redict/src/rax.h

#ifndef RAX_H
#define RAX_H

#include <stdint.h>

/* Representation of a radix tree as implemented in this file, that contains
 * the strings "foo", "foobar" and "footer" after the insertion of each
 * word. When the node represents a key inside the radix tree, we write it
 * between [], otherwise it is written between ().
 *
 * This is the vanilla representation:
 *
 *              (f) ""
 *                \
 *                (o) "f"
 *                  \
 *                  (o) "fo"
 *                    \
 *                  [t   b] "foo"
 *                  /     \
 *         "foot" (e)     (a) "foob"
 *                /         \
 *      "foote" (r)         (r) "fooba"
 *              /             \
 *    "footer" []             [] "foobar"
 *
 * However, this implementation implements a very common optimization where
 * successive nodes having a single child are "compressed" into the node
 * itself as a string of characters, each representing a next-level child,
 * and only the link to the node representing the last character node is
 * provided inside the representation. So the above representation is turend
 * into:
 *
 *                  ["foo"] ""
 *                     |
 *                  [t   b] "foo"
 *                  /     \
 *        "foot" ("er")    ("ar") "foob"
 *                 /          \
 *       "footer" []          [] "foobar"
 *
 * However this optimization makes the implementation a bit more complex.
 * For instance if a key "first" is added in the above radix tree, a
 * "node splitting" operation is needed, since the "foo" prefix is no longer
 * composed of nodes having a single child one after the other. This is the
 * above tree and the resulting node splitting after this event happens:
 *
 *
 *                    (f) ""
 *                    /
 *                 (i o) "f"
 *                 /   \
 *    "firs"  ("rst")  (o) "fo"
 *              /        \
 *    "first" []       [t   b] "foo"
 *                     /     \
 *           "foot" ("er")    ("ar") "foob"
 *                    /          \
 *          "footer" []          [] "foobar"
 *
 * Similarly after deletion, if a new chain of nodes having a single child
 * is created (the chain must also not include nodes that represent keys),
 * it must be compressed back into a single node.
 *
 */

#define RAX_NODE_MAX_SIZE ((1<<29)-1)
typedef struct raxNode {
    uint32_t iskey:1;     /* Does this node contain a key? */
    uint32_t isnull:1;    /* Associated value is NULL (don't store it). */
    uint32_t iscompr:1;   /* Node is compressed. */
    uint32_t size:29;     /* Number of children, or compressed string len. */
    /* Data layout is as follows:
     *
     * If node is not compressed we have 'size' bytes, one for each children
     * character, and 'size' raxNode pointers, point to each child node.
     * Note how the character is not stored in the children but in the
     * edge of the parents:
     *
     * [header strlen=0][abc][a-ptr][b-ptr][c-ptr](value-ptr?)
     *
     * if node is compressed (strlen != 0) the node has 1 children.
     * In that case the 'size' bytes of the string stored immediately at
     * the start of the data section, represent a sequence of successive
     * nodes linked one after the other, for which only the last one in
     * the sequence is actually represented as a node, and pointed to by
     * the current compressed node.
     *
     * [header strlen=3][xyz][z-ptr](value-ptr?)
     *
     * Both compressed and not compressed nodes can represent a key
     * with associated data in the radix tree at any level (not just terminal
     * nodes).
     *
     * If the node has an associated key (iskey=1) and is not NULL
     * (isnull=0), then after the raxNode pointers poiting to the
     * childen, an additional value pointer is present (as you can see
     * in the representation above as "value-ptr" field).
     */
    unsigned char data[];
} raxNode;

typedef struct rax {
    raxNode *head;
    uint64_t numele;
    uint64_t numnodes;
} rax;

/* Stack data structure used by raxLowWalk() in order to, optionally, return
 * a list of parent nodes to the caller. The nodes do not have a "parent"
 * field for space concerns, so we use the auxiliary stack when needed. */
#define RAX_STACK_STATIC_ITEMS 32
typedef struct raxStack {
    void **stack; /* Points to static_items or an heap allocated array. */
    size_t items, maxitems; /* Number of items contained and total space. */
    /* Up to RAXSTACK_STACK_ITEMS items we avoid to allocate on the heap
     * and use this static array of pointers instead. */
    void *static_items[RAX_STACK_STATIC_ITEMS];
    int oom; /* True if pushing into this stack failed for OOM at some point. */
} raxStack;

/* Radix tree iterator state is encapsulated into this data structure. */
#define RAX_ITER_STATIC_LEN 128
#define RAX_ITER_JUST_SEEKED (1<<0) /* Iterator was just seeked. Return current
                                       element for the first iteration and
                                       clear the flag. */
#define RAX_ITER_EOF (1<<1)    /* End of iteration reached. */
#define RAX_ITER_SAFE (1<<2)   /* Safe iterator, allows operations while
                                  iterating. But it is slower. */
typedef struct raxIterator {
    int flags;
    rax *rt;                /* Radix tree we are iterating. */
    unsigned char *key;     /* The current string. */
    void *data;             /* Data associated to this key. */
    size_t key_len;         /* Current key length. */
    size_t key_max;         /* Max key len the current key buffer can hold. */
    unsigned char key_static_string[RAX_ITER_STATIC_LEN];
    raxNode *node;          /* Current node. Only for unsafe iteration. */
    raxStack stack;         /* Stack used for unsafe iteration. */
} raxIterator;

/* A special pointer returned for not found items. */
extern void *raxNotFound;

/* Exported API. */
rax *raxNew(void);
int raxInsert(rax *rax, unsigned char *s, size_t len, void *data, void **old);
int raxRemove(rax *rax, unsigned char *s, size_t len, void **old);
void *raxFind(rax *rax, unsigned char *s, size_t len);
void raxFree(rax *rax);
void raxStart(raxIterator *it, rax *rt);
int raxSeek(raxIterator *it, const char *op, unsigned char *ele, size_t len);
int raxNext(raxIterator *it);
int raxPrev(raxIterator *it);
int raxRandomWalk(raxIterator *it, size_t steps);
int raxCompare(raxIterator *iter, const char *op, unsigned char *key, size_t key_len);
void raxStop(raxIterator *it);
int raxEOF(raxIterator *it);
void raxShow(rax *rax);
uint64_t raxSize(rax *rax);

#endif