package customschema

import (
	"bytes"
	"encoding/json"
	"fmt"

	"github.com/santhosh-tekuri/jsonschema/v6"
)

// SchemaValidator validates JSON Schema definitions and metadata payloads
// against the rules defined in Plan Section 6.2 / Tracking Section 1.2.
type SchemaValidator struct{}

// ValidateSchemaDefinition enforces the mutable-metadata schema rules on
// an incoming JSON Schema document. The rules are applied in this order
// so each failure mode surfaces a specific, greppable error substring:
//  1. Size ceiling (strict greater-than MaxSchemaDefinitionBytes).
//  2. JSON well-formedness.
//  3. JSON Schema draft 2020-12 meta-validation.
//  4. Root "type" must equal "object".
//  5. Root "additionalProperties" key must be present (true or false).
//
// Returns nil when every rule passes.
func (v *SchemaValidator) ValidateSchemaDefinition(schemaDef json.RawMessage) error {
	// Rule 1: size ceiling. Strict greater-than — a payload whose length
	// is exactly MaxSchemaDefinitionBytes must still be accepted.
	if len(schemaDef) > MaxSchemaDefinitionBytes {
		return fmt.Errorf(
			"schema definition exceeds maximum size of %d bytes",
			MaxSchemaDefinitionBytes,
		)
	}

	// Rule 2: JSON well-formedness. Decode into a generic map so we can
	// inspect the root keys after meta-validation.
	var root map[string]any
	if err := json.Unmarshal(schemaDef, &root); err != nil {
		return fmt.Errorf("invalid JSON: %w", err)
	}

	// Rule 3: JSON Schema draft 2020-12 meta-validation. Use the
	// santhosh-tekuri compiler, which validates the supplied schema
	// against the 2020-12 meta-schema during Compile().
	compiler := jsonschema.NewCompiler()
	parsed, err := jsonschema.UnmarshalJSON(bytes.NewReader(schemaDef))
	if err != nil {
		return fmt.Errorf("invalid JSON: %w", err)
	}
	if err := compiler.AddResource("schema.json", parsed); err != nil {
		return fmt.Errorf("schema does not compile as a valid JSON Schema (type check failed): %w", err)
	}
	if _, err := compiler.Compile("schema.json"); err != nil {
		return fmt.Errorf("schema does not compile as a valid JSON Schema (type check failed): %w", err)
	}

	// Rule 4: root "type" must equal "object". This check runs after
	// meta-validation so a clearly-malformed schema surfaces a type
	// error from the library rather than our generic "object" message.
	rawType, hasType := root["type"]
	if !hasType {
		return fmt.Errorf(`root schema must declare type "object" (type key missing)`)
	}
	typeStr, ok := rawType.(string)
	if !ok || typeStr != "object" {
		return fmt.Errorf(`root schema type must be "object"`)
	}

	// Rule 5: root "additionalProperties" must be present as a boolean.
	// Only true or false are accepted — a sub-schema object is not.
	// The feature's strict/open switch is a binary choice per plan Section 6.2.
	apVal, present := root["additionalProperties"]
	if !present {
		return fmt.Errorf(
			"root schema must explicitly declare additionalProperties (true or false)",
		)
	}
	if _, isBool := apVal.(bool); !isBool {
		return fmt.Errorf(
			"root schema must explicitly declare additionalProperties (true or false)",
		)
	}

	return nil
}

// ValidateMetadata validates a metadata payload against a schema definition.
// Both arguments must be well-formed JSON. The schema definition is assumed
// to have already passed ValidateSchemaDefinition — ValidateMetadata only
// runs the instance check, not the meta-validation pass.
//
// Returns nil when the payload conforms to the schema. Returns a wrapped
// error whose message contains "invalid JSON" for malformed inputs or the
// underlying jsonschema validation error for structural mismatches.
func (v *SchemaValidator) ValidateMetadata(metadata json.RawMessage, schemaDef json.RawMessage) error {
	// Reject empty payloads before touching the compiler. An empty body is
	// never a valid JSON Schema instance — the test expects a clear error.
	if len(metadata) == 0 {
		return fmt.Errorf("invalid JSON: metadata payload is empty")
	}

	// Compile the schema. The schema is re-parsed on every call because
	// the library's compiler holds references to the parsed resource.
	compiler := jsonschema.NewCompiler()
	parsedSchema, err := jsonschema.UnmarshalJSON(bytes.NewReader(schemaDef))
	if err != nil {
		return fmt.Errorf("invalid schema definition: %w", err)
	}
	if err := compiler.AddResource("schema.json", parsedSchema); err != nil {
		return fmt.Errorf("invalid schema definition: %w", err)
	}
	compiled, err := compiler.Compile("schema.json")
	if err != nil {
		return fmt.Errorf("invalid schema definition: %w", err)
	}

	// Parse the metadata payload.
	parsedMetadata, err := jsonschema.UnmarshalJSON(bytes.NewReader(metadata))
	if err != nil {
		return fmt.Errorf("invalid JSON: %w", err)
	}

	// Run instance validation.
	if err := compiled.Validate(parsedMetadata); err != nil {
		return fmt.Errorf("metadata does not conform to schema: %w", err)
	}
	return nil
}