package books

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"path/filepath"

	"gopkg.in/yaml.v3"

	"textmachine/platform/internal/runner"
)

// render.go: the platform writes a book's STARTING engine configuration, once, from a template the
// operator deploys (form Б, ratified D39.130).
//
// The shape follows from D39.110 §2b, which says the platform does not own `book.yaml`. It renders
// ONE file at the moment a book's directory is created and never reads or edits it again: after that
// the file belongs to the operator, and an operator who changes a ceiling or points the book at a
// different pipeline finds their change still there on the next run. That is why the write is
// O_EXCL — not as a race guard (though it is one), but because "the platform never overwrites this
// file" is the whole of the interpretation this form rests on.
//
// What the platform fills in is exactly what only IT knows: the book's identity, the languages the
// user declared at intake, and the name the intake wrote the source under. Everything else
// — the pipeline and models paths, the ceilings, the langpack roots, whatever the engine's schema
// grows next — comes from the template unread and unjudged. The platform does not know the engine's
// configuration schema and must not learn it: a key it has never heard of has to survive this file
// untouched, or every engine release would need a platform release.
//
// ⚠ Form В (`tmctl init`, unified backlog row 170) replaces this with a call to the engine. When it
// lands, the ONE place to change is renderConfig below — the seam that decides "there is no
// configuration here" is `provision`, and it does not care where the bytes come from.

// ErrNoTemplate is a deployment that has no book template configured, or one whose template cannot
// be used. It is deliberately the same CLASS as a book with no configuration at all — a deployment
// question a human answers — so it can never end an intake or spend its budget (see defer_).
var ErrNoTemplate = errors.New("books: the book template is not usable")

// maxTemplate bounds what will be read as a template. A book configuration is a page of keys; a
// megabyte of it means the path points at something else entirely.
const maxTemplate = 1 << 20

// bookConfig is what the PLATFORM knows about a book and the engine needs told. Nothing else about
// the engine's schema appears in this package.
type bookConfig struct {
	// ID is the platform's book id, which becomes the engine's `book_id`: one identity for the book
	// on both sides of the seam, so the handshake's book_id and the row agree without a translation
	// table.
	ID         string
	Title      string
	SourceLang string
	TargetLang string
	// SourceFile is the name — not the path — the intake wrote the upload under. The engine resolves
	// it against the directory of book.yaml, which is the book's own directory.
	SourceFile string
}

// provision makes sure the book's directory carries an engine configuration, rendering the starting
// one if it does not.
//
// It answers ErrNotProvisioned in every case where there is no usable configuration and no way to
// make one, because the CALLER's handling of that is already right: a book with no configuration
// waits for a human and never spends its attempt budget (defer_), which is the only safe answer to a
// deployment fault that would otherwise apply to every book on the host at once.
func (s *Service) provision(ctx context.Context, dir string, b bookConfig) error {
	path := filepath.Join(dir, runner.ConfigFile)
	switch _, err := os.Stat(path); {
	case err == nil:
		// The operator's file, or one this platform rendered earlier. Nothing here is read: where the
		// source is and what the ceilings are is what that file SAYS, and this platform does not get a
		// second opinion about its own book (D39.110 §2b).
		return nil
	case !errors.Is(err, fs.ErrNotExist):
		return fmt.Errorf("books: read book configuration: %w", err)
	}
	if s.Cfg.BookTemplate == "" {
		// A deployment where the operator drops the file in by hand is legitimate and is what every
		// deployment did before this existed. Nothing to do but wait for them.
		return ErrNotProvisioned
	}
	// Asked for only now, because only a render needs it: `source_file` is the one field whose value
	// is a fact about the directory rather than about the row.
	source, err := sourceFileIn(dir)
	if err != nil {
		// The directory is here and the upload is not. This platform does not produce that state — the
		// row is written before the bytes and removed with them — so it is read as a deployment fault
		// and waits, rather than as a verdict about a text nobody can see.
		s.log().ErrorContext(ctx, "a book directory has no source file to configure", "err", err)
		return ErrNotProvisioned
	}
	b.SourceFile = source
	rendered, err := s.renderConfig(b)
	if err != nil {
		// The engine is never asked, so the book learns nothing about itself here. Loud, because a
		// broken template stops EVERY intake on this host and the only other symptom is books that
		// quietly stay in `parsing`.
		s.log().ErrorContext(ctx, "the book template cannot be rendered; no uploaded book on this host can be parsed until it is fixed",
			"template", s.Cfg.BookTemplate, "err", err)
		return ErrNotProvisioned
	}
	// O_EXCL: this platform writes the file once and never again. Two passes of the intake can reach
	// here together — the queue worker and the backstop sweep — and the loser's EEXIST is the state it
	// wanted.
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o640)
	if errors.Is(err, fs.ErrExist) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("books: create book configuration: %w", err)
	}
	defer f.Close()
	if _, err := f.Write(rendered); err != nil {
		return fmt.Errorf("books: write book configuration: %w", err)
	}
	if err := f.Close(); err != nil {
		return fmt.Errorf("books: write book configuration: %w", err)
	}
	// Counts and identities, never the book id: the same rule as everywhere else on this side of the
	// log (ENGINEERING_STANDARDS §Наблюдаемость).
	s.log().InfoContext(ctx, "a starting engine configuration was rendered for a new book", "bytes", len(rendered))
	return nil
}

