seq: fix check for rounding error/truncation

[Todd C . Miller]

We need to compare the printable version of the last value displayed,
not the floating point representation.  Otherwise, we may print the
last value twice.

Old:

    $ seq 1050000 1050000
    1.05e+06
    1.05e+06

New:

    $ seq 1050000 1050000
    1.05e+06

We really need seq regression tests.  I have a few that I will
commit after this is in.

 - todd

Index: usr.bin/seq/seq.c
===================================================================
RCS file: /cvs/src/usr.bin/seq/seq.c,v
retrieving revision 1.6
diff -u -p -u -r1.6 seq.c
--- usr.bin/seq/seq.c   25 Feb 2022 16:00:39 -0000      1.6
+++ usr.bin/seq/seq.c   12 Jun 2023 17:13:44 -0000
@@ -89,13 +89,13 @@ main(int argc, char *argv[])
        double first = 1.0;
        double last = 0.0;
        double incr = 0.0;
-       double last_shown_value = 0.0;
+       double prev = 0.0;
        double cur, step;
        struct lconv *locale;
        char *fmt = NULL;
        const char *sep = "\n";
        const char *term = "\n";
-       char *cur_print, *last_print;
+       char *cur_print, *last_print, *prev_print;
        char pad = ZERO;
 
        if (pledge("stdio", NULL) == -1)
@@ -181,29 +181,31 @@ main(int argc, char *argv[])
                if (cur != first)
                        fputs(sep, stdout);
                printf(fmt, cur);
-               last_shown_value = cur;
+               prev = cur;
        }
 
        /*
         * Did we miss the last value of the range in the loop above?
         *
         * We might have, so check if the printable version of the last
-        * computed value ('cur') and desired 'last' value are equal.  If they
-        * are equal after formatting truncation, but 'cur' and
-        * 'last_shown_value' are not equal, it means the exit condition of the
-        * loop held true due to a rounding error and we still need to print
-        * 'last'.
+        * computed value ('cur') and desired 'last' value are equal.  If
+        * they are equal after formatting truncation, but 'cur' and 'prev'
+        * are different, it means the exit condition of the loop held true
+        * due to a rounding error and we still need to print 'last'.
         */
        if (asprintf(&cur_print, fmt, cur) == -1 ||
-           asprintf(&last_print, fmt, last) == -1)
+           asprintf(&last_print, fmt, last) == -1 ||
+           asprintf(&prev_print, fmt, prev) == -1)
                err(1, "asprintf");
-       if (strcmp(cur_print, last_print) == 0 && cur != last_shown_value) {
+       if (strcmp(cur_print, last_print) == 0 &&
+           strcmp(cur_print, prev_print) != 0) {
                if (cur != first)
                        fputs(sep, stdout);
                fputs(last_print, stdout);
        }
        free(cur_print);
        free(last_print);
+       free(prev_print);
 
        fputs(term, stdout);