Implement Set.At and Map.At in Go (#2903)

These get the set/map element at a specific index.
I haven't implemented it in JS yet because the JS code has no method to
create a cursor at an index. This exists in Go because a refactor was
done a few months ago to add it, but it hasn't been ported to JS.

go/types/map.go

@@ -5,6 +5,7 @@
 package types
 
 import (
+	"fmt"
 	"sort"
 
 	"github.com/attic-labs/noms/go/d"
@@ -140,6 +141,16 @@ func (m Map) Last() (Value, Value) {
 	return m.firstOrLast(true)
 }
 
+func (m Map) At(idx uint64) (key, value Value) {
+	if idx >= m.Len() {
+		panic(fmt.Errorf("Out of bounds: %s >= %s", idx, m.Len()))
+	}
+
+	cur := newCursorAtIndex(m.seq, idx)
+	entry := cur.current().(mapEntry)
+	return entry.key, entry.value
+}
+
 func (m Map) MaybeGet(key Value) (v Value, ok bool) {
 	cur := newCursorAtValue(m.seq, key, false, false)
 	if !cur.valid() {

go/types/map_test.go

@@ -1254,3 +1254,20 @@ func TestMapIterFrom(t *testing.T) {
 	assert.True(kvs[0:0].Equals(test(m1, Number(100), Number(1000))))
 	assert.True(kvs[50:60].Equals(test(m1, Number(0), Number(8))))
 }
+
+func TestMapAt(t *testing.T) {
+	assert := assert.New(t)
+
+	values := []Value{Bool(false), Number(42), String('a'), String('b'), String('c'), String('d')}
+	m := NewMap(values...)
+
+	for i := 0; i < len(values); i += 2 {
+		k, v := m.At(uint64(i / 2))
+		assert.Equal(values[i], k)
+		assert.Equal(values[i+1], v)
+	}
+
+	assert.Panics(func() {
+		m.At(42)
+	})
+}

go/types/set.go

@@ -5,6 +5,7 @@
 package types
 
 import (
+	"fmt"
 	"sort"
 
 	"github.com/attic-labs/noms/go/hash"
@@ -114,6 +115,15 @@ func (s Set) First() Value {
 	return cur.current().(Value)
 }
 
+func (s Set) At(idx uint64) Value {
+	if idx >= s.Len() {
+		panic(fmt.Errorf("Out of bounds: %s >= %s", idx, s.Len()))
+	}
+
+	cur := newCursorAtIndex(s.seq, idx)
+	return cur.current().(Value)
+}
+
 func (s Set) Insert(values ...Value) Set {
 	if len(values) == 0 {
 		return s

go/types/set_test.go

@@ -986,3 +986,18 @@ func TestSetRemoveLastWhenNotLoaded(t *testing.T) {
 		assert.True(ts.toSet().Equals(ns))
 	}
 }
+
+func TestSetAt(t *testing.T) {
+	assert := assert.New(t)
+
+	values := []Value{Bool(false), Number(42), String('a'), String('b'), String('c')}
+	s := NewSet(values...)
+
+	for i, v := range values {
+		assert.Equal(v, s.At(uint64(i)))
+	}
+
+	assert.Panics(func() {
+		s.At(42)
+	})
+}