simap/simap.h

#ifndef SIMAP_H
#define SIMAP_H
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <assert.h>
#include "amap.h"
#include "arena.h/arena.h"

/** This is to ensure 8byte storage of pointers (with possible padding) */
union simap_ptr64 {
	void *ptr;
	uint64_t u64;
};
typedef union simap_ptr64 simap_ptr64;

/**
 * A "peasantly" map data structure backed by arena.h - basically a toy data structure...
 *
 * This is very simple, no trees, no hashes, just (hopefully) autovectorized linear lookup.
 * Inserting NULLs to keys happens through tombstoning unless erase happens and we never
 * shrink memory so please do not add a lot of things then remove a lot of things.
 *
 * We also only do heuristics against data being the key so its not "sure" and can fail...
 *
 * XXX: So beware that this can FAIL just "statistically most often works"!
 *
 * The memory layout after at *base is as follows:
 *
 * 8 byte:
 * - void* value;
 * - ? padding (only for non-64 bit pointer machines)
 *
 * 8 byte:
 * - uint32_t previndex;
 * - uint32_t nextindex;
 *
 * K x 8 byte:
 * - char name[]; // inline stored
 * - padding (divisible by 8)
 *
 * ELEMENTS added to it...
 *
 * Because of it a lookup is basically via strstr-like with 8byte steps!
 * with few character names zero-padded in the search term parameter
 * and if you want check extra validity by jumping back&forth in it.
 */
struct simap_instance {
	arena a;
	uint32_t end;
	uint32_t usage_end; /* in bytes!!! */
	uint32_t next_previndex; /* in bytes!!! */
	/** see doc comment for layout and why uint64_t* is the type */
	uint64_t *base;
};
typedef struct simap_instance simap_instance;

static inline simap_instance simap_create() {
	simap_instance ret;
	ret.a = newarena((ptrdiff_t)1 << 33);
	ret.end = 0;
	ret.usage_end = 0;
	ret.next_previndex = 0;
	ret.base = ((uint64_t*) alloc(&(ret.a), sizeof(uint64_t), sizeof(uint64_t), 1)) /* addr divisible by 8 */
		+ 1; /* First really addressible thing */
	return ret;
}

union simap_c64 {
	char str8[8];
	uint64_t u64;
};
typedef union simap_char64 simap_char64;

static inline simap_ptr64 *simap_search_internal(simap_instance *map, const char *key) {
	/* Construct prefix (fast-key) */
	size_t keylen = strlen(key);
	char is_smallkey = (keylen < 8);

	simap_c64 prefix {0};
	size_t prefixlen =  is_smallkey ? keylen : 8;
	strncpy(prefix.str8, key, prefixlen);

	/* Construct keyremains - might point to the \0 terminator only if smallkey or 8 bytes exactly */
	const char *keyremains = key + prefixlen;

	/* TODO: Maybe I should create separate function for fast-lookup returning "next" pointer from a pointer to autovectorize? */
	/* Lookup prefix (fast-key) - hopefully this gets vectorized (should be)!!! */
	uint64_t *base = map->base;
	uint64_t *tipp = map->base;
	for(uint32_t i = 0; i < map->usage_end / 8; ++i, ++tipp) {
		/* Fast lookup */
		if(*tipp == prefix.u64) {
			/* First check the remains of the string (only if needed) */
			if(!is_smallkey) {
				char *tippremains = (char *)((uint8_t *)tipp + sizeof(uint64_t));
				if(strcmp(keyremains, tippremains) != 0) {
					continue;
				}
			}

			/* Check back & forth (jump validation) */
			uint32_t previ = *((uint32_t *)(tipp - 1));
			uint32_t prevnexi = *(uint32_t *)(((uint8_t *)base) + previ
					+ sizeof(simap_ptr64)
					+ sizeof(uint32_t));

			simap_ptr64 *ptr = (simap_ptr64 *)((uint8_t *)base + prevnexi);
			uint64_t *retipp = (uint64_t *)(((uint8_t *)base + prevnexi)
				+ sizeof(simap_ptr64) + sizeof(uint32_t) +
				+ sizeof(uint32_t));

			if(retipp != tipp) {
				continue;
			}

			/* Can have the (statistically checked) pointer */
			return ptr;
		}
	}

	/* Haven't found anything */
	return NULL;
}

/** Gets padding bytes for a size to be padded to divisible alignment */
static inline unsigned int get_size_padding(unsigned int size, unsigned int alignment) {
	// return (alignment - (size % alignment)) % alignment;
	return (size + alignment - 1) / alignment * alignment;
}

/** Returns the size of the storage needed for the given key */
static inline uint32_t simap_elem_storage_size(const char *key) {
	uint32_t keysize = strlen(key);
	uint32_t padding = get_size_padding(keysize, 8);

	return keysize +
		sizeof(simap_ptr64) +
		sizeof(uint32_t) +
		sizeof(uint32_t) +
		padding;
}

/** Force-add the (key,value) to the end of the map */
static inline void *simap_force_add_internal(simap_instance *map, const char *key, void *ptr) {
	uint32_t storage_needed = simap_elem_storage_size(key);
	if(map->end - map->usage_end > storage_needed) {
		/* Need storage */
		/* TODO: Implement */
		// Soemthin' like = ((T*) alloc(&a, sizeof(T), sizeof(T), 1)) + 1;
	}

	/* Already have the storage */
	/* TODO: Implement - beware that I NEED to store the first 8 characters as a simap_c64! */
	return NULL;
}

/**
 * A simple map data structure that fulfills amap.h
 *
 * Operations:
 *
 * 		AMAP_SET Saves a mapping from key->ptr in map. ptr==NULL "tombstones" the mapping to return NULL.
 * 		AMAP_GET Gets the symbol at key (the ptr parameter is unused). Returns NULL if there is no ptr for the key.
 * 		AMAP_ERASE Erases the symbol table so it becomes empty again. Can never fail, returns NULL.
 *
 * @param amap_instance The instance we operate upon.
 * @param op Defines which operation the caller wants.
 * @param key The key (both for SET and GET). This pointer can get easily invalidated so you might need a copy or you do Trie, etc.
 * @param ptr When adding a ptr (data) to the map / table, the key will point to this ptr.
 * @returns The ptr / data stored for the key, or NULL on tombstone or when not stored yet or op is SET and there was an error.
 */
static inline void* simap(void *amap_instance, AMAP_OP op, const char *key, void *ptr) {
	simap_instance *map = (simap_instance *) amap_instance;

	if(op == AMAP_ERASE) {
		map->usage_end = 0;
		return NULL;
	}

	/* Search for the key - also needed for SET in order to "re-set" */
	simap_ptr64 *found = simap_search_internal(map, key);

	if(op == AMAP_GET) {
		return found->ptr;
	} else {
		assert(op == AMAP_SET);

		if(found) {
			/* Just overwrite */
			found->ptr = ptr;
			return (void *) found;
		} else {
		}
	}

	return NULL;
}

#endif /* SIMAP_H */