A very common need is sorting data. Therefore libUCW contains few routines to accomplish that task. They are much more universal than qsort(), since they allow you to sort structures indexed by a macro, sort data externally, if they do not fit into memory, merge data with the same keys and sort data of variable length.

All routines described below are generic algorithms.

Simple array sorting

If you want to sort some data in memory and you aren’t too picky about setting how, you just use the routine defined in sorter/array-simple.h. It is an optimised hybrid quick-sort/insert-sort algorithm (quick-sort is used to split the input into small parts, each is then sorted by insert-sort). It is more than 2 times faster than stdlib’s qsort(), mostly because of inlining.

You need to define few macros and include the header. You get a sorting function in return. It will be called ASORT_PREFIX(sort).

Mandatory macros

  • ASORT_PREFIX(name) — The identifier generating macro.

  • ASORT_KEY_TYPE — Data type of a single array entry key.

Optional macros

  • ASORT_ELT(i) — Indexing macro. Returns the key of the corresponding entry. If not provided, usual array with sequential indexing is assumed.

  • ASORT_LT(x,y) — Comparing macro. If not provided, compares by the < operator.

  • ASORT_SWAP(i,j) — Swap elements with indices i and j. If not provided, it assumes ASORT_ELT is l-value and it just swaps keys.

  • ASORT_THRESHOLD — Sequences of at least this amount of elements are sorted by quick-sort, smaller are sorted by insert-sort. Defaults to 8 (result of experimentation).

  • ASORT_EXTRA_ARGS — Pass some extra arguments to the function. They are visible from all the macros. Must start with a comma.

static void ASORT_PREFIX(sort)(ASORT_ARRAY_ARG uint array_size ASORT_EXTRA_ARGS);

The generated sorting function. If ASORT_ELT macro is not provided, the ASORT_ARRAY_ARG is equal to ASORT_KEY_TYPE *array and is the array to be sorted. If the macro is provided, this parameter is omitted. In that case, you can sort global variables or pass your structure by ASORT_EXTRA_ARGS.


Let’s sort an array of integers, in the usual way.

#define ASORT_PREFIX(X) intarr_##X
#define ASORT_KEY_TYPE int
#include <ucw/sorter/array-simple.h>

This generates an intarr_sort(int *array, uint array_size) function that can be used the obvious way.

A more complicated example could be sorting a structure, where items with odd indices are stored in one array, even in another. Each item could be a structure containing a string and an integer. We would like to sort them by the strings.

struct elem {
  char *string;
  int integer;
#include <string.h>   // Because of strcmp
#define ASORT_PREFIX(X) complicated_##X
#define ASORT_KEY_TYPE struct elem
#define ASORT_ELT(i) ((i % 2 ? even_array : odd_array)[i / 2])
#define ASORT_LT(x, y) (strcmp((x).string, (y).string) < 0)
#define ASORT_EXTRA_ARGS , struct elem *odd_array, struct elem *even_array
#include <ucw/sorter/array-simple.h>

Now we got a complicated_sort(uint array_size, struct elem *odd_array, struct *even_array) function to perform our sorting.

Huge array sorting

This one is very similar to the simple array sorter, but it is optimised for huge arrays. It is used mostly by the external sorter machinery described below, but you can use it directly.

It is in the sorter/array.h header.

It differs in few details: - It supports only continuous arrays, no indexing macro can be provided. - It is able to sort in parallel on SMP systems. It assumes all callbacks you provide are thread-safe. - If you provide a monotone hash function (if hash(x) < hash(y), then x < y, but x and y may differ when hash(x) == hash(y)), it will use it to gain some more speed by radix-sort.

Mandatory macros

  • ASORT_PREFIX(x) — The identifier generating macro.

  • ASORT_KEY_TYPE — Type of elements in the array.

Optional macros

  • ASORT_LT(x,y) — Comparing macro. Uses the < operator if not provided.

  • ASORT_HASH(x) — A monotone hash function (or macro). Should return uint.

  • ASORT_LONG_HASH(x) — Like ASORT_HASH(x), but returns 64-bit number instead of 32-bit.

  • ASORT_THRESHOLD — How small should a chunk of data be to be sorted by insert-sort? Defaults to 8 elements.

  • ASORT_RADIX_BITS — How many bits of the hash function should be used at once for radix-sort? The default is guessed from your architecture.

static ASORT_KEY_TYPE *ASORT_PREFIX(sort)(ASORT_KEY_TYPE *array, uint num_elts ASORT_HASH_ARGS);

The generated function. The array is the data to be sorted, num_elts tells how many elements the array has. If you did not provide ASORT_HASH, then the ASORT_HASH_ARGS is empty (there are only the two parameters in that case). When you provide it, the function gains two more parameters in the ASORT_HASH_ARGS macro. They are ASORT_KEY_TYPE *@buffer, which must be a memory buffer of the same size as the input array, and uint @hash_bits, specifying how many significant bits the hash function returns.

The function returns pointer to the sorted data, either the array or the buffer argument.

External sorting

If you have too much data to fit into memory, you need to employ external sorting. This external sorter operates on fastbufs containing sequences of items. Each item consists of a key, optionally followed by data. Both the keys and data may be of variable length, but the keys must be represented by fixed-size type in memory. The length of data must be computable from the key. Data are just copied verbatim, unless you use the merging mode, in which data with the same keys get merged together.

All callbacks must be thread safe.

The sorter resides in the sorter/sorter.h header file.

