Re: [RFC/PATCH 1/3] teach --histogram to diff

[Junio C Hamano <gitster@pobox.com>]

This is just half-a-review (bottom half of the file).

> +static int histogram_diff(xpparam_t const *xpp, xdfenv_t *env,
> +	int line1, int count1, int line2, int count2)
> +{

What does this function return?

> +	struct histindex index;
> +	struct region lcs;
> +	int sz;
> +	int result = -1;
> +
> +	if (count1 <= 0 && count2 <= 0)
> +		return 0;
> +
> +	if (LINE_END(1) >= MAX_PTR)
> +		return -1;
> +
> +	if (!count1) {
> +		while(count2--)
> +			env->xdf2.rchg[line2++ - 1] = 1;
> +		return 0;
> +	} else if (!count2) {
> +		while(count1--)
> +			env->xdf1.rchg[line1++ - 1] = 1;
> +		return 0;
> +	}
> +
> +	memset(&index, 0, sizeof(index));
> +
> +	index.env = env;
> +	index.xpp = xpp;
> +
> +	index.records = NULL;
> +	index.line_map = NULL;
> +	/* in case of early xdl_cha_free() */
> +	index.rcha.head = NULL;
> +
> +	index.table_bits = xdl_hashbits(count1);
> +	sz = index.records_size = 1 << index.table_bits;
> +	sz *= sizeof(struct record *);
> +	if (!(index.records = (struct record **) xdl_malloc(sz)))
> +		goto cleanup;
> +	memset(index.records, 0, sz);
> +
> +	sz = index.line_map_size = count1;
> +	sz *= sizeof(struct record *);
> +	if (!(index.line_map = (struct record **) xdl_malloc(sz)))
> +		goto cleanup;
> +	memset(index.line_map, 0, sz);
> +
> +	sz = index.line_map_size;
> +	sz *= sizeof(unsigned int);
> +	if (!(index.next_ptrs = (unsigned int *) xdl_malloc(sz)))
> +		goto cleanup;
> +	memset(index.next_ptrs, 0, sz);
> +
> +	/* lines / 4 + 1 comes from xprepare.c:xdl_prepare_ctx() */
> +	if (xdl_cha_init(&index.rcha, sizeof(struct record), count1 / 4 + 1) < 0)
> +		goto cleanup;
> +
> +	index.ptr_shift = line1;
> +	index.max_chain_length = 64;
> +
> +	memset(&lcs, 0, sizeof(lcs));
> +	if (find_lcs(&index, &lcs, line1, count1, line2, count2))
> +		result = fall_back_to_classic_diff(&index, line1, count1, line2, count2);
> +	else {
> +		result = 0;
> +		if (lcs.begin1 == 0 && lcs.begin2 == 0) {
> +			int ptr;
> +			for (ptr = 0; ptr < count1; ptr++)
> +				env->xdf1.rchg[line1 + ptr - 1] = 1;
> +			for (ptr = 0; ptr < count2; ptr++)
> +				env->xdf2.rchg[line2 + ptr - 1] = 1;
> +		} else {
> +			result = histogram_diff(xpp, env,
> +				line1, lcs.begin1 - line1,
> +				line2, lcs.begin2 - line2);
> +			result = histogram_diff(xpp, env,
> +				lcs.end1 + 1, LINE_END(1) - lcs.end1,
> +				lcs.end2 + 1, LINE_END(2) - lcs.end2);

The result from the first half before lcs is discarded?

> +			result *= -1;

Again, what does this function (called recursively) return, and what does
flipping the sign of it do?

> +		}
> +	}
> +
> +cleanup:
> +	xdl_free(index.records);
> +	xdl_free(index.line_map);
> +	xdl_free(index.next_ptrs);
> +	xdl_cha_free(&index.rcha);
> +
> +	return result;
> +}
> +
> +int xdl_do_histogram_diff(mmfile_t *file1, mmfile_t *file2,
> +	xpparam_t const *xpp, xdfenv_t *env)
> +{
> +	int line1, line2, count1, count2;
> +
> +	if (xdl_prepare_env(file1, file2, xpp, env) < 0)
> +		return -1;
> +
> +	line1 = line2 = 1;
> +	count1 = env->xdf1.nrec;
> +	count2 = env->xdf2.nrec;
> +
> +	reduce_common_start_end(xpp, env, &line1, &count1, &line2, &count2);

What this does is logically not specific to histogram algorithm but can be
applied to other backends, no? And I vaguely recall that Linus did try
something like this once, found some issues with it when context is set to
non zero, and stopped doing it (sorry, I do not have any more details).

I am not suggesting you to remove this call or hoist the call to one level
up to xdl_do_diff(), but I do have to wonder how much of the performance
improvement you reported is due to this common head/tail reduction.

> +	return histogram_diff(xpp, env, line1, count1, line2, count2);
> +}