core: add Rust-backed MLT tile data by louwers · Pull Request #4428 · maplibre/maplibre-native

louwers · 2026-07-23T11:00:47Z

The Rust MLT decoder has a safe FastPFor implementation.

This PR is a vibe coded (GPT 5.6) integration with mlt-core.

for more information, see https://pre-commit.ci

…impl # Conflicts: # src/mbgl/tile/vector_mlt_tile_data.cpp

… vector-mlt-tile-data-impl

for more information, see https://pre-commit.ci

… vector-mlt-tile-data-impl

github-actions · 2026-07-23T12:31:53Z

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%    +592  +0.0%    +664    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4428-compared-to-main.txt

Compared to d387090 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +55% +63.3Mi  +472% +28.2Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4428-compared-to-legacy.txt

github-actions · 2026-07-23T12:33:06Z

Benchmark Results ⚡

Benchmark                                                          Time             CPU      Time Old      Time New       CPU Old       CPU New
-----------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                                 +0.4538         +0.4536             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-4428-compared-to-main.txt

CommanderStorm

Mostly LGTM, a few aspects are a bit weird though.

How is perf compared to cpp?

CommanderStorm · 2026-07-23T11:08:01Z

+    #[derive(Clone, Copy, PartialEq, Eq)]
+    enum MltValueType {
+        Missing = 0,
+        Null = 1,
+        Bool = 2,
+        I64 = 3,
+        U64 = 4,
+        F32 = 5,
+        F64 = 6,
+        String = 7,
+    }
+
+    #[derive(Clone, Copy)]
+    struct MltValue {
+        typ: MltValueType,
+        bool_value: bool,
+        i64_value: i64,
+        u64_value: u64,
+        f32_value: f32,
+        f64_value: f64,
+    }


A bit sad that there is no simpler way to do this.

CommanderStorm · 2026-07-23T11:08:28Z

+    #[derive(Clone, Copy)]
+    struct MltFeatureId {
+        has_id: bool,
+        value: u64,
+    }


Interesting that this is not yet implement in cxx

CommanderStorm · 2026-07-23T11:18:29Z


-        if (!testCase.fastPFOREnabled && testCase.useFastPFOR) {
-            // If fast PFOR is not enabled, the fast PFOR tile should fail to parse
+        if (!testCase.fastPFOREnabled && testCase.useFastPFOR && layerNames.empty()) {


If layerNames is empty, how can size be anything but zero?
Is this still testing something?

CommanderStorm · 2026-07-23T11:52:06Z

    GeometryCollection(const GeometryCollection&) = default;

-    std::shared_ptr<const mlt::MapLibreTile> triangleOwner;
+    std::shared_ptr<const void> triangleOwner;


Is this still save?
What is const void here?
Why do we need to keep it?

CommanderStorm · 2026-07-23T11:57:15Z

+std::string toStdString(rust::String value) {
+    return {value.data(), value.size()};
+}


rust::String implements the explicit converter constructor (not sure what the name is), so you can just std::string(value).
https://cxx.rs/binding/string.html

Also applies to below rust::Str.
Also, Inline afterwards ^^

CommanderStorm · 2026-07-23T12:25:33Z

+VectorMLTTileData::VectorMLTTileData(std::shared_ptr<const std::string> data, bool)
+    : impl(std::make_shared<Impl>(std::move(data))) {}
+
+VectorMLTTileData::VectorMLTTileData(const VectorMLTTileData&) = default;
+VectorMLTTileData::VectorMLTTileData(VectorMLTTileData&&) noexcept = default;
+VectorMLTTileData::~VectorMLTTileData() = default;
+
+std::unique_ptr<GeometryTileData> VectorMLTTileData::clone() const {
+    return std::make_unique<VectorMLTTileData>(*this);
+}
+
+std::unique_ptr<GeometryTileLayer> VectorMLTTileData::getLayer(const std::string& name) const {
+    return impl->getLayer(name);
+}
+
+std::vector<std::string> VectorMLTTileData::layerNames() const {
+    return impl->layerNames();
+}


Should they not also be in the class? Why are they below the class?

