# Project Euler 921 - Golden Recurrence
# S(1618034) mod 398874989
# Pure Flow port of native/p921.c.
const MOD: i64 = 398874989
const TARGET_M: i64 = 1618034
function powmod(a0: i64, b: i64, m: i64) -> i64 {
let mut r: i64 = 1
let mut a: i64 = a0 % m
if a < 0 { a = a + m }
while b > 0 {
if b % 2 == 1 {
r = (((r as i128) * (a as i128)) % (m as i128)) as i64
}
a = (((a as i128) * (a as i128)) % (m as i128)) as i64
b = b / 2
}
return r
}
function legendre(a: i64, p: i64) -> i64 {
return powmod(a, (p - 1) / 2, p)
}
function sqrt_mod_prime(n0: i64, p: i64) -> i64 {
let n: i64 = n0 % p
if n < 0 { n = n + p }
if n == 0 { return 0 }
if p == 2 { return n }
if legendre(n, p) != 1 { return -1 }
if p % 4 == 3 { return powmod(n, (p + 1) / 4, p) }
let mut q: i64 = p - 1
let mut s: i64 = 0
while q % 2 == 0 {
q = q / 2
s = s + 1
}
let mut z: i64 = 2
while legendre(z, p) != p - 1 {
z = z + 1
}
let c: i64 = powmod(z, q, p)
let mut x: i64 = powmod(n, (q + 1) / 2, p)
let mut t: i64 = powmod(n, q, p)
let mut m: i64 = s
while t != 1 {
let mut t2: i64 = t
let mut i: i64 = 1
while i < m {
t2 = (((t2 as i128) * (t2 as i128)) % (p as i128)) as i64
if t2 == 1 { break }
i = i + 1
}
let b: i64 = powmod(c, 1 << (m - i - 1), p)
x = (((x as i128) * (b as i128)) % (p as i128)) as i64
let bb: i64 = (((b as i128) * (b as i128)) % (p as i128)) as i64
t = (((t as i128) * (bb as i128)) % (p as i128)) as i64
c = bb
m = i
}
return x
}
function main() -> i32 {
let p: i64 = MOD
let exp_mod: i64 = p - 1
let inv2: i64 = powmod(2, p - 2, p)
let mut r: i64 = sqrt_mod_prime(5, p)
let mut inv_r: i64 = powmod(r, p - 2, p)
let mut phi: i64 = ((((1 + r) as i128) * (inv2 as i128)) % (p as i128)) as i64
let mut g: i64 = (((phi as i128) * (phi as i128)) % (p as i128)) as i64
g = (((g as i128) * (phi as i128)) % (p as i128)) as i64
# Check orientation: F_3 should be 2
let mut u: i64 = g
let mut uinv: i64 = powmod(u, p - 2, p)
let mut f3: i64 = ((((u + uinv) % p) as i128) * (inv_r as i128) % (p as i128)) as i64
if f3 != 2 {
r = (p - r) % p
inv_r = powmod(r, p - 2, p)
phi = ((((1 + r) as i128) * (inv2 as i128)) % (p as i128)) as i64
g = (((phi as i128) * (phi as i128)) % (p as i128)) as i64
g = (((g as i128) * (phi as i128)) % (p as i128)) as i64
u = g
uinv = powmod(u, p - 2, p)
}
let inv32: i64 = powmod(32, p - 2, p)
# Process i=2: E_1 = E_2 = 5
let mut e_im2: i64 = 5 % exp_mod
let mut e_im1: i64 = 5 % exp_mod
let mut total: i64 = 0
# i=2
let mut e: i64 = e_im1
u = powmod(g, e, p)
uinv = powmod(u, p - 2, p)
let mut f: i64 = ((((u + uinv) % p) as i128) * (inv_r as i128) % (p as i128)) as i64
let mut l: i64 = (u - uinv) % p
if l < 0 { l = l + p }
let f2: i64 = (((f as i128) * (f as i128)) % (p as i128)) as i64
let f4: i64 = (((f2 as i128) * (f2 as i128)) % (p as i128)) as i64
let f5: i64 = (((f4 as i128) * (f as i128)) % (p as i128)) as i64
let l2: i64 = (((l as i128) * (l as i128)) % (p as i128)) as i64
let l4: i64 = (((l2 as i128) * (l2 as i128)) % (p as i128)) as i64
let l5: i64 = (((l4 as i128) * (l as i128)) % (p as i128)) as i64
let mut term: i64 = (f5 + l5) % p
term = (((term as i128) * (inv32 as i128)) % (p as i128)) as i64
total = term
# i=3..m
let mut i: i64 = 3
while i <= TARGET_M {
e = (((e_im1 as i128) * (e_im2 as i128)) % (exp_mod as i128)) as i64
e_im2 = e_im1
e_im1 = e
