Add a MLT=0 build for a tippecanoe without MapLibre Tile support

MapLibre Tile support pulls in the maplibre-tile-spec submodule and needs
cmake to build it, which is a lot to ask of anyone who only wants to work
with Mapbox Vector Tiles.

`make MLT=0` compiles with -DNO_MLT, skips the submodule and its cmake
build entirely, and drops the MLT tests from `make test`. The result needs
no dependencies beyond the ones MVT already needed, and can be built from
a checkout with no submodules at all.

Everything that touches the MLT library is behind the #ifdef, which is
just the two files that were written for it. The option parsing and the
tile format helpers stay compiled either way, so nothing else needs to
know: --output-format=mlt reports that the build has no MLT support rather
than being an unrecognized value, and a tile that is recognized as MLT
reports the same instead of being misparsed as a protobuf, since the
format sniffing itself doesn't need the library.

CI builds and tests this configuration from a checkout without submodules,
so it can't quietly stop working.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LxhELiLtpUFwrPSWwTYdHG

.github/workflows/test.yml

@@ -21,3 +21,12 @@ jobs:
         run: brew install sqlite3
       - run: uname -a; BUILDTYPE=${{ matrix.version }} make
       - run: make test
+
+  test-without-mlt:
+    runs-on: ubuntu-latest
+    steps:
+      # No submodules: the MLT=0 build must not need them
+      - uses: actions/checkout@v3
+      - run: sudo apt-get install libsqlite3-dev
+      - run: uname -a; make MLT=0
+      - run: make MLT=0 test

Makefile

@@ -4,12 +4,36 @@ BUILDTYPE ?= Release
 BUILD_INFO ?=
 SHELL = /bin/sh
 
+# MapLibre Tile support requires the maplibre-tile-spec submodule and cmake
+# to build it. Use `make MLT=0` for a build with no dependencies beyond the
+# ones that Mapbox Vector Tiles need; it can neither read nor write MLT.
+MLT ?= 1
+
+MLT_BUILD_STAMP = mlt-build/.built
+
+ifeq ($(MLT),0)
+MLT_FLAGS = -DNO_MLT
+MLT_INCLUDES =
+MLT_STD =
+MLT_DEPS =
+MLT_DECODE_LIBS =
+MLT_LIBS =
+MLT_TESTS =
+else
+MLT_FLAGS =
+MLT_INCLUDES = -isystem maplibre-tile-spec/cpp/include -isystem maplibre-tile-spec/cpp/vendor/fsst
+MLT_STD = -std=c++20
+MLT_DEPS = $(MLT_BUILD_STAMP)
+MLT_DECODE_LIBS = mlt-build/libmlt-cpp.a mlt-build/libfsst-lib.a mlt-build/fastpfor/libFastPFOR.a
+MLT_LIBS = mlt-build/libmlt-cpp-encoder.a $(MLT_DECODE_LIBS)
+MLT_TESTS = mlt-test mlt-decode-test mlt-output-test
+endif
 
 # inherit from env if set
 CC := $(CC)
 CXX := $(CXX)
 CFLAGS := $(CFLAGS) -fPIE -DBUILD_INFO=$(BUILD_INFO)
-CXXFLAGS := $(CXXFLAGS) -std=c++17 -fPIE -DBUILD_INFO=$(BUILD_INFO)
+CXXFLAGS := $(CXXFLAGS) -std=c++17 -fPIE -DBUILD_INFO=$(BUILD_INFO) $(MLT_FLAGS)
 LDFLAGS := $(LDFLAGS)
 WARNING_FLAGS := -Wall -Wshadow -Wsign-compare -Wextra -Wunreachable-code -Wuninitialized -Wshadow
 RELEASE_FLAGS := -O3 -DNDEBUG
@@ -57,10 +81,6 @@ H = $(wildcard *.h) $(wildcard *.hpp)
 C = $(wildcard *.c) $(wildcard *.cpp)
 
 INCLUDES = -I/usr/local/include -I. -Iclipper2/include
-MLT_INCLUDES = -isystem maplibre-tile-spec/cpp/include -isystem maplibre-tile-spec/cpp/vendor/fsst
-MLT_DECODE_LIBS = mlt-build/libmlt-cpp.a mlt-build/libfsst-lib.a mlt-build/fastpfor/libFastPFOR.a
-MLT_LIBS = mlt-build/libmlt-cpp-encoder.a $(MLT_DECODE_LIBS)
-MLT_BUILD_STAMP = mlt-build/.built
 LIBS = -L/usr/local/lib
 
