package validate

import (
	"encoding/json"
	"fmt"
	"net/http"
	"sort"
	"strings"

	"gopkg.in/yaml.v3"

	"github.com/tegwick/fluid-core/internal/contract"
	"github.com/tegwick/fluid-core/internal/runtime"
)

// Document is the supported subset of an OpenAPI description.
type Document struct {
	OpenAPI    string               `json:"openapi" yaml:"openapi"`
	Paths      map[string]*PathItem `json:"paths" yaml:"paths"`
	Components *Components          `json:"components,omitempty" yaml:"components,omitempty"`
}

// Components holds reusable schemas.
type Components struct {
	Schemas map[string]*Schema `json:"schemas,omitempty" yaml:"schemas,omitempty"`
}

// PathItem holds the operations available at one path.
type PathItem struct {
	Get    *Operation `json:"get,omitempty" yaml:"get,omitempty"`
	Put    *Operation `json:"put,omitempty" yaml:"put,omitempty"`
	Post   *Operation `json:"post,omitempty" yaml:"post,omitempty"`
	Delete *Operation `json:"delete,omitempty" yaml:"delete,omitempty"`
	Patch  *Operation `json:"patch,omitempty" yaml:"patch,omitempty"`
	Head   *Operation `json:"head,omitempty" yaml:"head,omitempty"`
}

// operations returns the declared operations by method.
func (p *PathItem) operations() map[string]*Operation {
	out := map[string]*Operation{}
	for method, op := range map[string]*Operation{
		http.MethodGet: p.Get, http.MethodPut: p.Put, http.MethodPost: p.Post,
		http.MethodDelete: p.Delete, http.MethodPatch: p.Patch, http.MethodHead: p.Head,
	} {
		if op != nil {
			out[method] = op
		}
	}
	return out
}

// Operation is one method on one path.
type Operation struct {
	OperationID string       `json:"operationId,omitempty" yaml:"operationId,omitempty"`
	Parameters  []*Parameter `json:"parameters,omitempty" yaml:"parameters,omitempty"`
	RequestBody *RequestBody `json:"requestBody,omitempty" yaml:"requestBody,omitempty"`
}

// Parameter is a path, query or header parameter.
type Parameter struct {
	Name     string  `json:"name" yaml:"name"`
	In       string  `json:"in" yaml:"in"`
	Required bool    `json:"required,omitempty" yaml:"required,omitempty"`
	Schema   *Schema `json:"schema,omitempty" yaml:"schema,omitempty"`
}

// RequestBody describes an operation's body.
type RequestBody struct {
	Required bool                  `json:"required,omitempty" yaml:"required,omitempty"`
	Content  map[string]*MediaType `json:"content,omitempty" yaml:"content,omitempty"`
}

// MediaType binds a content type to a schema.
type MediaType struct {
	Schema *Schema `json:"schema,omitempty" yaml:"schema,omitempty"`
}

// Contract is a parsed, validated OpenAPI description ready to enforce.
type Contract struct {
	doc    *Document
	routes []route
}

// route is one compiled path template.
type route struct {
	template string
	segments []segment
	item     *PathItem
}

type segment struct {
	literal  string
	variable string
}

// ParseOpenAPI compiles an OpenAPI document.
//
// Parsing happens once, at revision publication, not per request. Compiling a
// contract on the hot path would put an unbounded amount of work between a
// consumer and their response for no benefit, since the contract cannot change
// without a new revision.
func ParseOpenAPI(raw []byte) (*Contract, error) {
	var doc Document
	if err := yaml.Unmarshal(raw, &doc); err != nil {
		return nil, fmt.Errorf("parse contract: %w", err)
	}
	if len(doc.Paths) == 0 {
		return nil, fmt.Errorf("contract declares no paths")
	}

	templates := make([]string, 0, len(doc.Paths))
	for t := range doc.Paths {
		templates = append(templates, t)
	}
	// Sorting makes route order deterministic. Two templates can match the same
	// request, and which one wins must not depend on map iteration.
	sort.Strings(templates)

	c := &Contract{doc: &doc}
	for _, t := range templates {
		c.routes = append(c.routes, route{
			template: t,
			segments: compile(t),
			item:     doc.Paths[t],
		})
	}

