Re: [PATCH v4 9/9] diff.c: add white space mode to move detection that allows indent changes

> The option of --color-moved has proven to be useful as observed on the
> mailing list. However when refactoring sometimes the indentation changes,
> for example when partitioning a functions into smaller helper functions
> the code usually mostly moved around except for a decrease in indentation.
> 
> To just review the moved code ignoring the change in indentation, a mode
> to ignore spaces in the move detection as implemented in a previous patch
> would be enough.  However the whole move coloring as motivated in commit
> 2e2d5ac (diff.c: color moved lines differently, 2017-06-30), brought
> up the notion of the reviewer being able to trust the move of a "block".
> 
> As there are languages such as python, which depend on proper relative
> indentation for the control flow of the program, ignoring any white space
> change in a block would not uphold the promises of 2e2d5ac that allows
> reviewers to pay less attention to the inside of a block, as inside
> the reviewer wants to assume the same program flow.
> 
> This new mode of white space ignorance will take this into account and will
> only allow the same white space changes per line in each block. This patch
> even allows only for the same change at the beginning of the lines.
> 
> As this is a white space mode, it is made exclusive to other white space
> modes in the move detection.
> 
> This patch brings some challenges, related to the detection of blocks.
> We need a white net the catch the possible moved lines, but then need to

> +
> +/**
> + * The struct ws_delta holds white space differences between moved lines, i.e.
> + * between '+' and '-' lines that have been detected to be a move.
> + * The string contains the difference in leading white spaces, before the
> + * rest of the line is compared using the white space config for move
> + * coloring. The current_longer indicates if the first string in the
> + * comparision is longer than the second.
> + */
> +struct ws_delta {
> +	char *string;
> +	unsigned int current_longer : 1;
>  };
> +#define WS_DELTA_INIT { NULL, 0 }
> +
> +static int compute_ws_delta(const struct emitted_diff_symbol *a,
> +			     const struct emitted_diff_symbol *b,
> +			     struct ws_delta *out)
> +{
> +	const struct emitted_diff_symbol *longer =  a->len > b->len ? a : b;
> +	const struct emitted_diff_symbol *shorter = a->len > b->len ? b : a;
> +	int d = longer->len - shorter->len;
> +
> +	out->string = xmemdupz(longer->line, d);
> +	out->current_longer = (a == longer);
> +
> +	return !strncmp(longer->line + d, shorter->line, shorter->len);
> +}