[PATCH] move LCS table away from the stack

[Lars Hjemli]

On Sun, Sep 11, 2011 at 02:44, Jamie Couture <jamie.couture at gmail.com> wrote:
> +/*
> + * ssdiff line limits
> + */
> +#ifndef MAX_SSDIFF_M
> +#define MAX_SSDIFF_M 1024
> +#endif
> +
> +#ifndef MAX_SSDIFF_N
> +#define MAX_SSDIFF_N 1024
> +#endif
> +#define MAX_SSDIFF_SIZE ((MAX_SSDIFF_M) * (MAX_SSDIFF_N))

I think this limit should be more like 128*128 for a few reasons:
* ss-diff for lines longer than 128 chars probably isn't very useful
(you'd need a very wide monitor)
* cpu time spent in LCS() seems to be propotional to avg(linelength)^2


>  static char *longest_common_subsequence(char *A, char *B)
>  {
>        int i, j, ri;
>        int m = strlen(A);
>        int n = strlen(B);
> -       int L[m + 1][n + 1];
> -       int tmp1, tmp2;
> +       int **L;
> +       int tmp1, tmp2, length;
>        int lcs_length;
>        char *result;
>
> +       length = (m + 1) * (n + 1);
> +
> +       // We bail if the lines are too long
> +       if (length > MAX_SSDIFF_SIZE)
> +               return NULL;
> +
> +       L = create_lcs_table(m + 1, n + 1);
> +
>        for (i = m; i >= 0; i--) {
>                for (j = n; j >= 0; j--) {
>                        if (A[i] == '\0' || B[j] == '\0') {
> @@ -59,6 +84,9 @@ static char *longest_common_subsequence(char *A, char *B)
>                        j += 1;
>                }
>        }
> +
> +       free(*L);
> +       free(L);
>        return result;
>  }

This function is potentially invoked for each diff-line, right? If so,
why not prepare a "shared" lcs-table in the caller (expecting
worst-case linelength) to avoid the setup/teardown of the table for
each line?

--
larsh