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write a function for skipping to the next NBT tag

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including a function for acquireing the array size, and bytesize of a
tag.
This commit is contained in:
Quinn
2025-07-15 13:02:08 +02:00
parent ee1811a3a5
commit 6c2f51929b
1 changed files with 61 additions and 0 deletions
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61
src/dat/nbt.c
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@@ -11,6 +11,11 @@ static inline u16 nbt_strlen(u8 const *restrict buf) {
return be16toh(*(u16 *)(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`. /* compares the string in `buf` to `matstr`.
* returns `=0` if equal, `>0` if buf is greater, `<0` if matstr is greater. */ * 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) { static int nbt_cmpstr(char const *restrict matstr, u8 const *restrict buf) {
@@ -24,6 +29,62 @@ static int nbt_cmpstr(char const *restrict matstr, u8 const *restrict buf) {
return strncmp(str, matstr, len); 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 const *restrict buf) {
u8 const *nxt = buf + 1;
nxt += nbt_strlen(nxt) + 2;
uint mems = 0;
switch (*buf) {
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:
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
}
}
int nbt_proc(void **restrict datout, u8 const *restrict buf, size_t len) { int nbt_proc(void **restrict datout, u8 const *restrict buf, size_t len) {
// first byte should be a compound tag // first byte should be a compound tag