Add packing for dynamic array and slice types (#18051)

* added tests for new abi encoding features (#4)

* added tests from bytes32[][] and string[]

* added offset to other types

* formatting

* Abi/dynamic types (#5)

* Revert "Abi/dynamic types (#5)" (#6)

This reverts commit dabca31d797623d43bd780721fc0ad461d24be53.

* Abi/dynamic types (#7)

* some cleanup

* Apply suggestions from code review

apply suggestions

Co-Authored-By: vedhavyas <vedhavyas.singareddi@gmail.com>

* added better formatting (#8)

* review chnages

* better comments

3 files changed, 117 insertions, 22 deletions

diff --git a/accounts/abi/argument.go b/accounts/abi/argument.go
index 93b513c34..f544c80db 100644
--- a/accounts/abi/argument.go
+++ b/accounts/abi/argument.go
@@ -243,11 +243,7 @@ func (arguments Arguments) Pack(args ...interface{}) ([]byte, error) {
 	// input offset is the bytes offset for packed output
 	inputOffset := 0
 	for _, abiArg := range abiArgs {
-		if abiArg.Type.T == ArrayTy {
-			inputOffset += 32 * abiArg.Type.Size
-		} else {
-			inputOffset += 32
-		}
+		inputOffset += getDynamicTypeOffset(abiArg.Type)
 	}
 	var ret []byte
 	for i, a := range args {
@@ -257,14 +253,13 @@ func (arguments Arguments) Pack(args ...interface{}) ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
-		// check for a slice type (string, bytes, slice)
-		if input.Type.requiresLengthPrefix() {
-			// calculate the offset
-			offset := inputOffset + len(variableInput)
+		// check for dynamic types
+		if isDynamicType(input.Type) {
 			// set the offset
-			ret = append(ret, packNum(reflect.ValueOf(offset))...)
-			// Append the packed output to the variable input. The variable input
-			// will be appended at the end of the input.
+			ret = append(ret, packNum(reflect.ValueOf(inputOffset))...)
+			// calculate next offset
+			inputOffset += len(packed)
+			// append to variable input
 			variableInput = append(variableInput, packed...)
 		} else {
 			// append the packed value to the input
diff --git a/accounts/abi/pack_test.go b/accounts/abi/pack_test.go
index 58a5b7a58..ddd2b7362 100644
--- a/accounts/abi/pack_test.go
+++ b/accounts/abi/pack_test.go
@@ -324,6 +324,66 @@ func TestPack(t *testing.T) {
 			"foobar",
 			common.Hex2Bytes("0000000000000000000000000000000000000000000000000000000000000006666f6f6261720000000000000000000000000000000000000000000000000000"),
 		},
+		{
+			"string[]",
+			[]string{"hello", "foobar"},
+			common.Hex2Bytes("0000000000000000000000000000000000000000000000000000000000000002" + // len(array) = 2
+				"0000000000000000000000000000000000000000000000000000000000000040" + // offset 64 to i = 0
+				"0000000000000000000000000000000000000000000000000000000000000080" + // offset 128 to i = 1
+				"0000000000000000000000000000000000000000000000000000000000000005" + // len(str[0]) = 5
+				"68656c6c6f000000000000000000000000000000000000000000000000000000" + // str[0]
+				"0000000000000000000000000000000000000000000000000000000000000006" + // len(str[1]) = 6
+				"666f6f6261720000000000000000000000000000000000000000000000000000"), // str[1]
+		},
+		{
+			"string[2]",
+			[]string{"hello", "foobar"},
+			common.Hex2Bytes("0000000000000000000000000000000000000000000000000000000000000040" + // offset to i = 0
+				"0000000000000000000000000000000000000000000000000000000000000080" + // offset to i = 1
+				"0000000000000000000000000000000000000000000000000000000000000005" + // len(str[0]) = 5
+				"68656c6c6f000000000000000000000000000000000000000000000000000000" + // str[0]
+				"0000000000000000000000000000000000000000000000000000000000000006" + // len(str[1]) = 6
+				"666f6f6261720000000000000000000000000000000000000000000000000000"), // str[1]
+		},
+		{
+			"bytes32[][]",
+			[][]common.Hash{{{1}, {2}}, {{3}, {4}, {5}}},
+			common.Hex2Bytes("0000000000000000000000000000000000000000000000000000000000000002" + // len(array) = 2
+				"0000000000000000000000000000000000000000000000000000000000000040" + // offset 64 to i = 0
+				"00000000000000000000000000000000000000000000000000000000000000a0" + // offset 160 to i = 1
+				"0000000000000000000000000000000000000000000000000000000000000002" + // len(array[0]) = 2
+				"0100000000000000000000000000000000000000000000000000000000000000" + // array[0][0]
+				"0200000000000000000000000000000000000000000000000000000000000000" + // array[0][1]
+				"0000000000000000000000000000000000000000000000000000000000000003" + // len(array[1]) = 3
+				"0300000000000000000000000000000000000000000000000000000000000000" + // array[1][0]
+				"0400000000000000000000000000000000000000000000000000000000000000" + // array[1][1]
+				"0500000000000000000000000000000000000000000000000000000000000000"), // array[1][2]
+		},
+
+		{
+			"bytes32[][2]",
+			[][]common.Hash{{{1}, {2}}, {{3}, {4}, {5}}},
+			common.Hex2Bytes("0000000000000000000000000000000000000000000000000000000000000040" + // offset 64 to i = 0
+				"00000000000000000000000000000000000000000000000000000000000000a0" + // offset 160 to i = 1
+				"0000000000000000000000000000000000000000000000000000000000000002" + // len(array[0]) = 2
+				"0100000000000000000000000000000000000000000000000000000000000000" + // array[0][0]
+				"0200000000000000000000000000000000000000000000000000000000000000" + // array[0][1]
+				"0000000000000000000000000000000000000000000000000000000000000003" + // len(array[1]) = 3
+				"0300000000000000000000000000000000000000000000000000000000000000" + // array[1][0]
+				"0400000000000000000000000000000000000000000000000000000000000000" + // array[1][1]
+				"0500000000000000000000000000000000000000000000000000000000000000"), // array[1][2]
+		},
+
+		{
+			"bytes32[3][2]",
+			[][]common.Hash{{{1}, {2}, {3}}, {{3}, {4}, {5}}},
+			common.Hex2Bytes("0100000000000000000000000000000000000000000000000000000000000000" + // array[0][0]
+				"0200000000000000000000000000000000000000000000000000000000000000" + // array[0][1]
+				"0300000000000000000000000000000000000000000000000000000000000000" + // array[0][2]
+				"0300000000000000000000000000000000000000000000000000000000000000" + // array[1][0]
+				"0400000000000000000000000000000000000000000000000000000000000000" + // array[1][1]
+				"0500000000000000000000000000000000000000000000000000000000000000"), // array[1][2]
+		},
 	} {
 		typ, err := NewType(test.typ)
 		if err != nil {
@@ -336,7 +396,7 @@ func TestPack(t *testing.T) {
 		}
 
