interpres/decode.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package interpres

import (
	"fmt"
	"math"
	"reflect"
	"strings"
	"time"
)

// decoder maps a parsed TOML tree onto Go values via reflection.
type decoder struct {
	disallowUnknown bool
}

func newDecoder() *decoder { return &decoder{} }

var timeType = reflect.TypeFor[time.Time]()

func (d *decoder) decode(tree map[string]any, v any) error {
	rv := reflect.ValueOf(v)
	if rv.Kind() != reflect.Pointer || rv.IsNil() {
		return fmt.Errorf("interpres: decode target must be a non-nil pointer")
	}
	return d.assign(tree, rv.Elem())
}

// assign stores data into dst, converting between the TOML value kinds and the
// destination's Go type.
func (d *decoder) assign(data any, dst reflect.Value) error {
	if dst.Kind() == reflect.Pointer {
		if dst.IsNil() {
			dst.Set(reflect.New(dst.Type().Elem()))
		}
		return d.assign(data, dst.Elem())
	}

	// An any destination takes the value as-is.
	if dst.Kind() == reflect.Interface && dst.NumMethod() == 0 {
		dst.Set(reflect.ValueOf(data))
		return nil
	}

	// Types implementing Unmarshaler get the parsed data wholesale and are
	// responsible for setting their own state. The decoder does not consult
	// any return value; whatever the receiver stores is kept. The lookup
	// covers both T and *T so a pointer-receiver UnmarshalTOML method is
	// invoked on an addressable struct field.
	if dst.CanInterface() {
		u, ok := dst.Interface().(Unmarshaler)
		if !ok && dst.CanAddr() {
			u, ok = dst.Addr().Interface().(Unmarshaler)
		}
		if ok {
			if err := u.UnmarshalTOML(data); err != nil {
				return fmt.Errorf("unmarshal: %w", err)
			}
			return nil
		}
	}

	switch v := data.(type) {
	case map[string]any:
		return d.assignTable(v, dst)
	case []map[string]any:
		return d.assignTableSlice(v, dst)
	case []any:
		return d.assignSlice(v, dst)
	case string:
		return setBasic(dst, reflect.ValueOf(v), "string")
	case bool:
		return setBasic(dst, reflect.ValueOf(v), "bool")
	case int64:
		return setInt(dst, v)
	case float64:
		return setFloat(dst, v)
	case time.Time:
		if dst.Type() != timeType {
			return fmt.Errorf("interpres: cannot assign datetime to %s", dst.Type())
		}
		dst.Set(reflect.ValueOf(v))
		return nil
	default:
		rv := reflect.ValueOf(data)
		if rv.IsValid() && dst.Type() == rv.Type() {
			dst.Set(rv)
			return nil
		}
		return fmt.Errorf("interpres: cannot assign %T to %s", data, dst.Type())
	}
}

func (d *decoder) assignTable(tbl map[string]any, dst reflect.Value) error {
	switch dst.Kind() {
	case reflect.Struct:
		return d.assignStruct(tbl, dst)
	case reflect.Map:
		return d.assignMap(tbl, dst)
	default:
		return fmt.Errorf("interpres: cannot assign table to %s", dst.Type())
	}
}

func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
	fields := structFields(dst.Type())
	for key, val := range tbl {
		field, ok := fields[strings.ToLower(key)]
		if !ok {
			if d.disallowUnknown {
				return fmt.Errorf("interpres: unknown field %q for %s", key, dst.Type())
			}
			continue
		}
		if err := d.assign(val, dst.Field(field)); err != nil {
			return fmt.Errorf("%s: %w", key, err)
		}
	}
	return nil
}

func (d *decoder) assignMap(tbl map[string]any, dst reflect.Value) error {
	if dst.Type().Key().Kind() != reflect.String {
		return fmt.Errorf("interpres: map key must be a string, got %s", dst.Type().Key())
	}
	if dst.IsNil() {
		dst.Set(reflect.MakeMap(dst.Type()))
	}
	elemType := dst.Type().Elem()
	for key, val := range tbl {
		elem := reflect.New(elemType).Elem()
		if err := d.assign(val, elem); err != nil {
			return fmt.Errorf("%s: %w", key, err)
		}
		dst.SetMapIndex(reflect.ValueOf(key), elem)
	}
	return nil
}

func (d *decoder) assignSlice(items []any, dst reflect.Value) error {
	if dst.Kind() != reflect.Slice {
		return fmt.Errorf("interpres: cannot assign array to %s", dst.Type())
	}
	out := reflect.MakeSlice(dst.Type(), len(items), len(items))
	for i, item := range items {
		if err := d.assign(item, out.Index(i)); err != nil {
			return fmt.Errorf("[%d]: %w", i, err)
		}
	}
	dst.Set(out)
	return nil
}

func (d *decoder) assignTableSlice(items []map[string]any, dst reflect.Value) error {
	if dst.Kind() != reflect.Slice {
		return fmt.Errorf("interpres: cannot assign array of tables to %s", dst.Type())
	}
	out := reflect.MakeSlice(dst.Type(), len(items), len(items))
	for i, item := range items {
		if err := d.assign(item, out.Index(i)); err != nil {
			return fmt.Errorf("[%d]: %w", i, err)
		}
	}
	dst.Set(out)
	return nil
}

// --- low-level setters -----------------------------------------------------

func setBasic(dst, val reflect.Value, kind string) error {
	if dst.Kind() != val.Kind() {
		return fmt.Errorf("interpres: cannot assign %s to %s", kind, dst.Type())
	}
	dst.Set(val)
	return nil
}

func setInt(dst reflect.Value, v int64) error {
	switch dst.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		if dst.OverflowInt(v) {
			return fmt.Errorf("interpres: integer %d overflows %s", v, dst.Type())
		}
		dst.SetInt(v)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		if v < 0 {
			return fmt.Errorf("interpres: cannot assign negative %d to %s", v, dst.Type())
		}
		var max uint64
		switch dst.Kind() {
		case reflect.Uint8:
			max = math.MaxUint8
		case reflect.Uint16:
			max = math.MaxUint16
		case reflect.Uint32:
			max = math.MaxUint32
		}
		if max != 0 && uint64(v) > max {
			return fmt.Errorf("interpres: integer %d overflows %s", v, dst.Type())
		}
		dst.SetUint(uint64(v))
	case reflect.Float32, reflect.Float64:
		dst.SetFloat(float64(v))
	default:
		return fmt.Errorf("interpres: cannot assign integer to %s", dst.Type())
	}
	return nil
}

func setFloat(dst reflect.Value, v float64) error {
	switch dst.Kind() {
	case reflect.Float32, reflect.Float64:
		dst.SetFloat(v)
		return nil
	default:
		return fmt.Errorf("interpres: cannot assign float to %s", dst.Type())
	}
}

// structFields builds a lower-cased lookup of field name → field index for the
// exported fields of t, honouring `toml:"name"` tags.
func structFields(t reflect.Type) map[string]int {
	fields := make(map[string]int, t.NumField())
	for i := range t.NumField() {
		f := t.Field(i)
		if f.PkgPath != "" { // unexported
			continue
		}
		name := f.Name
		if tag, ok := f.Tag.Lookup("toml"); ok {
			tag, _, _ = strings.Cut(tag, ",")
			if tag == "-" {
				continue
			}
			if tag != "" {
				name = tag
			}
		}
		fields[strings.ToLower(name)] = i
	}
	return fields
}