weatherfeeder/internal/normalizers/common/round.go

// FILE: ./internal/normalizers/common/round.go
package common

import (
	"math"
	"reflect"
)

// DefaultFloatPrecision is the project-wide wire-format policy for floating-point
// values in canonical payloads (weather.* schemas).
//
// Note: encoding/json will not necessarily print trailing zeros (e.g. 1.50 -> 1.5),
// but values will be *rounded* to this number of digits after the decimal point.
const DefaultFloatPrecision = 2

// RoundFloats returns a copy of v with all float32/float64 values (including pointers,
// slices, arrays, maps, and nested exported-struct fields) rounded to `decimals` digits
// after the decimal point.
//
// This is a best-effort helper meant for presentation stability. If reflection hits an
// unsupported/opaque type (e.g. structs with unexported fields like time.Time), that
// subtree is left unchanged.
func RoundFloats(v any, decimals int) any {
	if v == nil || decimals < 0 {
		return v
	}

	defer func() {
		// Never let presentation formatting crash the pipeline.
		_ = recover()
	}()

	rv := reflect.ValueOf(v)
	out := roundValue(rv, decimals)
	if !out.IsValid() {
		return v
	}
	return out.Interface()
}

func roundValue(v reflect.Value, decimals int) reflect.Value {
	if !v.IsValid() {
		return v
	}

	// Unwrap interfaces.
	if v.Kind() == reflect.Interface {
		if v.IsNil() {
			return v
		}
		elem := roundValue(v.Elem(), decimals)

		// Re-wrap in the same interface type.
		out := reflect.New(v.Type()).Elem()
		if elem.IsValid() && elem.Type().AssignableTo(v.Type()) {
			out.Set(elem)
			return out
		}
		if elem.IsValid() && elem.Type().ConvertibleTo(v.Type()) {
			out.Set(elem.Convert(v.Type()))
			return out
		}

		// If we can't sensibly re-wrap, just keep the original.
		return v
	}

	// Copy pointers (and round their targets).
	if v.Kind() == reflect.Pointer {
		if v.IsNil() {
			return v
		}

		// If the pointed-to type is an opaque struct (e.g. time.Time), keep as-is.
		if v.Elem().Kind() == reflect.Struct && isOpaqueStruct(v.Elem().Type()) {
			return v
		}

		elem := roundValue(v.Elem(), decimals)
		p := reflect.New(v.Type().Elem())
		if elem.IsValid() && elem.Type().AssignableTo(v.Type().Elem()) {
			p.Elem().Set(elem)
		} else if elem.IsValid() && elem.Type().ConvertibleTo(v.Type().Elem()) {
			p.Elem().Set(elem.Convert(v.Type().Elem()))
		} else {
			p.Elem().Set(v.Elem())
		}
		return p
	}

	switch v.Kind() {
	case reflect.Float32, reflect.Float64:
		f := v.Convert(reflect.TypeOf(float64(0))).Float()
		r := roundFloat64(f, decimals)
		return reflect.ValueOf(r).Convert(v.Type())

	case reflect.Struct:
		// Avoid reconstructing opaque structs (time.Time has unexported fields).
		if isOpaqueStruct(v.Type()) {
			return v
		}

		out := reflect.New(v.Type()).Elem()
		out.Set(v) // start from a copy, then replace rounded fields

		t := v.Type()
		for i := 0; i < v.NumField(); i++ {
			sf := t.Field(i)

			// Only exported fields are safely settable across packages.
			if sf.PkgPath != "" {
				continue
			}

			fv := v.Field(i)
			rf := roundValue(fv, decimals)

			of := out.Field(i)
			if !of.CanSet() {
				continue
			}

			if rf.IsValid() && rf.Type().AssignableTo(of.Type()) {
				of.Set(rf)
			} else if rf.IsValid() && rf.Type().ConvertibleTo(of.Type()) {
				of.Set(rf.Convert(of.Type()))
			}
		}
		return out

	case reflect.Slice:
		if v.IsNil() {
			return v
		}
		out := reflect.MakeSlice(v.Type(), v.Len(), v.Len())
		for i := 0; i < v.Len(); i++ {
			ev := v.Index(i)
			re := roundValue(ev, decimals)
			if re.IsValid() && re.Type().AssignableTo(out.Index(i).Type()) {
				out.Index(i).Set(re)
			} else if re.IsValid() && re.Type().ConvertibleTo(out.Index(i).Type()) {
				out.Index(i).Set(re.Convert(out.Index(i).Type()))
			} else {
				out.Index(i).Set(ev)
			}
		}
		return out

	case reflect.Array:
		out := reflect.New(v.Type()).Elem()
		out.Set(v)
		for i := 0; i < v.Len(); i++ {
			ev := v.Index(i)
			re := roundValue(ev, decimals)
			if re.IsValid() && re.Type().AssignableTo(out.Index(i).Type()) {
				out.Index(i).Set(re)
			} else if re.IsValid() && re.Type().ConvertibleTo(out.Index(i).Type()) {
				out.Index(i).Set(re.Convert(out.Index(i).Type()))
			} else {
				out.Index(i).Set(ev)
			}
		}
		return out

	case reflect.Map:
		if v.IsNil() {
			return v
		}
		out := reflect.MakeMapWithSize(v.Type(), v.Len())
		iter := v.MapRange()
		for iter.Next() {
			k := iter.Key()
			mv := iter.Value()
			rv := roundValue(mv, decimals)

			if rv.IsValid() && rv.Type().AssignableTo(v.Type().Elem()) {
				out.SetMapIndex(k, rv)
			} else if rv.IsValid() && rv.Type().ConvertibleTo(v.Type().Elem()) {
				out.SetMapIndex(k, rv.Convert(v.Type().Elem()))
			} else {
				out.SetMapIndex(k, mv)
			}
		}
		return out

	default:
		// ints, strings, bools, time.Time (handled as opaque), etc.
		return v
	}
}

func roundFloat64(f float64, decimals int) float64 {
	if decimals <= 0 {
		return math.Round(f)
	}
	pow := math.Pow10(decimals)
	return math.Round(f*pow) / pow
}

// isOpaqueStruct returns true for structs that are unsafe/unhelpful to reconstruct via reflection.
// Any struct containing unexported fields (e.g. time.Time) is treated as opaque.
func isOpaqueStruct(t reflect.Type) bool {
	if t.Kind() != reflect.Struct {
		return false
	}
	for i := 0; i < t.NumField(); i++ {
		if t.Field(i).PkgPath != "" {
			return true
		}
	}
	return false
}