Subject: Re: [PATCH v5 6/6] RFC blame: use a fingerprint heuristic to match ignored lines

From: SZEDER Gábor <[email-masked]>
Date: Thu, 4 Apr 2019 18:37:07 +0200

On Wed, Apr 03, 2019 at 12:02:07PM -0400, Barret Rhoden wrote:
> diff --git a/blame.c b/blame.c
> index c06cbd906658..50511a300059 100644
> --- a/blame.c
> +++ b/blame.c
> @@ -915,27 +915,109 @@ static int are_lines_adjacent(struct blame_line_tracker *first,
>  	       first->s_lno + 1 == second->s_lno;
>  }
>  
> -/*
> - * This cheap heuristic assigns lines in the chunk to their relative location in
> - * the parent's chunk.  Any additional lines are left with the target.
> +/* https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel */
> +static int bitcount(uint32_t v)
> +{
> +	v = v - ((v >> 1) & 0x55555555u);
> +	v = (v & 0x33333333u) + ((v >> 2) & 0x33333333u);
> +	return (((v + (v >> 4)) & 0xf0f0f0fu) * 0x1010101u) >> 24;
> +}
> +
> +#define FINGERPRINT_LENGTH (8 * 256)
> +#define FINGERPRINT_THRESHOLD 1
> +/* This is just a bitset indicating which byte pairs are present.
> + * e.g. the string "good goo" has pairs "go", "oo", "od", "d ", " g"
> + * String similarity is calculated as a bitwise or and counting the set bits.
> + *
> + * TODO for the string lengths we typically deal with, this would probably be
> + * implemented more efficiently with a set data structure.
>   */
> +struct fingerprint {
> +	uint32_t bits[FINGERPRINT_LENGTH];
> +};
> +
> +static void get_fingerprint(struct fingerprint *result, const char *line_begin,
> +			    const char *line_end)
> +{
> +	for (const char *p = line_begin; p + 1 < line_end; ++p) {

We still stick to C89, which doesn't support for loop initial
declarations yet.  Please declare the loop variable as a regular local
variable.  This also applies to the several 'for (int i = 0; ...)'
loops in the functions below.

> +		unsigned int c = tolower(*p) | (tolower(*(p + 1)) << 8);
> +
> +		result->bits[c >> 5] |= 1u << (c & 0x1f);
> +	}
> +}
> +
> +static int fingerprint_similarity(const struct fingerprint *a,
> +				  const struct fingerprint *b)
> +{
> +	int intersection = 0;
> +
> +	for (int i = 0; i < FINGERPRINT_LENGTH; ++i)
> +		intersection += bitcount(a->bits[i] & b->bits[i]);
> +	return intersection;
> +}
> +
> +static void get_chunk_fingerprints(struct fingerprint *fingerprints,
> +				   const char *content,
> +				   const int *line_starts,
> +				   long chunk_start,
> +				   long chunk_length)
> +{
> +	line_starts += chunk_start;
> +	for (int i = 0; i != chunk_length; ++i) {
> +		const char *linestart = content + line_starts[i];
> +		const char *lineend = content + line_starts[i + 1];
> +
> +		get_fingerprint(fingerprints + i, linestart, lineend);
> +	}
> +}
> +
>  static void guess_line_blames(struct blame_entry *e,
>  			      struct blame_origin *parent,
>  			      struct blame_origin *target,
>  			      int offset, int delta,
>  			      struct blame_line_tracker *line_blames)
>  {
> +	struct fingerprint *fp_parent, *fp_target;
>  	int nr_parent_lines = e->num_lines - delta;
>  
> +	fp_parent = xcalloc(sizeof(struct fingerprint), nr_parent_lines);
> +	fp_target = xcalloc(sizeof(struct fingerprint), e->num_lines);
> +
> +	get_chunk_fingerprints(fp_parent, parent->file.ptr,
> +			       parent->line_starts,
> +			       e->s_lno + offset, nr_parent_lines);
> +	get_chunk_fingerprints(fp_target, target->file.ptr,
> +			       target->line_starts,
> +			       e->s_lno, e->num_lines);
> +
>  	for (int i = 0; i < e->num_lines; i++) {
> -		if (i < nr_parent_lines) {
> +		int best_sim_val = FINGERPRINT_THRESHOLD;
> +		int best_sim_idx = -1;
> +		int sim;
> +
> +		for (int j = 0; j < nr_parent_lines; j++) {
> +			sim = fingerprint_similarity(&fp_target[i],
> +						     &fp_parent[j]);
> +			if (sim < best_sim_val)
> +				continue;
> +			/* Break ties with the closest-to-target line number */
> +			if (sim == best_sim_val && best_sim_idx != -1 &&
> +			    abs(best_sim_idx - i) < abs(j - i))
> +				continue;
> +			best_sim_val = sim;
> +			best_sim_idx = j;
> +		}
> +		if (best_sim_idx >= 0) {
>  			line_blames[i].is_parent = 1;
> -			line_blames[i].s_lno = e->s_lno + i + offset;
> +			line_blames[i].s_lno = e->s_lno + offset + best_sim_idx;
>  		} else {
>  			line_blames[i].is_parent = 0;
>  			line_blames[i].s_lno = e->s_lno + i;
>  		}
>  	}
> +
> +	free(fp_parent);
> +	free(fp_target);
>  }
>  
>  /*
> -- 
> 2.21.0.392.gf8f6787159e-goog
>