u = powmod(g, e, p)
uinv = powmod(u, p - 2, p)
f = ((((u + uinv) % p) as i128) * (inv_r as i128) % (p as i128)) as i64
l = (u - uinv) % p
if l < 0 { l = l + p }
let f2b: i64 = (((f as i128) * (f as i128)) % (p as i128)) as i64
let f4b: i64 = (((f2b as i128) * (f2b as i128)) % (p as i128)) as i64
let f5b: i64 = (((f4b as i128) * (f as i128)) % (p as i128)) as i64
let l2b: i64 = (((l as i128) * (l as i128)) % (p as i128)) as i64
let l4b: i64 = (((l2b as i128) * (l2b as i128)) % (p as i128)) as i64
let l5b: i64 = (((l4b as i128) * (l as i128)) % (p as i128)) as i64
term = (f5b + l5b) % p
term = (((term as i128) * (inv32 as i128)) % (p as i128)) as i64
total = total + term
if total >= p { total = total - p }
i = i + 1
}
printf("%lld\n", total)
return 0
}
Generated C
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* Flow runtime helpers */
typedef struct flow_temp_node { struct flow_temp_node* next; } flow_temp_node;
static flow_temp_node* flow_temp_head = NULL;
static int flow_temp_atexit_set = 0;
__attribute__((unused)) static void flow_temp_free_all(void) {
while (flow_temp_head) {
flow_temp_node* n = flow_temp_head;
flow_temp_head = n->next;
free(n);
}
}
__attribute__((unused)) static void* flow_temp_alloc(size_t nbytes) {
flow_temp_node* node = (flow_temp_node*)malloc(sizeof(flow_temp_node) + nbytes);
if (!node) return NULL;
node->next = flow_temp_head;
flow_temp_head = node;
if (!flow_temp_atexit_set) {
flow_temp_atexit_set = 1;
atexit(flow_temp_free_all);
}
return (void*)(node + 1);
}
#ifndef FLOW_DIAG
#define FLOW_DIAG(msg) fprintf(stderr, "%s", (msg))
#endif
#ifndef FLOW_LOG
#define FLOW_LOG(fmt, ...) printf(fmt, __VA_ARGS__)
#endif
#ifndef FLOW_LOG_EMPTY
#define FLOW_LOG_EMPTY(fmt) printf(fmt)
#endif
static char* flow_strcat(const char* a, const char* b) {
size_t la = strlen(a ? a : ""), lb = strlen(b ? b : "");
char* r = (char*)flow_temp_alloc(la + lb + 1);
if (!r) return NULL;
if (la) memcpy(r, a, la);
if (lb) memcpy(r + la, b, lb);
r[la + lb] = '\0';
return r;
}
#define __flow_in_arr(arr, val) __extension__ ({ \
int _found = 0; \
size_t _n = sizeof(arr)/sizeof((arr)[0]); \
for (size_t _i = 0; _i < _n; _i++) { \
if ((arr)[_i] == (val)) { _found = 1; break; } \
} _found; })
/* Unified fault handler (MISRA #279) — override with -DFLOW_FAULT_HANDLER=fn */
#ifndef FLOW_FAULT_HANDLER
__attribute__((unused)) static inline void flow_fault_handler(const char* msg) {
fprintf(stderr, "flow: %s\n", msg ? msg : "fault");
abort();
#if defined(__GNUC__) || defined(__clang__)
__builtin_unreachable();
#endif
}
#else
#define flow_fault_handler FLOW_FAULT_HANDLER
#endif
#define flow_div_by_zero_handler() flow_fault_handler("division by zero")
#define flow_shift_ub_handler() flow_fault_handler("invalid shift (amount out of range or left-shift of negative)")
#ifndef FLOW_CHECKED_DIV
#define FLOW_CHECKED_DIV(L, R) (((R) != 0) ? ((L) / (R)) : (flow_div_by_zero_handler(), (L) * 0))
#endif
#ifndef FLOW_CHECKED_MOD
#define FLOW_CHECKED_MOD(L, R) (((R) != 0) ? ((L) % (R)) : (flow_div_by_zero_handler(), (L) * 0))
#endif
#ifndef FLOW_CHECKED_SHL
#define FLOW_CHECKED_SHL(L, R) ((((R) >= 0) && ((unsigned long long)(R) < (sizeof(L) * 8ull)) && ((L) >= 0)) ? ((L) << (R)) : (flow_shift_ub_handler(), (L) * 0))
#endif
#ifndef FLOW_CHECKED_SHR
