/*
 *	The UCW Library -- Random numbers
 *
 *	(c) 2020--2022 Pavel Charvat <pchar@ucw.cz>
 *
 *	This software may be freely distributed and used according to the terms
 *	of the GNU Lesser General Public License.
 */

#ifndef _UCW_RANDOM_H
#define _UCW_RANDOM_H

#ifdef CONFIG_UCW_CLEAN_ABI
#define fastrand_new ucw_fastrand_new
#define fastrand_delete ucw_fastrand_delete
#define fastrand_set_seed ucw_fastrand_set_seed
#define fastrand_gen_seed_value ucw_fastrand_gen_seed_value
#define fastrand_gen_seed ucw_fastrand_gen_seed
#define fastrand_has_seed ucw_fastrand_has_seed
#define fastrand_reset ucw_fastrand_reset
#define fastrand_u32 ucw_fastrand_u32
#define fastrand_u64 ucw_fastrand_u64
#define fastrand_bits_u64 ucw_fastrand_bits_u64
#define fastrand_max ucw_fastrand_max
#define fastrand_max_u64 ucw_fastrand_max_u64
#define fastrand_mem ucw_fastrand_mem
#define fastrand_double ucw_fastrand_double
#define strongrand_close ucw_strongrand_close
#define strongrand_reset ucw_strongrand_reset
#define strongrand_mem ucw_strongrand_mem
#define strongrand_std_open ucw_strongrand_std_open
#define strongrand_getrandom_mem_try ucw_strongrand_getrandom_mem_try
#endif

/* random-fast.c */

/**
 * Context structure for a fast pseudo-random generator.
 * Useful for generating numbers in randomized algorithms, but not in critical things like cryptography.
 * The library is generally thread-safe, but each context must be used by at most one thread at one time.
 * Also be careful with fork() if you want to avoid the same numbers in subprocesses.
 **/
struct fastrand;

/**
 * Type for a seed value. Beware that some generators are only able to use the lowest 32 bits.
 **/
typedef u64 fastrand_seed_t;

/**
 * Allocate a new context. Should be paired with fastrand_delete().
 * Don't forget to supply a seed before started to read randomness.
 **/
struct fastrand *fastrand_new(void);
void fastrand_delete(struct fastrand *c);

/** (Re)seed the context with given value. **/
void fastrand_set_seed(struct fastrand *c, fastrand_seed_t seed);

/**
 * Generate a seed for fastrand_set_seed().
 *
 * Since glibc 2.25 and kernel 3.17, we use getrandom() syscall
 * to read kernel's urandom.
 *
 * Otherwise we mix these things:
 * -- current time
 * -- process id to deal with frequent fork() or startups
 * -- counter to deal with multiple contexts, for example in threaded application
 **/
fastrand_seed_t fastrand_gen_seed_value(void);

/** (Re)seed the context with fastrand_gen_seed_value(). **/
fastrand_seed_t fastrand_gen_seed(struct fastrand *c);

/** Is the context seeded? **/
bool fastrand_has_seed(struct fastrand *c);

/** Reset the context to its initial state. Useful after a fork() to assert forgotten reseed. **/
void fastrand_reset(struct fastrand *c);

/** Uniformly generate 32 random bits. **/
u32 fastrand_u32(struct fastrand *c);

/** Uniformly generate 64 random bits. **/
u64 fastrand_u64(struct fastrand *c);

/** Uniformly generate [0, 64] random bits. **/
u64 fastrand_bits_u64(struct fastrand *c, uint bits);

/** Uniformly generate [0, 8*sizeof(uint)] random bits. **/
static inline uint fastrand_bits(struct fastrand *c, uint bits)
{
  return fastrand_bits_u64(c, bits);
}

/** Uniformly generate a random number from range [0, bound-1], where @bound is [1, UINT_MAX]. **/
uint fastrand_max(struct fastrand *c, uint bound);

/** Uniformly generate a random number from range [0, bound-1], where @bound is [1, UINT64_MAX]. **/
u64 fastrand_max_u64(struct fastrand *c, u64 bound);

/** Generate @size random bytes. **/
void fastrand_mem(struct fastrand *c, void *ptr, size_t size);

/** Uniformly generate a random number from range [0.0, 1.0) + possible inaccuracies from basic floating point operations. **/
double fastrand_double(struct fastrand *c);

/* random-strong.c */

/**
 * Context structure for a strong random generator.
 * Useful for things like generating random ids or cryptography.
 * The library is generally thread-safe, but each context must be used by at most one thread at one time.
 *
 * Also be careful with fork(). Unless said otherwise, it's forbidden to use the context
 * in a forked child, we can even assert changes of pid. See strongrand_reset() if you
 * need that.
 **/
struct strongrand {
  const char *name;				/* (mandatory) Context name for logging. */
  void (*read)(struct strongrand *sr, byte *ptr, int size); /* (mandatory) Generate @size random bytes. */
  void (*close)(struct strongrand *sr);		/* Called from strongrand_close(). */
  void (*reset)(struct strongrand *sr);		/* Called from strongrand_reset(). */
};

/** Close a previously opened context. **/
void strongrand_close(struct strongrand *c);

/**
 * If you want to use the same context in multiple forked subprocesses, you must
 * call this function after the fork() to flush any buffered randomness.
 * Some future back-ends could use it for even more things, for example
 * to re-connect somewhere.
 **/
void strongrand_reset(struct strongrand *sr);

/** Generate @size random bytes. **/
void strongrand_mem(struct strongrand *c, void *ptr, size_t size);

/**
 * Open kernel's random generator.
 * Requires exactly one of these flags, among others:
 * -- STRONGRAND_STD_URANDOM
 * -- STRONGRAND_STD_RANDOM
 *
 * Since glibc 2.25 and kernel 3.17, we use getrandom() syscall,
 * otherwise we open and read from "/dev/[u]random" device.
 *
 * @buf_size can be 0 for unbuffered reading.
 *
 * STRONGRAND_STD_x_RESET flags are only useful for buffered reading.
 *
 * If at least one of the following conditions are met, strongrand_reset()
 * after fork() is not necessary:
 * -- zero @buf_size
 * -- STRONGRAND_STD_AUTO_RESET flag
 * -- no read since open or last reset (empty buffer)
 **/
struct strongrand *strongrand_std_open(uint flags, uint buf_size);

enum strongrand_std_flags {
  STRONGRAND_STD_URANDOM = 0x1,		// Use kernel's "urandom" source.
  STRONGRAND_STD_RANDOM = 0x2,		// Use kernel's "random" source.
  STRONGRAND_STD_DELAYED = 0x4,		// Delay non-trivial initializations like opening file descriptor to first read.
  STRONGRAND_STD_AUTO_RESET = 0x8,	// Automatically do strongrand_reset() after changes of getpid(); adds overhead to each read.
  STRONGRAND_STD_ASSERT_RESET = 0x10,	// Assert correct usage of strongrand_reset() after changes of getpid(); for debugging.
};

// Internals
bool strongrand_getrandom_mem_try(void *buf, size_t size);

#endif