// renderConfig reads the operator's template and returns it with this book's own fields set.
//
// The template is read at RENDER time and not at boot, and that is operational rather than
// stylistic: a template with a typo in it is discovered by the first upload, and an operator who
// fixes the file must not also have to restart the control plane for the books already waiting on it.
func (s *Service) renderConfig(b bookConfig) ([]byte, error) {
	raw, err := readTemplate(s.Cfg.BookTemplate)
	if err != nil {
		return nil, err
	}
	return renderBookConfig(raw, b)
}

func readTemplate(path string) ([]byte, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("%w: %w", ErrNoTemplate, err)
	}
	defer f.Close()
	raw, err := io.ReadAll(io.LimitReader(f, maxTemplate+1))
	if err != nil {
		return nil, fmt.Errorf("%w: %w", ErrNoTemplate, err)
	}
	if len(raw) > maxTemplate {
		return nil, fmt.Errorf("%w: it is over %d bytes, which no book configuration is", ErrNoTemplate, maxTemplate)
	}
	return raw, nil
}

// renderBookConfig sets this book's fields on the template's document and re-renders it.
//
// It works on the parsed NODE rather than on text, and that is what makes it safe to hand an
// operator's file: their comments and their key order survive, a key they wrote for a field the
// platform owns is REPLACED rather than duplicated (a duplicate key is a hard error to the engine's
// strict decoder), and a value that happens to look like a number or a boolean is written back as
// the string it is. Templating the file as text would get all three wrong, and the third silently:
// a book titled "123" or a language code the operator quoted would decode into the engine's string
// field as an int and fail the load.
func renderBookConfig(template []byte, b bookConfig) ([]byte, error) {
	dec := yaml.NewDecoder(bytes.NewReader(template))
	var doc yaml.Node
	if err := dec.Decode(&doc); err != nil {
		return nil, fmt.Errorf("%w: %w", ErrNoTemplate, err)
	}
	// A second document is REFUSED rather than dropped. The engine reads only the first, so a template
	// with two would load — and this render would silently emit half of what the operator wrote, which
	// is the kind of loss that is discovered by a book behaving strangely months later.
	if err := dec.Decode(new(yaml.Node)); !errors.Is(err, io.EOF) {
		return nil, fmt.Errorf("%w: it carries more than one YAML document", ErrNoTemplate)
	}
	if len(doc.Content) != 1 || doc.Content[0].Kind != yaml.MappingNode {
		return nil, fmt.Errorf("%w: it is not a mapping of configuration keys", ErrNoTemplate)
	}
	root := doc.Content[0]
	// The whole of what this platform claims about the engine's schema. Everything else in the
	// template is carried through unread — `genre` INCLUDED, since 0.3.0: the field left the contract
	// and the intake form (Б-23, owner 16.08), so the platform no longer knows a book's genre and the
	// operator's own value in the template is what stands. The engine keeps the key until its own
	// removal lands (unified backlog row 184).
	for _, kv := range [][2]string{
		{"book_id", b.ID},
		{"title", b.Title},
		{"source_lang", b.SourceLang},
		{"target_lang", b.TargetLang},
		{"source_file", b.SourceFile},
	} {
		setString(root, kv[0], kv[1])
	}
	out, err := yaml.Marshal(&doc)
	if err != nil {
		return nil, fmt.Errorf("books: render book configuration: %w", err)
	}
	return out, nil
}

// setString assigns a STRING value to a top-level key, replacing the value that was there.
//
// The !!str tag is load-bearing and not decoration: without it the encoder writes `title: 123` for a
// book called "123", and the engine's decoder — which is strict — then refuses to put an int in a
// string field. The tag makes the encoder quote whatever needs quoting and nothing that does not.
func setString(mapping *yaml.Node, key, value string) {
	v := &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: value}
	// A YAML mapping node is a flat list of alternating key and value nodes.
	for i := 0; i+1 < len(mapping.Content); i += 2 {
		if mapping.Content[i].Value == key {
			// The key node keeps its comments and its position; only what it points at changes.
			mapping.Content[i+1] = v
			return
		}
	}
	mapping.Content = append(mapping.Content,
		&yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: key}, v)
}

// sourceFileIn finds the name the intake wrote this book's upload under.
//
// The name is derived rather than stored because the intake is the only writer of this directory and
// it writes exactly one source: `SourceName` plus the extension the user's filename carried, which
// is kept because the engine dispatches its reader by it (extensionOf). Asking the directory keeps
// that one fact in one place instead of two that can disagree.
func sourceFileIn(dir string) (string, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return "", fmt.Errorf("books: read book directory: %w", err)
	}
	for _, e := range entries {
		name := e.Name()
		if e.IsDir() || name == runner.ConfigFile {
			continue
		}
		if base := name[:len(name)-len(filepath.Ext(name))]; base == SourceName {
			return name, nil
		}
	}
	return "", fmt.Errorf("books: no %s.* file in %s", SourceName, dir)
}