Re: [PATCH 4/8] add functions for memory-efficient bitmaps

[Eric Sunshine <sunshine@sunshineco.com>]

On Wed, Jun 25, 2014 at 7:40 PM, Jeff King <peff@peff.net> wrote:
> We already have a nice-to-use bitmap implementation in
> ewah/bitmap.c. It pretends to be infinitely long when asking
> for a bit (and just returns 0 for bits that haven't been
> allocated or set), and dynamically resizes as appropriate
> when you set bits.
>
> The cost to this is that each bitmap must store its own
> pointer and length, using up to 16 bytes per bitmap on top
> of the actual bit storage. This is a lot of storage (not to
> mention an extra level of pointer indirection) if you are
> going to store one bitmap per commit in a traversal.
>
> These functions provide an alternative bitmap implementation
> that can be used when you have a large number of fixed-size
> bitmaps. See the documentation in the header file for
> details and examples.
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> diff --git a/bitset.h b/bitset.h
> new file mode 100644
> index 0000000..5fbc956
> --- /dev/null
> +++ b/bitset.h
> @@ -0,0 +1,113 @@
> +#ifndef BITSET_H
> +#define BITSET_H
> +
> + * Return the number of unsigned chars required to store num_bits bits.
> + *
> + * This is mostly used internally by the bitset functions, but you may need it
> + * when allocating the bitset. Example:
> + *
> + *   bits = xcalloc(1, bitset_sizeof(nr));
> + */
> +static inline int bitset_sizeof(int num_bits)
> +{
> +       return (num_bits + CHAR_BIT - 1) / CHAR_BIT;
> +}
> +
> +/*
> + * Set the bit at position "n". "n" is counted from zero, and must be
> + * smaller than the num_bits given to bitset_sizeof when allocating the bitset.
> + */
> +static inline void bitset_set(unsigned char *bits, int n)
> +{
> +       bits[n / CHAR_BIT] |= 1 << (n % CHAR_BIT);
> +}

Is it intentional or an oversight that there is no way to clear a bit
in the set?

> +/*
> + * Return the bit at position "n" (see bitset_set for a description of "n").
> + */
> +static inline int bitset_get(unsigned char *bits, int n)
> +{
> +       return !!(bits[n / CHAR_BIT] & (1 << (n % CHAR_BIT)));
> +}
> +
> +/*
> + * Return true iff the bitsets contain the same bits. Each bitset should be the
> + * same size, and should have been allocated using bitset_sizeof(max).
> + *
> + * Note that it is not safe to check partial equality by providing a smaller
> + * "max" (we assume any bits beyond "max" up to the next CHAR_BIT boundary are
> + * zeroed padding).
> + */
> +static inline int bitset_equal(unsigned char *a, unsigned char *b, int max)
> +{
> +       int i;
> +       for (i = bitset_sizeof(max); i > 0; i--)
> +               if (*a++ != *b++)
> +                       return 0;
> +       return 1;
> +}
> +
> +/*
> + * Bitwise-or the bitsets in "dst" and "src", and store the result in "dst".
> + *
> + * See bitset_equal for the definition of "max".
> + */
> +static inline void bitset_or(unsigned char *dst, const unsigned char *src, int max)
> +{
> +       int i;
> +       for (i = bitset_sizeof(max); i > 0; i--)
> +               *dst++ |= *src++;
> +}
> +
> +/*
> + * Returns true iff the bitset contains all zeroes.
> + *
> + * See bitset_equal for the definition of "max".
> + */
> +static inline int bitset_empty(const unsigned char *bits, int max)
> +{
> +       int i;
> +       for (i = bitset_sizeof(max); i > 0; i--, bits++)
> +               if (*bits)
> +                       return 0;
> +       return 1;
> +}
> +
> +#endif /* BITSET_H */
> --
> 2.0.0.566.gfe3e6b2