Alternative map sorting
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- c
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- embedded-iot
调研方向
从 cborvalidation.c 第 475 行附近开始，检查现有的 map 排序和唯一性 flags。明确替代排序应取代还是补充当前行为，然后定义预期的顺序、key 处理方式以及重复 key 的行为；完成取决于对设计和实现范围达成一致。
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描述
Hello,
I've been looking into the CBOR validator and how maps are considered as sorted or not. In another project, I have previously been using another serialization standard that strictly defines maps in another way. Such as keys must only be strings, keys must be stored in lexicographical order and that no duplicates are allowed. Now we are looking into changing to cbor.
The CBOR specification does not strictly specifies what an ordered map is, but recommends that they should also be sorted in length. Meaning that "aa" > "b".
The alternatives are:
- Change sorting specification in our protocol (to order the fields like CBOR specification recommends)
- Not having sorted maps
- Finding/adapting a cbor library to allow for alternative sorting
Sorting is nice since is allows for linear parsing. Since we rely on this, we cannot directly swap to cbor. I would like to start a discussion of supporting an alternative sorting in tinycbor.
Wrote a short proof of concept for this (removes old sorting behavior) cborvalidation.c:475:
if (flags & CborValidateMapIsSorted) {
if (previous) {
uint64_t len1, len2;
const uint8_t *ptr;
/* extract the two lengths */
ptr = previous;
_cbor_value_extract_number(&ptr, it->parser->end, &len1);
ptr = current;
_cbor_value_extract_number(&ptr, it->parser->end, &len2);
size_t bytelen1 = (size_t)(previous_end - previous);
size_t bytelen2 = (size_t)(it->ptr - current);
/*
* Offset of actual key value (not including type information) is bytelenX - lenX??
* What if key value is indefinite??
*/
int r = memcmp(&previous[bytelen1 - len1], ¤t[bytelen2 - len2], len1 <= len2 ? len1 : len2);
if (r == 0 && len1 != len2)
r = len1 < len2 ? -1 : +1;
if (r > 0)
return CborErrorMapNotSorted;
if (r == 0 && (flags & CborValidateMapKeysAreUnique) == CborValidateMapKeysAreUnique)
return CborErrorMapKeysNotUnique;
}
Would it be possible to add a flag that would allow for this kind of sorting?
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