write a function for skipping to the next NBT tag

including a function for acquireing the array size, and bytesize of a
tag.

src/dat/nbt.c

@@ -11,6 +11,11 @@ static inline u16 nbt_strlen(u8 const *restrict buf) {
 	return be16toh(*(u16 *)(buf));
 }
 
+/* returns the length of an array from a specific location in the buffer */
+static inline i32 nbt_arrlen(u8 const *restrict buf) {
+	return be32toh(*(i32 *)(buf));
+}
+
 /* compares the string in `buf` to `matstr`.
  * returns `=0` if equal, `>0` if buf is greater, `<0` if matstr is greater. */
 static int nbt_cmpstr(char const *restrict matstr, u8 const *restrict buf) {
@@ -24,29 +29,60 @@ static int nbt_cmpstr(char const *restrict matstr, u8 const *restrict buf) {
 	return strncmp(str, matstr, len);
 }
 
+/* gets the tag size of primitive types, returns `>0` on success, `<0` on failure */
+int nbt_prim_tagsize(u8 tag) {
+	switch (tag) {
+	case NBT_I8:  return 1;
+	case NBT_I16: return 2;
+	case NBT_I32: return 4;
+	case NBT_I64: return 8;
+	case NBT_F32: return 4;
+	case NBT_F64: return 8;
+	default:      return -1;
+	}
+}
+
 /* returns the (expected) pointer of the tag following this one.
  * `NBT_COMPOUND` and `NBT_END` tags are not valid for this function and should be handled separately.
  * `NULL` is returned if anything went wrong. */
-static u8 const *nbt_nexttag(u8 *buf) {
-	u8 const *nxt = NULL;
+static u8 const *nbt_nexttag(u8 const *restrict buf) {
+	u8 const *nxt = buf + 1;
+	nxt += nbt_strlen(nxt) + 2;
 
+	uint mems = 0;
 	switch (*buf) {
-	case NBT_I8:      nxt = buf + 1 + 1; break; // add 1 for the tag size here, since the constant can be precomputed
-	case NBT_I16:     nxt = buf + 1 + 2; break;
-	case NBT_I32:     nxt = buf + 1 + 4; break;
-	case NBT_I64:     nxt = buf + 1 + 8; break;
-	case NBT_F32:     nxt = buf + 1 + 4; break;
-	case NBT_F64:     nxt = buf + 1 + 8; break;
+	case NBT_I8:
+	case NBT_I16:
+	case NBT_I32:
+	case NBT_I64:
+	case NBT_F32:
+	case NBT_F64:
+		nxt += nbt_prim_tagsize(*buf);
+		return nxt;
+
+	case NBT_ARR_I64:
+		mems += sizeof(i64) - sizeof(i32);
+	case NBT_ARR_I32:
+		mems += sizeof(i32) - sizeof(i8);
 	case NBT_ARR_I8:
-	case NBT_STR:     break;
-	case NBT_LIST:    break;
-	case NBT_ARR_I32: break;
-	case NBT_ARR_I64: break;
+		mems += 1;
+		nxt += mems * nbt_arrlen(nxt);
+		return nxt;
+	case NBT_STR:
+		nxt += nbt_strlen(nxt);
+		return nxt;
+
+	case NBT_LIST:
+		mems = nbt_prim_tagsize(*nxt);
+		if (mems > 0) {
+			nxt += 1;
+			nxt += mems * nbt_arrlen(nxt);
+			return nxt;
+		}
+		// let case escape to `default` when `mems` `≤0`
 
 	default: return NULL; // failure on compound/end tags; these require more nuanced logic
 	}
-
-	return nxt + nbt_strlen(buf + 1);
 }
 
 int nbt_proc(void **restrict datout, u8 const *restrict buf, size_t len) {

src/util/compat/endian.h

@@ -2,6 +2,9 @@
 // Licensed under the MIT Licence. See LICENSE for details
 #pragma once
 
+#if __has_include(<endian.h>)
+#include <endian.h>
+#else
 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #define le16toh(x) __uint16_identity(x)
 #define le32toh(x) __uint32_identity(x)
@@ -31,3 +34,4 @@
 #else
 #error machine architecture unsupported! Expected either big-endian or little-endian, make sure to use a compiler which defines __BYTE_ORDER__ (like clang or gcc)
 #endif
+#endif