// Package policy is the access-decision boundary for zddc-server.
//
// All ACL checks in handlers go through Decider.Allow rather than
// calling zddc.AllowedWithChain directly. This lets a deployment
// route policy decisions to an external OPA-compatible server
// (for federal customers running their own audited Rego policies)
// without changing handler code.
//
// Two implementations:
//
//   - InternalDecider — wraps zddc.AllowedWithChain. The default;
//     no new dependencies, identical semantics to the legacy code
//     path. This is what the docs in zddc/README.md describe.
//
//   - HTTPDecider — POSTs to OPA's canonical /v1/data/<package>/allow
//     endpoint over HTTP or a Unix-domain socket. Federal customers
//     deploy real OPA alongside zddc-server, write their own Rego,
//     and point ZDDC_OPA_URL at it.
//
// Configuration knob:
//
//	ZDDC_OPA_URL=                           # internal (default)
//	ZDDC_OPA_URL=internal                   # internal (explicit)
//	ZDDC_OPA_URL=http://127.0.0.1:8181      # external HTTP
//	ZDDC_OPA_URL=https://opa.example:8181   # external HTTPS
//	ZDDC_OPA_URL=unix:///run/opa/opa.sock   # external Unix socket
//
// Failure mode (external only): unreachable / non-2xx / malformed
// response → fail closed (deny), with a WARN log. Operators who
// prefer availability over correctness can set ZDDC_OPA_FAIL_OPEN=1
// to flip to fail-open with a WARN log instead.
package policy

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net"
	"net/http"
	"net/url"
	"strings"
	"time"

	"codeberg.org/VARASYS/ZDDC/zddc/internal/zddc"
)

// AllowInput is the canonical input shape for Decider.Allow. It
// matches OPA's input convention: a JSON object passed as the
// "input" field of a /v1/data/<package>/allow query.
//
// External Rego policies can:
//   - read input.user.email (string)
//   - read input.path (string)
//   - walk input.policy_chain.levels[].acl.{allow,deny} for
//     custom cascade semantics, or read the pre-resolved
//     input.policy_chain.has_any_file when implementing the
//     same default-deny rule we use internally.
type AllowInput struct {
	User struct {
		Email string `json:"email"`
	} `json:"user"`
	Path        string             `json:"path"`
	PolicyChain *SerializableChain `json:"policy_chain,omitempty"`
}

// SerializableChain is a JSON-friendly view of zddc.PolicyChain.
// We don't tag zddc.PolicyChain itself because it's tightly coupled
// to the parser; the duplication is one struct.
type SerializableChain struct {
	Levels     []zddc.ZddcFile `json:"levels"`
	HasAnyFile bool            `json:"has_any_file"`
}

func chainToSerializable(c zddc.PolicyChain) *SerializableChain {
	return &SerializableChain{Levels: c.Levels, HasAnyFile: c.HasAnyFile}
}

// Decider is the access-decision interface every handler uses.
type Decider interface {
	Allow(ctx context.Context, input AllowInput) (bool, error)
}

// Config selects and parameterizes the decider.
type Config struct {
	URL      string // raw value: "", "internal", "http(s)://...", "unix:///path"
	FailOpen bool   // external mode only: on transport error, allow instead of deny
}

// New constructs a Decider per cfg.URL semantics.
//   - "" or "internal" → InternalDecider
//   - "http(s)://..."   → HTTPDecider
//   - "unix:///..."     → HTTPDecider over a Unix socket
//
// Returns an error if URL is unrecognized.
func New(cfg Config) (Decider, error) {
	if cfg.URL == "" || strings.EqualFold(cfg.URL, "internal") {
		return &InternalDecider{}, nil
	}
	if strings.HasPrefix(cfg.URL, "http://") || strings.HasPrefix(cfg.URL, "https://") {
		return newHTTPDecider(cfg.URL, cfg.FailOpen, nil)
	}
	if strings.HasPrefix(cfg.URL, "unix://") {
		path := strings.TrimPrefix(cfg.URL, "unix://")
		dialer := &net.Dialer{Timeout: 2 * time.Second}
		transport := &http.Transport{
			DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {
				return dialer.DialContext(ctx, "unix", path)
			},
		}
		// HTTP host part is unused but the URL still has to parse.
		return newHTTPDecider("http://opa-unix-socket", cfg.FailOpen, transport)
	}
	return nil, fmt.Errorf("unrecognized ZDDC_OPA_URL %q (want \"internal\", http(s)://..., or unix:///...)", cfg.URL)
}

