fix: decode to values up to 255, not 256 - step

I don't know whether to consiser this a bugfix or a feature.
It does not increase color accuracy
(still four or eight values to one),
but it does increase range.

Use tables for encoding and decoding.
It makes the mapping of values visible and easier to understand.
It does not seem to affect performance on large files.

hicolor.h

@@ -25,6 +25,54 @@
 
 static const uint8_t hicolor_magic[7] = "HiColor";
 
+static const uint8_t hicolor_256_to_32[] = {
+    0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2,
+    3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5,
+    6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8,
+    9, 9, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 11, 11, 11, 11,
+    11, 11, 11, 11, 12, 12, 12, 12, 12, 12, 12, 12, 13, 13, 13, 13, 13, 13,
+    13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 15, 15, 15, 15, 15, 15, 15, 15,
+    16, 16, 16, 16, 16, 16, 16, 16, 17, 17, 17, 17, 17, 17, 17, 17, 18, 18,
+    18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19, 19, 20, 20, 20, 20,
+    20, 20, 20, 20, 21, 21, 21, 21, 21, 21, 21, 21, 22, 22, 22, 22, 22, 22,
+    22, 22, 23, 23, 23, 23, 23, 23, 23, 23, 24, 24, 24, 24, 24, 24, 24, 24,
+    25, 25, 25, 25, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26, 26, 27, 27,
+    27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 29, 29, 29, 29,
+    29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30, 30, 31, 31, 31, 31, 31, 31,
+    31, 31
+};
+
+static const uint8_t hicolor_256_to_64[] = {
+    0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5,
+    6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11,
+    11, 11, 12, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 15, 15,
+    16, 16, 16, 16, 17, 17, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20,
+    20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23, 24, 24, 24, 24,
+    25, 25, 25, 25, 26, 26, 26, 26, 27, 27, 27, 27, 28, 28, 28, 28, 29, 29,
+    29, 29, 30, 30, 30, 30, 31, 31, 31, 31, 32, 32, 32, 32, 33, 33, 33, 33,
+    34, 34, 34, 34, 35, 35, 35, 35, 36, 36, 36, 36, 37, 37, 37, 37, 38, 38,
+    38, 38, 39, 39, 39, 39, 40, 40, 40, 40, 41, 41, 41, 41, 42, 42, 42, 42,
+    43, 43, 43, 43, 44, 44, 44, 44, 45, 45, 45, 45, 46, 46, 46, 46, 47, 47,
+    47, 47, 48, 48, 48, 48, 49, 49, 49, 49, 50, 50, 50, 50, 51, 51, 51, 51,
+    52, 52, 52, 52, 53, 53, 53, 53, 54, 54, 54, 54, 55, 55, 55, 55, 56, 56,
+    56, 56, 57, 57, 57, 57, 58, 58, 58, 58, 59, 59, 59, 59, 60, 60, 60, 60,
+    61, 61, 61, 61, 62, 62, 62, 62, 63, 63, 63, 63
+};
+
+static const uint8_t hicolor_32_to_256[] = {
+    0, 8, 16, 24, 33, 41, 49, 57, 66, 74, 82, 90, 99, 107, 115, 123, 132,
+    140, 148, 156, 165, 173, 181, 189, 198, 206, 214, 222, 231, 239, 247,
+    255
+};
+
+static const uint8_t hicolor_64_to_256[] = {
+    0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 65, 69, 73,
+    77, 81, 85, 89, 93, 97, 101, 105, 109, 113, 117, 121, 125, 130, 134,
+    138, 142, 146, 150, 154, 158, 162, 166, 170, 174, 178, 182, 186, 190,
+    195, 199, 203, 207, 211, 215, 219, 223, 227, 231, 235, 239, 243, 247,
+    251, 255
+};
+
 static const double hicolor_bayer[HICOLOR_BAYER_SIZE * HICOLOR_BAYER_SIZE] = {
      0.0/64, 48.0/64, 12.0/64, 60.0/64,  3.0/64, 51.0/64, 15.0/64, 63.0/64,
     32.0/64, 16.0/64, 44.0/64, 28.0/64, 35.0/64, 19.0/64, 47.0/64, 31.0/64,
@@ -192,14 +240,14 @@ hicolor_result hicolor_value_to_rgb(
     switch (version) {
     case HICOLOR_VERSION_5:
         if (value & 0x8000) return HICOLOR_INVALID_VALUE;
-        rgb->r = (value & 0x1f) << 3;
-        rgb->g = (value & 0x3ff) >> (5 - 3);
-        rgb->b = (value & 0x7fff) >> (10 - 3);
+        rgb->r = hicolor_32_to_256[value & 0x1f];
+        rgb->g = hicolor_32_to_256[(value & 0x3ff) >> 5];
+        rgb->b = hicolor_32_to_256[(value & 0x7fff) >> 10];
         return HICOLOR_OK;
     case HICOLOR_VERSION_6:
-        rgb->r = (value & 0x1f) << 3;
-        rgb->g = (value & 0x7ff) >> (5 - 2);
-        rgb->b = value >> (11 - 3);
+        rgb->r = hicolor_32_to_256[value & 0x1f];
+        rgb->g = hicolor_64_to_256[(value & 0x7ff) >> 5];
+        rgb->b = hicolor_32_to_256[value >> 11];
         return HICOLOR_OK;
     default:
         return HICOLOR_UNKNOWN_VERSION;
@@ -214,10 +262,14 @@ hicolor_result hicolor_rgb_to_value(
 {
     switch (version) {
     case HICOLOR_VERSION_5:
-        *value = (rgb.r >> 3) | (rgb.g >> 3 << 5) | (rgb.b >> 3 << 10);
+        *value = hicolor_256_to_32[rgb.r]
+            | hicolor_256_to_32[rgb.g] << 5
+            | hicolor_256_to_32[rgb.b] << 10;
         return HICOLOR_OK;
     case HICOLOR_VERSION_6:
-        *value = (rgb.r >> 3) | (rgb.g >> 2 << 5) | (rgb.b >> 3 << 11);
+        *value = hicolor_256_to_32[rgb.r]
+            | hicolor_256_to_64[rgb.g] << 5
+            | hicolor_256_to_32[rgb.b] << 11;
         return HICOLOR_OK;
     default:
         return HICOLOR_UNKNOWN_VERSION;