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patch-ids: document intentional const-casting in patch_id_neq() #2226

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patch-ids: document intentional const-casting in patch_id_neq() #2226

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10 changes: 9 additions & 1 deletion patch-ids.c
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Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,15 @@ static int patch_id_neq(const void *cmpfn_data,
const struct hashmap_entry *entry_or_key,
const void *keydata UNUSED)
{
/* NEEDSWORK: const correctness? */
/*
* We drop the 'const' modifier here intentionally.
*
* The hashmap API requires us to treat the entries as const.
* However, to avoid performance regression, we lazily compute
* the patch IDs inside this comparison function. This fundamentally
* requires us to mutate the 'struct patch_id'. Therefore, we use
* container_of() to cast away the constness from the hashmap_entry.
*/
struct diff_options *opt = (void *)cmpfn_data;
struct patch_id *a, *b;

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