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big: Add choose.

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This commit is contained in:
Jeroen van Rijn
2021-08-02 02:07:14 +02:00
parent 320387c4ee
commit cd0ce7b76e
2 changed files with 43 additions and 26 deletions
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35
core/math/big/basic.odin
View File

@@ -773,8 +773,43 @@ int_factorial :: proc(res: ^Int, n: DIGIT) -> (err: Error) {
}
factorial :: proc { int_factorial, };
/*
Number of ways to choose `k` items from `n` items.
Also known as the binomial coefficient.
TODO: Speed up.
Could be done faster by reusing code from factorial and reusing the common "prefix" results for n!, k! and n-k!
We know that n >= k, otherwise we early out with res = 0.
So:
n-k, keep result
n, start from previous result
k, start from previous result
*/
int_choose_digit :: proc(res: ^Int, n, k: DIGIT) -> (err: Error) {
if res == nil { return .Invalid_Pointer; }
if err = clear_if_uninitialized(res); err != .None { return err; }
if k > n { return zero(res); }
/*
res = n! / (k! * (n - k)!)
*/
n_fac, k_fac, n_minus_k_fac := &Int{}, &Int{}, &Int{};
defer destroy(n_fac, k_fac, n_minus_k_fac);
if err = factorial(n_minus_k_fac, n - k); err != .None { return err; }
if err = factorial(k_fac, k); err != .None { return err; }
if err = mul(k_fac, k_fac, n_minus_k_fac); err != .None { return err; }
if err = factorial(n_fac, n); err != .None { return err; }
if err = div(res, n_fac, k_fac); err != .None { return err; }
return err;
}
choose :: proc { int_choose_digit, };
/*
==========================

34
core/math/big/example.odin
View File

@@ -54,7 +54,7 @@ print_timings :: proc() {
case avg < time.Millisecond:
avg_s = fmt.tprintf("%v µs", time.duration_microseconds(avg));
case:
avg_s = fmt.tprintf("%v", time.duration_milliseconds(avg));
avg_s = fmt.tprintf("%v ms", time.duration_milliseconds(avg));
}
total_s: string;
@@ -64,7 +64,7 @@ print_timings :: proc() {
case v.t < time.Millisecond:
total_s = fmt.tprintf("%v µs", time.duration_microseconds(v.t));
case:
total_s = fmt.tprintf("%v", time.duration_milliseconds(v.t));
total_s = fmt.tprintf("%v ms", time.duration_milliseconds(v.t));
}
fmt.printf("\t%v: %s (avg), %s (total, %v calls)\n", i, avg_s, total_s, v.c);
@@ -76,6 +76,7 @@ Category :: enum {
itoa,
atoi,
factorial,
choose,
lsb,
ctz,
};
@@ -114,30 +115,11 @@ demo :: proc() {
a, b, c, d, e, f := &Int{}, &Int{}, &Int{}, &Int{}, &Int{}, &Int{};
defer destroy(a, b, c, d, e, f);
set(a, 125);
set(b, 75);
err = gcd_lcm(c, d, a, b);
fmt.printf("gcd_lcm(");
print("a =", a, 10, false, true, false);
print(", b =", b, 10, false, true, false);
print("), gcd =", c, 10, false, true, false);
print(", lcm =", d, 10, false, true, false);
fmt.printf(" (err = %v)\n", err);
err = gcd(c, a, b);
fmt.printf("gcd(");
print("a =", a, 10, false, true, false);
print(", b =", b, 10, false, true, false);
print(") =", c, 10, false, true, false);
fmt.printf(" (err = %v)\n", err);
err = lcm(c, a, b);
fmt.printf("lcm(");
print("a =", a, 10, false, true, false);
print(", b =", b, 10, false, true, false);
print(") =", c, 10, false, true, false);
fmt.printf(" (err = %v)\n", err);
s := time.tick_now();
err = choose(a, 65535, 255);
Timings[.choose].t += time.tick_since(s); Timings[.choose].c += 1;
print("choose", a);
fmt.println(err);
}
main :: proc() {