1 files changed, 281 insertions, 0 deletions
diff --git a/starlarkstruct/struct.go b/starlarkstruct/struct.go
new file mode 100644
index 0000000..1982cc0
--- /dev/null
+++ b/starlarkstruct/struct.go
@@ -0,0 +1,281 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package starlarkstruct defines the Starlark types 'struct' and
+// 'module', both optional language extensions.
+//
+package starlarkstruct // import "go.starlark.net/starlarkstruct"
+
+// It is tempting to introduce a variant of Struct that is a wrapper
+// around a Go struct value, for stronger typing guarantees and more
+// efficient and convenient field lookup. However:
+// 1) all fields of Starlark structs are optional, so we cannot represent
+//    them using more specific types such as String, Int, *Depset, and
+//    *File, as such types give no way to represent missing fields.
+// 2) the efficiency gain of direct struct field access is rather
+//    marginal: finding the index of a field by binary searching on the
+//    sorted list of field names is quite fast compared to the other
+//    overheads.
+// 3) the gains in compactness and spatial locality are also rather
+//    marginal: the array behind the []entry slice is (due to field name
+//    strings) only a factor of 2 larger than the corresponding Go struct
+//    would be, and, like the Go struct, requires only a single allocation.
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+
+	"go.starlark.net/starlark"
+	"go.starlark.net/syntax"
+)
+
+// Make is the implementation of a built-in function that instantiates
+// an immutable struct from the specified keyword arguments.
+//
+// An application can add 'struct' to the Starlark environment like so:
+//
+// 	globals := starlark.StringDict{
+// 		"struct":  starlark.NewBuiltin("struct", starlarkstruct.Make),
+// 	}
+//
+func Make(_ *starlark.Thread, _ *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+	if len(args) > 0 {
+		return nil, fmt.Errorf("struct: unexpected positional arguments")
+	}
+	return FromKeywords(Default, kwargs), nil
+}
+
+// FromKeywords returns a new struct instance whose fields are specified by the
+// key/value pairs in kwargs.  (Each kwargs[i][0] must be a starlark.String.)
+func FromKeywords(constructor starlark.Value, kwargs []starlark.Tuple) *Struct {
+	if constructor == nil {
+		panic("nil constructor")
+	}
+	s := &Struct{
+		constructor: constructor,
+		entries:     make(entries, 0, len(kwargs)),
+	}
+	for _, kwarg := range kwargs {
+		k := string(kwarg[0].(starlark.String))
+		v := kwarg[1]
+		s.entries = append(s.entries, entry{k, v})
+	}
+	sort.Sort(s.entries)
+	return s
+}
+
+// FromStringDict returns a whose elements are those of d.
+// The constructor parameter specifies the constructor; use Default for an ordinary struct.
+func FromStringDict(constructor starlark.Value, d starlark.StringDict) *Struct {
+	if constructor == nil {
+		panic("nil constructor")
+	}
+	s := &Struct{
+		constructor: constructor,
+		entries:     make(entries, 0, len(d)),
+	}
+	for k, v := range d {
+		s.entries = append(s.entries, entry{k, v})
+	}
+	sort.Sort(s.entries)
+	return s
+}
+
+// Struct is an immutable Starlark type that maps field names to values.
+// It is not iterable and does not support len.
+//
+// A struct has a constructor, a distinct value that identifies a class
+// of structs, and which appears in the struct's string representation.
+//
+// Operations such as x+y fail if the constructors of the two operands
+// are not equal.
+//
+// The default constructor, Default, is the string "struct", but
+// clients may wish to 'brand' structs for their own purposes.
+// The constructor value appears in the printed form of the value,
+// and is accessible using the Constructor method.
+//
+// Use Attr to access its fields and AttrNames to enumerate them.
+type Struct struct {
+	constructor starlark.Value
+	entries     entries // sorted by name
+}
+
+// Default is the default constructor for structs.
+// It is merely the string "struct".
+const Default = starlark.String("struct")
+
+type entries []entry
+
+func (a entries) Len() int           { return len(a) }
+func (a entries) Less(i, j int) bool { return a[i].name < a[j].name }
+func (a entries) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+type entry struct {
+	name  string
+	value starlark.Value
+}
+
+var (
+	_ starlark.HasAttrs  = (*Struct)(nil)
+	_ starlark.HasBinary = (*Struct)(nil)
+)
+
+// ToStringDict adds a name/value entry to d for each field of the struct.
+func (s *Struct) ToStringDict(d starlark.StringDict) {
+	for _, e := range s.entries {
+		d[e.name] = e.value
+	}
+}
+
+func (s *Struct) String() string {
+	buf := new(strings.Builder)
+	if s.constructor == Default {
+		// NB: The Java implementation always prints struct
+		// even for Bazel provider instances.
