// Abbreviations resolve to the first entry they prefix. `s` prefixes show,
+// statistics, set and save, and show is first - which is the original's
+// behaviour and the reason the list is ordered rather than sorted.
+static_assert(terminal::lookup("s") == 1);
+static_assert(terminal::lookup("st") == 6);
+static_assert(terminal::lookup("se") == 9);
+static_assert(terminal::lookup("sa") == 12);
+static_assert(terminal::lookup("up") == 5);
+static_assert(terminal::lookup("b") == 4);
+
+// `reset` is the exception: it must be typed in full, so no abbreviation of it
+// resolves - and none of its prefixes names anything else either.
+static_assert(terminal::lookup("reset") == 8);
+static_assert(terminal::lookup("rese") == terminal::no_command);
+static_assert(terminal::lookup("res") == terminal::no_command);
+static_assert(terminal::lookup("r") == terminal::no_command);
+
+// Nothing, and nothing that matches.
+static_assert(terminal::lookup("") == terminal::no_command);
+static_assert(terminal::lookup("xyzzy") == terminal::no_command);
+
+// Longer than the name it prefixes is not a match: `helpful` is not `help`.
+static_assert(terminal::lookup("helpful") == terminal::no_command);
+
 } // namespace