CommanderStorm · 2026-07-23T12:28:49Z

+                    .collect(),
+                property_values: Vec::new(),
+                ffi_property_values: Vec::new(),
+                part_offsets: vec![0],


This will mean we always alloc here, can we make the first index implied 0 in the cpp code or in the decoding instead?

I assume this is reasonably hot

CommanderStorm · 2026-07-23T12:36:54Z

+}
+
+fn value(typ: ffi::MltValueType) -> ffi::MltValue {
+    ffi::MltValue {


nit: I'd inline these, derive Default for the last and .. Default::default(), but that is likely fine too.

CommanderStorm · 2026-07-23T12:39:48Z

+            f64_value: *f64_value,
+            ..value(ffi::MltValueType::F64)
+        },
+        MltPropertyValue::String(_) => value(ffi::MltValueType::String),


Is this what we want? Where is the string stored?

CommanderStorm · 2026-07-23T12:43:55Z

+        let Some(start) = layer.part_offsets.get(offset_index) else {
+            return 0;
+        };
+        let Some(end) = layer.part_offsets.get(offset_index + 1) else {
+            return 0;


Not 💯 sure if this compiles, but my intuition would be to do this.

Suggested change

let Some(start) = layer.part_offsets.get(offset_index) else {

return 0;

};

let Some(end) = layer.part_offsets.get(offset_index + 1) else {

return 0;

let Some([start,end]) = layer.part_offsets.get(offset_index..offset_index +1) else {

return 0;

};

louwers · 2026-07-23T12:52:10Z

How is perf compared to cpp?

About the same.

Regarding your Rust comments, this was 100% vibe coded as a soft of proof of concept. I don't really know Rust.

github-actions · 2026-07-23T13:59:00Z

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +80  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-4428-compared-to-main.txt

louwers and others added 5 commits July 23, 2026 00:59

Refactor VectorMLTTileData behind impl

ccaa831

[pre-commit.ci] auto fixes from pre-commit.com hooks

5df7959

for more information, see https://pre-commit.ci

Merge remote-tracking branch 'origin/main' into vector-mlt-tile-data-…

fbcccf9

…impl # Conflicts: # src/mbgl/tile/vector_mlt_tile_data.cpp

Add Rust-backed MLT tile data

07dc863

Merge remote-tracking branch 'louwers/vector-mlt-tile-data-impl' into…

6c6e862

… vector-mlt-tile-data-impl

github-actions Bot added build Related to build, configuration or CI/CD core Changes that affect the C++ core of MapLibre Native labels Jul 23, 2026

pre-commit-ci Bot and others added 6 commits July 23, 2026 11:01

[pre-commit.ci] auto fixes from pre-commit.com hooks

a788c31

for more information, see https://pre-commit.ci

Use published mlt-core crate

8fa68f0

Merge remote-tracking branch 'louwers/vector-mlt-tile-data-impl' into…

a8ec3de

… vector-mlt-tile-data-impl

Fix Clippy lifetime warning

24b65d0

Restore C++ MLT tile data implementation

ee2bb25

Raise rustutils Rust version for mlt-core

b547835

louwers changed the title ~~Add Rust-backed MLT tile data~~ core: add Rust-backed MLT tile data Jul 23, 2026

CommanderStorm approved these changes Jul 23, 2026

View reviewed changes

CommanderStorm mentioned this pull request Jul 24, 2026

fix(rust): Skip encoding competition for single-value int streams; pick smaller of VarInt/plain maplibre/maplibre-tile-spec#1466

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github-actions Bot commented Jul 23, 2026

Bloaty Results 🐋

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-4428-compared-to-main.txt

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github-actions Bot commented Jul 23, 2026

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louwers commented Jul 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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github-actions Bot commented Jul 23, 2026

Bloaty Results (iOS) 🐋

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-4428-compared-to-main.txt

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louwers commented Jul 23, 2026 •

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