feat: mapping slice of complex struct (#312)

* feat: mapping slice of complex struct #298

* feat: support predefined values and pre initialized structs

* refactor: some improvement

* refactor: some improvement

* test: support normal features for nested fields

* test: trying to fix `gofumpt` lint issues

* chore: add sample for complex struct in readme

---------

Co-authored-by: Hamid Reza Ranjbar <hamidreza.ranjbar@snapp.cab>

README.md

@@ -476,6 +476,61 @@ func main() {
 }
 ```
 
+### Complex objects inside array (slice)
+
+You can set sub-struct field values inside a slice by naming the environment variables with sequential numbers starting from 0 (without omitting numbers in between) and an underscore.
+It is possible to use prefix tag too.
+
+Here's an example with and without prefix tag:
+
+```go
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/caarlos0/env/v11"
+)
+
+type Test struct {
+	Str string `env:"STR"`
+	Num int    `env:"NUM"`
+}
+type ComplexConfig struct {
+	Baz []Test  `env:",init"`
+	Bar []Test  `envPrefix:"BAR"`
+	Foo *[]Test `envPrefix:"FOO_"`
+}
+
+func main() {
+	cfg := &ComplexConfig{}
+	opts := env.Options{
+		Environment: map[string]string{
+			"0_STR":     "bt",
+			"1_NUM":     "10",
+
+			"FOO_0_STR": "b0t",
+			"FOO_1_STR": "b1t",
+			"FOO_1_NUM": "212",
+
+			"BAR_0_STR": "f0t",
+			"BAR_0_NUM": "101",
+			"BAR_1_STR": "f1t",
+			"BAR_1_NUM": "111",
+		},
+	}
+
+	// Load env vars.
+	if err := env.ParseWithOptions(cfg, opts); err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the loaded data.
+	fmt.Printf("%+v\n", cfg)
+}
+```
+
 ### On set hooks
 
 You might want to listen to value sets and, for example, log something or do

env.go

@@ -168,6 +168,19 @@ func customOptions(opt Options) Options {
 	return opt
 }
 
+func optionsWithSliceEnvPrefix(opts Options, index int) Options {
+	return Options{
+		Environment:           opts.Environment,
+		TagName:               opts.TagName,
+		RequiredIfNoDef:       opts.RequiredIfNoDef,
+		OnSet:                 opts.OnSet,
+		Prefix:                fmt.Sprintf("%s%d_", opts.Prefix, index),
+		UseFieldNameByDefault: opts.UseFieldNameByDefault,
+		FuncMap:               opts.FuncMap,
+		rawEnvVars:            opts.rawEnvVars,
+	}
+}
+
 func optionsWithEnvPrefix(field reflect.StructField, opts Options) Options {
 	return Options{
 		Environment:           opts.Environment,
@@ -313,6 +326,104 @@ func doParseField(refField reflect.Value, refTypeField reflect.StructField, proc
 		return doParse(refField, processField, optionsWithEnvPrefix(refTypeField, opts))
 	}
 
+	if isSliceOfStructs(refTypeField, opts) {
+		return doParseSlice(refField, processField, optionsWithEnvPrefix(refTypeField, opts))
+	}
+
+	return nil
+}
+
+func isSliceOfStructs(refTypeField reflect.StructField, opts Options) bool {
+	field := refTypeField.Type
+	if reflect.Ptr == field.Kind() {
+		field = field.Elem()
+	}
+
+	if reflect.Slice != field.Kind() {
+		return false
+	}
+
+	field = field.Elem()
+
+	if reflect.Ptr == field.Kind() {
+		field = field.Elem()
+	}
+
+	_, ignore := defaultBuiltInParsers[field.Kind()]
+
+	if !ignore {
+		_, ignore = opts.FuncMap[field]
+	}
+
+	if !ignore {
+		_, ignore = reflect.New(field).Interface().(encoding.TextUnmarshaler)
+	}
+
+	if !ignore {
+		ignore = reflect.Struct != field.Kind()
+	}
+	return !ignore