 $(MLT_BUILD_STAMP): maplibre-tile-spec/cpp/CMakeLists.txt
@@ -76,7 +96,9 @@ $(MLT_BUILD_STAMP): maplibre-tile-spec/cpp/CMakeLists.txt
 	cmake --build mlt-build --target mlt-cpp mlt-cpp-encoder $(if $(VERBOSE),,-- -s) > /dev/null
 	touch $@
 
+ifneq ($(MLT),0)
 $(MLT_LIBS): $(MLT_BUILD_STAMP)
+endif
 
 tippecanoe: geojson.o jsonpull/jsonpull.o tile.o pool.o mbtiles.o geometry.o projection.o memfile.o mvt.o mlt.o mlt_decode.o serial.o main.o platform.o text.o dirtiles.o pmtiles_file.o plugin.o read_json.o write_json.o geobuf.o flatgeobuf.o evaluator.o geocsv.o csv.o geojson-loop.o json_logger.o visvalingam.o compression.o clip.o sort.o attribute.o thread.o shared_borders.o clipper2/src/clipper.engine.o $(MLT_LIBS)
 	$(CXX) $(PG) $(LIBS) $(FINAL_FLAGS) $(CXXFLAGS) -o $@ $^ $(LDFLAGS) -lm -lz -lsqlite3 -lpthread
@@ -104,8 +126,8 @@ tippecanoe-overzoom: overzoom.o mvt.o mlt.o mlt_decode.o clip.o evaluator.o json
 %.o: %.c
 	$(CC) -MMD $(PG) $(INCLUDES) $(FINAL_FLAGS) $(CFLAGS) -c -o $@ $<
 
-mlt.o mlt_decode.o: %.o: %.cpp $(MLT_BUILD_STAMP)
-	$(CXX) -MMD $(PG) $(INCLUDES) $(MLT_INCLUDES) $(FINAL_FLAGS) $(CXXFLAGS) -std=c++20 -c -o $@ $<
+mlt.o mlt_decode.o: %.o: %.cpp $(MLT_DEPS)
+	$(CXX) -MMD $(PG) $(INCLUDES) $(MLT_INCLUDES) $(FINAL_FLAGS) $(CXXFLAGS) $(MLT_STD) -c -o $@ $<
 
 %.o: %.cpp
 	$(CXX) -MMD $(PG) $(INCLUDES) $(FINAL_FLAGS) $(CXXFLAGS) -c -o $@ $<
@@ -119,7 +141,7 @@ indent:
 TESTS = $(wildcard tests/*/out/*.json)
 SPACE = $(NULL) $(NULL)
 
-test: tippecanoe tippecanoe-decode $(addsuffix .check,$(TESTS)) raw-tiles-test parallel-test pbf-test join-test enumerate-test decode-test join-filter-test unit json-tool-test allow-existing-test csv-test layer-json-test pmtiles-test decode-pmtiles-test overzoom-test flatgeobuf-test mlt-test mlt-decode-test mlt-output-test
+test: tippecanoe tippecanoe-decode $(addsuffix .check,$(TESTS)) raw-tiles-test parallel-test pbf-test join-test enumerate-test decode-test join-filter-test unit json-tool-test allow-existing-test csv-test layer-json-test pmtiles-test decode-pmtiles-test overzoom-test flatgeobuf-test $(MLT_TESTS)
 	./unit
 
