## Relations and Tuples

Relations and Tuples — tables of data which can be indexed on any number of fields.

## Synopsis

    #include <glib.h>

    struct      GRelation;
    GRelation*  g_relation_new                  (gint fields);
    void        g_relation_index                (GRelation *relation,
                                                 gint field,
                                                 GHashFunc hash_func,
                                                 GEqualFunc key_equal_func);
    void        g_relation_insert               (GRelation *relation,
                                                 ...);
    gboolean    g_relation_exists               (GRelation *relation,
                                                 ...);
    gint        g_relation_count                (GRelation *relation,
                                                 gconstpointer key,
                                                 gint field);
    GTuples*    g_relation_select               (GRelation *relation,
                                                 gconstpointer key,
                                                 gint field);
    gint        g_relation_delete               (GRelation *relation,
                                                 gconstpointer key,
                                                 gint field);
    void        g_relation_destroy              (GRelation *relation);

    void        g_relation_print                (GRelation *relation);

    struct      GTuples;
    void        g_tuples_destroy                (GTuples *tuples);
    gpointer    g_tuples_index                  (GTuples *tuples,
                                                 gint index_,
                                                 gint field);

## Description

A GRelation is a table of data which can be indexed on any number of fields, rather like simple database tables. A GRelation contains a number of records, called tuples. Each record contains a number of fields. Records are not ordered, so it is not possible to find the record at a particular index.

Note that GRelation tables are currently limited to 2 fields.

To create a GRelation, use `g_relation_new()`.

To specify which fields should be indexed, use `g_relation_index()`. Note that this must be called before any tuples are added to the GRelation.

To add records to a GRelation use `g_relation_insert()`.

To determine if a given record appears in a GRelation, use `g_relation_exists()`. Note that fields are compared directly, so pointers must point to the exact same position (i.e. different copies of the same string will not match.)

To count the number of records which have a particular value in a given field, use `g_relation_count()`.

To get all the records which have a particular value in a given field, use `g_relation_select()`. To access fields of the resulting records, use `g_tuples_index()`. To free the resulting records use `g_tuples_destroy()`.

To delete all records which have a particular value in a given field, use `g_relation_delete()`.

To destroy the GRelation, use `g_relation_destroy()`.

To help debug GRelation objects, use `g_relation_print()`.

## Details

### struct GRelation

    struct GRelation;

The GRelation struct is an opaque data structure to represent a Relation. It should only be accessed via the following functions.

### g_relation_new ()

    GRelation*  g_relation_new                  (gint fields);

Creates a new GRelation with the given number of fields. Note that currently the number of fields must be 2.

`fields` : | the number of fields.
Returns : | a new GRelation.

### g_relation_index ()

    void        g_relation_index                (GRelation *relation,
                                                 gint field,
                                                 GHashFunc hash_func,
                                                 GEqualFunc key_equal_func);

Creates an index on the given field. Note that this must be called before any records are added to the GRelation.

`relation` : | a GRelation.
`field` : | the field to index, counting from 0.
`hash_func` : | a function to produce a hash value from the field data.
`key_equal_func` : | a function to compare two values of the given field.

### g_relation_insert ()

    void        g_relation_insert               (GRelation *relation,
                                                 ...);

Inserts a record into a GRelation.

`relation` : | a GRelation.
`...` : | the fields of the record to add. This must match the number of fields in the GRelation.

### g_relation_exists ()

    gboolean    g_relation_exists               (GRelation *relation,
                                                 ...);

Returns `TRUE` if a record with the given values exists in a GRelation. Note that the values are compared directly, so that, for example, two copies of the same string will not match.

`relation` : | a GRelation.
`...` : | the fields of the record to compare. The number must match the number of fields in the GRelation.
Returns : | `TRUE` if a record matches.

### g_relation_count ()

    gint        g_relation_count                (GRelation *relation,
                                                 gconstpointer key,
                                                 gint field);

Returns the number of tuples in a GRelation that have the given value in the given field.

`relation` : | a GRelation.
`key` : | the value to compare with.
`field` : | the field of each record to match.
Returns : | the number of matches.

### g_relation_select ()

    GTuples*    g_relation_select               (GRelation *relation,
                                                 gconstpointer key,
                                                 gint field);

Returns all of the tuples which have the given key in the given field. Use `g_tuples_index()` to access the returned records. The returned records should be freed with `g_tuples_destroy()`.

`relation` : | a GRelation.
`key` : | the value to compare with.
`field` : | the field of each record to match.
Returns : | the records (tuples) that matched.

### g_relation_delete ()

    gint        g_relation_delete               (GRelation *relation,
                                                 gconstpointer key,
                                                 gint field);

Deletes any records from a GRelation that have the given key value in the given field.

`relation` : | a GRelation.
`key` : | the value to compare with.
`field` : | the field of each record to match.
Returns : | the number of records deleted.

### g_relation_destroy ()

    void        g_relation_destroy              (GRelation *relation);

Destroys the GRelation, freeing all memory allocated. However, it does not free memory allocated for the tuple data, so you should free that first if appropriate.

`relation` : | a GRelation.

### g_relation_print ()

    void        g_relation_print                (GRelation *relation);

Outputs information about all records in a GRelation, as well as the indexes. It is for debugging.

`relation` : | a GRelation.

### struct GTuples

    struct GTuples {

      guint len;
    };

The GTuples struct is used to return records (or tuples) from the GRelation by `g_relation_select()`. It only contains one public member - the number of records that matched. To access the matched records, you must use `g_tuples_index()`.

guint `len` | the number of records that matched.

### g_tuples_destroy ()

    void        g_tuples_destroy                (GTuples *tuples);

Frees the records which were returned by `g_relation_select()`. This should always be called after `g_relation_select()` when you are finished with the records. The records are not removed from the GRelation.

`tuples` : | the tuple data to free.

### g_tuples_index ()

    gpointer    g_tuples_index                  (GTuples *tuples,
                                                 gint index_,
                                                 gint field);

Gets a field from the records returned by `g_relation_select()`. It returns the given field of the record at the given index. The returned value should not be changed.

`tuples` : | the tuple data, returned by `g_relation_select()`.
`index_` : | the index of the record.
`field` : | the field to return.
Returns : | the field of the record.