#define FLOW_CHECKED_SHR(L, R) ((((R) >= 0) && ((unsigned long long)(R) < (sizeof(L) * 8ull))) ? ((L) >> (R)) : (flow_shift_ub_handler(), (L) * 0))
#endif
#include <math.h>
void* _ui_state = NULL;
static inline float i32_to_f32(int32_t v) { return (float)v; }
/* Host stub for @gpu kernels (device codegen replaces this). */
static inline int32_t gpu_thread_id(void) { return 0; }
int64_t powmod_i64_i64_i64(int64_t a0, int64_t b, int64_t m);
int64_t legendre_i64_i64(int64_t a, int64_t p);
int64_t sqrt_mod_prime_i64_i64(int64_t n0, int64_t p);
int32_t main(void);
static const int64_t MOD = 398874989;
static const int64_t TARGET_M = 1618034;
int64_t powmod_i64_i64_i64(int64_t a0, int64_t b, int64_t m) {
int64_t r = 1;
int64_t a = FLOW_CHECKED_MOD((a0), (m));
if (a < 0) {
a = (a + m);
}
while (b > 0) {
if (FLOW_CHECKED_MOD((b), (2)) == 1) {
r = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(r)) * ((__int128)(a)))), (((__int128)(m))))));
}
a = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(a)) * ((__int128)(a)))), (((__int128)(m))))));
b = FLOW_CHECKED_DIV((b), (2));
}
return r;
}
int64_t legendre_i64_i64(int64_t a, int64_t p) {
return powmod_i64_i64_i64(a, FLOW_CHECKED_DIV(((p - 1)), (2)), p);
}
int64_t sqrt_mod_prime_i64_i64(int64_t n0, int64_t p) {
int64_t n = FLOW_CHECKED_MOD((n0), (p));
if (n < 0) {
n = (n + p);
}
if (n == 0) {
return 0;
}
if (p == 2) {
return n;
}
if (legendre_i64_i64(n, p) != 1) {
return (-1);
}
if (FLOW_CHECKED_MOD((p), (4)) == 3) {
return powmod_i64_i64_i64(n, FLOW_CHECKED_DIV(((p + 1)), (4)), p);
}
int64_t q = (p - 1);
int64_t s = 0;
while (FLOW_CHECKED_MOD((q), (2)) == 0) {
q = FLOW_CHECKED_DIV((q), (2));
s = (s + 1);
}
int64_t z = 2;
while (legendre_i64_i64(z, p) != (p - 1)) {
z = (z + 1);
}
int64_t c = powmod_i64_i64_i64(z, q, p);
int64_t x = powmod_i64_i64_i64(n, FLOW_CHECKED_DIV(((q + 1)), (2)), p);
int64_t t = powmod_i64_i64_i64(n, q, p);
int64_t m = s;
while (t != 1) {
int64_t t2 = t;
int64_t i = 1;
while (i < m) {
t2 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(t2)) * ((__int128)(t2)))), (((__int128)(p))))));
if (t2 == 1) {
break;
}
i = (i + 1);
}
int64_t b = powmod_i64_i64_i64(c, FLOW_CHECKED_SHL((1), (((m - i) - 1))), p);
x = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(x)) * ((__int128)(b)))), (((__int128)(p))))));
int64_t bb = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(b)) * ((__int128)(b)))), (((__int128)(p))))));
t = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(t)) * ((__int128)(bb)))), (((__int128)(p))))));
c = bb;
m = i;
}
return x;
}
int32_t main(void) {
int64_t p = MOD;
int64_t exp_mod = (p - 1);
int64_t inv2 = powmod_i64_i64_i64(2, (p - 2), p);
int64_t r = sqrt_mod_prime_i64_i64(5, p);
int64_t inv_r = powmod_i64_i64_i64(r, (p - 2), p);
int64_t phi = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)((1 + r))) * ((__int128)(inv2)))), (((__int128)(p))))));
int64_t g = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(phi)) * ((__int128)(phi)))), (((__int128)(p))))));
g = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(g)) * ((__int128)(phi)))), (((__int128)(p))))));
int64_t u = g;
int64_t uinv = powmod_i64_i64_i64(u, (p - 2), p);
int64_t f3 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(FLOW_CHECKED_MOD(((u + uinv)), (p)))) * ((__int128)(inv_r)))), (((__int128)(p))))));
