Go: flatten an array of arbitrarily nested arrays of integers into a flat array of integers

theorem_test.go

// The MIT License (MIT)
//
// Copyright (c) 2019, @srfrog
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
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//
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// Package main
//
// This is an example of how to use Go to flatten an arbitrarily nested array of integers into
// a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]
//
// Issues:
//
// - We can't use Go arrays because they can't be nested.
// - Also Go does not support nested types per-se.
//
// Solution:
//
// Although Go has a static type system, all type representations are described internally by
// an interface dynamic type (IDT). The IDT can be assignable to any type including itself.
// We can use this property to mimic a nested type using slices.
//
// In other words, we must abuse the Go type system.
package main

import (
	"reflect"
	"testing"
)

// flattenNested receives nested values and flattens them to a one-dimension
// int slice. If any value in nested is not an int, this function will return nil.
//
// The original problem did not specify what to do with invalid values (non-int), so we
// take the high road and end quickly. To return a partial slice of int values just
// remove the default case in the switch.
//
// nested is a slice of interface dynamic types. That includes any type representation
// but for this exercise we only need to accept two types: int and []interface{}
// NOTE: It would be trivial to support other types and/or convert values to int.
//
// Returns a slice of ints from flatted nested values, or nil.
func flattenNested(nested []interface{}) []int {
	var out []int
	for _, value := range nested {
		switch value.(type) {
		case int:
			out = append(out, value.(int))
		case []interface{}:
			values := flattenNested(value.([]interface{}))
			// We must cascade nils otherwise we might end up with partial slice output.
			// Because appending nil to a slice is a no-op.
			if values == nil {
				return nil
			}
			out = append(out, values...)
		default:
			// Type not accepted, invalidate result.
			// NOTE: remove this case to return a partial slice output.
			return nil
		}
	}
	return out
}

// TestFlattenNested tests flattenNested for various nested inputs and their expected
// output. We must use reflect.DeepEqual to compare outputs because slices can't be
// compared.
func TestFlattenNested(t *testing.T) {
	// Test cases
	tests := []struct {
		in  []interface{} // input
		out []int         // expected output
	}{
		{
			in:  nil,
			out: nil,
		},
		{
			in:  []interface{}{},
			out: nil,
		},
		{
			in:  []interface{}{"1", "2", "3"},
			out: nil,
		},
		{
			in:  []interface{}{1, "2", "3"},
			out: nil,
		},
		{
			in:  []interface{}{1, "2", 3},
			out: nil,
		},
		{
			in:  []interface{}{1, 2, 3, 4},
			out: []int{1, 2, 3, 4},
		},
		{
			in:  []interface{}{1, 2, []interface{}{3, 4}},
			out: []int{1, 2, 3, 4},
		},
		{
			in: []interface{}{1,
				[]interface{}{2,
					[]interface{}{3,
						[]interface{}{4}}}},
			out: []int{1, 2, 3, 4},
		},
		{
			in: []interface{}{1, 2,
				[]interface{}{3, 4,
					[]interface{}{5, 6}, []interface{}{7, 8}},
				[]interface{}{9, 10}},
			out: []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
		},
		// Edge case: cascade nils
		{
			in:  []interface{}{1, []interface{}{}},
			out: nil,
		},
		// Edge case: cascade nils
		{
			in:  []interface{}{1, []interface{}{[]interface{}{}}},
			out: nil,
		},
		// Edge case: cascade nils
		{
			in:  []interface{}{1, []interface{}{[]interface{}{"2"}}},
			out: nil,
		},
	}

	for _, tc := range tests {
		out := flattenNested(tc.in)
		if !reflect.DeepEqual(out, tc.out) {
			t.Errorf("Unexpected output: %v != %v", out, tc.out)
		}
	}
}