Basic macros

You need to provide some basic macros. Some of them are optional.

  • SORT_PREFIX(x) — Identifier generating macro. This one is mandatory.

  • SORT_KEY — Data structure holding the key of item in memory. The representation on disk may be different. Either this one or SORT_KEY_REGULAR must be provided.

  • SORT_KEY_REGULAR — You may use this instead of SORT_KEY, when the keys have the same representation both in memory and on disk. Then the sorter uses bread() and bwrite() to load and store them. It also assumes the keys are not very long.

  • SORT_KEY_SIZE(key) — Returns the real size of the key. The sorter can use this to save space and truncate the key to the given number of bytes, when the keys have variable lengths. If the keys have fixed sizes, there is no need for this macro.

  • SORT_DATA_SIZE(key) — Returns the amount of data following this key. If you do not provide this one, the sorter assumes there are only keys and no data.


Furthermore, you need to provide these callback functions (make sure they are thread safe):

  • int SORT_PREFIX(compare)(SORT_KEY *a, SORT_KEY *b) — Comparing function. It should act like strcmp(). Mandatory unless provided by integer sorting.

  • int SORT_PREFIX(read_key)(struct fastbuf *f, SORT_KEY *k) —  Should read a key from the provided fastbuf f and store it into k. Returns nonzero when ok and zero when an EOF was met. Mandatory unless SORT_KEY_REGULAR is defined.

  • void SORT_PREFIX(write_key)(struct fastbuf *f, SORT_KEY *k) —  Should store key k into f. Mandatory unless SORT_KEY_REGULAR is defined.

Integer sorting

If you sort by an integer value (either computed or available from the key), you can use this to save yourself some functions. It also activates the hashing automatically.

  • SORT_INT(key) — This macro returns the integer to sort by. When you provide it, the compare function is automatically provided for you and the sorting function gets another parameter specifying the range of the integers. The better the range fits, the faster the sorting runs.

  • SORT_INT64(key) — The same, but with 64-bit integers.


If you have a monotone hash function for your keys, you may speed the sorting up by providing it. Monotone hashing function must satisfy if hash(x) < hash(y), then x < y. It should be approximately uniformly distributed.

When you want to use it, define SORT_HASH_BITS and set it to the number of significant bits the hashing function provides. Then provide a callback function uint SORT_PREFIX(hash)(SORT_KEY *key).

Merging items with identical keys

The sorter is able to merge items with the same keys (the compare function returns 0 for them). To use it, define SORT_UNIFY macro and provide these functions:

  • void SORT_PREFIX(write_merged)(struct fastbuf \*dest, SORT_KEY \*\*keys, void \*\*data, uint n, void *buf)  — This function takes n records in memory and writes a single record into the dest fastbuf. The keys and data are just the records. The buf parameter points to a workspace memory. It is guaranteed to hold at last the sum of SUM_UNIFY_WORKSPACE() macro over all the keys. The function is allowed to modify all its parameters.

  • void SORT_PREFIX(copy_merged)(SORT_KEY \*\*keys, struct fastbuf \*\*data, uint n, struct fastbuf \*dest)  — This one is similar to the above one, but the data are still in the fastbufs data and no workspace is provided. This is only used when SORT_DATA_SIZE or SORT_UNIFY_WORKSPACE is provided.

  • SORT_UNIFY_WORKSPACE(key) — Returns the amount of workspace needed when merging this record. Defaults to 0.

Specifying input

To tell the sorter where is the input, you specify one of these macros:

  • SORT_INPUT_FILE — The function takes a filename.

  • SORT_INPUT_FB — The input is a seekable fastbuf stream.

  • SORT_INPUT_PIPE — The input is a non-seekable fastbuf stream.

  • SORT_INPUT_PRESORT — The input is a custom presorter. In this case, you need to write a presorting function int SORT_PREFIX(presort)(struct fastbuf *dest, void *buf, size_t bufsize). The function gets a buffer buf of size buf_size to presort in and is supposed to write presorted bunch of data into the dest buffer. Should return 1 on success or 0 on EOF (all it could was already written, no more data). In this case, you can safely pass NULL as the input parameter. The function may be used to generate the data on the fly. The function does not have to be thread safe (it can access global variables).

If you define SORT_DELETE_INPUT and it evaluates to true (nonzero), the input files are deleted as soon as possible.

Specifying output

You can configure the output in a similar way. Define one of macros:

  • SORT_OUTPUT_FILE — The function takes a filename.

  • SORT_OUTPUT_FB — The function should be provided with NULL and the fastbuf with data is returned.

  • SORT_THIS_FB — A fastbuf is provided to the function and it writes into it. It can already contain some data.

Other switches

You may define the SORT_UNIQUE macro if all keys are distinct. It is checked in debug mode.

The generated function

A SORT_PREFIX(sort)() function is generated after you include the sorter/sorter.h header. It has up to three parameters:

  • Input. It is either a string (a filename) if you use SORT_INPUT_FILE or a fastbuf (otherwise). It should be set to NULL if you use the SORT_INPUT_PRESORT input.

  • Output. It is either a string (a filename) if you defined the SORT_OUTPUT_FILE or a fastbuf. It must be NULL if you defined SORT_OUTPUT_FB.

  • Integer range. The maximum value of integers that are used in the integer sorting. This parameter is here only if you defined SORT_INT or SORT_INT64.

The function returns a fastbuf you can read the data from.