//
// Copyright 2017 Garrett D'Amore <garrett@damore.org>
//
// This software is supplied under the terms of the MIT License, a
// copy of which should be located in the distribution where this
// file was obtained (LICENSE.txt).  A copy of the license may also be
// found online at https://opensource.org/licenses/MIT.
//

#include "core/nng_impl.h"

// This is ISAAC, a (reputedly) cryptographically secure PRNG that is also
// quite efficient.  While the particular adjustments to fit in our code
// base are under our copyright, the actual algorithm itself, as well as
// sample implementations, are part of the public domain.  See this:
// http://www.burtleburtle.net/bob/c/readable.c
//
// Our changes include making this code thread safe/reentrant, and naming
// and style changes, to fit C99.

typedef struct {
	// the rsl is the actual results, and the randcnt is the length
	// of the results.
	uint32_t	randrsl[256];
	uint32_t	randcnt;

	// lock to protect concurrent access
	nni_mtx		mx;

	// more or less internal state
	uint32_t	mm[256];
	uint32_t	aa;
	uint32_t	bb;
	uint32_t	cc;
} nni_isaac_ctx;


static void
nni_isaac(nni_isaac_ctx *ctx)
{
	register uint32_t i, x, y;

	ctx->cc++;              // cc incremented once per 256 results
	ctx->bb += ctx->cc;     // then combined with bb

	for (i = 0; i < 256; ++i) {
		x = ctx->mm[i];
		switch (i%4) {
		case 0:
			ctx->aa ^= (ctx->aa<<13);
			break;
		case 1:
			ctx->aa ^= (ctx->aa>>6);
			break;
		case 2:
			ctx->aa ^= (ctx->aa<<2);
			break;
		case 3:
			ctx->aa ^= (ctx->aa>>16);
			break;
		}
		ctx->aa += ctx->mm[(i+128)%256];
		ctx->mm[i] = y = ctx->mm[(x>>2)%256] + ctx->aa + ctx->bb;
		ctx->randrsl[i] = ctx->bb = ctx->mm[(y>>10)%256] + x;

		// Note that bits 2..9 are chosen from x but 10..17 are chosen
		// from y.  The only important thing here is that 2..9 and
		// 10..17 don't overlap.  2..9 and 10..17 were then chosen
		// for speed in the optimized version (rand.c)

		// See http://burtleburtle.net/bob/rand/isaac.html
		// for further explanations and analysis.
	}
}


// if (flag!=0), then use the contents of randrsl[] to initialize mm[].
#define nni_isaac_mix(a, b, c, d, e, f, g, h) \
	{				      \
		a ^= b<<11; d += a; b += c;   \
		b ^= c>>2;  e += b; c += d;   \
		c ^= d<<8;  f += c; d += e;   \
		d ^= e>>16; g += d; e += f;   \
		e ^= f<<10; h += e; f += g;   \
		f ^= g>>4;  a += f; g += h;   \
		g ^= h<<8;  b += g; h += a;   \
		h ^= a>>9;  c += h; a += b;   \
	}

static void
nni_isaac_randinit(nni_isaac_ctx *ctx, int flag)
{
	int i;
	uint32_t a, b, c, d, e, f, g, h;

	ctx->aa = ctx->bb = ctx->cc = 0;
	a = b = c = d = e = f = g = h = 0x9e3779b9;     // the golden ratio

	for (i = 0; i < 4; ++i) {                       // scramble it
		nni_isaac_mix(a, b, c, d, e, f, g, h);
	}

	for (i = 0; i < 256; i += 8) {  // fill in mm[] with messy stuff
		if (flag) {             // use all the information in the seed
			a += ctx->randrsl[i];
			b += ctx->randrsl[i+1];
			c += ctx->randrsl[i+2];
			d += ctx->randrsl[i+3];
			e += ctx->randrsl[i+4];
			f += ctx->randrsl[i+5];
			g += ctx->randrsl[i+6];
			h += ctx->randrsl[i+7];
		}
		nni_isaac_mix(a, b, c, d, e, f, g, h);
		ctx->mm[i] = a;
		ctx->mm[i+1] = b;
		ctx->mm[i+2] = c;
		ctx->mm[i+3] = d;
		ctx->mm[i+4] = e;
		ctx->mm[i+5] = f;
		ctx->mm[i+6] = g;
		ctx->mm[i+7] = h;
	}

	if (flag) {
		// do a second pass to make all of the seed affect all of mm
		for (i = 0; i < 256; i += 8) {
			a += ctx->mm[i];
			b += ctx->mm[i+1];
			c += ctx->mm[i+2];
			d += ctx->mm[i+3];
			e += ctx->mm[i+4];
			f += ctx->mm[i+5];
			g += ctx->mm[i+6];
			h += ctx->mm[i+7];
			nni_isaac_mix(a, b, c, d, e, f, g, h);
			ctx->mm[i] = a;
			ctx->mm[i+1] = b;
			ctx->mm[i+2] = c;
			ctx->mm[i+3] = d;
			ctx->mm[i+4] = e;
			ctx->mm[i+5] = f;
			ctx->mm[i+6] = g;
			ctx->mm[i+7] = h;
		}
	}

	nni_isaac(ctx);         // fill in the first set of results
	ctx->randcnt = 256;     // prepare to use the first set of results
}


static nni_isaac_ctx nni_random_ctx;

int
nni_random_init(void)
{
	// minimally, grab the system clock
	nni_isaac_ctx *ctx = &nni_random_ctx;
	int rv;

	if ((rv = nni_mtx_init(&ctx->mx)) != 0) {
		return (rv);
	}

	nni_plat_seed_prng(ctx->randrsl, sizeof (ctx->randrsl));
	nni_isaac_randinit(ctx, 1);
	return (0);
}


uint32_t
nni_random(void)
{
	uint32_t rv;
	nni_isaac_ctx *ctx = &nni_random_ctx;

	nni_mtx_lock(&ctx->mx);
	if (ctx->randcnt < 1) {
		nni_isaac(ctx);
		ctx->randcnt = 256;
	}
	ctx->randcnt--;
	rv = ctx->randrsl[ctx->randcnt];
	nni_mtx_unlock(&ctx->mx);

	return (rv);
}


void
nni_random_fini(void)
{
	nni_mtx_fini(&nni_random_ctx.mx);
}