+		buf.WriteString("struct") // avoid String()'s quotation
+	} else {
+		buf.WriteString(s.constructor.String())
+	}
+	buf.WriteByte('(')
+	for i, e := range s.entries {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		buf.WriteString(e.name)
+		buf.WriteString(" = ")
+		buf.WriteString(e.value.String())
+	}
+	buf.WriteByte(')')
+	return buf.String()
+}
+
+// Constructor returns the constructor used to create this struct.
+func (s *Struct) Constructor() starlark.Value { return s.constructor }
+
+func (s *Struct) Type() string         { return "struct" }
+func (s *Struct) Truth() starlark.Bool { return true } // even when empty
+func (s *Struct) Hash() (uint32, error) {
+	// Same algorithm as Tuple.hash, but with different primes.
+	var x, m uint32 = 8731, 9839
+	for _, e := range s.entries {
+		namehash, _ := starlark.String(e.name).Hash()
+		x = x ^ 3*namehash
+		y, err := e.value.Hash()
+		if err != nil {
+			return 0, err
+		}
+		x = x ^ y*m
+		m += 7349
+	}
+	return x, nil
+}
+func (s *Struct) Freeze() {
+	for _, e := range s.entries {
+		e.value.Freeze()
+	}
+}
+
+func (x *Struct) Binary(op syntax.Token, y starlark.Value, side starlark.Side) (starlark.Value, error) {
+	if y, ok := y.(*Struct); ok && op == syntax.PLUS {
+		if side == starlark.Right {
+			x, y = y, x
+		}
+
+		if eq, err := starlark.Equal(x.constructor, y.constructor); err != nil {
+			return nil, fmt.Errorf("in %s + %s: error comparing constructors: %v",
+				x.constructor, y.constructor, err)
+		} else if !eq {
+			return nil, fmt.Errorf("cannot add structs of different constructors: %s + %s",
+				x.constructor, y.constructor)
+		}
+
+		z := make(starlark.StringDict, x.len()+y.len())
+		for _, e := range x.entries {
+			z[e.name] = e.value
+		}
+		for _, e := range y.entries {
+			z[e.name] = e.value
+		}
+
+		return FromStringDict(x.constructor, z), nil
+	}
+	return nil, nil // unhandled
+}
+
+// Attr returns the value of the specified field.
+func (s *Struct) Attr(name string) (starlark.Value, error) {
+	// Binary search the entries.
+	// This implementation is a specialization of
+	// sort.Search that avoids dynamic dispatch.
+	n := len(s.entries)
+	i, j := 0, n
+	for i < j {
+		h := int(uint(i+j) >> 1)
+		if s.entries[h].name < name {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	if i < n && s.entries[i].name == name {
+		return s.entries[i].value, nil
+	}
+
+	var ctor string
+	if s.constructor != Default {
+		ctor = s.constructor.String() + " "
+	}
+	return nil, starlark.NoSuchAttrError(
+		fmt.Sprintf("%sstruct has no .%s attribute", ctor, name))
+}
+
+func (s *Struct) len() int { return len(s.entries) }
+
+// AttrNames returns a new sorted list of the struct fields.
+func (s *Struct) AttrNames() []string {
+	names := make([]string, len(s.entries))
+	for i, e := range s.entries {
+		names[i] = e.name
+	}
+	return names
+}
+
+func (x *Struct) CompareSameType(op syntax.Token, y_ starlark.Value, depth int) (bool, error) {
+	y := y_.(*Struct)
+	switch op {
+	case syntax.EQL:
+		return structsEqual(x, y, depth)
+	case syntax.NEQ:
+		eq, err := structsEqual(x, y, depth)
+		return !eq, err
+	default:
+		return false, fmt.Errorf("%s %s %s not implemented", x.Type(), op, y.Type())
+	}
+}
+
+func structsEqual(x, y *Struct, depth int) (bool, error) {
+	if x.len() != y.len() {
+		return false, nil
+	}
+
+	if eq, err := starlark.Equal(x.constructor, y.constructor); err != nil {
+		return false, fmt.Errorf("error comparing struct constructors %v and %v: %v",
+			x.constructor, y.constructor, err)
+	} else if !eq {
+		return false, nil
+	}
+
+	for i, n := 0, x.len(); i < n; i++ {
+		if x.entries[i].name != y.entries[i].name {
+			return false, nil
+		} else if eq, err := starlark.EqualDepth(x.entries[i].value, y.entries[i].value, depth-1); err != nil {
+			return false, err
+		} else if !eq {
+			return false, nil
+		}
+	}
+	return true, nil
+}