interpres/decode.go

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

package interpres

import (
	"encoding"
	"fmt"
	"reflect"
	"slices"
	"strings"
	"sync"
	"sync/atomic"
	"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]()

var (
	unmarshalerType     = reflect.TypeFor[Unmarshaler]()
	textUnmarshalerType = reflect.TypeFor[encoding.TextUnmarshaler]()
)

// The per-type flags record which interface lookups a decode into that type
// can succeed at, so the hot path consults the cache instead of boxing every
// value into an interface to ask. The bits name the receiver the method is
// found on: the value itself, or its address.
const (
	flagUnmarshaler uint8 = 1 << iota
	flagAddrUnmarshaler
	flagTextUnmarshaler
	flagAddrTextUnmarshaler
)

// typeFlagCache holds one flag entry per destination type. A set is immutable
// once published, the same trade-off structSchemaCache makes; the cache grows
// with the number of distinct types decoded, never per document. The hint
// below re-points at these published entries, so a hot lookup allocates
// nothing.
var typeFlagCache sync.Map // reflect.Type -> *flagHintEntry

// flagHintEntry pairs a type with its cached flags for the monomorphic hint
// below. Both caches share the entry shape.
type flagHintEntry struct {
	typ   reflect.Type
	flags uint8
}

// typeFlagHint remembers the entry resolved last, because a decode walks one
// type across consecutive fields and elements. A lost race loses only the
// hint: every value it can hold came from the cache.
var typeFlagHint atomic.Pointer[flagHintEntry]

func typeFlags(t reflect.Type) uint8 {
	if e := typeFlagHint.Load(); e != nil && e.typ == t {
		return e.flags
	}
	if v, ok := typeFlagCache.Load(t); ok {
		entry := v.(*flagHintEntry)
		typeFlagHint.Store(entry)
		return entry.flags
	}
	var f uint8
	if t.Implements(unmarshalerType) {
		f |= flagUnmarshaler
	}
	pt := reflect.PointerTo(t)
	if pt.Implements(unmarshalerType) {
		f |= flagAddrUnmarshaler
	}
	// The date-time types are excluded from the text path: they carry
	// time.Time's UnmarshalText through an embedded field while their only
	// accepted form is a bare timestamp.
	if !isDateTimeType(t) {
		if t.Implements(textUnmarshalerType) {
			f |= flagTextUnmarshaler
		}
		if pt.Implements(textUnmarshalerType) {
			f |= flagAddrTextUnmarshaler
		}
	}
	actual, _ := typeFlagCache.LoadOrStore(t, &flagHintEntry{t, f})
	published := actual.(*flagHintEntry)
	typeFlagHint.Store(published)
	return published.flags
}

// unmarshalerOf resolves the Unmarshaler for dst through the flag cache, so
// an interface value is built only where the cache says the assertion can
// succeed. An interface destination is asked dynamically, because the value
// it will hold may implement the interface even when the interface type
// itself does not.
func unmarshalerOf(dst reflect.Value) (Unmarshaler, bool) {
	if dst.Kind() == reflect.Interface {
		u, ok := dst.Interface().(Unmarshaler)
		return u, ok
	}
	f := typeFlags(dst.Type())
	if f&flagUnmarshaler != 0 {
		u, ok := dst.Interface().(Unmarshaler)
		return u, ok
	}
	if f&flagAddrUnmarshaler != 0 && dst.CanAddr() {
		u, ok := dst.Addr().Interface().(Unmarshaler)
		return u, ok
	}
	return nil, false
}

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
		}
	}

	// A TOML string fills a destination that implements
	// encoding.TextUnmarshaler, the rule encoding/json follows. Every other
	// value kind keeps its own rule, so an integer still reaches a numeric
	// destination.
	if s, isString := data.(string); isString {
		if tu, ok := textUnmarshalerOf(dst); ok {
			if err := tu.UnmarshalText([]byte(s)); err != nil {
				return fmt.Errorf("unmarshal text: %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:
		if dst.Type() == durationType {
			return setDuration(dst, v)
		}
		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 OffsetDateTime:
		return setOffsetDateTime(v, dst)
	case time.Time:
		return setDateTime(v, dst)
	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())
	}
}

// textUnmarshalerOf is the same resolution for encoding.TextUnmarshaler,
// with the date-time types excluded for the reason typeFlags records.
func textUnmarshalerOf(dst reflect.Value) (encoding.TextUnmarshaler, bool) {
	if !dst.CanInterface() || isDateTimeType(dst.Type()) {
		return nil, false
	}
	if dst.Kind() == reflect.Interface {
		tu, ok := dst.Interface().(encoding.TextUnmarshaler)
		return tu, ok
	}
	f := typeFlags(dst.Type())
	if f&flagTextUnmarshaler != 0 {
		tu, ok := dst.Interface().(encoding.TextUnmarshaler)
		return tu, ok
	}
	if f&flagAddrTextUnmarshaler != 0 && dst.CanAddr() {
		tu, ok := dst.Addr().Interface().(encoding.TextUnmarshaler)
		return tu, ok
	}
	return nil, false
}

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[key]; ok {
				continue
			}
			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 {
		// A key that is already lowercase, which document keys usually are,
		// hits the map directly; only a miss pays for the case fold.
		field, ok := schema.byName[key]
		if !ok {
			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 -----------------------------------------------------

// setOffsetDateTime stores an offset date-time: in a wrapper destination as it
// is, and in a plain time.Time, which takes the instant with the offset the
// document wrote, so a timestamp field does not have to name the wrapper.
func setOffsetDateTime(v OffsetDateTime, dst reflect.Value) error {
	switch dst.Type() {
	case offsetDateTimeType:
		dst.Set(reflect.ValueOf(v))
	case timeType:
		dst.Set(reflect.ValueOf(v.Time))
	default:
		return fmt.Errorf("interpres: cannot assign datetime to %s", dst.Type())
	}
	return nil
}

// setDateTime stores a time.Time that reached the tree directly, which is the
// shape a tree built by hand carries. Dates the parser produced arrive as
// OffsetDateTime instead.
func setDateTime(v time.Time, dst reflect.Value) error {
	switch dst.Type() {
	case timeType:
		dst.Set(reflect.ValueOf(v))
	case offsetDateTimeType:
		dst.Set(reflect.ValueOf(OffsetDateTime{v}))
	default:
		return fmt.Errorf("interpres: cannot assign datetime to %s", dst.Type())
	}
	return nil
}

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())
	}
	// Convert rather than assign: a value of the predeclared type is not
	// assignable to a defined type of the same kind, so a plain Set panics on
	// a destination such as `type Name string`.
	dst.Set(val.Convert(dst.Type()))
	return nil
}

// setDuration reads a duration literal into a time.Duration destination. TOML
// has no duration type, so the encoder writes the canonical Go form and the
// decoder reads that back; a bare integer stays the nanosecond count it has
// always been, and reaches the destination through setInt.
func setDuration(dst reflect.Value, s string) error {
	d, err := time.ParseDuration(s)
	if err != nil {
		return fmt.Errorf("interpres: invalid duration %q", s)
	}
	dst.SetInt(int64(d))
	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
}