From f86529d65944016631efe09218c4828feed32f03 Mon Sep 17 00:00:00 2001
From: Gleb Minaev <43728100+lounres@users.noreply.github.com>
Date: Tue, 15 Mar 2022 15:35:17 +0300
Subject: [PATCH] Optimized `optimizedMultiply` and `optimizedAddMultiplied`
 for cases of negative value of `other` and `multiplier`

---
 .../kscience/kmath/functions/algebraicStub.kt | 59 +++++++++++++------
 1 file changed, 42 insertions(+), 17 deletions(-)

diff --git a/kmath-functions/src/commonMain/kotlin/space/kscience/kmath/functions/algebraicStub.kt b/kmath-functions/src/commonMain/kotlin/space/kscience/kmath/functions/algebraicStub.kt
index 9e5043b8c..3c37a01ec 100644
--- a/kmath-functions/src/commonMain/kotlin/space/kscience/kmath/functions/algebraicStub.kt
+++ b/kmath-functions/src/commonMain/kotlin/space/kscience/kmath/functions/algebraicStub.kt
@@ -16,17 +16,11 @@ import space.kscience.kmath.operations.Group
  * @receiver the multiplicand.
  * @param other the multiplier.
  * @return the difference.
+ * @author Gleb Minaev
  */
-internal tailrec fun <C> Group<C>.optimizedMultiply(arg: C, other: Int): C =
-    when {
-        other == 0 -> zero
-        other == 1 -> arg
-        other == -1 -> -arg
-        other % 2 == 0 -> optimizedMultiply(arg + arg, other / 2)
-        other % 2 == 1 -> optimizedAddMultiplied(arg, arg + arg, other / 2)
-        other % 2 == -1 -> optimizedAddMultiplied(-arg, arg + arg, other / 2)
-        else -> error("Error in multiplication group instant by integer: got reminder by division by 2 different from 0, 1 and -1")
-    }
+internal fun <C> Group<C>.optimizedMultiply(arg: C, other: Int): C =
+    if (other >= 0) optimizedMultiply(arg, other.toUInt())
+    else optimizedMultiply(arg, (-other).toUInt())
 
 // TODO: Move receiver to context receiver
 /**
@@ -40,12 +34,43 @@ internal tailrec fun <C> Group<C>.optimizedMultiply(arg: C, other: Int): C =
  * @author Gleb Minaev
  */
 internal tailrec fun <C> Group<C>.optimizedAddMultiplied(base: C, arg: C, multiplier: Int): C =
+    if (multiplier >= 0) optimizedAddMultiplied(base, arg, multiplier.toUInt())
+    else optimizedAddMultiplied(base, arg, (-multiplier).toUInt())
+
+// TODO: Move receiver to context receiver
+/**
+ * Multiplication of element and integer.
+ *
+ * @receiver the multiplicand.
+ * @param other the multiplier.
+ * @return the difference.
+ * @author Gleb Minaev
+ */
+internal tailrec fun <C> Group<C>.optimizedMultiply(arg: C, other: UInt): C =
     when {
-        multiplier == 0 -> base
-        multiplier == 1 -> base + arg
-        multiplier == -1 -> base - arg
-        multiplier % 2 == 0 -> optimizedAddMultiplied(base, arg + arg, multiplier / 2)
-        multiplier % 2 == 1 -> optimizedAddMultiplied(base + arg, arg + arg, multiplier / 2)
-        multiplier % 2 == -1 -> optimizedAddMultiplied(base + arg, arg + arg, multiplier / 2)
-        else -> error("Error in multiplication group instant by integer: got reminder by division by 2 different from 0, 1 and -1")
+        other == 0u -> zero
+        other == 1u -> arg
+        other % 2u == 0u -> optimizedMultiply(arg + arg, other / 2u)
+        other % 2u == 1u -> optimizedAddMultiplied(arg, arg + arg, other / 2u)
+        else -> error("Error in multiplication group instant by unsigned integer: got reminder by division by 2 different from 0 and 1")
+    }
+
+// TODO: Move receiver to context receiver
+/**
+ * Adds product of [arg] and [multiplier] to [base].
+ *
+ * @receiver the algebra to provide multiplication.
+ * @param base the augend.
+ * @param arg the multiplicand.
+ * @param multiplier the multiplier.
+ * @return sum of the augend [base] and product of the multiplicand [arg] and the multiplier [multiplier].
+ * @author Gleb Minaev
+ */
+internal tailrec fun <C> Group<C>.optimizedAddMultiplied(base: C, arg: C, multiplier: UInt): C =
+    when {
+        multiplier == 0u -> base
+        multiplier == 1u -> base + arg
+        multiplier % 2u == 0u -> optimizedAddMultiplied(base, arg + arg, multiplier / 2u)
+        multiplier % 2u == 1u -> optimizedAddMultiplied(base + arg, arg + arg, multiplier / 2u)
+        else -> error("Error in multiplication group instant by unsigned integer: got reminder by division by 2 different from 0 and 1")
     }
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