 		if !bytes.Equal(output, test.output) {
-			t.Errorf("%d failed. Expected bytes: '%x' Got: '%x'", i, test.output, output)
+			t.Errorf("input %d for typ: %v failed. Expected bytes: '%x' Got: '%x'", i, typ.String(), test.output, output)
 		}
 	}
 }
diff --git a/accounts/abi/type.go b/accounts/abi/type.go
index dce89d2b4..6bfaabf5a 100644
--- a/accounts/abi/type.go
+++ b/accounts/abi/type.go
@@ -183,23 +183,39 @@ func (t Type) pack(v reflect.Value) ([]byte, error) {
 		return nil, err
 	}
 
-	if t.T == SliceTy || t.T == ArrayTy {
-		var packed []byte
+	switch t.T {
+	case SliceTy, ArrayTy:
+		var ret []byte
 
+		if t.requiresLengthPrefix() {
+			// append length
+			ret = append(ret, packNum(reflect.ValueOf(v.Len()))...)
+		}
+
+		// calculate offset if any
+		offset := 0
+		offsetReq := isDynamicType(*t.Elem)
+		if offsetReq {
+			offset = getDynamicTypeOffset(*t.Elem) * v.Len()
+		}
+		var tail []byte
 		for i := 0; i < v.Len(); i++ {
 			val, err := t.Elem.pack(v.Index(i))
 			if err != nil {
 				return nil, err
 			}
-			packed = append(packed, val...)
-		}
-		if t.T == SliceTy {
-			return packBytesSlice(packed, v.Len()), nil
-		} else if t.T == ArrayTy {
-			return packed, nil
+			if !offsetReq {
+				ret = append(ret, val...)
+				continue
+			}
+			ret = append(ret, packNum(reflect.ValueOf(offset))...)
+			offset += len(val)
+			tail = append(tail, val...)
 		}
+		return append(ret, tail...), nil
+	default:
+		return packElement(t, v), nil
 	}
-	return packElement(t, v), nil
 }
 
 // requireLengthPrefix returns whether the type requires any sort of length
@@ -207,3 +223,27 @@ func (t Type) pack(v reflect.Value) ([]byte, error) {
 func (t Type) requiresLengthPrefix() bool {
 	return t.T == StringTy || t.T == BytesTy || t.T == SliceTy
 }
+
+// isDynamicType returns true if the type is dynamic.
+// StringTy, BytesTy, and SliceTy(irrespective of slice element type) are dynamic types
+// ArrayTy is considered dynamic if and only if the Array element is a dynamic type.
+// This function recursively checks the type for slice and array elements.
+func isDynamicType(t Type) bool {
+	// dynamic types
+	// array is also a dynamic type if the array type is dynamic
+	return t.T == StringTy || t.T == BytesTy || t.T == SliceTy || (t.T == ArrayTy && isDynamicType(*t.Elem))
+}
+
+// getDynamicTypeOffset returns the offset for the type.
+// See `isDynamicType` to know which types are considered dynamic.
+// If the type t is an array and element type is not a dynamic type, then we consider it a static type and
+// return 32 * size of array since length prefix is not required.
+// If t is a dynamic type or element type(for slices and arrays) is dynamic, then we simply return 32 as offset.
+func getDynamicTypeOffset(t Type) int {
+	// if it is an array and there are no dynamic types
+	// then the array is static type
+	if t.T == ArrayTy && !isDynamicType(*t.Elem) {
+		return 32 * t.Size
+	}
+	return 32
+}