From 63409ef59a63c49fcfd71c0c0cf723fd7aa3a14b Mon Sep 17 00:00:00 2001
From: Corin Lawson <corin@responsight.com>
Date: Fri, 19 Oct 2018 09:36:21 +1100
Subject: [PATCH] Ensure that gen.Struct is reproducible for a seed

Prior to this change, `gen.Struct` produced a value by ranging over
items in a map, which in turn draws values from the `gopter.Gen`s from
the items, which in turn presumedly makes calls to the PRNG in the
`gopter.GenParameters`.  Ranging over items in a map is intentionally
non-deterministic, therefore the calls to the PRNG is also
non-deterministic.

This change extracts the keys from the map, sorts them and then ranges
over the keys.  This ensures that the calls to PRNG is deterministic.

fixes #44

Signed-off-by: Corin Lawson <corin@responsight.com>
---
 gen/struct.go      | 11 ++++++++++-
 gen/struct_test.go | 48 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/gen/struct.go b/gen/struct.go
index d57a829..e2f22b8 100644
--- a/gen/struct.go
+++ b/gen/struct.go
@@ -2,6 +2,7 @@ package gen
 
 import (
 	"reflect"
+	"sort"
 
 	"github.com/leanovate/gopter"
 )
@@ -20,7 +21,15 @@ func Struct(rt reflect.Type, gens map[string]gopter.Gen) gopter.Gen {
 	return func(genParams *gopter.GenParameters) *gopter.GenResult {
 		result := reflect.New(rt)
 
-		for name, gen := range gens {
+		names := make([]string, len(gens))
+		i := 0
+		for name := range gens {
+			names[i] = name
+			i++
+		}
+		sort.Strings(names)
+		for _, name := range names {
+			gen := gens[name]
 			field, ok := rt.FieldByName(name)
 			if !ok {
 				continue
diff --git a/gen/struct_test.go b/gen/struct_test.go
index 62ea546..516f09f 100644
--- a/gen/struct_test.go
+++ b/gen/struct_test.go
@@ -35,6 +35,54 @@ func TestStruct(t *testing.T) {
 	}
 }
 
+func TestStructDeterminism(t *testing.T) {
+	structGen := gen.Struct(reflect.TypeOf(&testStruct{}), map[string]gopter.Gen{
+		"Value1": gen.Identifier(),
+		"Value2": gen.Int64(),
+		"Value3": gen.SliceOf(gen.Int8()),
+	})
+	for i := 0; i < 100; i++ {
+		parameters := gopter.DefaultGenParameters().CloneWithSeed(1234)
+		for _, expected := range []testStruct{
+			testStruct{
+				Value1: "hUeNzDbtiF4xxkidfvLaiczgpwsqfyvbbuhrjjoez4jtewulIKwzMguttazo3qwi5ufIfi6izpqT4evzrmgtmk1gQo",
+				Value2: -2282921689139609493,
+				Value3: []int8{-93, -96, -23, -58, 65, -108, 56, 63, -64, 26, -69, 62, 61, -93, -107, 52, -95},
+			},
+			testStruct{
+				Value1: "ubJrJEawwnoh63jv1lxd7xhtaqqrEnjawudgiixhhkw6sdmqdgxbabyoxcoE0uviwDupccvYvxcqOv0z8opjk",
+				Value2: -1611599231975617329,
+				Value3: []int8{15, -41, -106, 37, 3, 76, -65, -87, 113, -115, 76, 61, 41, 65, 11, -90, -4, 43, 110, -121, 65, 112, -128, 51, -86, 50, 30, 33, -73, -88, 94, 101, 63, -113, 45, 110, 46, 21, 115, 78, -58, 47, -110, 7, -14, -18, 2, -26, 63, -33, 77, 82, -52, -57, -105},
+			},
+			testStruct{
+				Value1: "axnbggD6Hgsxyxd6ZwcZ4Bn1uM7hzd0azvsuLvj3wvfvoramjcltivmditt5qhmHYfn0egagcFpuAffzaWxvalEaniojczez",
+				Value2: -345052727922296584,
+				Value3: []int8{-61, 94, 67, 9, 39, 119, 23, 1, 57, -66, 57, -94, 38, -122, 16, 82, -119, 21, -74, -66, -111, 55, -96, 8, -79, 13, -41, 124, 71, -63, 56, 16, 62, 55, -13, -35, -27, 68, -82, 22, -63, -76, 96, 60, -89, -10, -65, -102, -97, 45, 124, 117, -37, 21, 58, -87, 116, 60, -111, 27, 102, -102, -81, -123, -86, -95},
+			},
+			testStruct{
+				Value1: "lr",
+				Value2: -6442088894944465291,
+				Value3: []int8{-20, 38, 25, -76, -110, -98, -61, 65, -14, 52, -47, 22, -90},
+			},
+			testStruct{
+				Value1: "un23mggozHs4txZtydz6mIBymnxjxklkjyNzf",
+				Value2: -26686468742269553,
+				Value3: []int8{78, 91, -22, -126, -93, 35, -14, 67, -97, 13, -25, 73, -111, 26, 14, -67, 50, -23, -15, -63, -40, -103, 126, 60, -63, -83, -126, 64, 52, -50, 86, -25, -1, 108, 7, 62, 79, 89, 45, -73, 52, -7, -85, -111, -120, -21, 116, -8, -22, 34, 85, 36, 124, 12, -111, -114, -115, 91, -94, 82, -3, -46, 94, -73, 62, -117, -7, 84, -94, 13, 71, -5, 21, 32, 106, -44, 46},
+			},
+		} {
+			value, ok := structGen(parameters).Retrieve()
+
+			if !ok {
+				t.Errorf("Invalid value: %#v", value)
+			}
+			v, ok := value.(testStruct)
+			if !reflect.DeepEqual(expected, v) {
+				t.Errorf("Invalid value: %#v; expected: %#v", v, expected)
+			}
+		}
+	}
+}
+
 func TestStructPropageEmpty(t *testing.T) {
 	fail := gen.Struct(reflect.TypeOf(&testStruct{}), map[string]gopter.Gen{
 		"Value1": gen.Identifier().SuchThat(func(str string) bool {