1 files changed, 136 insertions, 0 deletions
diff --git a/src/bin/xi/xidec.c b/src/bin/xi/xidec.c
new file mode 100644
index 0000000..cb4c1bb
--- /dev/null
+++ b/src/bin/xi/xidec.c
@@ -0,0 +1,136 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <string.h>
+#include <assert.h>
+#include <err.h>
+
+static uint8_t
+rotate_right(uint8_t b, const uint8_t count)
+{
+   for (size_t i = 0; i < count; ++i) {
+      const bool drop = ((b & 0x01) == 0x01);
+      b = (b >> 1) | (drop ? 0x80 : 0);
+   }
+   return b;
+}
+
+static void
+decode(uint8_t *data, const size_t size, const uint8_t count)
+{
+   assert(data);
+
+   for (size_t i = 0; i < size; ++i)
+      data[i] = rotate_right(data[i], count);
+}
+
+static size_t
+count_bits(const uint8_t byte)
+{
+   static const uint8_t lut[16] = {
+      0, 1, 1, 2, 1, 2, 2, 3,
+      1, 2, 2, 3, 2, 3, 3, 4
+   };
+
+   return lut[byte & 0x0F] + lut[byte >> 4];
+}
+
+static uint8_t
+text_rotation(const uint8_t *data, const size_t size)
+{
+   assert(data);
+
+   if (size < 2 || (data[0] == 0 && data[1] == 0))
+      return 0;
+
+   const int seed = count_bits(data[1]) - count_bits(data[0]);
+   switch (abs(seed) % 5) {
+      case 0: return 1;
+      case 1: return 7;
+      case 2: return 2;
+      case 3: return 6;
+      case 4: return 3;
+      default:break;
+   }
+
+   assert(0 && "failed to detect rotation");
+   return 0;
+}
+
+static uint8_t
+other_rotation(const uint8_t *data, const size_t size)
+{
+   assert(data);
+
+   if (size < 13)
+      return 0;
+
+   const int seed = count_bits(data[2]) - count_bits(data[11]) + count_bits(data[12]);
+   switch (abs(seed) % 5) {
+      case 0: return 7;
+      case 1: return 1;
+      case 2: return 6;
+      case 3: return 2;
+      case 4: return 5;
+      default:break;
+   }
+
+   assert(0 && "failed to detect rotation");
+   return 0;
+}
+
+static uint8_t
+item_rotation(const uint8_t *data, const size_t size)
+{
+   assert(data);
+   (void)data, (void)size;
+   return 5;
+}
+
+int
+main(int argc, char *argv[])
+{
+   if (argc < 2)
+      errx(EXIT_FAILURE, "usage: %s (name | ability | spell | item | text) < data", argv[0]);
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
+
+   const struct info {
+      const char *name;
+      size_t chunk;
+      uint8_t (*rotation)(const uint8_t *data, const size_t size);
+   } map[] = {
+      { .name = "name", .chunk = 32 },
+      { .name = "ability", .chunk = 1024, .rotation = other_rotation },
+      { .name = "spell", .chunk = 1024, .rotation = other_rotation },
+      { .name = "item",  .chunk = 3072, .rotation = item_rotation },
+      { .name = "text",  .chunk = 255, .rotation = text_rotation },
+   };
+
+   const struct info *info = NULL;
+   for (size_t i = 0; i < ARRAY_SIZE(map); ++i) {
+      if (strcmp(map[i].name, argv[1]))
+         continue;
+
+      info = &map[i];
+      break;
+   }
+
+   if (!info)
+      errx(EXIT_FAILURE, "unknown file type '%s'", argv[1]);
+
+   uint8_t buf[4096];
+   assert(sizeof(buf) >= info->chunk);
+
+   size_t bytes;
+   while ((bytes = fread(buf, 1, info->chunk, stdin)) > 0) {
+      if (info->rotation)
+         decode(buf, bytes, info->rotation(buf, bytes));
+
+      fwrite(buf, 1, bytes, stdout);
+   }
+
+   return EXIT_SUCCESS;
+}