interpres/decode.go

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

package interpres

import (
	"fmt"
	"reflect"
	"slices"
	"strings"
	"sync"
	"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 {
	schema := cachedStructSchema(dst.Type())
	if d.disallowUnknown {
		// Map iteration order is random, so pick the unknown key to report
		// deterministically: the smallest one.
		unknown := ""
		for key := range tbl {
			if _, ok := schema.byName[strings.ToLower(key)]; ok {
				continue
			}
			if unknown == "" || key < unknown {
				unknown = key
			}
		}
		if unknown != "" {
			return fmt.Errorf("interpres: unknown field %q for %s", unknown, dst.Type())
		}
	}
	for key, val := range tbl {
		field, ok := schema.byName[strings.ToLower(key)]
		if !ok {
			if schema.embedMaps != nil {
				// Leftover keys land in an untagged embedded map, the inverse
				// of the encoder inlining that map's entries.
				mv, err := fieldByIndex(dst, schema.embedMaps[0])
				if err != nil {
					return newDecodeError(key, err)
				}
				if err := d.assignMap(map[string]any{key: val}, mv); err != nil {
					return newDecodeError(key, err)
				}
			}
			continue
		}
		fv, err := fieldByIndex(dst, field.index)
		if err != nil {
			return newDecodeError(key, err)
		}
		if err := d.assign(val, fv); err != nil {
			return newDecodeError(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 newDecodeError(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 newDecodeError(fmt.Sprintf("[%d]", 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 newDecodeError(fmt.Sprintf("[%d]", 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())
		}
		// OverflowUint knows every width, uint included on platforms where it
		// is narrower than uint64; SetUint would silently truncate instead.
		if dst.OverflowUint(uint64(v)) {
			return fmt.Errorf("interpres: integer %d overflows %s", v, dst.Type())
		}
		dst.SetUint(uint64(v))
	case reflect.Float32, reflect.Float64:
		// A finite value beyond the float32 range would silently become ±Inf;
		// infinities and NaN themselves pass through. An int64 never
		// overflows either float width.
		f := float64(v)
		if dst.OverflowFloat(f) {
			return fmt.Errorf("interpres: integer %d overflows %s", v, dst.Type())
		}
		dst.SetFloat(f)
	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:
		if dst.OverflowFloat(v) {
			return fmt.Errorf("interpres: float %g overflows %s", v, dst.Type())
		}
		dst.SetFloat(v)
		return nil
	default:
		return fmt.Errorf("interpres: cannot assign float to %s", dst.Type())
	}
}

// structFieldLoc locates one destination field by its index path from the
// struct root and by the depth the field sits at, which breaks name clashes
// in favour of the shallower field.
type structFieldLoc struct {
	index []int
	depth int
}

// structSchema flattens the exported fields of t for decode, mirroring the
// encoder: an untagged embedded struct is inlined, so its own fields match
// keys of the same table, and an untagged embedded map is recorded in
// embedMaps (first declaration first) as the destination for leftover keys.
// When two fields resolve to one name, the shallower wins, then the later
// declaration.
type structSchema struct {
	byName    map[string]structFieldLoc
	embedMaps [][]int
}

// structSchemaCache holds one schema per struct type. A schema is immutable
// once published, so concurrent callers only race to build an identical value,
// the same trade-off encoding/json's field cache makes. The cache grows with
// the number of distinct types decoded or encoded, never per document.
var structSchemaCache sync.Map // reflect.Type -> structSchema

func cachedStructSchema(t reflect.Type) structSchema {
	if s, ok := structSchemaCache.Load(t); ok {
		return s.(structSchema)
	}
	s := newStructSchema(t)
	actual, _ := structSchemaCache.LoadOrStore(t, s)
	return actual.(structSchema)
}

func newStructSchema(t reflect.Type) structSchema {
	s := structSchema{byName: make(map[string]structFieldLoc, t.NumField())}
	// A struct may embed a pointer to itself, which is legal Go, so the walk
	// tracks the struct types on the current path and stops when one repeats;
	// without the guard the recursion never terminates. A self-promoted key
	// always loses to the shallower original, so skipping it changes nothing.
	visiting := map[reflect.Type]bool{}
	var walk func(t reflect.Type, prefix []int, depth int)
	walk = func(t reflect.Type, prefix []int, depth int) {
		visiting[t] = true
		defer delete(visiting, t)
		for i := range t.NumField() {
			f := t.Field(i)
			if f.PkgPath != "" { // unexported
				continue
			}
			path := append(append([]int{}, prefix...), i)
			name := ""
			if tag, ok := f.Tag.Lookup("toml"); ok {
				name, _, _ = strings.Cut(tag, ",")
				if name == "-" {
					continue
				}
			}
			if f.Anonymous && name == "" {
				ft := f.Type
				for ft.Kind() == reflect.Pointer {
					ft = ft.Elem()
				}
				switch {
				case ft.Kind() == reflect.Struct && !isScalarStruct(ft):
					if !visiting[ft] {
						walk(ft, path, depth+1)
					}
					continue
				case ft.Kind() == reflect.Map && ft.Key().Kind() == reflect.String:
					s.embedMaps = append(s.embedMaps, path)
					continue
				}
				name = f.Name
			}
			if name == "" {
				name = f.Name
			}
			key := strings.ToLower(name)
			if existing, ok := s.byName[key]; !ok || depth <= existing.depth {
				s.byName[key] = structFieldLoc{index: path, depth: depth}
			}
		}
	}
	walk(t, nil, 0)
	return s
}

// ownsKey reports whether the field at path is the one that resolves key.
// The encoder consults it to emit exactly the field the decoder would fill,
// so a struct with two fields mapping to one key does not marshal into a
// duplicate TOML key.
func (s structSchema) ownsKey(key string, path []int) bool {
	loc, ok := s.byName[key]
	return ok && slices.Equal(loc.index, path)
}

// fieldByIndex walks an index path from a struct value, allocating nil
// pointers along the way so a key can reach through an embedded pointer
// struct. Every field on the path is exported, so each step is settable.
func fieldByIndex(v reflect.Value, path []int) (reflect.Value, error) {
	for i, x := range path {
		v = v.Field(x)
		if i < len(path)-1 && v.Kind() == reflect.Pointer {
			if v.IsNil() {
				if !v.CanSet() {
					return reflect.Value{}, fmt.Errorf("cannot allocate nil embedded pointer")
				}
				v.Set(reflect.New(v.Type().Elem()))
			}
			v = v.Elem()
		}
	}
	return v, nil
}