func slice()

in starlark/eval.go [1268:1329]
48 lines of code
13 McCabe index (conditional complexity)

func slice(x, lo, hi, step_ Value) (Value, error) {
	sliceable, ok := x.(Sliceable)
	if !ok {
		return nil, fmt.Errorf("invalid slice operand %s", x.Type())
	}

	n := sliceable.Len()
	step := 1
	if step_ != None {
		var err error
		step, err = AsInt32(step_)
		if err != nil {
			return nil, fmt.Errorf("invalid slice step: %s", err)
		}
		if step == 0 {
			return nil, fmt.Errorf("zero is not a valid slice step")
		}
	}

	// TODO(adonovan): opt: preallocate result array.

	var start, end int
	if step > 0 {
		// positive stride
		// default indices are [0:n].
		var err error
		start, end, err = indices(lo, hi, n)
		if err != nil {
			return nil, err
		}

		if end < start {
			end = start // => empty result
		}
	} else {
		// negative stride
		// default indices are effectively [n-1:-1], though to
		// get this effect using explicit indices requires
		// [n-1:-1-n:-1] because of the treatment of -ve values.
		start = n - 1
		if err := asIndex(lo, n, &start); err != nil {
			return nil, fmt.Errorf("invalid start index: %s", err)
		}
		if start >= n {
			start = n - 1
		}

		end = -1
		if err := asIndex(hi, n, &end); err != nil {
			return nil, fmt.Errorf("invalid end index: %s", err)
		}
		if end < -1 {
			end = -1
		}

		if start < end {
			start = end // => empty result
		}
	}

	return sliceable.Slice(start, end, step), nil
}