Re: [PATCH 08/12] add oidset API

> This is similar to many of our uses of sha1-array, but it
> overcomes one limitation of a sha1-array: when you are
> de-duplicating a large input with relatively few unique
> entries, sha1-array uses 20 bytes per non-unique entry.
> Whereas this set will use memory linear in the number of
> unique entries (albeit a few more than 20 bytes due to
> hashmap overhead).
> 
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> This may be overkill. You can get roughly the same thing by making
> actual object structs via lookup_unknown_object(). But see the next
> patch for some comments on that.
> 
>  Makefile |  1 +
>  oidset.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  oidset.h | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 95 insertions(+)
>  create mode 100644 oidset.c
>  create mode 100644 oidset.h
> 
> diff --git a/Makefile b/Makefile
> index 27afd0f37..e41efc2d8 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -774,6 +774,7 @@ LIB_OBJS += notes-cache.o
>  LIB_OBJS += notes-merge.o
>  LIB_OBJS += notes-utils.o
>  LIB_OBJS += object.o
> +LIB_OBJS += oidset.o
>  LIB_OBJS += pack-bitmap.o
>  LIB_OBJS += pack-bitmap-write.o
>  LIB_OBJS += pack-check.o
> diff --git a/oidset.c b/oidset.c
> new file mode 100644
> index 000000000..6094cff8c
> --- /dev/null
> +++ b/oidset.c
> @@ -0,0 +1,49 @@
> +#include "cache.h"
> +#include "oidset.h"
> +
> +struct oidset_entry {
> +	struct hashmap_entry hash;
> +	struct object_id oid;
> +};
> +
> +int oidset_hashcmp(const void *va, const void *vb,