tv: hack gridlayoutmanager to fill columns before row if we're not scrolling

If we're horizontally scrolling, it makes sense to fill rows before
columns. But if it all fits in one page and we don't need to scroll
horizontally, it looks ridiculous. So, in this case, rearrange the tiles
so that it appears to fill columns before rows. But we don't want things
suddenly jumping around, so actually, keep the same ordering as
rows-before-columns, but add invisible spaces after certain items, so
that the fill area makes it look as though it's columns-before-rows.
This winds up being much more visually pleasing.

We do this by figuring out this kind of transformation:

If we convert this matrix:

	0 3 6
	1 4 _
	2 5 _

To this one:

	0 2 4 6
	1 3 5 _
	_ _ _ _

For a given index, how many spaces are under it? This changes depending
on how many total are in a grid. Going from 3x3 to 4x3, for example, we
have:

	count == 12, index =
	count == 11, index = 10
	count == 10, index = 7,9
	count == 9, index = 4,6,8
	count == 8, index = 1,3,5,7
	count == 7, index = 1,3,5,6!
	count == 6, index = 1,3,4!,5!
	count == 5, index = 1,2!,3!,4!
	count == 4, index = 0!,1!,2!,3!
	count == 3, index = 0!,1!,2!
	count == 2, index = 0!,1!
	count == 1, index = 0!
	count == 0, index =

The '!' means two blanks below, no '!' means one blank below, and no
mention means no blanks below.

This commit adds code to compute such a table on the fly.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

ui/src/main/java/com/wireguard/android/activity/TvMainActivity.kt

@@ -25,6 +25,8 @@ import androidx.databinding.DataBindingUtil
 import androidx.databinding.ObservableBoolean
 import androidx.databinding.ObservableField
 import androidx.lifecycle.lifecycleScope
+import androidx.recyclerview.widget.GridLayoutManager
+import androidx.recyclerview.widget.GridLayoutManager.SpanSizeLookup
 import com.wireguard.android.Application
 import com.wireguard.android.R
 import com.wireguard.android.backend.GoBackend
@@ -94,6 +96,8 @@ class TvMainActivity : AppCompatActivity() {
         binding.isDeleting = isDeleting
         binding.files = files
         binding.filesRoot = filesRoot
+        val gridManager = binding.tunnelList.layoutManager as GridLayoutManager
+        gridManager.spanSizeLookup = SlatedSpanSizeLookup(gridManager)
         binding.tunnelRowConfigurationHandler = object : ObservableKeyedRecyclerViewAdapter.RowConfigurationHandler<TvTunnelListItemBinding, ObservableTunnel> {
             override fun onConfigureRow(binding: TvTunnelListItemBinding, item: ObservableTunnel, position: Int) {
                 binding.isDeleting = isDeleting
@@ -339,6 +343,48 @@ class TvMainActivity : AppCompatActivity() {
             get() = forcedKey ?: if (file.isDirectory) "${file.name}/" else file.name
     }
 
+    private class SlatedSpanSizeLookup(private val gridManager: GridLayoutManager) : SpanSizeLookup() {
+        private val originalHeight = gridManager.spanCount
+        private var newWidth = 0
+        private lateinit var sizeMap: Array<IntArray?>
+
+        private fun emptyUnderIndex(index: Int, size: Int): Int {
+            sizeMap[size - 1]?.let { return it[index] }
+            val sizes = IntArray(size)
+            val oh = originalHeight
+            val nw = newWidth
+            var empties = 0
+            for (i in 0 until size) {
+                val ox = (i + empties) / oh
+                val oy = (i + empties) % oh
+                var empty = 0
+                for (j in oy + 1 until oh) {
+                    val ni = nw * j + ox
+                    if (ni < size)
+                        break
+                    empty++
+                }
+                empties += empty
+                sizes[i] = empty
+            }
+            sizeMap[size - 1] = sizes
+            return sizes[index]
+        }
+
+        override fun getSpanSize(position: Int): Int {
+            if (newWidth == 0) {
+                val child = gridManager.getChildAt(0) ?: return 1
+                if (child.width == 0) return 1
+                newWidth = gridManager.width / child.width
+                sizeMap = Array(originalHeight * newWidth - 1) { null }
+            }
+            val total = gridManager.itemCount
+            if (total >= originalHeight * newWidth || total == 0)
+                return 1
+            return emptyUnderIndex(position, total) + 1
+        }
+    }
+
     companion object {
         private const val TAG = "WireGuard/TvMainActivity"
     }