	// Literal routes are matched before templated ones, so /entries/latest
	// wins over /entries/{id} regardless of alphabetical order.
	sort.SliceStable(c.routes, func(i, j int) bool {
		return variableCount(c.routes[i].segments) < variableCount(c.routes[j].segments)
	})
	return c, nil
}

func compile(template string) []segment {
	parts := strings.Split(strings.Trim(template, "/"), "/")
	out := make([]segment, 0, len(parts))
	for _, p := range parts {
		if strings.HasPrefix(p, "{") && strings.HasSuffix(p, "}") {
			out = append(out, segment{variable: strings.Trim(p, "{}")})
			continue
		}
		out = append(out, segment{literal: p})
	}
	return out
}

func variableCount(segs []segment) int {
	n := 0
	for _, s := range segs {
		if s.variable != "" {
			n++
		}
	}
	return n
}

// match finds the route serving a path, with its extracted variables.
func (c *Contract) match(path string) (route, map[string]string, bool) {
	parts := strings.Split(strings.Trim(path, "/"), "/")

	for _, r := range c.routes {
		if len(r.segments) != len(parts) {
			continue
		}
		vars := map[string]string{}
		ok := true
		for i, seg := range r.segments {
			if seg.variable != "" {
				if parts[i] == "" {
					ok = false
					break
				}
				vars[seg.variable] = parts[i]
				continue
			}
			if seg.literal != parts[i] {
				ok = false
				break
			}
		}
		if ok {
			return r, vars, true
		}
	}
	return route{}, nil, false
}

// Resolve implements Resolver for local component references.
func (c *Contract) Resolve(ref string) (*Schema, error) {
	const prefix = "#/components/schemas/"
	if !strings.HasPrefix(ref, prefix) {
		// Remote references would make validation depend on a network fetch,
		// which the request path must never do.
		return nil, fmt.Errorf("%w: only local %s references are supported", ErrUnsupported, prefix)
	}
	if c.doc.Components == nil {
		return nil, fmt.Errorf("contract declares no components")
	}
	s, ok := c.doc.Components.Schemas[strings.TrimPrefix(ref, prefix)]
	if !ok {
		return nil, fmt.Errorf("no such component schema")
	}
	return s, nil
}

// Operations lists the operations the contract declares, for complexity
// measurement and for reporting surface area.
func (c *Contract) Operations() []string {
	var out []string
	for _, r := range c.routes {
		for method := range r.item.operations() {
			out = append(out, method+" "+r.template)
		}
	}
	sort.Strings(out)
	return out
}

// OpenAPIValidator enforces a compiled contract at the gateway.
//
// It implements runtime.ContractValidator. Contracts are compiled per revision
// and cached by contract digest: two revisions sharing a contract share the
// compiled form, and a changed contract is a different digest and so a
// different entry.
type OpenAPIValidator struct {
	contracts map[contract.Digest]*Contract
}

// NewOpenAPIValidator returns a validator holding no contracts.
func NewOpenAPIValidator() *OpenAPIValidator {
	return &OpenAPIValidator{contracts: map[contract.Digest]*Contract{}}
}

// Register compiles and stores the contract for a digest.
func (v *OpenAPIValidator) Register(digest contract.Digest, raw []byte) error {
	c, err := ParseOpenAPI(raw)
	if err != nil {
		return err
	}
	v.contracts[digest] = c
	return nil
}

// Contract returns a registered contract.
func (v *OpenAPIValidator) Contract(digest contract.Digest) (*Contract, bool) {
	c, ok := v.contracts[digest]
	return c, ok
}

