// Package schema turns nested JSON objects into flat key/value maps and
// classifies field types for conflict detection.
//
// Rules:
//   - keys are joined with "_" : {"a": {"b": 1}} -> {"a_b": 1}
//   - arrays are preserved as-is and not descended into
//   - keys are sanitized: lowercase, non-[a-z0-9_] replaced with "_"
package schema

import (
	"strings"
	"unicode"
)

type DataType string

const (
	TypeString    DataType = "string"
	TypeNumber    DataType = "number"
	TypeBoolean   DataType = "boolean"
	TypeObject    DataType = "object"
	TypeArray     DataType = "array"
	TypeTimestamp DataType = "timestamp"
	TypeNull      DataType = "null"
)

// Flatten flattens nested objects under a snake_case prefix.
// Returns a new map, never mutates input.
func Flatten(in map[string]any) map[string]any {
	out := make(map[string]any, len(in))
	for k, v := range in {
		flattenInto(out, sanitize(k), v)
	}
	return out
}

func flattenInto(out map[string]any, prefix string, v any) {
	switch x := v.(type) {
	case map[string]any:
		if len(x) == 0 {
			out[prefix] = x
			return
		}
		for k, child := range x {
			flattenInto(out, prefix+"_"+sanitize(k), child)
		}
	default:
		out[prefix] = v
	}
}

// sanitize replaces characters outside [a-z0-9_] with "_" and lowercases.
// Leading underscores are kept; trailing underscores are trimmed.
func sanitize(k string) string {
	if k == "" {
		return k
	}
	var b strings.Builder
	b.Grow(len(k))
	for _, r := range k {
		switch {
		case unicode.IsLetter(r):
			b.WriteRune(unicode.ToLower(r))
		case unicode.IsDigit(r) || r == '_':
			b.WriteRune(r)
		default:
			b.WriteRune('_')
		}
	}
	return strings.TrimRight(b.String(), "_")
}

// Classify maps a Go value (from json.Unmarshal) to a DataType.
func Classify(v any) DataType {
	switch x := v.(type) {
	case nil:
		return TypeNull
	case bool:
		return TypeBoolean
	case float64, float32, int, int32, int64, uint, uint32, uint64:
		return TypeNumber
	case string:
		_ = x
		return TypeString
	case []any:
		return TypeArray
	case map[string]any:
		return TypeObject
	default:
		return TypeString
	}
}