sometimes i forgot why i write code like this.
-- S.T.M.L

  • follow linux coding style

  • priority of source code (4S) : + stable + simple + small + secure (this option does not need for this program)

  • keep as small as possible: + remove unneeded space + remove unneeded variable

  • write comment/documentation as clear as possible

  • learn to use: + if (1 == var)

  • learn to avoid: + (i < strlen(str)) on loop statement because strlen() need temporary variable. try, l = strlen(str); while (i < l) { …​ }

  • use function in libc as much as possible; if not, wrap it!

001 - I/O Relation between Statement

LOAD is an input statement.

SORT, CREATE, JOIN is an output statement, but it can be an input. i.e:

1 - load abc ( ... ) as x;
2 - sort x by a, b;
3 - create ghi ( x.field, ... ) as out_x;

file output created by sort statement in line 2 will be an input by create statement in line 3.

002 - Why we need '2nd-loser’

to minimize comparison and insert in merge tree.

003 - Why we need 'level’ on tree node

list of input file to merge is A, B, C contain sorted data :

A : 10, 11, 12, 13      (1st file)
B : 1, 12, 100, 101     (2nd file)
C : 2, 13, 200, 201     (3rd file)

if we use tree insert algorithm:

if (root < node)
	insert to left
else
	insert to right

after several step we will get:

B-12
    \
    C-13
    /
A-12

which result in not-a-stable sort,

B-1 C-2 A-10 A-11 B-12 A-12 ...

they should be,

B-1 C-2 A-10 A-11 A-12 B-12 ...

Even if we choose different algorithm in insert:

if (root <= node)
	insert to left
else
	insert to right

there is also input data that will violate this, i.e:

A : 2, 13, 200, 201     (1st file)
B : 1, 12, 100, 101     (2nd file)
C : 10, 11, 12, 13      (3rd file)

004 - recursives call + thread + free on SunOS 5.10

i did not investigate much, but doing a recursive call + thread + free cause SIGSEGV on SunOS 5.10 system, but not in GNU/Linux system. This odd’s found whee testing on Solaris and by using dbx the SIGSEGV "sometimes" catched in str_destroy,

if (str->buf)
	free(str->buf); <= dbx catch here

and "sometimes" below that (but not in vos function/stack).

i.e:

list_destroy(**ptr)
{
	if (! (*ptr))
		return;
	list_destroy((*ptr)->next);
	free((*ptr));
}

and no, it’s not about double free.