// InternalDecider routes Allow through zddc.AllowedWithChain. No
// network, no Rego, no new dependencies — same Go evaluator the
// existing test suite covers.
type InternalDecider struct{}

func (d *InternalDecider) Allow(_ context.Context, input AllowInput) (bool, error) {
	chain := zddc.PolicyChain{}
	if input.PolicyChain != nil {
		chain.Levels = input.PolicyChain.Levels
		chain.HasAnyFile = input.PolicyChain.HasAnyFile
	}
	return zddc.AllowedWithChain(chain, input.User.Email), nil
}

// HTTPDecider POSTs to /v1/data/zddc/access/allow on the configured
// endpoint. Spec:
//   - request body  {"input": <AllowInput>}
//   - response body {"result": true|false}
//   - 5-second per-request timeout
//   - non-2xx, transport error, missing/malformed result → policy
//     decision is "deny" unless FailOpen=true
//
// The path "/v1/data/zddc/access/allow" is the OPA convention; the
// "zddc.access" Rego package on an external server would expose
// `allow` for this endpoint.
type HTTPDecider struct {
	endpoint string
	client   *http.Client
	failOpen bool
}

func newHTTPDecider(endpoint string, failOpen bool, transport http.RoundTripper) (*HTTPDecider, error) {
	if _, err := url.Parse(endpoint); err != nil {
		return nil, fmt.Errorf("invalid OPA URL %q: %w", endpoint, err)
	}
	c := &http.Client{Timeout: 5 * time.Second}
	if transport != nil {
		c.Transport = transport
	}
	return &HTTPDecider{
		endpoint: strings.TrimRight(endpoint, "/") + "/v1/data/zddc/access/allow",
		client:   c,
		failOpen: failOpen,
	}, nil
}

type opaResponse struct {
	Result *bool `json:"result"`
}

func (d *HTTPDecider) Allow(ctx context.Context, input AllowInput) (bool, error) {
	body, err := json.Marshal(struct {
		Input AllowInput `json:"input"`
	}{Input: input})
	if err != nil {
		return d.failResult(fmt.Errorf("marshal input: %w", err))
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, d.endpoint, bytes.NewReader(body))
	if err != nil {
		return d.failResult(fmt.Errorf("build request: %w", err))
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Accept", "application/json")

	resp, err := d.client.Do(req)
	if err != nil {
		return d.failResult(fmt.Errorf("opa request: %w", err))
	}
	defer resp.Body.Close()

	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		// Read up to 512 bytes of the error body for the log without
		// blowing up on a verbose OPA error page.
		snippet, _ := io.ReadAll(io.LimitReader(resp.Body, 512))
		return d.failResult(fmt.Errorf("opa returned %d: %s", resp.StatusCode, strings.TrimSpace(string(snippet))))
	}
	var parsed opaResponse
	if err := json.NewDecoder(resp.Body).Decode(&parsed); err != nil {
		return d.failResult(fmt.Errorf("decode opa response: %w", err))
	}
	if parsed.Result == nil {
		return d.failResult(errors.New("opa response missing 'result' field"))
	}
	return *parsed.Result, nil
}

// failResult logs the failure and returns the configured fail-mode
// decision. Logged at WARN so a healthy run is silent but a sick OPA
// is loud.
func (d *HTTPDecider) failResult(err error) (bool, error) {
	if d.failOpen {
		slog.Warn("policy decision failed; failing open (allow)", "endpoint", d.endpoint, "err", err)
		return true, nil
	}
	slog.Warn("policy decision failed; failing closed (deny)", "endpoint", d.endpoint, "err", err)
	return false, nil
}

// AllowFromChain is a convenience for callers that already have a
// PolicyChain in hand. Equivalent to constructing AllowInput manually
// from (chain, email, path) and calling d.Allow.
func AllowFromChain(ctx context.Context, d Decider, chain zddc.PolicyChain, email, path string) (bool, error) {
	in := AllowInput{Path: path, PolicyChain: chainToSerializable(chain)}
	in.User.Email = email
	return d.Allow(ctx, in)
}