math: sqrti, powi, factoriali (#12072)

vlib/math/pow.v

@@ -35,6 +35,41 @@ pub fn pow10(n int) f64 {
 	return 0.0
 }
 
+// powi returns base raised to power (a**b) as an integer (i64)
+//
+// special case:
+// powi(a, b) = -1 for a = 0 and b < 0
+pub fn powi(a i64, b i64) i64 {
+	mut b_ := b
+	mut p := a
+	mut v := i64(1)
+
+	if b_ < 0 { // exponent < 0
+		if a == 0 {
+			return -1 // division by 0
+		}
+		return if a * a != 1 {
+			0
+		} else {
+			if (b_ & 1) > 0 {
+				a
+			} else {
+				1
+			}
+		}
+	}
+
+	for ; b_ > 0; {
+		if b_ & 1 > 0 {
+			v *= p
+		}
+		p *= p
+		b_ >>= 1
+	}
+
+	return v
+}
+
 // pow returns base raised to the provided power.
 //
 // todo(playXE): make this function work on JS backend, probably problem of JS codegen that it does not work.