# string-list API

The string_list API offers a data structure and functions to handle sorted and unsorted string lists. A "sorted" list is one whose entries are sorted by string value in `strcmp()` order.

The string_list struct used to be called path_list, but was renamed because it is not specific to paths.

The caller:

1. Allocates and clears a `struct string_list` variable.

2. Initializes the members. You might want to set the flag `strdup_strings` if the strings should be strdup()ed. For example, this is necessary when you add something like git_path("…"), since that function returns a static buffer that will change with the next call to git_path().

   If you need something advanced, you can manually malloc() the `items` member (you need this if you add things later) and you should set the `nr` and `alloc` members in that case, too.

3. Adds new items to the list, using `string_list_append`, `string_list_append_nodup`, `string_list_insert`, `string_list_split`, and/or `string_list_split_in_place`.

4. Can check if a string is in the list using `string_list_has_string` or `unsorted_string_list_has_string` and get it from the list using `string_list_lookup` for sorted lists.

5. Can sort an unsorted list using `sort_string_list`.

6. Can remove duplicate items from a sorted list using `string_list_remove_duplicates`.

7. Can remove individual items of an unsorted list using `unsorted_string_list_delete_item`.

8. Can remove items not matching a criterion from a sorted or unsorted list using `filter_string_list`, or remove empty strings using `string_list_remove_empty_items`.

9. Finally it should free the list using `string_list_clear`.

Example:

    struct string_list list = STRING_LIST_INIT_NODUP;
    int i;

    string_list_append(&list, "foo");
    string_list_append(&list, "bar");
    for (i = 0; i < list.nr; i++)
            printf("%s\n", list.items[i].string)

Note | It is more efficient to build an unsorted list and sort it afterwards, instead of building a sorted list (`O(n log n)` instead of `O(n^2)`).

+ However, if you use the list to check if a certain string was added already, you should not do that (using unsorted_string_list_has_string()), because the complexity would be quadratic again (but with a worse factor).

## Functions

- General ones (works with sorted and unsorted lists as well)

  `string_list_init`

    Initialize the members of the string_list, set `strdup_strings` member according to the value of the second parameter.

  `filter_string_list`

    Apply a function to each item in a list, retaining only the items for which the function returns true. If free_util is true, call free() on the util members of any items that have to be deleted. Preserve the order of the items that are retained.

  `string_list_remove_empty_items`

    Remove any empty strings from the list. If free_util is true, call free() on the util members of any items that have to be deleted. Preserve the order of the items that are retained.

  `print_string_list`

    Dump a string_list to stdout, useful mainly for debugging purposes. It can take an optional header argument and it writes out the string-pointer pairs of the string_list, each one in its own line.

  `string_list_clear`

    Free a string_list. The `string` pointer of the items will be freed in case the `strdup_strings` member of the string_list is set. The second parameter controls if the `util` pointer of the items should be freed or not.

- Functions for sorted lists only

  `string_list_has_string`

    Determine if the string_list has a given string or not.

  `string_list_insert`

    Insert a new element to the string_list. The returned pointer can be handy if you want to write something to the `util` pointer of the string_list_item containing the just added string. If the given string already exists the insertion will be skipped and the pointer to the existing item returned.

    Since this function uses xrealloc() (which die()s if it fails) if the list needs to grow, it is safe not to check the pointer. I.e. you may write `string_list_insert(...)->util = ...;`.

  `string_list_lookup`

    Look up a given string in the string_list, returning the containing string_list_item. If the string is not found, NULL is returned.

  `string_list_remove_duplicates`

    Remove all but the first of consecutive entries that have the same string value. If free_util is true, call free() on the util members of any items that have to be deleted.

- Functions for unsorted lists only

  `string_list_append`

    Append a new string to the end of the string_list. If `strdup_string` is set, then the string argument is copied; otherwise the new `string_list_entry` refers to the input string.

  `string_list_append_nodup`

    Append a new string to the end of the string_list. The new `string_list_entry` always refers to the input string, even if `strdup_string` is set. This function can be used to hand ownership of a malloc()ed string to a `string_list` that has `strdup_string` set.

  `sort_string_list`

    Sort the list’s entries by string value in `strcmp()` order.

  `unsorted_string_list_has_string`

    It’s like `string_list_has_string()` but for unsorted lists.

  `unsorted_string_list_lookup`

    It’s like `string_list_lookup()` but for unsorted lists.

    The above two functions need to look through all items, as opposed to their counterpart for sorted lists, which performs a binary search.

  `unsorted_string_list_delete_item`

    Remove an item from a string_list. The `string` pointer of the items will be freed in case the `strdup_strings` member of the string_list is set. The third parameter controls if the `util` pointer of the items should be freed or not.

  `string_list_split`
  `string_list_split_in_place`

    Split a string into substrings on a delimiter character and append the substrings to a `string_list`. If `maxsplit` is non-negative, then split at most `maxsplit` times. Return the number of substrings appended to the list.

    `string_list_split` requires a `string_list` that has `strdup_strings` set to true; it leaves the input string untouched and makes copies of the substrings in newly-allocated memory. `string_list_split_in_place` requires a `string_list` that has `strdup_strings` set to false; it splits the input string in place, overwriting the delimiter characters with NULs and creating new string_list_items that point into the original string (the original string must therefore not be modified or freed while the `string_list` is in use).

## Data structures

- `struct string_list_item`

Represents an item of the list. The `string` member is a pointer to the string, and you may use the `util` member for any purpose, if you want.

- `struct string_list`

Represents the list itself.

1. The array of items are available via the `items` member.

2. The `nr` member contains the number of items stored in the list.

3. The `alloc` member is used to avoid reallocating at every insertion. You should not tamper with it.

4. Setting the `strdup_strings` member to 1 will strdup() the strings before adding them, see above.

5. The `compare_strings_fn` member is used to specify a custom compare function, otherwise `strcmp()` is used as the default function.