// Validate checks a request against its revision's contract.
//
// An unregistered contract is a refusal, not a pass. A gateway that served
// traffic for a revision whose contract it could not find would be serving
// undeclared semantics, which is the thing minimal conformance forbids.
func (v *OpenAPIValidator) Validate(rev contract.Revision, r *http.Request, body []byte) error {
	c, ok := v.contracts[rev.Contract.Digest]
	if !ok {
		return &runtime.ValidationError{
			Message: fmt.Sprintf("no contract registered for revision %s", rev.ID),
		}
	}

	matched, pathVars, found := c.match(r.URL.Path)
	if !found {
		return &runtime.ValidationError{
			Message: fmt.Sprintf("%s is not a path in this contract", r.URL.Path),
		}
	}

	op, ok := matched.item.operations()[r.Method]
	if !ok {
		allowed := make([]string, 0)
		for method := range matched.item.operations() {
			allowed = append(allowed, method)
		}
		sort.Strings(allowed)
		return &runtime.ValidationError{
			Message: fmt.Sprintf("%s is not allowed on %s; the contract declares %s",
				r.Method, matched.template, strings.Join(allowed, ", ")),
		}
	}

	result := &Result{}
	validateParameters(op, r, pathVars, result, c)
	validateBody(op, r, body, result, c)

	if !result.OK() {
		return &runtime.ValidationError{
			Field:   result.FirstPath(),
			Message: result.Error(),
		}
	}
	return nil
}

func validateParameters(op *Operation, r *http.Request, pathVars map[string]string, result *Result, resolver Resolver) {
	query := r.URL.Query()

	for _, p := range op.Parameters {
		if p == nil {
			continue
		}
		var (
			raw     string
			present bool
		)
		switch p.In {
		case "path":
			raw, present = pathVars[p.Name]
		case "query":
			present = query.Has(p.Name)
			raw = query.Get(p.Name)
		case "header":
			raw = r.Header.Get(p.Name)
			present = raw != ""
		default:
			// Cookie parameters and anything else are reported rather than
			// ignored: silently skipping a constraint is how a validator
			// reports success it did not earn.
			result.add(p.In+"."+p.Name, "%v: parameter location %q", ErrUnsupported, p.In)
			continue
		}

		if !present {
			if p.Required {
				result.add(p.In+"."+p.Name, "is required")
			}
			continue
		}
		if p.Schema != nil {
			validateValue(coerce(raw, p.Schema), p.Schema, p.In+"."+p.Name, result, resolver, 0)
		}
	}
}

// coerce turns a string parameter into the type its schema declares.
//
// Query and path parameters arrive as text; comparing them against a numeric
// schema without conversion would fail every well-formed request.
func coerce(raw string, schema *Schema) any {
	names := typeNames(schema.Type)
	if len(names) == 0 {
		return raw
	}
	switch names[0] {
	case "integer", "number":
		var f float64
		if _, err := fmt.Sscanf(raw, "%g", &f); err == nil {
			return f
		}
		// Left as a string so the type mismatch is reported honestly rather
		// than becoming a confusing zero.
		return raw
	case "boolean":
		switch raw {
		case "true":
			return true
		case "false":
			return false
		}
		return raw
	}
	return raw
}

func validateBody(op *Operation, r *http.Request, body []byte, result *Result, resolver Resolver) {
	if op.RequestBody == nil {
		return
	}
	if len(body) == 0 {
		if op.RequestBody.Required {
			result.add("body", "is required")
		}
		return
	}

	mediaType := "application/json"
	if ct := r.Header.Get("Content-Type"); ct != "" {
		mediaType = strings.TrimSpace(strings.Split(ct, ";")[0])
	}

	media, ok := op.RequestBody.Content[mediaType]
	if !ok {
		declared := make([]string, 0, len(op.RequestBody.Content))
		for m := range op.RequestBody.Content {
			declared = append(declared, m)
		}
		sort.Strings(declared)
		result.add("body", "content type %q is not declared; the contract accepts %s",
			mediaType, strings.Join(declared, ", "))
		return
	}
	if media == nil || media.Schema == nil {
		return
	}

	var decoded any
	if err := json.Unmarshal(body, &decoded); err != nil {
		result.add("body", "is not valid JSON: %v", err)
		return
	}
	validateValue(decoded, media.Schema, "body", result, resolver, 0)
}