 suffixes = json json.gz

README.md

@@ -50,6 +50,12 @@ $ make -j
 $ make install
 ```
 
+The submodule is the [MapLibre Tile](https://github.com/maplibre/maplibre-tile-spec)
+implementation, which is built with `cmake`. If you don't need MapLibre Tile support,
+build with `make MLT=0` instead, which needs neither the submodule nor `cmake`. Such a
+build can't read or write MLT, and rejects `--output-format=mlt`, but is otherwise the same.
+Run `make clean` first if you are switching an existing build directory between the two.
+
 See [Development](#development) below for how to upgrade your
 C++ compiler or install prerequisite packages if you get
 compiler errors.

mlt.cpp

@@ -1,8 +1,6 @@
 #include "mlt.hpp"
 #include "errors.hpp"
 
-#include <mlt/encoder.hpp>
-
 #include <algorithm>
 #include <cstdint>
 #include <cstdio>
@@ -12,7 +10,11 @@
 #include <string>
 #include <vector>
 
+#ifndef NO_MLT
+#include <mlt/encoder.hpp>
+
 using Vertex = mlt::Encoder::Vertex;
+#endif
 
 int output_format = OUTPUT_MVT;
 bool mlt_sort_features = true;
@@ -22,7 +24,12 @@ void set_output_format(char **argv, const char *format) {
 	if (strcmp(format, "mvt") == 0 || strcmp(format, "pbf") == 0) {
 		output_format = OUTPUT_MVT;
 	} else if (strcmp(format, "mlt") == 0) {
+#ifdef NO_MLT
+		fprintf(stderr, "%s: this build was compiled without MapLibre Tile support\n", argv[0]);
+		exit(EXIT_ARGS);
+#else
 		output_format = OUTPUT_MLT;
+#endif
 	} else {
 		fprintf(stderr, "%s: --output-format must be 'mvt' or 'mlt'\n", argv[0]);
 		exit(EXIT_ARGS);
@@ -38,13 +45,19 @@ const char *tile_format_extension(int format) {
 }
 
 std::string encode_tile(mvt_tile &tile, int format) {
+#ifndef NO_MLT
 	if (format == OUTPUT_MLT) {
 		return encode_as_mlt(tile, mlt_sort_features, mlt_pretessellate);
-	} else {
-		return tile.encode();
 	}
+#else
+	(void) format;
+#endif
+
+	return tile.encode();
 }
 
+#ifndef NO_MLT
+
 // An MLT property column has a single type for the whole layer, while MVT
 // values each carry their own type, so a type that can hold every value of
 // an attribute has to be chosen before the layer can be converted.
@@ -327,3 +340,5 @@ std::string encode_as_mlt(const mvt_tile &tile, bool sort_features, bool pretess
 	auto bytes = encoder.encode(layers, config);
 	return std::string(reinterpret_cast<const char *>(bytes.data()), bytes.size());
 }
+
+#endif

mlt.hpp

@@ -24,14 +24,17 @@ const char *tile_format_extension(int format);
 // options were selected on the command line
 std::string encode_tile(mvt_tile &tile, int format);
 
+#ifndef NO_MLT
 std::string encode_as_mlt(const mvt_tile &tile, bool sort_features, bool pretessellate);
+#endif
 
 // Does this (already decompressed) tile look like MapLibre Tile rather than
 // Mapbox Vector Tile data?
 bool is_mlt(const std::string &message);
 
 // Decode a MapLibre Tile into the equivalent vector tile, returning false
-// (and complaining to stderr) if it can't be parsed.
+// (and complaining to stderr) if it can't be parsed, or if this build
+// doesn't have MLT support compiled in.
 bool decode_mlt(const std::string &message, mvt_tile &out);
 
 #endif

mlt_decode.cpp

@@ -1,11 +1,5 @@
 #include "mlt.hpp"
 
-#include <mlt/decoder.hpp>
-#include <mlt/geometry.hpp>
-#include <mlt/layer.hpp>
-#include <mlt/properties.hpp>
-#include <mlt/tile.hpp>
-
 #include <algorithm>
 #include <cmath>
 #include <cstdint>
@@ -18,6 +12,14 @@
 #include <variant>
 #include <vector>
 
+#ifndef NO_MLT
+#include <mlt/decoder.hpp>
+#include <mlt/geometry.hpp>
+#include <mlt/layer.hpp>
+#include <mlt/properties.hpp>
+#include <mlt/tile.hpp>
+#endif
+
 namespace {
 
 bool read_varint(const char *&p, const char *end, unsigned long long &out) {
@@ -39,6 +41,8 @@ bool read_varint(const char *&p, const char *end, unsigned long long &out) {
 	return false;
 }
 
+#ifndef NO_MLT
+
 long long round_coord(float v) {
 	return (long long) std::llround(v);
 }
@@ -237,6 +241,8 @@ void convert_layer(const mlt::Layer &in, mvt_layer &out, const std::shared_ptr<s
 	}
 }
 
+#endif
+
 }  // namespace
 
 bool is_mlt(const std::string &message) {
@@ -267,6 +273,18 @@ bool is_mlt(const std::string &message) {
 	return layer_tag == 1;
 }
 
+#ifdef NO_MLT
+
+bool decode_mlt(const std::string &message, mvt_tile &out) {
+	(void) message;
+	(void) out;
+
+	fprintf(stderr, "This build was compiled without MapLibre Tile support\n");
+	return false;
+}
+
+#else
+
 bool decode_mlt(const std::string &message, mvt_tile &out) {
 	try {
 		mlt::Decoder decoder(true);
@@ -289,3 +307,5 @@ bool decode_mlt(const std::string &message, mvt_tile &out) {
 		return false;
 	}
 }
+
+#endif