Re: [PATCH 1/3] implement generic key/value map

> It is frequently useful to have a fast, generic data
> structure mapping keys to values. We already have something
> like this in the "decorate" API, but it has two downsides:
>
>  1. The key type must always be a "struct object *".
>
>  2. The value type is a void pointer, which means it is
>     inefficient and cumbersome for storing small values.
>     One must either encode their value inside the void
>     pointer, or allocate additional storage for the pointer
>     to point to.
>
> This patch introduces a generic map data structure, mapping
> keys of arbitrary type to values of arbitrary type.
>
> One possible strategy for implementation is to have a struct
> that points to a sequence of bytes for each of the key and
> the value, and to try to treat them as opaque in the code.
> However, this code gets complex, has a lot of casts, and
> runs afoul of violating alignment and strict aliasing rules.
>
> This patch takes a different approach. We parameterize the
> types in each map by putting the declarations and
> implementations inside macros, and expand the macros with
> the correct types. This lets the compiler see the actual
> code, with its real types, and figure out things like struct
> packing and alignment itself.
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> In addition to switching from using void pointers to macro expansion,
> this has one other difference from my previous patch: it handles
> arbitrary types for keys, not just object pointers. This was mentioned
> by Jakub, and would allow things like a fast bi-directional map for SVN
> revision numbers and commits.
>
> I tried to keep the implementation simple. Two things that could be changed:
>
>  1. We can't assume that the map key is a pointer. So the sentinel
>     "NULL" value isn't necessarily available to use, and we have to
>     keep a separate bit in each hash entry to say "is this valid".
>     This means when we _do_ store a pointer, we end up with an extra
>     32 bits or so in each hash entry for the "used" flag.
>
>     We could add a macro parameter for sentinel values, so that types
>     which _can_ handle this efficiently don't have to pay the price.
>     Or we could decide that mapping arbitrary keys isn't worth the
>     hassle. I wrote this way to be flexible for future use; I don't
>     personally have plans to add svn revision number mappings.
>
>  2. It assumes values are assignable. That means storing something like
>     "unsigned char sha1[20]" doesn't work. You can wrap it in a struct,
>     but do we assume that struct assignment works everywhere? I didn't
>     check, but I think it is in C89 but some antique compilers didn't
>     allow it. Switching it to use memcpy() would be easy enough (or
>     again, parameterizing so that assignable things don't have to pay
>     the price).
>
>  Makefile |    2 +
>  map.c    |   86 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  map.h    |   24 +++++++++++++++++
>  3 files changed, 112 insertions(+), 0 deletions(-)
>  create mode 100644 map.c
>  create mode 100644 map.h
>
> diff --git a/Makefile b/Makefile
> index 46793d1..6242321 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -530,6 +530,7 @@ LIB_H += list-objects.h
>  LIB_H += ll-merge.h
>  LIB_H += log-tree.h
>  LIB_H += mailmap.h
> +LIB_H += map.h
>  LIB_H += merge-file.h
>  LIB_H += merge-recursive.h
>  LIB_H += notes.h
> @@ -621,6 +622,7 @@ LIB_OBJS += ll-merge.o
>  LIB_OBJS += lockfile.o
>  LIB_OBJS += log-tree.o
>  LIB_OBJS += mailmap.o
> +LIB_OBJS += map.o
>  LIB_OBJS += match-trees.o
>  LIB_OBJS += merge-file.o
>  LIB_OBJS += merge-recursive.o
> diff --git a/map.c b/map.c
> new file mode 100644
> index 0000000..378cecb
> --- /dev/null
> +++ b/map.c
> @@ -0,0 +1,86 @@
> +#include "cache.h"
> +#include "map.h"
> +#include "object.h"
> +
> +static unsigned int hash_obj(const struct object *obj, unsigned int n)
> +{
> +       unsigned int hash;
> +
> +       memcpy(&hash, obj->sha1, sizeof(unsigned int));
> +       return hash % n;
> +}
> +
> +static unsigned int cmp_obj(const struct object *a, const struct object *b)
> +{
> +       return b == a;
> +}
> +
> +#define MAP_IMPLEMENT(name, ktype, vtype, cmp_fun, hash_fun) \