Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/chunked-vector.hh

@@ -0,0 +1,75 @@
+#pragma once
+///@file
+
+#include <cstdint>
+#include <cstdlib>
+#include <vector>
+#include <limits>
+
+#include "error.hh"
+
+namespace nix {
+
+/**
+ * Provides an indexable container like vector<> with memory overhead
+ * guarantees like list<> by allocating storage in chunks of ChunkSize
+ * elements instead of using a contiguous memory allocation like vector<>
+ * does. Not using a single vector that is resized reduces memory overhead
+ * on large data sets by on average (growth factor)/2, mostly
+ * eliminates copies within the vector during resizing, and provides stable
+ * references to its elements.
+ */
+template<typename T, size_t ChunkSize>
+class ChunkedVector {
+private:
+    uint32_t size_ = 0;
+    std::vector<std::vector<T>> chunks;
+
+    /**
+     * Keep this out of the ::add hot path
+     */
+    [[gnu::noinline]]
+    auto & addChunk()
+    {
+        if (size_ >= std::numeric_limits<uint32_t>::max() - ChunkSize)
+            unreachable();
+        chunks.emplace_back();
+        chunks.back().reserve(ChunkSize);
+        return chunks.back();
+    }
+
+public:
+    ChunkedVector(uint32_t reserve)
+    {
+        chunks.reserve(reserve);
+        addChunk();
+    }
+
+    uint32_t size() const { return size_; }
+
+    std::pair<T &, uint32_t> add(T value)
+    {
+        const auto idx = size_++;
+        auto & chunk = [&] () -> auto & {
+            if (auto & back = chunks.back(); back.size() < ChunkSize)
+                return back;
+            return addChunk();
+        }();
+        auto & result = chunk.emplace_back(std::move(value));
+        return {result, idx};
+    }
+
+    const T & operator[](uint32_t idx) const
+    {
+        return chunks[idx / ChunkSize][idx % ChunkSize];
+    }
+
+    template<typename Fn>
+    void forEach(Fn fn) const
+    {
+        for (const auto & c : chunks)
+            for (const auto & e : c)
+                fn(e);
+    }
+};
+}