pixops: Replace shellsort with radix sort

Enabled by the fact that color channels are now guaranteed to be
8-bit. Improves symbol conversion speed by over 20% in my tests.

chafa/internal/chafa-pixops.c

@@ -798,38 +798,21 @@ chafa_prepare_pixel_data_for_symbols (const ChafaPalette *palette,
 void
 chafa_sort_pixel_index_by_channel (guint8 *index, const ChafaPixel *pixels, gint n_pixels, gint ch)
 {
-    const gint gaps [] = { 57, 23, 10, 4, 1 };
-    gint g, i, j;
+    guint8 buckets [256] [64];
+    guint8 bucket_size [256] = { 0 };
+    gint i, j, k;
 
-    /* Since we don't care about stability and the number of elements
-     * is small and known in advance, use a simple in-place shellsort.
-     *
-     * Due to locality and callback overhead this is probably faster
-     * than qsort(), although admittedly I haven't benchmarked it.
-     *
-     * Another option is to use radix, but since we support multiple
-     * color spaces with fixed-point reals, we could get more buckets
-     * than is practical. */
+    g_assert (n_pixels <= 64);
 
-    for (g = 0; ; g++)
+    for (i = 0; i < n_pixels; i++)
     {
-        gint gap = gaps [g];
-
-        for (i = gap; i < n_pixels; i++)
-        {
-            guint8 ptemp = index [i];
-
-            for (j = i; j >= gap && pixels [index [j - gap]].col.ch [ch]
-                                  > pixels [ptemp].col.ch [ch]; j -= gap)
-            {
-                index [j] = index [j - gap];
-            }
-
-            index [j] = ptemp;
-        }
+        guint8 bucket = pixels [i].col.ch [ch];
+        buckets [bucket] [bucket_size [bucket]++] = i;
+    }
 
-        /* After gap == 1 the array is always left sorted */
-        if (gap == 1)
-            break;
+    for (i = 0, k = 0; i < 256; i++)
+    {
+        for (j = 0; j < bucket_size [i]; j++)
+            index [k++] = buckets [i] [j];
     }
 }

chafa/internal/chafa-work-cell.c

@@ -163,22 +163,13 @@ static const guint8 *
 work_cell_get_sorted_pixels (ChafaWorkCell *wcell, gint ch)
 {
     guint8 *index;
-    const guint8 index_init [CHAFA_SYMBOL_N_PIXELS] =
-    {
-        0,   1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15,
-        16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
-        32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
-        48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63
-    };
 
     index = &wcell->pixels_sorted_index [ch] [0];
 
     if (wcell->have_pixels_sorted_by_channel [ch])
         return index;
 
-    memcpy (index, index_init, CHAFA_SYMBOL_N_PIXELS);
     chafa_sort_pixel_index_by_channel (index, wcell->pixels, CHAFA_SYMBOL_N_PIXELS, ch);
-
     wcell->have_pixels_sorted_by_channel [ch] = TRUE;
     return index;
 }