Add asin() and acos() to runtime

SPEC.md

@@ -173,6 +173,8 @@ These are the right primitives, but we can build much more sophisticated systems
 
 - `sin(number) => number`: sine
 - `cos(number) => number`: cosine
+- `asin(number) => number`: arcsine (inverse sin)
+- `acos(number) => number`: arccosine (inverse cos)
 - `pow(number, number) => number`: power, also stands in for finding roots with exponent < 1
 - `ln(number) => number`: natural log
 - `floor(number) => number`: floor / truncation

pkg/ink/runtime.go

@@ -69,6 +69,8 @@ func (ctx *Context) LoadEnvironment() {
 	// math
 	ctx.LoadFunc("sin", inkSin)
 	ctx.LoadFunc("cos", inkCos)
+	ctx.LoadFunc("asin", inkAsin)
+	ctx.LoadFunc("acos", inkAcos)
 	ctx.LoadFunc("pow", inkPow)
 	ctx.LoadFunc("ln", inkLn)
 	ctx.LoadFunc("floor", inkFloor)
@@ -1388,6 +1390,42 @@ func inkCos(ctx *Context, in []Value) (Value, error) {
 	return NumberValue(math.Cos(float64(inNum))), nil
 }
 
+func inkAsin(ctx *Context, in []Value) (Value, error) {
+	if len(in) != 1 {
+		return nil, Err{
+			ErrRuntime,
+			"sin() takes exactly one number argument",
+		}
+	}
+	inNum, isNum := in[0].(NumberValue)
+	if !isNum {
+		return nil, Err{
+			ErrRuntime,
+			fmt.Sprintf("sin() takes a number argument, got %s", in[0]),
+		}
+	}
+
+	return NumberValue(math.Asin(float64(inNum))), nil
+}
+
+func inkAcos(ctx *Context, in []Value) (Value, error) {
+	if len(in) != 1 {
+		return nil, Err{
+			ErrRuntime,
+			"cos() takes exactly one number argument",
+		}
+	}
+	inNum, isNum := in[0].(NumberValue)
+	if !isNum {
+		return nil, Err{
+			ErrRuntime,
+			fmt.Sprintf("cos() takes a number argument, got %s", in[0]),
+		}
+	}
+
+	return NumberValue(math.Acos(float64(inNum))), nil
+}
+
 func inkPow(ctx *Context, in []Value) (Value, error) {
 	if len(in) != 2 {
 		return nil, Err{