[experiment] add alternative wasm sqlite3 implementation available via build-tag (#2863)

This allows for building GoToSocial with [SQLite transpiled to WASM](https://github.com/ncruces/go-sqlite3) and accessed through [Wazero](https://wazero.io/).

vendor/github.com/tetratelabs/wazero/internal/descriptor/table.go

@@ -0,0 +1,164 @@
+package descriptor
+
+import "math/bits"
+
+// Table is a data structure mapping 32 bit descriptor to items.
+//
+// # Negative keys are invalid.
+//
+// Negative keys (e.g. -1) are invalid inputs and will return a corresponding
+// not-found value. This matches POSIX behavior of file descriptors.
+// See https://pubs.opengroup.org/onlinepubs/9699919799/functions/dirfd.html#tag_16_90
+//
+// # Data structure design
+//
+// The data structure optimizes for memory density and lookup performance,
+// trading off compute at insertion time. This is a useful compromise for the
+// use cases we employ it with: items are usually accessed a lot more often
+// than they are inserted, each operation requires a table lookup, so we are
+// better off spending extra compute to insert items in the table in order to
+// get cheaper lookups. Memory efficiency is also crucial to support scaling
+// with programs that maintain thousands of items: having a high or non-linear
+// memory-to-item ratio could otherwise be used as an attack vector by
+// malicious applications attempting to damage performance of the host.
+type Table[Key ~int32, Item any] struct {
+	masks []uint64
+	items []Item
+}
+
+// Len returns the number of items stored in the table.
+func (t *Table[Key, Item]) Len() (n int) {
+	// We could make this a O(1) operation if we cached the number of items in
+	// the table. More state usually means more problems, so until we have a
+	// clear need for this, the simple implementation may be a better trade off.
+	for _, mask := range t.masks {
+		n += bits.OnesCount64(mask)
+	}
+	return n
+}
+
+// grow ensures that t has enough room for n items, potentially reallocating the
+// internal buffers if their capacity was too small to hold this many items.
+func (t *Table[Key, Item]) grow(n int) {
+	// Round up to a multiple of 64 since this is the smallest increment due to
+	// using 64 bits masks.
+	n = (n*64 + 63) / 64
+
+	if n > len(t.masks) {
+		masks := make([]uint64, n)
+		copy(masks, t.masks)
+
+		items := make([]Item, n*64)
+		copy(items, t.items)
+
+		t.masks = masks
+		t.items = items
+	}
+}
+
+// Insert inserts the given item to the table, returning the key that it is
+// mapped to or false if the table was full.
+//
+// The method does not perform deduplication, it is possible for the same item
+// to be inserted multiple times, each insertion will return a different key.
+func (t *Table[Key, Item]) Insert(item Item) (key Key, ok bool) {
+	offset := 0
+insert:
+	// Note: this loop could be made a lot more efficient using vectorized
+	// operations: 256 bits vector registers would yield a theoretical 4x
+	// speed up (e.g. using AVX2).
+	for index, mask := range t.masks[offset:] {
+		if ^mask != 0 { // not full?
+			shift := bits.TrailingZeros64(^mask)
+			index += offset
+			key = Key(index)*64 + Key(shift)
+			t.items[key] = item
+			t.masks[index] = mask | uint64(1<<shift)
+			return key, key >= 0
+		}
+	}
+
+	offset = len(t.masks)
+	n := 2 * len(t.masks)
+	if n == 0 {
+		n = 1
+	}
+
+	t.grow(n)
+	goto insert
+}
+
+// Lookup returns the item associated with the given key (may be nil).
+func (t *Table[Key, Item]) Lookup(key Key) (item Item, found bool) {
+	if key < 0 { // invalid key
+		return
+	}
+	if i := int(key); i >= 0 && i < len(t.items) {
+		index := uint(key) / 64
+		shift := uint(key) % 64
+		if (t.masks[index] & (1 << shift)) != 0 {
+			item, found = t.items[i], true
+		}
+	}
+	return
+}
+
+// InsertAt inserts the given `item` at the item descriptor `key`. This returns
+// false if the insert was impossible due to negative key.
+func (t *Table[Key, Item]) InsertAt(item Item, key Key) bool {
+	if key < 0 {
+		return false
+	}
+	if diff := int(key) - t.Len(); diff > 0 {
+		t.grow(diff)
+	}
+	index := uint(key) / 64
+	shift := uint(key) % 64
+	t.masks[index] |= 1 << shift
+	t.items[key] = item
+	return true
+}
+
+// Delete deletes the item stored at the given key from the table.
+func (t *Table[Key, Item]) Delete(key Key) {
+	if key < 0 { // invalid key
+		return
+	}
+	if index, shift := key/64, key%64; int(index) < len(t.masks) {
+		mask := t.masks[index]
+		if (mask & (1 << shift)) != 0 {
+			var zero Item
+			t.items[key] = zero
+			t.masks[index] = mask & ^uint64(1<<shift)
+		}
+	}
+}
+
+// Range calls f for each item and its associated key in the table. The function
+// f might return false to interupt the iteration.
+func (t *Table[Key, Item]) Range(f func(Key, Item) bool) {
+	for i, mask := range t.masks {
+		if mask == 0 {
+			continue
+		}
+		for j := Key(0); j < 64; j++ {
+			if (mask & (1 << j)) == 0 {
+				continue
+			}
+			if key := Key(i)*64 + j; !f(key, t.items[key]) {
+				return
+			}
+		}
+	}
+}
+
+// Reset clears the content of the table.
+func (t *Table[Key, Item]) Reset() {
+	for i := range t.masks {
+		t.masks[i] = 0
+	}
+	var zero Item
+	for i := range t.items {
+		t.items[i] = zero
+	}
+}