+}
+
+func doParseSlice(ref reflect.Value, processField processFieldFn, opts Options) error {
+	if opts.Prefix != "" && !strings.HasSuffix(opts.Prefix, string(underscore)) {
+		opts.Prefix += string(underscore)
+	}
+
+	var environments []string
+	for environment := range opts.Environment {
+		if strings.HasPrefix(environment, opts.Prefix) {
+			environments = append(environments, environment)
+		}
+	}
+
+	if len(environments) > 0 {
+		counter := 0
+		for finished := false; !finished; {
+			finished = true
+			prefix := fmt.Sprintf("%s%d%c", opts.Prefix, counter, underscore)
+			for _, variable := range environments {
+				if strings.HasPrefix(variable, prefix) {
+					counter++
+					finished = false
+					break
+				}
+			}
+		}
+
+		sliceType := ref.Type()
+		var initialized int
+		if reflect.Ptr == ref.Kind() {
+			sliceType = sliceType.Elem()
+			// Due to the rest of code the pre-initialized slice has no chance for this situation
+			initialized = 0
+		} else {
+			initialized = ref.Len()
+		}
+
+		var capacity int
+		if capacity = initialized; counter > initialized {
+			capacity = counter
+		}
+		result := reflect.MakeSlice(sliceType, capacity, capacity)
+		for i := 0; i < capacity; i++ {
+			item := result.Index(i)
+			if i < initialized {
+				item.Set(ref.Index(i))
+			}
+			if err := doParse(item, processField, optionsWithSliceEnvPrefix(opts, i)); err != nil {
+				return err
+			}
+		}
+
+		if reflect.Ptr == ref.Kind() {
+			resultPtr := reflect.New(sliceType)
+			resultPtr.Elem().Set(result)
+			result = resultPtr
+		}
+		ref.Set(result)
+	}
+
 	return nil
 }
 

env_test.go

@@ -2151,3 +2151,67 @@ func TestMultipleTagOptions(t *testing.T) {
 		isEqual(t, "", os.Getenv("URL"))
 	})
 }
+
+func TestIssue298(t *testing.T) {
+	type Test struct {
+		Str string `env:"STR"`
+		Num int    `env:"NUM"`
+	}
+	type ComplexConfig struct {
+		Foo *[]Test `envPrefix:"FOO_"`
+		Bar []Test  `envPrefix:"BAR"`
+		Baz []Test  `env:",init"`
+	}
+
+	t.Setenv("FOO_0_STR", "f0t")
+	t.Setenv("FOO_0_NUM", "101")
+	t.Setenv("FOO_1_STR", "f1t")
+	t.Setenv("FOO_1_NUM", "111")
+
+	t.Setenv("BAR_0_STR", "b0t")
+	// t.Setenv("BAR_0_NUM", "202") // Not overridden
+	t.Setenv("BAR_1_STR", "b1t")
+	t.Setenv("BAR_1_NUM", "212")
+
+	t.Setenv("0_STR", "bt")
+	t.Setenv("1_NUM", "10")
+
+	sample := make([]Test, 1)
+	sample[0].Str = "overridden text"
+	sample[0].Num = 99999999
+	cfg := ComplexConfig{Bar: sample}
+
+	isNoErr(t, Parse(&cfg))
+
+	isEqual(t, "f0t", (*cfg.Foo)[0].Str)
+	isEqual(t, 101, (*cfg.Foo)[0].Num)
+	isEqual(t, "f1t", (*cfg.Foo)[1].Str)
+	isEqual(t, 111, (*cfg.Foo)[1].Num)
+
+	isEqual(t, "b0t", cfg.Bar[0].Str)
+	isEqual(t, 99999999, cfg.Bar[0].Num)
+	isEqual(t, "b1t", cfg.Bar[1].Str)
+	isEqual(t, 212, cfg.Bar[1].Num)
+
+	isEqual(t, "bt", cfg.Baz[0].Str)
+	isEqual(t, 0, cfg.Baz[0].Num)
+	isEqual(t, "", cfg.Baz[1].Str)
+	isEqual(t, 10, cfg.Baz[1].Num)
+}
+
+func TestIssue298ErrorNestedFieldRequiredNotSet(t *testing.T) {
+	type Test struct {
+		Str string `env:"STR,required"`
+		Num int    `env:"NUM"`
+	}
+	type ComplexConfig struct {
+		Foo *[]Test `envPrefix:"FOO"`
+	}
+
+	t.Setenv("FOO_0_NUM", "101")
+
+	cfg := ComplexConfig{}
+	err := Parse(&cfg)
+	isErrorWithMessage(t, err, `env: required environment variable "FOO_0_STR" is not set`)
+	isTrue(t, errors.Is(err, EnvVarIsNotSetError{}))
+}