From d4d863c4db6fcc893ff3a04ff3ef4ca1ef757f4c Mon Sep 17 00:00:00 2001
From: Jeroen van Rijn <Kelimion@users.noreply.github.com>
Date: Thu, 22 Jul 2021 18:12:19 +0200
Subject: [PATCH] big: Add `mod_power_of_two`.

---
 core/math/big/basic.odin   | 59 ++++++++++++++++++++++++++++++++------
 core/math/big/example.odin |  8 +++---
 2 files changed, 55 insertions(+), 12 deletions(-)

diff --git a/core/math/big/basic.odin b/core/math/big/basic.odin
index 7a3bbf751..42db8b88e 100644
--- a/core/math/big/basic.odin
+++ b/core/math/big/basic.odin
@@ -322,7 +322,7 @@ int_sub_digit :: proc(dest, a: ^Int, digit: DIGIT) -> (err: Error) {
 			carry := dest.digit[i] >> ((size_of(DIGIT) * 8) - 1);
 			dest.digit[i] &= _MASK;
 		}
-   	}
+	}
 
 	zero_count := old_used - dest.used;
 	/*
@@ -374,13 +374,13 @@ int_halve :: proc(dest, src: ^Int) -> (err: Error) {
 			Shift the current digit, add in carry and store.
 		*/
 		dest.digit[x] = (src_digit >> 1) | (fwd_carry << (_DIGIT_BITS - 1));
-      	/*
-      		Forward carry to next iteration.
-      	*/
-      	fwd_carry = carry;
-   	}
+		/*
+			Forward carry to next iteration.
+		*/
+		fwd_carry = carry;
+	}
 
- 	zero_count := old_used - dest.used;
+	zero_count := old_used - dest.used;
 	/*
 		Zero remainder.
 	*/
@@ -447,7 +447,7 @@ int_double :: proc(dest, src: ^Int) -> (err: Error) {
 		*/
 		dest.digit[dest.used] = 1;
 	}
- 	zero_count := old_used - dest.used;
+	zero_count := old_used - dest.used;
 	/*
 		Zero remainder.
 	*/
@@ -463,6 +463,49 @@ int_double :: proc(dest, src: ^Int) -> (err: Error) {
 double :: proc { int_double, };
 shl1   :: double;
 
+
+/*
+	dest = src % (2^power);
+*/
+int_mod_power_of_two :: proc(dest, src: ^Int, power: int) -> (err: Error) {
+	dest := dest; src := src;
+	if err = clear_if_uninitialized(dest); err != .None {
+		return err;
+	}
+	if err = clear_if_uninitialized(src); err != .None {
+		return err;
+	}
+
+	if power  < 0 { return .Invalid_Argument; }
+	if power == 0 { return zero(dest); }
+
+	/*
+		If the modulus is larger than the value, return the value.
+	*/
+	err = copy(dest, src);
+	if power >= (src.used * _DIGIT_BITS) || err != .None {
+		return;
+	}
+
+	/*
+		Zero digits above the last digit of the modulus.
+	*/
+	zero_count := (power / _DIGIT_BITS) + 0 if (power % _DIGIT_BITS == 0) else 1;
+	/*
+		Zero remainder.
+	*/
+	if zero_count > 0 {
+		mem.zero_slice(dest.digit[zero_count:]);
+	}
+
+	/*
+		Clear the digit that is not completely outside/inside the modulus.
+	*/
+	dest.digit[power / _DIGIT_BITS] &= DIGIT(1 << DIGIT(power % _DIGIT_BITS)) - DIGIT(1);
+	return clamp(dest);
+}
+mod_power_of_two :: proc { int_mod_power_of_two, };
+
 /*
 	==========================
 		Low-level routines    
diff --git a/core/math/big/example.odin b/core/math/big/example.odin
index 014e19af6..1b8503233 100644
--- a/core/math/big/example.odin
+++ b/core/math/big/example.odin
@@ -57,16 +57,16 @@ demo :: proc() {
 	a, b, c := &Int{}, &Int{}, &Int{};
 	defer destroy(a, b, c);
 
-	err = set(a, 0);
-	a.used = 2;
-	a.digit[1] = 1;
+	err = set(a, -512);
 	err = set(b, 1);
 	err = set(c, -4);
 
+
+	err = mod_power_of_two(a, a, 10);
 	fmt.printf("%v (%v)\n", int_get_float(a));
 
 
-	print("a", a, 16);
+	print("a", a, 10);
 	print("b", b, 10);
 	print("c", c, 10);