if (f3 != 2) {
r = FLOW_CHECKED_MOD(((p - r)), (p));
inv_r = powmod_i64_i64_i64(r, (p - 2), p);
phi = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)((1 + r))) * ((__int128)(inv2)))), (((__int128)(p))))));
g = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(phi)) * ((__int128)(phi)))), (((__int128)(p))))));
g = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(g)) * ((__int128)(phi)))), (((__int128)(p))))));
u = g;
uinv = powmod_i64_i64_i64(u, (p - 2), p);
}
int64_t inv32 = powmod_i64_i64_i64(32, (p - 2), p);
int64_t e_im2 = FLOW_CHECKED_MOD((5), (exp_mod));
int64_t e_im1 = FLOW_CHECKED_MOD((5), (exp_mod));
int64_t total = 0;
int64_t e = e_im1;
u = powmod_i64_i64_i64(g, e, p);
uinv = powmod_i64_i64_i64(u, (p - 2), p);
int64_t f = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(FLOW_CHECKED_MOD(((u + uinv)), (p)))) * ((__int128)(inv_r)))), (((__int128)(p))))));
int64_t l = FLOW_CHECKED_MOD(((u - uinv)), (p));
if (l < 0) {
l = (l + p);
}
int64_t f2 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(f)) * ((__int128)(f)))), (((__int128)(p))))));
int64_t f4 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(f2)) * ((__int128)(f2)))), (((__int128)(p))))));
int64_t f5 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(f4)) * ((__int128)(f)))), (((__int128)(p))))));
int64_t l2 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(l)) * ((__int128)(l)))), (((__int128)(p))))));
int64_t l4 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(l2)) * ((__int128)(l2)))), (((__int128)(p))))));
int64_t l5 = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(l4)) * ((__int128)(l)))), (((__int128)(p))))));
int64_t term = FLOW_CHECKED_MOD(((f5 + l5)), (p));
term = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(term)) * ((__int128)(inv32)))), (((__int128)(p))))));
total = term;
int64_t i = 3;
while (i <= TARGET_M) {
e = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(e_im1)) * ((__int128)(e_im2)))), (((__int128)(exp_mod))))));
e_im2 = e_im1;
e_im1 = e;
u = powmod_i64_i64_i64(g, e, p);
uinv = powmod_i64_i64_i64(u, (p - 2), p);
f = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(FLOW_CHECKED_MOD(((u + uinv)), (p)))) * ((__int128)(inv_r)))), (((__int128)(p))))));
l = FLOW_CHECKED_MOD(((u - uinv)), (p));
if (l < 0) {
l = (l + p);
}
int64_t f2b = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(f)) * ((__int128)(f)))), (((__int128)(p))))));
int64_t f4b = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(f2b)) * ((__int128)(f2b)))), (((__int128)(p))))));
int64_t f5b = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(f4b)) * ((__int128)(f)))), (((__int128)(p))))));
int64_t l2b = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(l)) * ((__int128)(l)))), (((__int128)(p))))));
int64_t l4b = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(l2b)) * ((__int128)(l2b)))), (((__int128)(p))))));
int64_t l5b = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(l4b)) * ((__int128)(l)))), (((__int128)(p))))));
term = FLOW_CHECKED_MOD(((f5b + l5b)), (p));
term = ((int64_t)(FLOW_CHECKED_MOD(((((__int128)(term)) * ((__int128)(inv32)))), (((__int128)(p))))));
total = (total + term);
if (total >= p) {
total = (total - p);
}
i = (i + 1);
}
printf("%lld\n", total);
return 0;
}